Session Item

MR-guided radiotherapy (MRgRT) provides the opportunity of daily plan adaptation based on information about the patient’s current anatomy. Automatic MRgRT planning approaches may be used to realize fast plan optimization during online adaptation. The aim of this work was to evaluate the differences in terms of dosimetric quality comparing automatic online plan adaptation starting from a reference plan to complete re-optimization both based on the anatomy of the day.

Clinical data of ten prostate carcinoma patients treated with online adaptive MRgRT (20 x 3 Gy) at the 1.5 T MR-Linac were included into this study. Optimization of the MRgRT plans was carried out by an automated planning approach using a particle swarm optimization (PSO). For each treatment fraction, two adapted plans were optimized using (1) a plan re-optimization with the optimal planning constraints achieved for the reference PSO plan at baseline and (2) a completely new automatic PSO planning approach to take the anatomical variations best possible into account. Method 1 consists of a fast plan optimization exploring a limited search space defined by a set of plan constraints determined upfront whereas method 2 allows for unlimited exploitation of plan constraints for optimal adaptation at the cost of increased calculation times.

The daily dose distributions obtained with the two automatic planning approaches were evaluated by comparing the relative adherence to a set of clinically relevant dosimetric criteria (cf. table 1). The significance of the differences was considered using a t-test at the 5% significance level.

For the ten patients, a total of 154 adaptive MRgRT fractions were available for evaluation. On average (range) method 1 fulfilled 73.8% (67.7-79.3%) of the dosimetric criteria (cf. figure 1). For method 2, the mean relative adherence to the dosimetric criteria was 74.1% (69.2-79.3%). The two methods yielded no significant differences.

This study compared automatic online plan optimization with a set of baseline plan constraints to a complete re-optimization. Regarding dosimetric plan criteria, no significant differences between the two approaches were found. Therefore, daily plan adaptation with a set of pre-defined planning constraints does not compromise plan quality even though anatomical variations may occur.