ITV or tumour tracking for dose painting of NSCLC? A comparison of target coverage and out-of-target dose in proton treatments

Abstract

Introduction: Dose-escalation radiotherapy is used to calculate biologically adapted plan. When treating moving tumours the boost on the dose can be lost. We investigated the possibility to use tumour tracking for high precision dose painting in lung cancers. We chose a case where the target motion was below 5mm, normally recruited under free-breathing/rescanning-only protocol. We performed a comparative 4D dose calculation (4DDC) analysis for two extreme planning approaches: tracking and free-breathing. Material and Methods: The plan has been calculated on a Mid-Position reference image with a 17% dose boost on the dose escalated volume (DE). The planning CT has been warped into a different 4DCT and animated thanks to deformable image registration. The margins applied to the PTV were 5 mm for the tracking plan and 10 mm for the free-breathing one. For both the cases, the 4DDC included a different numbers of rescans RS (1-6). Results: Considering ICRU definitions, we calculated the conformity index (CI) for DE, the tumour coverage for the CTV and the hot-spot index (HS). CI and HS improved for the tracking plan with the number or rescans. These proportionality were not found in free-breathing. CI is respectively 1.41 and 1.48 for the tracking and the free-breathing plan considering RS=6. V95 is respectively 98.7 and 99.7 for the tracking and the free-breathing plan, on average and for RS=1,4,6. For the tracking plan, HS is 1.41,0.9,0.86 for RS=1,4,6 and 58.19,56.98,58,4 for free-breathing. Summary: Tracking the tumour allowed to keep a better conformity on the boosted volume while giving less dose to healthy tissues in the lungs. The tracking plan shown a loss of coverage in the CTV. This effect is due to the fact that fully continuous energy modulation is not provided from the used treatment-planning-system and can be improved working on its dept-dose-curves resolution.