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• Definition & Introduction
• Classification / Types (where applicable)
• Mechanism / Radiobiology (physics + biology basis)
• Clinical details (indications, technique, dose, fractionation)
• Complications & Management
• Key exam points / mnemonics to remember

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• Ask me one topic at a time for deep coverage, or give me a list and I'll prioritize
• Tell me if you want short notes (5-10 marks) or long answers (15-20 marks)
• I'll flag "examiner favorites" - points that frequently appear in MD exams

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PERCENTAGE DEPTH DOSE (PDD)

Definition

• D_d = dose at depth d
• D_dmax = dose at the reference depth (depth of maximum dose)

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Reference Conditions for PDD Measurement

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Factors Affecting PDD

1. Beam Energy (Most Important Factor)

• Higher energy → Higher PDD (better depth dose)
• As energy increases, the depth of maximum dose (d_max) shifts deeper
• d_max values:
  • Cobalt-60: ~0.5 cm
  • 6 MV: ~1.5 cm
  • 10 MV: ~2.5 cm
  • 15 MV: ~3.0 cm
  • 18 MV: ~3.5 cm
• Higher energy beams are skin-sparing due to deeper d_max

2. Field Size

• Larger field size → Higher PDD
• Reason: More scatter radiation reaches the central axis from larger fields
• Relationship is significant at depth but less at d_max

3. Depth

• PDD decreases with increasing depth beyond d_max
• Caused by:
  • Inverse square law (beam divergence)
  • Attenuation and scatter in tissue

4. Source-to-Skin Distance (SSD)

• Larger SSD → Higher PDD
• At greater SSD, the beam geometry is more parallel (less divergence)
• Governed by the inverse square law
• PDD at extended SSD is calculated using the Mayneord F-factor:

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PDD Curve - Key Characteristics

100% |----peak (d_max)
     |   \
     |    \
PDD  |     \
     |      \
     |       \
  0% |________\______
        Depth in tissue →

• Build-up region: From surface to d_max - dose increases due to secondary electron buildup
• d_max: Point of maximum dose = 100% reference point
• Beyond d_max: Exponential fall-off of dose with depth

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Skin-Sparing Effect

• A unique property of megavoltage beams (>1 MV)
• At the skin surface, dose is lower than d_max because:
  • Secondary electrons (from Compton interactions) travel forward
  • They deposit maximum energy at a depth (d_max), not at the surface
• Clinical importance: Allows delivery of high tumor doses while sparing skin
• Lost when:
  • Bolus material is applied
  • Very large field sizes used
  • Beam obliquity increases
  • Beam modifiers (trays, blocks) are close to skin

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PDD vs. TAR vs. TMR - Comparison (Exam Favorite)

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Clinical Applications of PDD

1. Treatment planning - To calculate dose at tumor depth from surface dose
2. Choosing beam energy - Higher energy for deep tumors
3. Calculating MU (Monitor Units):

1. Comparing beam quality of different machines
2. Bolus prescription - To deliberately reduce PDD and increase surface dose

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PDD at Specific Depths - Approximate Values

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Extended SSD Technique and PDD Correction

• If SSD is increased (e.g., large patient, special techniques like TBI - Total Body Irradiation):
  • PDD increases slightly
  • Must recalculate using F-factor

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Relationship Between PDD and Other Dosimetric Quantities

• TAR = PDD × (SSD + d)² / (SSD + d_max)² × (1/BSF)
  • BSF = Backscatter Factor
• TMR = PDD × (SSD + d)² / (SSD + d_max)²

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Key Exam Points / Mnemonics

• Field size ↑
• Energy ↑
• Extended SSD ↑
• Depth... wait, depth decreases PDD (the exception!)

