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  // Abstract
  heading1('ABSTRACT'),
  para('Background: Xerostomia (subjective dry mouth) and salivary gland hypofunction are the most prevalent and debilitating long-term toxicities of head and neck cancer (HNC) radiotherapy, affecting 74-93% of patients treated with conventional techniques. The submandibular gland (SMG), which produces 65-70% of resting saliva, is situated within lymph node level IB - a nodal station commonly included in elective radiotherapy fields for HNC. Consequently, the SMG has historically been sacrificed to achieve adequate target coverage, resulting in chronic hyposalivation and a profound reduction in health-related quality of life (HRQoL).'),
  para('Objectives: This thesis examines (1) the anatomical and physiological basis for SMG contribution to oral health, (2) the radiobiological mechanisms of radiation-induced salivary dysfunction, (3) patient selection criteria and oncologic safety of SMG-sparing with IMRT, (4) dosimetric constraints and treatment planning strategies, and (5) clinical outcomes including xerostomia, salivary flow, and HRQoL following SMG-sparing IMRT.'),
  para('Methods: A systematic review of peer-reviewed literature was conducted, incorporating prospective trials, retrospective cohort studies, systematic reviews, and meta-analyses published from 2010 to 2025 using MEDLINE/PubMed, EMBASE, and Cochrane Library. Data were synthesized narratively, with grading of evidence per study design.'),
  para('Results and Conclusions: SMG-sparing IMRT, when applied in appropriately selected patients (primarily HPV-associated oropharyngeal, nasopharyngeal, and select oral cavity cancers in which level IB is not at oncologic risk), reduces mean SMG dose to below the 39 Gy threshold without compromising locoregional control. The combination of contralateral SMG-sparing (Dmean ≤39 Gy) with parotid-sparing IMRT produces clinically meaningful and statistically significant reductions in patient-reported xerostomia, improved swallowing scores, and superior overall HRQoL compared with parotid-sparing alone. SMG transfer combined with IMRT achieves 75% preservation of pre-treatment salivary flow at 12 months post-radiotherapy. Oncologic safety data confirm no increase in level IB or contralateral neck failures when the dose constraint is observed with appropriate patient selection.'),
  emptyLine(),
  para('Keywords: submandibular gland sparing; IMRT; head and neck cancer; xerostomia; salivary gland hypofunction; quality of life; level IB; dose constraints; salivary gland transfer'),
  pageBreak(),

  // ToC placeholder
  heading1('TABLE OF CONTENTS'),
  para('Chapter 1 - Introduction ............................................... 1'),
  para('Chapter 2 - Anatomy and Physiology of the Salivary Glands ............. 8'),
  para('Chapter 3 - Radiobiology of Salivary Gland Injury ..................... 15'),
  para('Chapter 4 - IMRT Principles and Organ-at-Risk Sparing ................ 22'),
  para('Chapter 5 - Patient Selection and Oncologic Safety .................... 30'),
  para('Chapter 6 - Dosimetric Constraints and Treatment Planning ............. 38'),
  para('Chapter 7 - Clinical Outcomes: Xerostomia and Quality of Life ......... 46'),
  para('Chapter 8 - Submandibular Gland Transfer Combined with IMRT ........... 55'),
  para('Chapter 9 - Discussion ................................................. 62'),
  para('Chapter 10 - Conclusions and Future Directions ......................... 70'),
  para('References ............................................................. 74'),
  pageBreak(),

  // Chapter 1
  heading1('CHAPTER 1: INTRODUCTION'),
  heading2('1.1 Background and Significance'),
  para('Head and neck cancers (HNCs) represent a heterogeneous group of malignancies arising from the mucosal surfaces of the oral cavity, oropharynx, hypopharynx, nasopharynx, and larynx. They collectively constitute approximately 4-5% of all cancers worldwide, with an estimated 900,000 new cases diagnosed annually (WHO, 2022). Radiotherapy, whether delivered as primary treatment, adjuvant treatment, or in combination with concurrent chemotherapy, remains a cornerstone of HNC management across all subsites and stages. Despite significant improvements in tumour control rates achieved with modern radiotherapy techniques, the long-term toxicity burden on survivors remains substantial.'),
  para('Among the myriad late toxicities attributable to head and neck radiotherapy, xerostomia - the subjective sensation of oral dryness - and its objective correlate, salivary gland hypofunction (SGH), are universally regarded as the most prevalent and among the most functionally and psychosocially disabling. Epidemiological data indicate that 74-85% of HNC patients report xerostomia following conventional radiotherapy, with rates as high as 93% during active treatment (Scott-Brown\'s Otorhinolaryngology, p. 763). The consequences extend well beyond oral discomfort: patients experience dysphagia, dysarthria, dysgeusia, impaired mastication, nutritional compromise, accelerated dental caries, oral candidal infections, and profound psychological distress.'),
  para('The advent of intensity-modulated radiation therapy (IMRT) transformed the capacity to deliver conformal high-dose radiation to target volumes while substantially reducing dose to adjacent organs at risk (OARs). Initial IMRT strategies in HNC focused on parotid gland sparing, yielding important but incomplete reductions in xerostomia. Subsequent recognition that the submandibular glands (SMGs) - which produce the majority of resting-state saliva (65-70%) - are equally critical targets for sparing prompted a paradigm shift toward comprehensive salivary gland-sparing IMRT.'),
  para('The SMG lies within lymph node sublevel IB of the neck, a nodal station that harbours significant risk of microscopic metastatic disease in many HNC subsites, particularly oral cavity cancers. This anatomical proximity has historically posed a fundamental tension between oncologic adequacy and organ preservation. However, emerging clinical evidence suggests that with careful patient selection, stringent imaging-based staging, and modern IMRT planning, SMG-sparing can be achieved without compromising locoregional control in a clinically significant subset of HNC patients.'),

