A lung prescription reads 60 Gy in 30 fractions with 6 MV photons, and the plan defines GTV, CTV, ITV, and PTV structures. The physician wants the structure that already accounts for breathing motion but excludes the setup margin. Which target volume matches that description?
Select an answer to reveal the explanation.
Short Explanation
Margins get added in order, and each one answers a different problem. The ITV includes the CTV in the target’s positions across breathing motion but precedes the setup margin. Add setup uncertainty and you have crossed into the next volume.
Full Explanation
Target volumes are built in a fixed order, and each layer answers a different question about where the dose should go. The ITV includes the CTV in the positions it occupies due to internal motion, so it accounts for breathing motion while leaving setup error outside the definition; that is exactly the structure requested. The GTV identifies gross tumor, while the CTV includes the GTV and tissue at risk for subclinical disease; neither definition by itself accounts for breathing motion. The PTV includes margins for internal motion and setup uncertainties, so it is one step beyond what the physician asked for. Not every department names an explicit ITV, and some combine internal and setup margins directly into a PTV, so read the structure list in the plan.