• PDD is an SSD-dependent quantity - this is why TAR/TMR (SSD-independent) are preferred in isocentric techniques
• 10 MV at 10 cm depth ≈ 74% - a commonly asked value
• Cobalt-60 d_max = 0.5 cm - shallowest d_max among megavoltage beams
• Skin sparing is a megavoltage phenomenon - not seen with orthovoltage (kV) beams

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Quick Recall Summary (For Last-Minute Revision)

PDD = (Dose at depth / Dose at d_max) × 100 Measured in water phantom, 10×10 field, 100 cm SSD Increases with: ↑ Energy, ↑ Field size, ↑ SSD Decreases with: ↑ Depth SSD-dependent (unlike TAR/TMR) Skin sparing = d_max shifts deeper with higher energy

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DOSE CONSTRAINTS IN CRANIOSPINAL IRRADIATION (CSI) FOR EPENDYMOMA - ORGANS AT RISK (Excluding Head & Neck)

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Context: CSI Dose in Ependymoma

• CSI dose: 23.4 - 36 Gy (standard risk) / 36 - 39.6 Gy (high risk/disseminated)
• Boost to primary site: total 54 - 59.4 Gy (1.8 Gy/fraction)
• The spinal field traverses thorax, abdomen, and pelvis - hence anterior visceral organs receive low-to-intermediate dose bath, and this is the primary concern

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WHY VISCERAL OARs MATTER IN CSI

• Lungs (posterior portions)
• Heart (especially with cervico-thoracic spine irradiation)
• Kidneys (posterior, close to spine)
• Liver (right lobe, adjacent to lower thoracic spine)
• Bowel / small intestine (lumbar spine level)
• Gonads / ovaries (lumbosacral irradiation)
• Vertebral bodies and growth plates (pediatric concern)

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OAR DOSE CONSTRAINTS IN CSI - COMPLETE TABLE

1. LUNGS

• Complication feared: Radiation pneumonitis, pulmonary fibrosis
• Note: In proton CSI, lung dose dramatically reduced vs. photon CSI - major advantage of protons

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2. HEART

• Complication feared: Radiation-induced pericarditis, cardiomyopathy, coronary artery disease
• Pediatric note: Children are far more susceptible to late cardiac effects - mean heart dose should be minimized as much as possible (ideally < 5 Gy with proton therapy)
• The upper thoracic spine field junction region is critical here

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3. KIDNEYS

• Complication feared: Radiation nephropathy, hypertension, renal failure (TD5/5 for whole kidney = 20 Gy)
• Kidneys lie posterior and are directly in the path of spinal fields at L1-L2 level
• This is one of the most critical OARs in CSI - especially in pediatric patients who have long life expectancy

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4. LIVER

• Complication feared: RILD (veno-occlusive disease), hepatic failure
• Right lobe of liver is adjacent to lower thoracic spine (T8-T12 level)
• TD5/5 for whole liver = 30 Gy; TD50/5 = 40 Gy

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5. SMALL BOWEL / LARGE BOWEL

• In CSI, lumbar spine fields are adjacent to small bowel and large bowel loops
• Bowel irradiation leads to: enteritis, malabsorption, adhesions, obstruction (late)
• In lumbosacral ependymoma with boost to cauda equina, this becomes even more critical

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6. SPINAL CORD (within CSI field - applies to boosted cord segments)

• The cord within the CSI field receives ~36 Gy from the whole-spine dose
• Boost to spinal ependymoma site can bring total cord dose to 50.4 - 54 Gy
• Below the conus medullaris (cauda equina level), tolerance is higher - doses up to 59.4 Gy considered acceptable

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7. GONADS / OVARIES / TESTES

• Lumbosacral field irradiation exposes gonads
• Oophoropexy (surgical ovarian transposition) may be done pre-radiotherapy in young female patients to move ovaries out of the field
• Proton CSI virtually eliminates gonadal dose vs. photon CSI

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8. ESOPHAGUS

• At risk during cervico-thoracic junction field in CSI

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9. THYROID GLAND

• Radiation-induced hypothyroidism common after CSI
• Thyroid lies at the cervico-thoracic junction - within the field junction overlap zone
• Screen TSH annually post-treatment (endocrine follow-up mandatory)

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10. VERTEBRAL BODIES / GROWTH PLATES (Pediatric - Special Concern)

• Uneven irradiation of vertebral epiphyses causes radiation-induced scoliosis and kyphosis
• CTV must include entire width of vertebral body to prevent asymmetric growth
• Hemi-vertebral irradiation is contraindicated

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SUMMARY TABLE - QUICK RECALL