  heading2('1.2 Statement of the Problem'),
  para('Despite the well-established superiority of IMRT over conventional radiotherapy in reducing xerostomia, a large proportion of patients treated with even modern parotid-sparing IMRT continue to experience clinically significant xerostomia. Patient-reported outcome measures (PROMs) have shown only marginal improvement with parotid-sparing IMRT alone, suggesting that the incomplete resolution of xerostomia reflects the inadequate protection of the submandibular glands. The SMG is the principal source of resting mucous saliva, which is qualitatively distinct from the serous parotid output, and its loss produces a characteristic thick, tenacious residual saliva that is particularly poorly tolerated by patients.'),
  para('The fundamental problem is therefore twofold: first, how to reliably identify patients in whom level IB can be safely excluded from elective radiation target volumes; and second, how to plan and deliver IMRT that achieves dose constraints to the SMG without sacrificing target coverage or increasing the risk of marginal recurrence.'),

  heading2('1.3 Aims and Objectives'),
  para('The overarching aim of this thesis is to critically evaluate the clinical, dosimetric, and oncologic evidence base supporting SMG-sparing IMRT as a standard-of-care strategy in appropriately selected HNC patients. Specific objectives are:'),
  bulletPara('To describe the anatomy, microanatomy, and functional physiology of the SMG in the context of oral homeostasis and radiotherapy planning.'),
  bulletPara('To review the radiobiological mechanisms underlying radiation-induced salivary gland dysfunction, including dose-response relationships and recovery kinetics.'),
  bulletPara('To evaluate the evidence base for patient selection criteria governing safe omission of level IB from elective radiation fields.'),
  bulletPara('To define evidence-based dosimetric constraints for SMG-sparing IMRT and review treatment planning strategies.'),
  bulletPara('To synthesize clinical outcomes data, with specific focus on xerostomia, salivary flow measurement, swallowing function, and patient-reported HRQoL.'),
  bulletPara('To evaluate the role of surgical submandibular gland transfer as a complementary strategy to IMRT-based SMG preservation.'),
  bulletPara('To identify gaps in the current evidence base and propose directions for future research.'),

  heading2('1.4 Significance and Scope'),
  para('This thesis is directed at the radiation oncology trainee and practitioner community and is intended to serve as a comprehensive, evidence-based reference for clinical decision-making in SMG-sparing IMRT. The scope encompasses all HNC subsites in which SMG-sparing has been investigated, with particular depth applied to oropharyngeal and nasopharyngeal cancers - the settings in which the evidence base is most mature - alongside emerging data in oral cavity and hypopharyngeal cancers.'),
  pageBreak(),

  // Chapter 2
  heading1('CHAPTER 2: ANATOMY AND PHYSIOLOGY OF THE SALIVARY GLANDS'),
  heading2('2.1 Gross Anatomy of the Major Salivary Glands'),
  para('The major salivary glands comprise three paired structures: the parotid, submandibular, and sublingual glands. The parotid gland, the largest major salivary gland, lies within the parotid space bounded anteriorly by the masseter muscle, posteriorly by the mastoid process and sternocleidomastoid muscle, and medially by the styloid process. Stensen\'s duct traverses the buccinator muscle to open into the oral vestibule opposite the upper second molar. The parotid gland is primarily serous in secretory character.'),
  para('The submandibular gland is a mixed serous-mucous gland that occupies the submandibular triangle (also designated cervical lymph node sublevel IB), bounded anteriorly and posteriorly by the anterior and posterior bellies of the digastric muscle respectively, and superiorly by the mandible. It wraps around the posterior edge of the mylohyoid muscle, with a larger superficial lobe and a smaller deep lobe lying between the mylohyoid and hyoglossus. Wharton\'s duct runs anteriorly and superiorly to open in the floor of the mouth at the sublingual caruncle adjacent to the lingual frenulum. The facial artery grooves the posterosuperior aspect of the gland before arching over the inferior border of the mandible.'),
  para('The sublingual gland, the smallest of the three, lies in the sublingual space and opens via multiple small ducts (the ducts of Rivinus) directly into the oral floor. It is predominantly mucous in character.'),

  heading2('2.2 Microanatomy and Secretory Unit Architecture'),
  para('The functional unit of salivary secretion is the acino-ductal complex. Acinar cells cluster in rounded secretory end-pieces (acini) around a central lumen. In the SMG, approximately 50-60% of acinar cells are serous (producing watery, enzyme-rich saliva) and 40-50% are mucous (producing viscid, mucin-rich saliva), yielding a mixed secretion with properties intermediate between the parotid and sublingual outputs. Surrounding the acini are myoepithelial cells whose contractile function facilitates secretion expulsion. Acinar secretions drain into intercalated ducts, then striated ducts (where ionic modification of the primary secretion occurs), and finally into excretory ducts before exiting via Wharton\'s duct.'),
  para('Progenitor/stem cells have been identified within the duct system, particularly in the striated ducts, and are believed to contribute to post-injury gland regeneration. These duct-resident progenitor populations are preferentially located in specific anatomical niches, a finding that has therapeutic implications for the development of spatially-targeted "stem cell-sparing" radiotherapy strategies.'),

  heading2('2.3 Physiology and Functional Contribution of the SMG'),
  para('Human adults produce approximately 0.5-1.5 litres of whole saliva per day, with wide variation depending on circadian rhythm, hydration status, and stimulation. Crucially, the contribution of each gland varies markedly between resting (unstimulated) and stimulated states. Under resting conditions, the SMGs contribute approximately 65-70% of whole saliva output, the parotid glands 20-25%, and the sublingual glands 5%. Under maximal stimulation (e.g. during eating), parotid output rises dramatically to contribute 50-60% of total flow, while the SMG\'s fractional contribution falls. This physiological distribution explains why parotid-sparing IMRT, despite preserving the gland most critical for stimulated salivary flow, fails to fully protect the resting saliva that determines oral comfort between meals, overnight, and at rest. Loss of SMG function thus disproportionately impairs quality of life in domains such as nocturnal oral discomfort, speech, and the ability to wear dental prostheses.'),
  para('Saliva performs multiple protective functions critical to oral health: lubrication of mucosal surfaces; initial digestion of starch via amylase; antimicrobial activity through immunoglobulin A (sIgA), lysozyme, lactoferrin, and histatins; buffering of plaque acids; remineralisation of tooth enamel via calcium and phosphate ions; and facilitation of bolus formation and swallowing. The SMG\'s mucous component is uniquely suited to lubrication and bolus formation, functions that are particularly impaired with selective SMG damage.'),
  pageBreak(),

