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.
TH- External Beam- Particle/high LET therapy: Proton therapy – treatment planning/virtual clinical studies