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EXAM KEY POINTS

1. Kidneys are the most critical OAR in the spinal field - they lie directly posterior, within the treatment volume at L1-L2 level
2. Proton therapy advantage: Dramatically reduces dose to all anterior visceral OARs (heart, lungs, liver, bowel, gonads) compared to photon CSI - this is the prime clinical indication for proton CSI in pediatric ependymoma
3. Vertebral body rule: Always irradiate full vertebral width in pediatric CSI to prevent scoliosis - partial vertebral irradiation is a known cause of radiation-induced spinal deformity
4. QUANTEC source: These constraints are derived from the landmark QUANTEC (Quantitative Analysis of Normal Tissue Effects in the Clinic) guidelines - published in Int J Radiat Oncol Biol Phys, 2010
5. Below conus medullaris (cauda equina): Spinal cord tolerance is higher, allowing doses up to 59.4 Gy for lumbosacral ependymoma
6. Gonadal protection: Oophoropexy before CSI in young females with lumbosacral involvement is a valid clinical decision to preserve fertility

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OAR DOSE CONSTRAINTS - CSI 36 Gy / 20 Fractions (1.8 Gy/fraction)

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Fractionation Context

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OAR CONSTRAINTS (Excluding Head & Neck) - For 36 Gy / 20 # CSI

LUNGS

• At 36 Gy CSI, posterior lung portions receive low-intermediate scatter dose
• Actual mean lung dose observed in photon VMAT CSI planning studies: ~6-10 Gy (well within limits)
• With proton CSI: mean lung dose drops to < 1 Gy - major advantage

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HEART

• In 36 Gy CSI VMAT planning studies, actual mean heart dose achieved: ~9-10 Gy
• Thoracic spine irradiation is the main source of cardiac dose
• Pediatric patients: aim for mean heart dose < 15 Gy (ideally < 5 Gy) due to lifetime cardiac risk

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KIDNEYS

• Kidneys lie directly posterior (L1-L2 level) - in the path of the lumbar spine field
• In 36 Gy CSI, actual mean kidney dose in photon planning: ~11-13 Gy (close to but within limit)
• This is the tightest constraint to meet in photon CSI at 36 Gy - requires careful planning
• Proton CSI reduces kidney dose to near zero

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LIVER

• Right lobe of liver adjacent to T8-T12 vertebral levels
• Actual mean liver dose in 36 Gy CSI photon planning: ~6-7 Gy - well within limits
• Constraint easily met at this dose level

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BOWEL (Small & Large Intestine)

• At CSI dose of only 36 Gy, the bowel Dmax will be well below 54 Gy from the spine field alone
• Constraint becomes critical only when lumbosacral boost is added (total dose approaching 54 Gy)
• Lumbar spine field overlap with bowel loops is the main concern

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GONADS

• Lumbosacral field at 36 Gy puts gonads at significant risk
• Even 4-6 Gy to ovaries can cause premature ovarian failure in young girls
• Oophoropexy prior to CSI is standard recommendation when lumbosacral field is used
• In male patients: testicular shielding and sperm banking discussed pre-treatment

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ESOPHAGUS

• At risk in cervico-thoracic junction - the field junction overlap zone in CSI
• At 36 Gy CSI, esophageal dose is well within tolerance unless junction overlap occurs

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THYROID

• Sits at cervical/upper thoracic junction
• Even at 36 Gy CSI, thyroid receives significant dose (~16 Gy mean in VMAT planning studies)
• Post-CSI hypothyroidism in 30-60% of patients - screen TSH every 6-12 months

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VERTEBRAL BODIES / GROWTH PLATES

• Asymmetric vertebral irradiation → radiation-induced scoliosis (most common long-term skeletal complication)
• All vertebral bodies within CSI field must receive homogeneous dose

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ACTUAL DOSES ACHIEVED IN 36 Gy / 20 # VMAT CSI PLANNING STUDIES

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QUICK RECALL - ONE-LINER CONSTRAINTS (Exam Viva Format)

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EXAM KEY POINT

At 36 Gy / 20 fractions, the kidneys are the most challenging OAR to protect in photon CSI because they lie directly posterior within the lumbar spine field. The mean kidney dose (~12-13 Gy) approaches but stays within the QUANTEC limit of 18 Gy. This is one of the strongest arguments for proton beam CSI in pediatric ependymoma - protons virtually eliminate kidney, gonadal, and cardiac dose.

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