Evaluation of plan adaptation strategies for stereotactic radiotherapy of lymph node oligometastases using online magnetic resonance image guidance.

BACKGROUND AND PURPOSE: Recent studies have shown that the use of magnetic resonance (MR) guided online plan adaptation yields beneficial dosimetric values and reduces unplanned violations of the dose constraints for stereotactic body radiation therapy (SBRT) of lymph node oligometastases. The purpose of this R-IDEAL stage 0 study was to determine the optimal plan adaptation approach for MR-guided SBRT treatment of lymph node oligometastases. MATERIALS AND METHODS: Using pre-treatment computed tomography (CT) and repeated MR data from five patients with in total 17 pathological lymph nodes, six different methods of plan adaptation were performed on the daily MRI and contours. To determine the optimal plan adaptation approach for treatment of lymph node oligometastases, the adapted plans were evaluated using clinical dose criteria and the time required for performing the plan adaptation. RESULTS: The average time needed for the different plan adaptation methods ranged between 11 and 119 s. More advanced adaptation methods resulted in more plans that met the clinical dose criteria [range, 0-16 out of 17 plans]. The results show a large difference between target coverage achieved by the different plan adaptation methods. CONCLUSION: Results suggested that multiple plan adaptation methods, based on plan adaptation on the daily anatomy, were feasible for MR-guided SBRT treatment of lymph node oligometastases. The most advanced method, in which a full online replanning was performed by segment shape and weight optimization after fluence optimization, yielded the most favourable dosimetric values and could be performed within a time-frame acceptable (<5 min) for MR-guided treatment.

Evaluation of Online Plan Adaptation Strategies for the 1.5T MR-linac Based on "First-In-Man" Treatments.

The superior soft tissue contrast provided by magnetic resonance (MR) images on the 1.5T MR-linac allows for the incorporation of patient anatomy information. In this retrospective case study, we present the simulated dosimetric effects and timings of full online replanning as compared to the five plan adaptation methods currently available on the 1.5T MR-linac treatment system. For this case, it is possible to create treatment plans with all six methods within a time slot suitable for an online treatment procedure. However, large dosimetric differences between the plan adaptation methods and full online replanning are present with regards to target coverage and dose to organs at risk (OARs).