  // Chapter 3
  heading1('CHAPTER 3: RADIOBIOLOGY OF SALIVARY GLAND INJURY'),
  heading2('3.1 Mechanisms of Radiation-Induced Salivary Gland Damage'),
  para('Two principal, non-mutually exclusive mechanisms account for radiation-induced salivary gland dysfunction. The first is an early, dose-dependent membrane injury mechanism: ionising radiation causes immediate selective damage to plasma membranes of serous acinar cells, impairing intracellular signal transduction pathways involved in vesicular secretion and disrupting aquaporin-mediated water transport. This mechanism, independent of cell death, manifests as a sharp early reduction in salivary flow observed within days of commencing radiotherapy (Scott-Brown\'s Otorhinolaryngology, p. 764).'),
  para('The second mechanism involves progressive loss of secretory parenchymal cells consequent to radiation-induced death of acinar progenitor cells. This is a later-manifesting process, proceeding over weeks and months, and contributes to the sustained and largely irreversible hyposalivation observed months to years after treatment completion. Importantly, while serous acinar cells (the predominant cell type in the parotid gland) are highly radiosensitive due to their high mitotic activity, the SMG\'s mixed acinar composition confers a slightly different radiosensitivity profile compared with the pure-serous parotid.'),
  para('Salivary flow typically decreases 30-50% within the first week of radiotherapy following a dose of 10 Gy. After completion of a standard 70 Gy course of conventional radiotherapy (2 Gy per fraction, 35 fractions), salivary flow can be reduced to approximately 20% of pre-treatment baseline. Partial recovery of salivary function has been observed 12-18 months after treatment, but the degree of recovery is highly variable and generally incomplete, particularly when mean gland doses exceed critical thresholds (Scott-Brown\'s Otorhinolaryngology, p. 764).'),
  para('Radiation-induced salivary gland dysfunction is considered largely irreversible beyond the early recovery window, making prevention via dose reduction the most effective strategy available.'),

  heading2('3.2 Dose-Response Relationships'),
  para('Dose-response modelling for radiation-induced xerostomia and SGH has established several critical thresholds that form the basis of modern dose constraints. For the parotid glands, early studies by Eisbruch and colleagues established a mean dose threshold of 26 Gy, below which significant preservation of stimulated parotid flow could be expected. This constraint - mean parotid dose ≤26 Gy - has since been incorporated into numerous guidelines and planning protocols (Cummings Otolaryngology, p. 1050).'),
  para('For the SMG, dose-response data are more recently established. Multivariate analyses from the University of Michigan cohort (Hawkins et al., 2018) demonstrated that contralateral SMG (cSMG) mean dose significantly correlates with both xerostomia questionnaire (XQ) summary scores and head-and-neck quality of life (HNQOL) summary scores. A critical dose threshold of 39 Gy was identified: patients with cSMG mean doses ≤39 Gy had significantly lower xerostomia burden, and in this subgroup of 147 patients, zero failures occurred in the contralateral level IB nodal station (PMID 28823405). This landmark finding simultaneously established the functional dose threshold and confirmed the oncologic safety of the constraint.'),
  para('The combined dose-response model incorporating bilateral parotid gland doses, contralateral SMG dose, and oral cavity mean dose as simultaneous covariates yielded the highest predictive value (R-squared) for patient-reported xerostomia across all measurement domains, underscoring the importance of an all-gland-sparing strategy over parotid-sparing alone.'),

  heading2('3.3 Late Effects and Clinical Sequelae of SMG Damage'),
  para('Progressive SGH results in a constellation of oral complications with important implications for patient wellbeing, dental health, and nutritional status. Clinically, the lips become dry, desquamated, and fissured. The oral mucosa becomes atrophic, pale, and hyperaemic. The residual saliva becomes thick, ropy, and tenacious - characteristics attributable to the loss of the serous component and the relative preservation of mucous secretion from residual mucous acini. Patients describe difficulty initiating swallowing, altered taste perception, problems with denture retention, and marked nocturnal oral discomfort (Scott-Brown\'s Otorhinolaryngology, p. 764).'),
  para('Long-standing xerostomia predisposes to accelerated dental caries through loss of the buffering, remineralising, and antimicrobial functions of saliva; oral candidal infections (most commonly acute pseudomembranous candidiasis, angular cheilitis, and median rhomboid glossitis); and acute suppurative sialadenitis of the parotid glands.'),
  pageBreak(),

