Role of Daily Plan Adaptation in MR-Guided Stereotactic Ablative Radiation Therapy for Adrenal Metastases

AbstractPurpose To study inter-fractional organ changes during MR-guided stereotactic ablative radiotherapy (SABR) for adrenal metastases and to evaluate the dosimetric advantages of online plan adaptation. Methods and Materials Seventeen patients underwent a total of 84 fractions of video-assisted, respiration-gated MR-guided adaptive radiotherapy to deliver either 50 Gy (5 fractions), 60 Gy (8 fractions) or 24 Gy (3 fractions). An MR scan was repeated prior to each fraction, followed by rigid co-registration to the GTV on the pre-treatment MR scan. Contour deformation, PTV (GTV+3mm) expansion and online plan re-optimization were then performed. Re-optimized plans were compared to baseline treatment plans recalculated on the anatomy-of-the-day (‘predicted plans'). Inter-fractional changes in OARs were quantified according to OAR volume changes within a 3 cm distance from PTV surface, centre of mass (COM) displacements and the Dice Similarity Coefficient (DSC). Plan quality evaluation was based on target coverage (GTV and PTV), and also high dose sparing of all OARs (V36Gy, V33Gy and V25Gy). Results Substantial COM displacements were observed for stomach, bowel and duodenum of 17, 27 and 36 mm, respectively. Maximum volume changes for the stomach, bowel and duodenum within 3 cm of PTV were 23.8, 20.5 and 20.9 cc, respectively. DSC values for OARs ranged from 0.0 to 0.9 for all fractions. Baseline plans recalculated on anatomy-of-the-day revealed underdosage of target volumes, and variable OAR sparing, leading to a failure to meet institutional constraints in a third of fractions. Online re-optimization improved target coverage in 63% of fractions, and reduced the number of fractions not meeting the V95% objective for GTV and PTV. Re-optimized plans exhibited significantly better sparing of OAR. Conclusions Significant inter-fractional changes in OARs positions were observed despite breath-hold SABR delivery under MR-guidance. Online re-optimization of treatment plans led to significant improvements in target coverage and OAR sparing