Dosimetric impact of baseline shift plan in a magnetic resonance linear accelerator according to target motion and plan characteristics

Abstract

Purpose

In this study, the dosimetric effect of baseline shift (BLS) in a magnetic resonance linear accelerator was investigated and compared with adapt-to-position (ATP), adapt-to-shape (ATS), and an uncorrected plan without adaptation.

Methods

In a simulation dataset consisting of 30 patients (10 prostate, 10 liver, 10 pancreatic), all structures were rigidly shifted by 1, 3, 5, and 10 mm separately in the superior–inferior and posterior directions, and four plans (ATS reoptimization, ATP reoptimization, BLS using the segment aperture morphing algorithm, and an uncorrected plan without adaptation) were generated for each shift condition. Planning target volume (PTV) coverage and PTV maximum dose (Dmax) were then compared. The dosimetric effects of BLS on the targets and organs at risk (OARs) under non-rigid anatomic changes were also evaluated using a clinical dataset based on magnetic resonance imaging acquired during treatment.

Results

In the simulation analysis of superior–inferior shifts, target coverage using BLS was comparable to that using ATS and ATP across all disease sites and improved compared with that using the uncorrected plan. For posterior shifts, target coverage remained comparable to that using ATS and ATP; however, BLS increased the PTV Dmax at all posterior shift magnitudes (1–10 mm) in prostate and pancreatic cases, with limited increases in liver cases. In the clinical dataset, OAR doses upon left–right or anterior–posterior displacement were greater using BLS than using ATP, ATS, and uncorrected plans in the prostate and pancreatic cases, with limited increases in the liver cases.

Conclusions

The dosimetric impact of BLS relies on motion direction and plan characteristics. BLS was stable for superior–inferior shifts, whereas posterior shifts may increase PTV hotspots and OAR doses. Therefore, clinical application of BLS should consider motion direction and plan design rather than applying a uniform strategy.