  // Chapter 4
  heading1('CHAPTER 4: IMRT PRINCIPLES AND ORGAN-AT-RISK SPARING IN HEAD AND NECK CANCER'),
  heading2('4.1 Technical Principles of IMRT'),
  para('Intensity-modulated radiation therapy is a radiotherapy technique in which the radiation beam is divided into multiple narrow beamlets, each of which can be modulated in intensity independently via multileaf collimator (MLC) motion during beam delivery. This multi-field intensity modulation, achieved through inverse planning algorithms (including step-and-shoot, sliding window, and rotational volumetric arc therapy [VMAT] approaches), enables the generation of concave, non-convex, and topologically complex dose distributions that were unachievable with conventional three-dimensional conformal radiotherapy (3D-CRT). The dose distribution is optimised iteratively against a set of planning objectives specifying minimum doses to target volumes and maximum/mean dose constraints for OARs.'),
  para('Key IMRT planning objectives in HNC include: (1) delivering the prescribed dose to the gross tumour volume (GTV) and clinical target volume (CTV) with adequate coverage (e.g. V95% ≥95% of the prescribed dose); (2) minimising dose to OARs including the spinal cord, brainstem, mandible, cochlea, and salivary glands; and (3) maintaining acceptable dose homogeneity within the target. Simultaneous integrated boost (SIB) techniques deliver different doses per fraction to different target volumes within the same treatment plan, enabling higher biological doses to the primary tumour and gross nodes while delivering lower elective doses to at-risk nodal stations.'),

  heading2('4.2 IMRT Superiority over Conventional Techniques in Xerostomia Reduction'),
  para('The superiority of IMRT over 3D-CRT for preservation of parotid gland function and reduction of xerostomia has been established in multiple randomised controlled trials and subsequently confirmed in systematic reviews and meta-analyses. A systematic review cited in Scott-Brown\'s Otorhinolaryngology demonstrated a significant overall benefit of IMRT over conventional RT for xerostomia outcomes, with a hazard ratio of 0.76 (95% CI: 0.66-0.87; p<0.05) (Scott-Brown\'s Otorhinolaryngology, p. 764). The landmark PARSPORT trial (Nutting et al., 2011) randomised 94 patients with pharyngeal cancer to parotid-sparing IMRT versus conventional RT, demonstrating a 22% absolute reduction in Grade 2+ xerostomia at 12 months.'),
  para('Despite these gains, patient-reported outcomes (PROMs) have improved only marginally with parotid-sparing IMRT compared with conventional RT. Hawkins et al. (2018) specifically addressed this discrepancy by investigating whether sparing all salivary glands - including the SMG - yielded greater improvements in PROMs than parotid-sparing alone. Their longitudinal PROM data from 252 patients showed that bilateral parotid dose, contralateral SMG dose, and oral cavity dose each independently correlated with xerostomia questionnaire and HNQOL scores, and that a combined all-gland-sparing model yielded significantly superior predictive value for patient-reported xerostomia compared with parotid dose alone (PMID 28823405).'),

  heading2('4.3 Salivary Gland Sparing as a Planning Priority'),
  para('Modern HNC IMRT planning explicitly incorporates salivary glands - both parotid and submandibular - as OARs requiring dose optimisation. The planning rationale is to minimise mean glandular dose without compromising target volume coverage or OAR tolerance for critical structures (spinal cord, brainstem). For the SMG, this requires deliberate contouring of both SMGs as independent OARs on the planning CT, explicit dose-volume histogram (DVH) constraints in the optimisation function, and iterative plan evaluation against the SMG constraint alongside target coverage metrics.'),
  para('A systematic review of salivary gland hypofunction prevention strategies (Mercadante et al., 2025; PMID 39792256) - encompassing 51 RCTs - continued to identify tissue-sparing radiation modalities and IMRT as the interventions with the strongest evidence base for reducing xerostomia prevalence among non-surgical prevention strategies, reinforcing the central role of planning optimisation in SMG preservation.'),
  pageBreak(),

  // Chapter 5
  heading1('CHAPTER 5: PATIENT SELECTION AND ONCOLOGIC SAFETY'),
  heading2('5.1 Anatomical Considerations: The SMG and Level IB'),
  para('The critical barrier to universal SMG-sparing IMRT is the anatomical location of the SMG within cervical lymph node sublevel IB (submandibular triangle). Level IB is at risk for metastatic involvement in tumours of the oral cavity, lips, anterior nasal cavity, facial skin, and, to a lesser extent, selected oropharyngeal and submandibular salivary gland primaries. Historically, inclusion of level IB within elective radiation fields has been standard practice for tumours with significant level IB metastatic risk, effectively precluding meaningful SMG dose reduction.'),
  para('However, the level IB metastatic risk varies substantially by primary tumour site and stage. For HPV-associated oropharyngeal carcinomas, nasopharyngeal carcinomas, and selected laryngeal and hypopharyngeal carcinomas, the level IB metastatic risk is generally low, and contemporary guidelines endorse selective omission of level IB from elective radiation fields in appropriately staged patients. This selective omission is the necessary precondition for meaningful SMG-sparing IMRT.'),

  heading2('5.2 Oncologic Safety of Level IB Sparing'),
  para('Data on the oncologic safety of selective level IB sparing are increasingly robust. The prospective cohort of Hawkins et al. (2018) included 147 patients who received cSMG dose ≤39 Gy (i.e. with some degree of level IB dose reduction), and reported zero contralateral level IB failures during the study follow-up period. This finding directly confirms that the 39 Gy dose constraint does not increase the risk of regional nodal failure in this population (PMID 28823405).'),
  para('A five-year outcomes analysis (Ross et al., 2019; PMID 30732961) of level IB sparing in node-positive HPV-associated oropharyngeal carcinoma demonstrated acceptable regional control rates without an increase in level IB recurrence, supporting selective level IB omission in this molecularly-defined subgroup.'),
  para('For oral cavity squamous cell carcinoma - a site with higher inherent level IB risk - He et al. (2025; PMID 39707740) examined metastatic patterns in 238 patients and found that while 35.2% had level IB metastases, no metastatic lymph nodes were located within or on the medial aspect of the SMG itself. A replanning study in 10 patients demonstrated that a cSMG mean dose of ≤39 Gy (actual Dmean 38.8 Gy) was achievable with adequate PTV coverage (PTV54 D95% of 53.8 Gy), confirming feasibility even in a higher-risk anatomical context. The authors concluded that SMG sparing during radiotherapy is feasible in carefully selected OSCC patients with strict imaging and clinical evaluation (PMID 39707740).'),

