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Sinonasal Undifferentiated Carcinoma (SNUC)

1. Introduction and Historical Background

Sinonasal undifferentiated carcinoma (SNUC) was first described as a distinct pathological entity by Frierson et al. in 1986, who recognized it as an aggressive neoplasm derived from Schneiderian (sinonasal) epithelium, distinct from olfactory neuroblastoma (esthesioneuroblastoma). It was subsequently characterized by Levine et al. in 1987 as "a distinctive and highly aggressive neoplasm." SNUC is rare, constituting a small fraction of all sinonasal malignancies, and retrospective clinical series often span decades given its rarity.

In recent years, advances in molecular pathology have identified several distinct subtypes previously grouped under SNUC, including:

IDH2-mutant SNUC
INI-1-deficient sinonasal carcinoma
NUT-midline carcinoma (NMC)

This underscores the importance of current molecular classification, as older clinical outcome data may describe mixed histological entities.

Cummings Otolaryngology Head and Neck Surgery, p. 2275

2. Epidemiology

SNUC is an extremely rare tumor with no established gender or ethnic predilection.
Primarily affects middle-aged to older adults.
Due to rarity, most published data come from single-institution retrospective case series or pooled analyses.
Approximately 80% of patients present with T4 disease at diagnosis - emphasizing its characteristically late presentation.
Cummings Otolaryngology, citing Morand et al. meta-analysis of 390 patients

3. Etiopathogenesis and Molecular Pathology

SNUC arises from the Schneiderian (pseudostratified columnar) epithelium lining the sinonasal cavity. The exact carcinogenic trigger remains unknown; no consistent association with EBV (unlike NPC), HPV, or occupational exposure has been firmly established.

Key molecular findings:

IDH2 mutations (R172 hotspot): An emerging subtype with distinct morphology and potentially better prognosis; these IDH2-mutant tumors may respond to IDH inhibitors.
NUT rearrangements: NUT-midline carcinomas involving the NUT gene on chromosome 15q14 can mimic SNUC.
INI-1 (SMARCB1) loss: INI-1-deficient sinonasal carcinoma is another distinct entity.
SMARCA4-deficient tumors are also recognized.
Importantly, SNUC is EBV-negative, which helps distinguish it from undifferentiated nasopharyngeal carcinoma (WHO Type 3).

Biomarker research highlights potential roles for PD-L1 expression and tumor mutational burden as targets for immunotherapy.

Cummings Otolaryngology, p. 2275; Scott-Brown's Otorhinolaryngology HNS; Turri-Zanoni et al., Curr Oncol Rep 2022 [PMID: 35059992]

4. Pathology

Gross Features

Bulky, often necrotic tumor arising most commonly from the ethmoid sinuses and nasal cavity
Frequently invades adjacent structures at presentation
Areas of hemorrhage and necrosis are characteristic

Histology

Sheets, trabeculae, nests, or ribbons of undifferentiated cells with large nuclei, prominent nucleoli, and frequent mitoses
High nuclear-to-cytoplasmic ratio
Extensive tumor necrosis and angioinvasion are common
No squamous, glandular, or neuroendocrine differentiation on light microscopy

Immunohistochemistry (IHC)

SNUC is a diagnosis of exclusion requiring IHC to rule out other high-grade sinonasal tumors:

Marker	SNUC	Notes
Cytokeratin (AE1/AE3, CAM5.2)	Positive	Key positive marker
EMA	Variable positive
Synaptophysin / Chromogranin	Negative	Excludes neuroendocrine carcinoma
S100 / CD56	Negative	Excludes ONB
EBV (EBER)	Negative	Distinguishes from undifferentiated NPC
p63 / p40	Negative	Excludes squamous cell carcinoma
HMB-45, Melan-A	Negative	Excludes melanoma

Scott-Brown's, p. 6861-6868; Cummings, p. 2275

5. Clinical Features

Symptoms

SNUC characteristically presents late with symptoms that are initially non-specific:

Unilateral nasal obstruction
Epistaxis
Facial pain or pressure
Proptosis (orbital invasion)
Diplopia
Cranial nerve deficits (perineural or intracranial spread)
Headache

A hallmark is the paradox of extensive disease with relatively few early symptoms - Scott-Brown's notes the tumor "often produces few symptoms despite its extensive nature," leading to diagnosis at an advanced stage.

