Oblique Beam Angles Lead to Better Rectal Protection But Worse Bladder Sparing in Prostate Cancer Receiving Pencil-Beam-Scanning Proton Therapy (PBS)
Y Yang, J-C Rwigema, C Vargas, N Yu, S Keole, W Wong, S Schild, M Bues*, W Liu, J Shen, Mayo Clinic Arizona, Phoenix, AZ
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PO-GePV-T-165 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To investigate the dosimetric impact of beam angle changes in prostate cancer treated with pencil-beam-scanning proton therapy (PBS).
Methods: Ten prostate patients were included in this study. Four plans for each patient were generated, one plan with left and right lateral parallel-opposed beams and three ones from posterior oblique angles of 5°, 10°, and 15° respectively. Dose-Linear-Energy-Transfer (LET)-volume histogram was employed to study the changes in dose and LET after beam angle changes. Volume metrics, V(d,l), defined as the cumulative normalized volume that has a dose of at least d (Gy[RBE]) and an LET of at least l (keV/µm), were calculated for both rectum and bladder. To evaluate the relative volume metrics changes, the relative volume change (RVC) was used. Three relative-biological-effective (RBE) models were employed to study the biological effect from dose and LET. Effect sizes for volume metrics of all RBE doses were evaluated.
Results: Both high dose and LET volumes were significantly reduced in rectum using oblique angles. High RBE dose volume was effectively minimized at high dose in rectum with a maximum RVC of -69.7%. LETs but not doses in bladders significantly increased, which caused a maximum RVC of 34.6% at high dose. Both magnitudes and effect sizes of the increase in bladder were less than those of the decrease in rectum.
Conclusion: Oblique angle plans achieved better dose and LET sparing of rectum while sacrificing LET sparing in bladder. Changing beam angles could be a good strategy to redistribute LET distributions.
Funding Support, Disclosures, and Conflict of Interest: This research was supported by Arizona Biomedical Research Commission Investigator Award (ADHS16-162521), the Lawrence W. and Marilyn W. Matteson Fund for Cancer Research, and the Kemper Marley Foundation.
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