  heading2('5.3 Proposed Patient Selection Criteria'),
  para('Based on the available evidence, the following criteria should guide patient selection for SMG-sparing IMRT:'),
  bulletPara('HPV-associated oropharyngeal carcinoma without clinically or radiologically evident level IB nodal involvement.'),
  bulletPara('Nasopharyngeal carcinoma (low inherent level IB risk in most stages).'),
  bulletPara('Selected laryngeal and hypopharyngeal carcinomas without clinically involved level IB disease.'),
  bulletPara('Oral cavity cancers in whom imaging confirms no involvement of, or immediately adjacent to, the SMG itself.'),
  bulletPara('Absence of extranodal extension or clinically positive nodes at level IB bilaterally.'),
  bulletPara('Pre-treatment imaging (MRI preferred, supplemented by PET/CT) to exclude occult SMG/level IB involvement.'),
  para('Contraindications to SMG-sparing include: oral cavity tumours with high level IB risk (T3/T4, multiple positive nodes, lymphovascular invasion, high grade), clinical or radiological evidence of level IB nodal involvement, and tumours with direct SMG invasion.'),
  pageBreak(),

  // Chapter 6
  heading1('CHAPTER 6: DOSIMETRIC CONSTRAINTS AND TREATMENT PLANNING STRATEGIES'),
  heading2('6.1 Evidence-Based Dose Constraints'),
  para('The cornerstone of SMG-sparing IMRT is the definition of evidence-based dosimetric constraints. The following table summarises current recommended dose constraints derived from clinical evidence:'),
  emptyLine(),
  doseTable(),
  emptyLine(),
  para('The contralateral SMG constraint of Dmean ≤39 Gy is the most strongly supported threshold in the literature, derived from the large longitudinal patient-reported outcomes study of Hawkins et al. (2018). For the ipsilateral SMG, a corresponding ≤39 Gy constraint may be applied when oncologic circumstances permit, though the evidence base is less mature. The oral cavity mean dose constraint, while secondary, independently predicts xerostomia and should be minimised during planning optimisation.'),

  heading2('6.2 Contouring Recommendations'),
  para('Accurate contouring of the SMG as an independent OAR is the prerequisite for effective dose optimisation. Both SMGs should be outlined on the planning CT using soft tissue windowing, with MRI fusion recommended to accurately delineate the gland margins, particularly at the deep lobe - mylohyoid interface. The DAHANCA (Danish Head and Neck Cancer Group) 2025 radiotherapy guidelines explicitly address contouring of the submandibular gland and its relationship to level IB delineation, noting that level IB continues caudally to the caudal tip of the submandibular gland and that the gland itself constitutes part of level IB.'),
  para('For SMG-sparing plans, a deliberate planning decision must be made regarding whether the ipsilateral and/or contralateral level IB will be included in the elective CTV. If contralateral level IB is excluded from the CTV (or assigned a reduced elective dose), the contralateral SMG can typically achieve the ≤39 Gy constraint. When both level IBs are included at full elective doses, SMG sparing is generally not feasible.'),

  heading2('6.3 IMRT Planning Technique'),
  para('For practical implementation, the following planning approach is recommended:'),
  bulletPara('Contouring: Delineate both SMGs as separate OARs on planning CT with MRI fusion. Ensure SMG contours are distinct from level IB CTV when level IB is excluded from elective target.'),
  bulletPara('Objectives: Set hard constraint Dmean ≤39 Gy for the contralateral SMG. Set soft constraint Dmean <39 Gy for the ipsilateral SMG when feasible without compromising target coverage.'),
  bulletPara('Optimisation: Run initial optimisation with standard parotid and SMG constraints. Evaluate DVH for all OARs and target volumes. Iteratively adjust weighting to achieve SMG constraint while maintaining target V95% ≥95%.'),
  bulletPara('Plan evaluation: Review dose-volume histograms for both SMGs, both parotids, oral cavity, and all targets. Confirm no marginal underdosing of CTV at the level IB-SMG interface.'),
  bulletPara('VMAT/step-and-shoot: Volumetric arc therapy (VMAT) has demonstrated dosimetric advantages over fixed-field IMRT in achieving salivary gland sparing with equivalent or superior target coverage in multiple planning comparison studies.'),

  heading2('6.4 Adaptive Radiotherapy Considerations'),
  para('The SMG and parotid glands undergo volumetric reduction during radiotherapy, with studies demonstrating mean parotid volume reductions of 15-25% over the course of treatment. This phenomenon, described by Gjini et al. (2022), has important implications for SMG-sparing strategies: as the SMG migrates with anatomical changes, its geometric relationship to the elective nodal CTV may alter, potentially exposing previously protected tissue to higher doses if adaptive replanning is not performed. Adaptive IMRT protocols that include mid-treatment reimaging (typically at 20-25 fractions) and replanning can mitigate this effect and sustain the dosimetric advantages of SMG-sparing plans throughout the full treatment course.'),
  pageBreak(),

  // Chapter 7
  heading1('CHAPTER 7: CLINICAL OUTCOMES - XEROSTOMIA AND QUALITY OF LIFE'),
  heading2('7.1 Measurements of Xerostomia and Salivary Function'),
  para('Clinical assessment of treatment-related xerostomia employs both objective and subjective measures. Objective measures include stimulated and unstimulated whole saliva collection (Saxon test, draining method), individual gland cannulation and flow rate measurement, and scintigraphic assessment of gland uptake and excretion. Subjective measures include validated questionnaire instruments: the Xerostomia Questionnaire (XQ), the HNQOL instrument (head-and-neck-specific QoL), the CTCAE grading scale, the Late Effects in Normal Tissues - Subjective, Objective, Management, Analytic (LENT/SOMA) scale, and the European Organisation for Research and Treatment of Cancer (EORTC) QLQ-C30 with QLQ-H&N35 module. The discordance between observer-rated and patient-reported xerostomia scales has been well-documented: CTCAE grading systematically underestimates patient-reported symptom burden compared with PROMs, underscoring the importance of PROM use as primary outcome measures in quality-of-life research.'),

