Radiation Therapy: Fractionation, Dosimetry, and Delivery Verification
Dose and fractionation
Absorbed dose is measured in gray (1 Gy = 1 J/kg). Conventional fractionation (~2 Gy per fraction, 5 days/week) exploits the R's of radiobiology: repair of sublethal damage, repopulation, redistribution through the cell cycle, reoxygenation, and intrinsic radiosensitivity. The α/β ratio predicts fractionation sensitivity: tumours and early-responding tissues have high α/β (≈10 Gy), late-responding tissues low (≈3 Gy), so late effects are spared more by small fractions. A common trap is assuming a higher total dose is always better without regard to fraction size and normal-tissue constraints.
Beam data you must distinguish
Percentage depth dose (PDD) depends on SSD - decreasing SSD raises PDD. TMR is independent of SSD, which is why isocentric calculations use it. Photon beams show a build-up region with dmax near 1.5 cm for 6 MV; electrons show a rapid fall-off and are chosen for shallow targets. Penumbra widens with larger source size, greater distance from collimator, and shallower depth.
Planning and delivery
Volumes run GTV → CTV → PTV with margins for microscopic spread and setup error. IMRT/VMAT use inverse planning and MLC segmentation: better conformity but more monitor units, more low-dose integral dose, and more leakage - and above ~10 MV, neutron production becomes relevant. Brachytherapy dose falls steeply with distance, so source-position accuracy dominates (HDR Ir-192, LDR I-125). Verification tools include portal imaging, CBCT, and in-vivo dosimetry.
Traps
- Confusing PDD with TMR when SSD changes.
- Forgetting that DVH analysis describes dose to a structure, not spatial distribution.
- Overlooking that a larger field at fixed isocentre increases scatter and integral dose.
Rule of thumb
Inverse square law governs output change with distance; set-up errors of a few millimetres matter more than small dosimetric differences.