Medical Physics for Radiation Oncology
Core concepts
A candidate must be fluent in photon and electron interactions, dosimetry formalisms (TG-51/TG-21 calibration, absorbed dose to water), and treatment-planning physics. Know the difference between percent depth dose and tissue-air ratio, and how field size, SSD, energy, and depth shift each. Understand inverse-square falloff, buildup region, and why dmax moves shallower with higher energy. For brachytherapy, know TG-43 formalism, dose-rate constant, and anisotropy. Imaging physics (kV/MV, CBCT, CT number–to–electron density) and QA tolerances (TG-142) are fair game.
Traps
Questions love to flip variables: a larger field increases PDD; a longer SSD increases PDD; higher energy increases depth of dmax. Watch for confusion between output factor and Scp (collimator + phantom scatter). Electron questions often hinge on the practical range and the rapid distal falloff, plus the inverse relationship between energy and surface dose. MU calculations are a classic trap when inverse-square, tray, and wedge factors are omitted.
Rule of thumb
For a quick MU sanity check at dmax with a single field: MU ≈ prescribed dose ÷ (dose rate × output factor × PDD/100). Always confirm the reference depth and calibration geometry (TG-51 uses dref = 10 cm for photons).
Worked example
A 6 MV beam, 100 cm SSD, 10×10 field, delivers 1 cGy/MU at dmax. If PDD at 10 cm is 67%, to give 200 cGy at 10 cm: MU = 200 ÷ (1 × 0.67) ≈ 299 MU. Add inverse-square if SSD differs from calibration.