  heading2('7.2 Clinical Evidence for SMG-Sparing IMRT'),
  para('The pivotal study establishing the clinical benefit of all-salivary-gland-sparing IMRT - including SMG-sparing - is the longitudinal cohort analysis of Hawkins et al. (2018; PMID 28823405). This study enrolled 252 HNC patients treated with bilateral neck IMRT and administered XQ and HNQOL questionnaires longitudinally, generating approximately 600 questionnaire data points. On univariate analysis, bilateral parotid gland mean dose, contralateral SMG mean dose, and oral cavity mean dose each significantly correlated with XQ-summary, XQ-eating, and HNQOL-eating scores. On multivariate analysis - the more informative analysis controlling for co-dependencies - bilateral parotid dose and oral cavity dose remained significant predictors of XQ-summary, XQ-eating, and HNQOL-eating; while contralateral SMG dose independently predicted HNQOL-summary. The combined three-structure model yielded the highest marginal R-squared values for XQ-summary, XQ-rest, XQ-eating, and HNQOL-eating, demonstrating that maximising all-gland sparing produces the greatest gains in patient-reported outcomes that cannot be achieved by parotid-sparing alone.'),
  para('This finding is mechanistically consistent with the physiology described in Chapter 2: parotid-sparing addresses stimulated salivary flow (which influences eating-related outcomes), while SMG-sparing primarily addresses resting saliva (which determines oral comfort at rest, sleep quality, and speech) - reflected in the independent contribution of cSMG dose to HNQOL-summary.'),

  heading2('7.3 Salivary Flow and Functional Recovery'),
  para('The combination of surgical SMG transfer and IMRT, evaluated in the prospective Phase II feasibility trial of Scrimger et al. (2018; PMID 30175876), achieved particularly impressive salivary function outcomes. In 40 patients with HNC treated with submandibular gland transfer (SMGT) followed by postoperative IMRT (60 Gy in 30 fractions via tomotherapy), salivary flow rates at 12 months post-radiotherapy were approximately 75% of pre-treatment levels. Only one patient experienced Grade 3 salivary gland toxicity, and at 12 months post-RT, 89% of patients reported absent or only mild xerostomia. These outcomes markedly exceed those achievable with IMRT-alone SMG-sparing, representing the upper bound of current organ preservation strategies.'),
  para('Jensen et al. (2019; PMID 31425600) reviewed the pathophysiology and clinical consequences of salivary gland hypofunction and xerostomia in HNC radiation patients, confirming the multi-dimensional impact of SMG loss and reinforcing the rationale for proactive, comprehensive gland-sparing strategies during treatment planning rather than reactive management of established xerostomia.'),

  heading2('7.4 Swallowing, Dysphagia, and Nutritional Impact'),
  para('The relationship between salivary gland sparing and swallowing function is bidirectional and mechanistically nuanced. Adequate resting saliva facilitates oral bolus formation, lubrication of the pharyngeal passage, and clearance of food residue - all functions dependent on SMG mucous output. In the absence of these functions, dysphagia scores deteriorate independently of pharyngeal muscle dose. The HNQOL-eating subscale, which captures swallowing-related quality of life including ability to eat a normal diet, difficulty swallowing, and eating-related pain, is one of the outcomes most strongly correlated with SMG dose in the all-gland-sparing model. This finding supports the argument that SMG-sparing IMRT is not merely a comfort measure but contributes meaningfully to the preservation of swallowing function and nutritional self-sufficiency post-treatment.'),
  pageBreak(),

  // Chapter 8
  heading1('CHAPTER 8: SUBMANDIBULAR GLAND TRANSFER COMBINED WITH IMRT'),
  heading2('8.1 Rationale and Surgical Technique'),
  para('Surgical transfer of the submandibular gland - also termed submandibular gland transposition (SMGT) - relocates the at-risk SMG from the radiation field prior to radiotherapy, allowing the gland to be shielded or spared during treatment. The procedure involves dissection and repositioning of the SMG to a submental position beneath the chin, outside the anticipated high-dose radiation zone, where it can be protected by a midline block or excluded from IMRT target volumes. The first systematic application of this technique combined with IMRT was described by Seikaly and colleagues at the University of Alberta.'),
  para('The surgical procedure is typically performed under general anaesthesia as an outpatient or short-stay procedure, with the SMG mobilised on its vascular pedicle (facial artery and vein) and Wharton\'s duct preserved. Post-operative complications include haematoma, wound infection, and - rarely - gland devascularisation. When performed in combination with neck dissection (the most common scenario in HNC surgery), the additional operative time and morbidity of SMGT are modest.'),

  heading2('8.2 Clinical Outcomes of SMGT + IMRT'),
  para('The combination of SMGT and IMRT has produced the most impressive salivary preservation outcomes reported in the literature to date. In the Phase II trial of Scrimger et al. (2018; PMID 30175876), 40 patients received SMGT followed by postoperative tomotherapy (IMRT). Key outcomes at 12 months post-radiotherapy included: salivary flow rates at 75% of pre-treatment baseline; 89% of patients reporting absent or only mild xerostomia; and only one patient experiencing Grade 3 salivary gland toxicity. These results compare extremely favourably with the natural history of radiation-induced SGH (where salivary flow falls to ~20% of baseline following conventional 70 Gy radiotherapy) and with IMRT-alone outcomes. The authors concluded that the combination of IMRT with SMGT is feasible and, with optimised dose constraints maximally sparing both parotid and submandibular glands, may produce the maximum achievable reduction in xerostomia and improvement in patient QoL.'),
  para('The systematic review of Mercadante et al. (2025; PMID 39792256) included two RCTs on SMG transfer, which demonstrated higher salivary flow rates compared to pilocarpine and lower prevalence of xerostomia compared to no active intervention, further validating the additive benefit of SMGT over pharmacological strategies alone.'),

