Treatment delivery
From plan to machine
Delivery accuracy begins with a correct monitor-unit calculation: MU = prescribed dose ÷ (output × Sc × Sp × depth factor × inverse-square × accessory factors). Every factor propagates linearly, so one mis-entered value shifts the whole treatment.
IMRT and VMAT specifics
Multileaf collimator transmission (about 1–2% at 6 MV), rounded leaf ends, tongue-and-groove leakage, and the dosimetric leaf gap (typically 0.5–1.5 mm, model-specific) all bias delivered dose; the gap is measured and entered into the planning system. Small segments and low MU per segment amplify leaf-position error, so planners impose minimum segment area and MU constraints.
Verification and motion
Patient-specific QA is judged by gamma analysis; TG-218 recommends 3%/2 mm global normalization, 10% dose threshold, 95% tolerance and 90% action limit. A passing gamma only proves agreement with the planning system, not clinical quality. Motion management (respiratory gating, deep-inspiration breath hold, compression, tracking) requires that the beam-on window match the gating threshold and that the phase or amplitude binning used for planning match the delivery system.
Worked example
A 6 MV beam gives 1.000 cGy/MU at dmax. Prescribe 200 cGy to a point at 10 cm depth, 100 cm SSD, 10 × 10 cm², PDD 66.5%, Sc·Sp = 1.000, no other factors: MU = 200 ÷ (1.000 × 0.665) = 301 MU. At 120 cm SSD the PDD rises to 68.2% and the answer becomes 293 MU, because the inverse-square change is already inside the PDD.