Interfractional Geometric Variations and Dosimetric Benefits of Online Adaptive Stereotactic Body Radiotherapy of Prostate Bed After Radical Prostatectomy

Abstract

To evaluate volumetric and shape variations of target and organs-at-risk (OAR) of prostate patients receiving stereotactic body radiotherapy (SBRT) after radical prostatectomy and to investigate potential dosimetric benefits of MRI guided online adaptive planning (MRgART) to compensate for these variations.CTV and OAR were contoured by a single physician on the setup MRI images of 11 patients treated with an MR-Linac and enrolled on a phase II trial of post-prostatectomy SBRT. All patients followed institutional bladder and rectum filling protocols and received 5 fractions of 6-6.8 Gy to the prostate bed. Changes in interfractional volume were calculated and shape deformation was quantified by the Dice similar index (DSC). Dosimetric changes in CTV V95%, bladder maximum dose, V32.5 Gy, and rectal maximum dose, V32.5Gy and V27.5Gy were predicated by recalculating the initial treatment plan on the daily MRI. MRgART was retrospectively simulated if the predicted dose exceeds pre-set dosimetric criteria and the potential dosimetric improvement of MRgART was evaluated.Interfractional CTV volume and shape remained stable with median volumetric change of 3.0% (IQR -3.0% to 11.5%) and median DSC of 0.83 (IQR 0.79 to 0.88). Relatively large volumetric changes in bladder (median -24.5%, IQR -34.8% to 14.5%) and rectum (median 5.5%, IQR -9.7% to 20.7%) were observed while shape changes were moderate (median DSC of 0.79 and 0.67, respectively). Median CTV V95% was 98.4% (IQR 94.9% to 99.6%) for the predicted doses. However, MRgART would have been deemed necessary for 78% of the 55 fractions based on CTV undercoverage (16%), exceeding OAR constraints (50%), or both (11%). Simulated online ART improved the plan dosimetry and met dosimetric criteria in all adaptive fractions. Moderate correlations between CTV V95% and target DSC (R-squared = 0.73) and bladder mean dose versus bladder volumetric changes (R-squared = 0.61) were observed.Interfractional dosimetric variations resulting from target and OAR deformation are commonly encountered with post-prostatectomy RT and can be mitigated with MRgART.