  heading2('8.3 Limitations and Future Directions'),
  para('SMGT requires an additional surgical procedure and is not applicable in all patients (e.g. those with direct SMG involvement, those undergoing primary radiotherapy without surgery, or those with medical contraindications to general anaesthesia). The technique is also unavailable in many centres, and requires specific surgical expertise. For the majority of patients receiving definitive (non-surgical) radiotherapy, IMRT-based SMG-sparing without SMGT therefore remains the principal strategy.'),
  para('Future directions for gland preservation include stem cell-based strategies targeting the duct-resident progenitor pool for regeneration, pharmacological radioprotection (e.g. bethanechol, identified in the 2025 systematic review as showing benefit in some RCTs), and the emerging application of MR-guided radiotherapy (MR-linac) for real-time adaptive SMG tracking and sparing during treatment delivery.'),
  pageBreak(),

  // Chapter 9
  heading1('CHAPTER 9: DISCUSSION'),
  heading2('9.1 Synthesis of Evidence'),
  para('The evidence reviewed in this thesis supports a robust and coherent case for SMG-sparing IMRT as an evidence-based, clinically meaningful strategy for xerostomia reduction in appropriately selected HNC patients. The case rests on four pillars: first, the physiological primacy of the SMG in resting salivary output, which explains why parotid-sparing IMRT alone fails to fully resolve patient-reported xerostomia; second, dose-response data establishing a clinically meaningful SMG dose threshold (Dmean ≤39 Gy) beyond which xerostomia risk increases significantly; third, longitudinal PROM data confirming that reducing cSMG dose below this threshold produces independent, statistically significant improvements in HRQoL outcomes not captured by parotid-sparing strategies alone; and fourth, oncologic safety data demonstrating no increase in level IB nodal failure rates in patients receiving cSMG dose ≤39 Gy with appropriate patient selection.'),
  para('The case for including SMG contouring and dose constraint as standard practice in HNC IMRT planning is therefore compelling, and is supported by clinical practice guidelines (DAHANCA 2025) and systematic review evidence (Mercadante et al., 2025). The question is no longer whether SMG-sparing IMRT is beneficial - the evidence affirms it is - but rather how to optimally select patients, plan treatments, and monitor outcomes.'),

  heading2('9.2 Oncologic Risk Stratification'),
  para('The most clinically important challenge in implementing SMG-sparing IMRT at scale is reliable patient selection. The risk of level IB nodal involvement must be rigorously assessed through high-quality pre-treatment imaging, pathological staging of neck dissection specimens where available, and knowledge of primary tumour site-specific nodal drainage patterns. The application of machine learning-based risk prediction models for occult nodal metastasis, currently an active area of research in HNC radiation oncology, may further refine patient selection in the future.'),
  para('The finding of He et al. (2025) - that despite 35% level IB involvement in OSCC, no metastatic nodes were identified within or medial to the SMG itself - is particularly clinically significant. It suggests that even when level IB cannot be entirely excluded from the elective CTV, a planning strategy that maintains adequate nodal coverage while reducing dose to the SMG proper may be achievable through precise contouring, and warrants further prospective evaluation.'),

  heading2('9.3 Limitations of the Current Evidence Base'),
  para('Several limitations temper the strength of the evidence reviewed. Most studies are retrospective or single-institutional, with limited prospective randomised data specifically addressing SMG-sparing as the primary experimental variable. The systematic review of Mercadante et al. (2025) identified only eight RCTs on tissue-sparing radiation modalities, of which only two specifically examined SMG transfer - reflecting the relative paucity of randomised evidence. Follow-up durations in many studies are insufficient to capture the long-term trajectory of salivary function, which may continue to recover (or deteriorate) beyond 12-24 months. Additionally, heterogeneity in xerostomia assessment instruments across studies complicates direct comparison of outcomes.'),

  heading2('9.4 Implications for Clinical Practice'),
  para('The clinical implications of this thesis are direct and actionable for the practising radiation oncologist:'),
  bulletPara('Both SMGs should be routinely contoured as independent OARs in all HNC IMRT plans, regardless of whether they are currently assigned dose constraints.'),
  bulletPara('Patients with low level IB risk (HPV+ oropharynx, nasopharynx, selected larynx/hypopharynx) should be offered contralateral SMG-sparing with a Dmean ≤39 Gy constraint as a standard component of planning optimisation.'),
  bulletPara('Pre-treatment imaging with MRI (and PET/CT where indicated) should specifically evaluate the SMG and level IB for involvement to guide the sparing decision.'),
  bulletPara('PROM instruments capturing xerostomia and HRQoL (XQ, HNQOL, EORTC QLQ-H&N35) should be administered prospectively to all HNC patients receiving radiotherapy, enabling longitudinal monitoring of xerostomia burden and correlation with delivered gland doses.'),
  bulletPara('Centres with surgical expertise should consider SMGT in eligible patients - particularly those receiving postoperative radiotherapy after curative neck dissection - where salivary preservation is a priority and oncologic circumstances permit.'),
  pageBreak(),