Sites of Origin

Most commonly arises from the ethmoid sinuses, followed by the nasal cavity, maxillary sinus, and sphenoid sinus.

Scott-Brown's Otorhinolaryngology, p. 6861-6868

6. Staging

SNUC is staged using the Kadish staging system (originally developed for olfactory neuroblastoma) or the AJCC TNM system for nasal cavity and paranasal sinuses. A dedicated SNUC staging system proposed by Reiersen and colleagues is:

Stage	Description
T1	Tumor involving nasal cavity or paranasal sinuses (excluding sphenoid), sparing superior ethmoidal cells
T2	Tumor involving nasal cavity or paranasal sinuses (including sphenoid) with extension to or erosion of cribriform plate
T3	Tumor extending into orbit or protruding into anterior cranial fossa without dural invasion
T4	Tumor involving brain
N0	No cervical lymph node metastases
N1	Any cervical lymph node metastases
M0	No distant metastases
M1	Distant metastases

The majority of patients (approximately 80%) present at T4 stage.

Scott-Brown's Otorhinolaryngology, p. 6856 (staging table)

7. Investigations

Imaging

MRI (brain and face with gadolinium): Investigation of choice - best delineates soft tissue extent, intracranial/orbital invasion, dural involvement, and perineural spread
CT of sinuses: Complements MRI for bone erosion assessment; useful for surgical planning
CT chest/abdomen/pelvis or PET-CT: Staging for distant metastases, which are common at presentation

Pathological Diagnosis

Endoscopic biopsy under general or local anesthesia
IHC panel as described above - essential to exclude other entities
Molecular testing: IDH2, SMARCB1, SMARCA4, NUT gene rearrangement should be sent in all cases given therapeutic implications

8. Differential Diagnosis

SNUC must be distinguished from other high-grade sinonasal malignancies:

Olfactory neuroblastoma (ONB/Esthesioneuroblastoma): Neuroendocrine markers (synaptophysin, chromogranin) positive; S100 positive peripheral sustentacular cells; Hyams grading
Sinonasal neuroendocrine carcinoma: Positive neuroendocrine markers; distinct histology
Undifferentiated nasopharyngeal carcinoma (NPC WHO Type 3): EBV-positive (EBER ISH); lymphocytic stroma
NUT-midline carcinoma: NUT immunostaining positive; highly aggressive
High-grade sinonasal adenocarcinoma: Glandular differentiation
Mucosal melanoma: HMB-45, Melan-A, S100 positive
Lymphoma (T/NK cell, DLBCL): CD45, CD3, CD20 positive; cytokeratin negative

Scott-Brown's, p. 6861; Cummings, p. 2275

9. Management

SNUC is managed with multimodality therapy given its aggressive nature. No prospective randomized trials exist due to rarity; evidence is from retrospective series and meta-analyses.

a. Induction (Neoadjuvant) Chemotherapy

Platinum-based regimens (cisplatin + 5-fluorouracil, or cisplatin/etoposide) are standard
Allows assessment of chemosensitivity, potential tumor downstaging, and early treatment of micrometastases
A 2024 meta-analysis by Burggraf et al. (Head & Neck, PMID: 39162231) of 192 patients found 5-year OS of 72.6% with induction chemotherapy vs 44.5% without - a significant difference. The authors conclude IC should be considered in every SNUC patient.
KJ Lee's recommends: neoadjuvant chemotherapy followed by chemoradiation or surgery + postoperative IMRT

