Technical note: Investigation of dose and LETd effect to rectum and bladder by using non-straight laterals in prostate cancer receiving proton therapy

Background Parallel-opposed lateral beams are the conventional beam arrangements in proton therapy for prostate cancer. However, when considering linear energy transfer (LET) and RBE effects, alternative beam arrangements should be investigated. Purpose To investigate the dose and dose averaged LET (LETd) impact of using new beam arrangements rotating beams 5 degrees-15 degrees posteriorly to the laterals in prostate cancer treated with pencil-beam-scanning (PBS) proton therapy. Methods Twenty patients with localized prostate cancer were included in this study. Four proton treatment plans for each patient were generated utilizing 0 degrees, 5 degrees, 10 degrees, and 15 degrees posterior oblique beam pairs relative to parallel-opposed lateral beams. Dose-volume histograms (DVHs) from posterior oblique beams were analyzed. Dose-LETd-volume histogram (DLVH) was employed to study the difference in dose and LETd with each beam arrangement. DLVH indices, V(d,l)$V( {d,l} )$, defined as the cumulative absolute volume that has a dose of at least d (Gy[RBE]) and a LETd of at least l (keV/mu m), were calculated for both the rectum and bladder to the whole group of patients and two-sub groups with and without hydrogel spacer. These metrics were tested using Wilcoxon signed-rank test. Results Rotating beam angles from laterals to slightly posterior by 5 degrees-15 degrees reduced high LETd volumes while it increased the dose volume in the rectum and increased LETd in bladders. Beam angles rotated five degrees posteriorly from laterals (i.e., gantry in 95 degrees and 265 degrees) are proposed since they achieved the optimal balance of better LETd sparing and minimal dose increase in the rectum. A reduction of V(50 Gy[RBE], 2.6 keV/mu m) from 7.41 to 3.96 cc (p < 0.01), and a slight increase of V(50 Gy[RBE], 0 keV/mu m) from 20.1 to 21.6 cc (p < 0.01) were observed for the group without hydrogel spacer. The LETd sparing was less effective for the group with hydrogel spacer, which achieved the reduction of V(50 Gy[RBE], 2.6 keV/mu m) from 4.28 to 2.10 cc (p < 0.01). Conclusions Posterior oblique angle plans improved LETd sparing of the rectum while sacrificing LETd sparing in the bladder in the treatment of prostate cancer with PBS. Beam angle modification from laterals to slightly posterior may be a strategy to redistribute LETd and perhaps reduce rectal toxicity risks in prostate cancer patients treated with PBS. However, the effect is reduced for patients with hydrogel spacer.