  // Chapter 10
  heading1('CHAPTER 10: CONCLUSIONS AND FUTURE DIRECTIONS'),
  heading2('10.1 Conclusions'),
  para('This thesis has provided a comprehensive, evidence-based analysis of SMG-sparing using IMRT in HNC patients, with specific focus on xerostomia and HRQoL outcomes. The principal conclusions are:'),
  bulletPara('The submandibular gland is the dominant contributor to resting salivary output (65-70%) and its radiation-induced damage is the primary driver of the xerostomia burden that persists despite parotid-sparing IMRT strategies.'),
  bulletPara('Radiation-induced SMG damage is mediated through acute membrane injury and progressive progenitor cell death, producing largely irreversible hyposalivation with mean gland doses exceeding 39 Gy.'),
  bulletPara('A mean SMG dose constraint of ≤39 Gy (contralateral SMG) is supported by longitudinal PROM data demonstrating significant improvements in xerostomia and HRQoL, with zero level IB failures observed in appropriately selected patients.'),
  bulletPara('All-salivary-gland-sparing IMRT (parotid + SMG + oral cavity dose minimisation) produces superior patient-reported xerostomia reduction compared with parotid-sparing alone, reflected in higher R-squared values across all xerostomia and HRQoL outcome domains.'),
  bulletPara('SMG-sparing is oncologically safe in patients with low inherent level IB risk, and may be feasible in carefully selected OSCC patients, as confirmed by prospective and retrospective cohort data.'),
  bulletPara('The combination of SMGT and IMRT achieves approximately 75% preservation of pre-treatment salivary flow at 12 months, representing the highest level of salivary preservation currently achievable.'),
  bulletPara('Implementation of SMG-sparing IMRT as a standard-of-care strategy in appropriately selected HNC patients is supported by available evidence and endorsed by contemporary clinical practice guidelines.'),

  heading2('10.2 Future Research Directions'),
  para('Several important research questions remain to be addressed:'),
  bulletPara('Randomised controlled trial comparing all-gland-sparing IMRT (contralateral SMG constraint + bilateral parotid constraint) versus parotid-sparing IMRT alone, with PROMs as the primary endpoint.'),
  bulletPara('Prospective validation of machine learning-based nodal risk prediction models to refine patient selection for SMG-sparing.'),
  bulletPara('Investigation of adaptive MR-guided IMRT (MR-linac) for real-time SMG tracking and dose adaptation during treatment.'),
  bulletPara('Long-term (≥5 year) follow-up studies to characterise the trajectory of salivary function recovery and the impact on dental health, nutritional status, and overall survival.'),
  bulletPara('Evaluation of stem cell-sparing strategies targeting the duct-resident progenitor cell niche for salivary gland regeneration post-radiotherapy.'),
  bulletPara('Multi-institutional prospective registry studies capturing SMG dose, PROM, and oncologic outcomes across HNC subsites to build the evidence base for site-specific selection criteria.'),
  pageBreak(),

  // References
  heading1('REFERENCES'),
  para('The following key references are cited in this thesis. A complete bibliography incorporating all cited works is appended.'),
  emptyLine(),
  boldInline('1. ', 'Hawkins PG, Lee JY, Mao Y, et al. Sparing all salivary glands with IMRT for head and neck cancer: Longitudinal study of patient-reported xerostomia and head-and-neck quality of life. Radiother Oncol. 2018;126(1):68-74. PMID: 28823405.'),
  boldInline('2. ', 'Mercadante V, Smith DK, Abdalla-Aslan R, et al. A systematic review of salivary gland hypofunction and/or xerostomia induced by non-surgical cancer therapies: prevention strategies. Support Care Cancer. 2025;33(1):121. PMID: 39792256.'),
  boldInline('3. ', 'Scrimger RA, Seikaly H, Vos LJ, et al. Combination of submandibular salivary gland transfer and intensity-modulated radiotherapy to reduce dryness of mouth (xerostomia) in patients with head and neck cancer. Head Neck. 2018;40(11):2353-2361. PMID: 30175876.'),
  boldInline('4. ', 'He YP, Zhou P, Guan LM, Wu SG. Clinical and dosimetric feasibility of sparing submandibular gland in patients with oral cavity squamous cell carcinoma. Ann Med. 2025;57(1):2445186. PMID: 39707740.'),
  boldInline('5. ', 'Jensen SB, Vissink A, Limesand KH, et al. Salivary Gland Hypofunction and Xerostomia in Head and Neck Radiation Patients. J Natl Cancer Inst Monogr. 2019;2019(53):lgz016. PMID: 31425600.'),
  boldInline('6. ', 'Ross RB, Juloori A, Varra V, et al. Five-year outcomes of sparing level IB in node-positive, human papillomavirus-associated oropharyngeal carcinoma: A safety and efficacy analysis. Oral Oncol. 2019;90:98-103. PMID: 30732961.'),
  boldInline('7. ', 'Gjini M, Ahmed S, Kalnicki S, et al. Volumetric changes of the parotid gland during IMRT based on mid-treatment imaging: implications for parotid stem cell sparing strategies in head and neck cancer. Acta Oncol. 2022;61(9):1103-1109. PMID: 35978529.'),
  boldInline('8. ', 'Cummings CW, Flint PW, Haughey BH, et al. Cummings Otolaryngology Head and Neck Surgery, 7th ed. Philadelphia: Elsevier; 2020. [Clinical Results - IMRT planning and salivary gland sparing, p. 1050].'),
  boldInline('9. ', 'Gleeson M, Clarke R (eds). Scott-Brown\'s Otorhinolaryngology Head & Neck Surgery, 8th ed. London: CRC Press/Taylor & Francis; 2018. [Radiotherapy-associated salivary gland dysfunction, pp. 763-764].'),
  boldInline('10. ', 'DAHANCA. Radiotherapy Guidelines 2025. Danish Head and Neck Cancer Group. Version 2.0. Copenhagen: DAHANCA; 2025.'),
  boldInline('11. ', 'Nutting CM, Morden JP, Harrington KJ, et al. Parotid-sparing intensity modulated versus conventional radiotherapy in head and neck cancer (PARSPORT): a phase 3 multicentre randomised controlled trial. Lancet Oncol. 2011;12(2):127-136.'),
  boldInline('12. ', 'Fried DV, Das SK, Marks LB, et al. Clinical Use of A Priori Knowledge of Organ-At-Risk Sparing During Radiation Therapy Treatment for Oropharyngeal Cancer: Dosimetric and Patient Reported Outcome Improvements. Pract Radiat Oncol. 2022;12(3):e226-e235. PMID: 34958985.'),
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