Purpose: To evaluate the dosimetric impact of air cavity electron density correction for MR-guided Online Adaptive Radiation Therapy (MRgART) of prostate bed patients after radical prostatectomy.
Methods: Five prostate bed patients treated on a 0.35T MR Linac following institutional bladder and rectum filling protocol were retrospectively included to simulate the MRgART workflow. For two fractions of each patient, workflow1 ‘Recalc’: the initial clinical plan was calculated on the daily setup MRI with electron density directly obtained from simulation CT deformably registered to daily MRI; and workflow2 ‘Override’: the initial clinical plan was calculated with one additional step, air cavities were manually contoured with bulky air density assignment based on daily MRI anatomy. Dosimetric metrics for targets and OAR were compared between the calculated dose with and without air cavity electron density correction and its impact on clinical decision for the necessity of adaptive planning was evaluated for each fraction.
Results: The average air cavity change between simulation CT and daily MRI was 8.15±9.16cc. Clinical dosimetric endpoints between the ‘Recalc’ and ‘Override’ workflow showed little clinical dosimetric differences, with the maximum deviations being 3.47% and 2.82% for the rectum (V27.5Gy) and bladder (V32.5Gy). Furthermore, the dosimetric difference did not lead to change in clinical decision for adaptive planning except for only one fraction, where the dose calculation with air cavity density correction indicated that the bladder Dmax dose exceeded the constraint while the calculated dose without correction showed acceptable dose, although the absolute difference was only 0.4Gy.
Conclusion: With strict bladder and rectum filling protocol, air cavity changes between simulation and daily treatment did not result in large dosimetric changes on our 0.35T MRgRT system. However, caution may need to be paid for potential large air cavity variations or under higher magnetic fields.
Funding Support, Disclosures, and Conflict of Interest: Dr. Yang has received honoraria and consulting fees from ViewRay. Dr. Kishan has received honoraria and research funding from ViewRay.