Reduction in Rectal Dose in Prostate Cancer Patients Using Hydrogel Spacer During Proton Therapy

Presented by Jesse Conterato, BA (Northwestern Medicine Chicago Proton Center) – at the PTCOG-NA October 2016

Objectives: Injection of hydrogel spacer (HS) can create space between the prostate and rectum and has been demonstrated to reduce rectal dose in men receiving intensity modulated radiotherapy for prostate cancer (CaP). Placement of HS should also reduce rectal dose in CaP patients treated with definitive proton beam radiotherapy (PBRT). This single institution review compares reduction of rectal dose using HS versus daily endo-rectal balloon (RB) in men receiving definitive PBRT for CaP.

Methods: From April 2015 to February 2016, 63 men with localized CaP had placement of HS and were treated with PBRT. This cohort was matched to a cohort of CaP patients treated with PBRT using RB (N=65) based on the following criteria: prostate cancer risk stratification and treatment era of January to December 2014. All patients completed PBRT, receiving 79.2 Cobalt Gray Equivalent (CGE) in 44 fractions. Records were retrospectively reviewed to obtain dosimetric variables assessing bladder, rectal and penile bulb doses. Dosimetric variables were compared using a two-tailed, independent t-test and calculated using Microsoft Excel. Of the 63 patients receiving HS, PBRT was delivered for low risk (N=15), intermediate risk (N=33) and high risk (N=15) disease. PBRT targets were prostate (P) only (N=14), prostate and seminal vesicles (P+SV) (N=36) and P+SV with elective inclusion of pelvic lymph nodes (PLN) (N=13). In this cohort, median prostate size was 63.9cc (35.0-253.0cc). Of the 65 men receiving RB, PBRT was delivered for low risk (N=17), intermediate risk (N=33) and high risk (N=15) disease. PBRT targets were P only (N=15), P+SV (N=41) and P+SV with elective inclusion of PLN (N=9). In this cohort, median prostate size was 58.3cc (30.7-134.0cc).

Results: Median Rectum Eval V50 and V70 were reduced in men using HS compared to RB (21.2% vs. 16.8%, p=0.00003; 11.78% vs. 5.66%, p<0.00001, respectively). Median Rectum V81 was also reduced in men using HS compared to RB (0.02cc vs. 0.00cc, p=0.0004). There were no significant differences in median Bladder V81, V80, V50 and mean penile bulb dose.

Conclusion: HS during definitive PBRT for CaP resulted in lower rectal dose compared to RB. Additional follow-up is needed to assess rectal toxicity.