Treatment Planning, Delivery Techniques, and Quality Assurance
What to know
Simulation, target delineation (GTV/CTV/PTV), margin concepts, plan evaluation (DVH, conformity, homogeneity), and delivery techniques (3D-CRT, IMRT/VMAT, SBRT/SRS, protons). Plus QA (machine QA, patient-specific QA, in vivo dosimetry, chart checks).
Concepts to master
- ICRU volumes: GTV (visible), CTV (microscopic), PTV (setup/motion margin), OARs.
- Setup margin accounts for systematic/random error; internal margin for organ motion.
- DVH metrics: D95, Dmax, V20 (lung), mean dose (parotid, heart).
- SBRT/SRS requires high geometric accuracy, immobilization, image guidance, and steep dose gradients.
- QA tiers: daily/monthly/annual machine QA; patient-specific IMRT QA; independent dose check.
Traps
- Confusing CTV vs PTV (microscopic disease vs geometric uncertainty).
- Assuming larger PTV is always safer - it increases normal-tissue dose.
- Forgetting motion management for lung/pancreas SBRT.
Rule of thumb
PTV = CTV + setup + internal margin. If a stem emphasizes breathing motion, add an internal margin or use gating/tracking - do not just enlarge the PTV blindly.
Worked example
A lung SBRT plan shows V20 = 35% and mean lung dose 22 Gy. That exceeds typical tolerances (V20 ~30-35% borderline; mean lung dose ~20 Gy). The correct move is re-plan to spare lung, not proceed - plan evaluation guides safe delivery.