On-line dose-guidance to account for inter-fractional motion during proton therapy

Abstract

Background and purpose: Proton therapy (PT) of extra-cranial tumour sites is challenged by density changes caused by inter-fractional organ motion. In this study we investigate on-line dose-guided PT (DGPT) to account inter-fractional target motion, exemplified by internal motion in the pelvis.

Materials and methods: On-line DGPT involved re-calculating dose distributions with the isocenter shifted up to 15 mm from the position corresponding to conventional soft-tissue based image-guided PT (IGPT). The method was applied to patient models with simulated prostate/seminal vesicle target motion of ±3, ±5 and ±10 mm along the three cardinal axes. Treatment plans were created using either two lateral (gantry angles of 90°/270°) or two lateral oblique fields (gantry angles of 35°/325°). Target coverage and normal tissue doses from DGPT were compared to both soft-tissue and bony anatomy based IGPT.

Results: DGPT improved the dose distributions relative to soft-tissue based IGPT for 39 of 90 simulation scenarios using lateral fields and for 50 of 90 scenarios using lateral oblique fields. The greatest benefits of DGPT were seen for large motion, e.g. a median target coverage improvement of 13% was found for 10 mm anterior motion with lateral fields. DGPT also improved the dose distribution in comparison to bony anatomy IGPT in all cases. The best strategy was often to move the fields back towards the original target position prior to the simulated target motion.

Conclusion: DGPT has the potential to better account for large inter-fractional organ motion in the pelvis than IGPT.

Fig. 1

Dose distribution for 10 mm anterior motion for a) bony based IGPT, i.e., the plan was moved from the pCT to the rCT based on bony anatomy, b) soft-tissue based IGPT, i.e. the fields are at the isocenter of the moved target after a soft-tissue registration and c) DGPT, where the preferred dose distribution have been chosen, which in this case is when the fields are moved 5 mm posterior towards the uncorrected position prior to IGPT.

Fig. 2

V98%/D1cc for the soft-tissue (black circles) and bony anatomy (black crosses) based IGPT plans and the best DGPT (open circles) dose distribution for the CTVs and normal tissue using two lateral fields. Motion of 3, 5 and 10 mm in the anterior and posterior direction in all CT-scans were simulated (pCT and four rCTs). The vertical line depicts the V98% value from the original pCT plan. Normal tissue was defined as everything except CTV targets and bones.

Fig. 3

V98%/D1cc for the soft-tissue (black circles) and bony anatomy (black crosses) based IGPT plans and the best DGPT (open circles) dose distribution for the CTVs and normal tissue using two lateral oblique fields. Motion of 3, 5 and 10 mm in the posterior direction in all CT-scans were simulated (pCT and four rCTs). The vertical line depicts the V98% value from the original pCT plan. Normal tissue was defined as everything except CTV targets and bones.