Treatment Planning and Simulation
Treatment planning and simulation tests how imaging, immobilization, volume definition, margin design, and dose calculation produce a deliverable plan. Expect CT simulation, coordinate systems, ICRU volumes, DVH interpretation, and forward versus inverse planning.
Simulation and volumes
CT simulation establishes the treatment position, immobilization, reference marks, and planning images. The patient coordinate system is linked to isocentre; left/right, anterior/posterior, and superior/inferior must be confirmed. GTV is visible tumour; CTV adds suspected microscopic spread; PTV adds geometric uncertainty. OARs may have planning organ-at-risk volumes (PRVs) when they move or are close to the target. Margins should be based on measured systematic and random error, not habit. Bolus increases surface dose by reducing build-up; it does not simply protect skin.
Planning and evaluation
Forward planning sets fields and weights manually; inverse planning uses objectives and constraints to optimize fluence, as in IMRT or VMAT. Dose algorithms must account for tissue heterogeneity, especially in lung. DVHs show dose-volume relationships: D95, D98, D2, V20, mean dose, and maximum dose answer different questions. Homogeneity describes dose uniformity within the target; conformity describes how well the prescription isodose encloses the PTV while sparing OARs. A plan can be conformal but inhomogeneous, or homogeneous but poorly conformal.
Worked example
A department measures systematic set-up error Σ=3 mm and random error σ=2 mm. Using the van Herk margin formula M=2.5Σ+0.7σ gives 2.5×3+0.7×2=7.5+1.4=8.9 mm, often rounded to 9 mm. Reasoning: systematic error shifts the whole treatment and needs a larger margin; random error averages out but still needs coverage. If image guidance reduces Σ and σ, the margin can be reviewed, but only with evidence and protocol.
Traps
Do not add a PTV margin to every OAR; use PRVs only where appropriate. Do not confuse CTV with PTV or GTV with the whole organ. A 95% PTV coverage does not by itself prove an acceptable plan; check hot spots, OAR constraints, and dose algorithm. Do not apply a fixed 5 mm margin to all sites. Bolus changes build-up, not beam penetration.