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HPTC RT Exam study materials
HPTC RT Exam study resources

HPTC RT Exam: Radiation Oncology Principles

HPTC RT Exam · Work through your exam, one topic at a time.

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Radiation Oncology Principles

Radiation Oncology Principles tests the biology and physics reasoning behind fractionation, dose prescription, and normal-tissue tolerance. Expect questions that ask you to compare schedules quantitatively or to identify which tissue constraint is at risk.

The 5 Rs and the LQ model

  • Repair of sublethal damage, repopulation, redistribution through the cell cycle, reoxygenation, and intrinsic radiosensitivity explain why fractionation spares late tissues.
  • Cell survival follows S = exp(-(alpha D + beta D squared)). The alpha/beta ratio is the dose at which the linear and quadratic contributions are equal.
  • Early-responding tumors and acute tissues have alpha/beta near 10 Gy; late tissues near 3 Gy; prostate about 1.5 Gy; breast about 4 Gy.

BED and fractionation

  • Biologically effective dose: BED = n d (1 + d / (alpha/beta)), where n is the number of fractions and d the dose per fraction. Compare regimens only at the same alpha/beta.
  • Hypofractionation raises dose per fraction, which inflates the BED of low alpha/beta late tissues fastest. That is why large fractions suit prostate tumor control but threaten a nearby cord.

Normal-tissue tolerances

  • Spinal cord maximum about 50 Gy; brainstem 54 Gy; optic nerve and chiasm 55 Gy; parotid mean 26 Gy (or 20 Gy for a single gland); lung V20 of 30-35 percent or less; heart mean below 26 Gy; kidney mean 18 Gy or less; liver mean 28-30 Gy; rectum V70 under 20 percent.

Worked example

Compare 60 Gy in 30 fractions with 40 Gy in 5 fractions. With alpha/beta = 10: BED1 = 30 x 2 x (1 + 0.2) = 72 Gy10; BED2 = 5 x 8 x (1 + 0.8) = 72 Gy10. The tumor effect is equivalent. For late tissue with alpha/beta = 3: BED1 = 60 x 1.667 = 100 Gy3, while BED2 = 40 x 3.667 = 146.7 Gy3. The five-fraction schedule delivers roughly 47 percent more late-tissue BED, so it is not safe adjacent to a serial structure.

Traps

  • Comparing BED values calculated at different alpha/beta ratios.
  • Forgetting that the LQ model assumes complete repair between fractions and fails at very large single doses.
  • Treating TCP and NTCP model outputs as clinical certainties.
  • Dropping the Gy10 or Gy3 subscript when reporting BED.