b. Surgery

En-bloc or craniofacial resection with endoscopic or open approaches depending on extent
Craniofacial resection (transfacial + frontal craniotomy) is the historical gold standard for skull base involvement; allows en-bloc removal with pericranial flap reconstruction
Endoscopic endonasal approaches increasingly used for selected cases
Local control: 74% at 5 years with gross total resection vs 24% with subtotal resection (UCSF series, cited in Cummings)

c. Radiation Therapy

IMRT (Intensity-Modulated Radiation Therapy) is standard, allowing dose escalation to tumor while sparing adjacent critical structures
Doses typically 60-70 Gy
Elective neck irradiation should be considered - regional failure occurs in ~27% at 2 years even in clinically N0 neck (Morand meta-analysis)

d. Concurrent Chemoradiation (CRT)

Platinum-based concurrent CRT for unresectable disease or as definitive treatment
Rischin et al. reported 2-year OS of 64% in patients treated with induction cisplatin/5-FU followed by concurrent platinum CRT
MD Anderson reported 5-year OS of 64.2% with primary combination therapy

e. Summary of Modality Outcomes (Morand Meta-analysis, 390 patients)

Treatment	Risk of death vs. dual/triple modality
Surgery alone	~3x increased risk (HR 2.97)
RT alone	~3x increased risk (HR 2.80)
Surgery + RT	Best outcomes in operable disease
CRT (definitive)	Best outcomes in unresectable disease

A 2024 updated meta-analysis by See et al. (Clin Otolaryngol, PMID: 37859617, 208 patients) found: overall cumulative survival 30% at 95 months; chemoradiotherapy had highest cumulative survival (42% at 40 months); trimodality did not confer survival advantage over bimodality.

Cummings Otolaryngology, p. 2275-2276; KJ Lee's Essential Otolaryngology, p. 9247

10. Prognosis

SNUC carries a poor prognosis, with 5-year overall survival rates of approximately 20-40% in most series:

5-year OS: 22% in University of Michigan experience (older series without chemotherapy)
5-year OS: ~43-44% in contemporary pooled analyses (Burggraf et al. 2024)
5-year OS with induction chemotherapy: ~72.6% in selected patients (Burggraf et al. 2024)
An international collaborative study reported 5-year OS of 0% for undifferentiated sinonasal tumors (n=14) in an older mixed-histology series

Poor prognostic factors:

T4 stage (brain invasion)
Positive surgical margins
Subtotal resection
Single-modality treatment
Distant metastases at presentation

Patterns of failure: Locoregional failure is common; distant metastases (lung, bone, brain) occur at high rates. Elective neck treatment is recommended to reduce regional recurrence.

Cummings Otolaryngology, p. 2275; See et al. [PMID: 37859617]; Burggraf et al. [PMID: 39162231]

Summary Table

Feature	Detail
Cell of origin	Schneiderian epithelium
First described	Frierson et al., 1986
EBV status	Negative (vs NPC)
Stage at presentation	~80% T4
IHC	CK+, EMA+/-, synaptophysin-, EBER-
Treatment	Multimodality: IC + surgery + RT/CRT
5-year OS (contemporary)	~40-45% (up to ~72% with IC)
Key prognostic factor	Extent of resection; use of IC

Key References:

Cummings Otolaryngology Head and Neck Surgery, 7th ed., Chapter on Skull Base Tumors
Scott-Brown's Otorhinolaryngology Head & Neck Surgery, 8th ed.
KJ Lee's Essential Otolaryngology, 11th ed.
See ASY et al. Clin Otolaryngol 2024 - [Meta-analysis, PMID: 37859617]
Burggraf M et al. Head Neck 2024 - [Meta-analysis with induction chemotherapy focus, PMID: 39162231]
Turri-Zanoni M et al. Curr Oncol Rep 2022 - [Molecular biomarkers, PMID: 35059992]

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