Treatment Planning
Core concepts
Planning questions test volume definitions and plan evaluation. Know ICRU concepts: GTV (visible disease), CTV (GTV plus presumed microscopic spread), ITV (CTV plus internal motion), PTV (CTV/ITV plus setup and penumbral margin - a geometric, not biologic, concept), plus OAR and PRV. Understand simulation and immobilization, isocenter placement, beam arrangement, wedges/bolus/compensators, field matching, and when each modality (3D conformal, IMRT/VMAT, SBRT, electrons, protons, brachytherapy) is chosen.
Be able to read a DVH: target coverage, hot spots, dose to OARs, homogeneity and conformity indices, and plan sums. Know fractionation vocabulary (conventional, hypofractionation, hyperfractionation, SBRT) and that prescriptions specify total dose, dose per fraction, and the reference point or volume.
Traps
- Calling the PTV a biologic margin, or placing microscopic disease in the GTV.
- Thinking bolus pushes dose deeper - it pulls the high-dose region superficially.
- Confusing internal motion (ITV) with setup uncertainty (PTV).
- Reporting a beautiful DVH line without checking which structure was contoured or whether the plan sum is complete.
Worked example / rule of thumb
A common institutional convention is that ≥95% of the PTV receives ≥95% of the prescription while no more than a small volume exceeds 107%. If a plan shows PTV V95 = 88%, the plan fails coverage regardless of how good the OAR sparing looks. Margin rule of thumb (van Herk): PTV margin ≈ 2.5Σ + 0.7σ for systematic (Σ) and random (σ) error.