Dose accumulation for personalized stereotactic MR-guided adaptive radiation therapy in prostate cancer

Background and purpose Adaptive MR-guided radiotherapy ( MRgRT) is an innovative approach for delivering stereotactic body radiotherapy (SBRT) in prostate cancer (PC). Despite the increased clinical use of SBRT for PC, there is limited data on the relation between the actual delivered dose and toxicity. We aimed to identify dose parameters based on the total accumulated delivered bladder dose (DOSE ACC TX ). Furthermore, for future personalization, we studied whether prospective accumulation of the first 3 of 5 fractions (DOSE ACC 3FR ) could be used as a representative of DOSE ACC TX . Materials and methods We deployed a recently validated deformable image registration-based dose accumulation strategy to reconstruct DOSE ACC TX and DOSE ACC 3FR in 101 PC patients treated with stereotactic MRgRT. IPSS scores at baseline, end of MRgRT, at 6 and 12 weeks after treatment were analyzed to identify a clinically relevant increase of acute urinary symptoms. A receiver operator characteristic curve analysis was used to investigate the correlation of an increase in IPSS and bladder DOSE ACC TX (range V 5 –V 36.25 Gy, D 1cc , D 5cc ) and DOSE ACC 3FR (range V 6 –V 21.8 Gy, D 1cc, D 5cc ) parameters. Results A clinically relevant increase in IPSS in the three months following MRgRT was observed in 25 patients. The V 20Gy-32Gy from DOSE ACC TX and V 15Gy-18Gy from DOSE ACC 3FR showed good correlation with IPSS increase with area under the curve (AUC) values ranging from 0.71 to 0.75. In contrast, baseline dosimetry showed a poor correlation with AUC values between 0.53 and 0.62. Conclusion DOSE ACC TX was superior to baseline dosimetry in predicting acute urinary symptoms. Because DOSE ACC 3FR also showed good correlation, this can potentially be used to optimize MRgRT for the remaining fractions.