Radiation Therapy: Treatment Planning, Delivery, and Radiobiology
Core concepts
Radiation therapy questions test the link between prescription, planning, and delivery. Know absorbed dose units (gray), fractionation, and the therapeutic ratio. Understand photon depth-dose behavior, build-up, and how beam energy and field arrangement shape dose distribution. Study target volumes (GTV, CTV, PTV) and organs at risk, plus planning constraints and dose-volume histograms. Know delivery techniques including three-dimensional conformal, IMRT, VMAT, and stereotactic approaches, and the role of image guidance. Radiobiology essentials include the 4 Rs (repair, repopulation, redistribution, reoxygenation), the alpha/beta model, and tolerance doses.
Traps to expect
Examiners often confuse GTV with PTV, or ask which margin accounts for setup error versus microscopic disease. Another trap is assuming higher energy always improves sparing; it shifts build-up deeper. Distinguish acute from late effects and remember that fractionation spares late-responding tissues more than tumor.
Worked example
A prescription of 60 Gy in 30 fractions means 2 Gy per fraction. If a plan delivers 95% of the dose to 98% of the PTV, report it as PTV V95% = 98%. For a serial organ at risk such as the spinal cord, maximum dose matters more than mean dose, so evaluate Dmax against tolerance rather than the DVH mean. Always state whether a constraint is a maximum, mean, or volume-based threshold.