Rectal dose-volume differences for scanning proton beam therapy with or without an endorectal balloon for the treatment of prostate cancer.

Abstract

243 Background: Endorectal balloons (ERB) are routinely used in proton radiation for prostate cancer (PCa). The dosimetric implications, however, in conjunction with scanning proton beam therapy (SPT) have not been studied. We sought to describe dose-volume values with the use of an ERB vs no ERB for SPT for PCa. Methods: We analyzed 10 patients with localized PCa. Each patient underwent two CT simulations, one with a water-filled ERB (50-60 ml) and one with no ERB. For 8 patients, the prostate and proximal 1 cm of the seminal vesicles was the clinical target volume (CTV), with the prostate only as CTV for 2 patients. We defined 3 structures: rectum (R), anterior rectal wall (ARW) as a 3 mm rim of the anterior half of the rectum, and AR-CTV as the ARW only on slices with a contoured CTV. SPT plans were created in both cases for each patient, with the creation of a scanning target volume (STV), which expanded the CTV by 6 mm in all dimensions except 4 mm posteriorly and 12 mm laterally. Two opposed lateral beams were used to plan to the STV. Total dose was 78 Co-60 Gy equivalent in 39 fractions, to cover 95% of the STV. RBE was 1.1. We compared the dose-volume values for the R, ARW, and AR-CTV between the two arms. Results: The ERB significantly increased the R volume for all cases (61 ± 11 ml vs 105 ± 10 ml (p<0.01)). Rectal volumes radiated for all cases: V10, 55%; V50, 23%; and V70, 11%. The ERB significantly decreased the R volume radiated for all dose levels V5-V82 (p<0.05). The absolute difference was larger at low dose levels (8% at V10) and intermediate dose levels (5% at V50) compared to high dose levels (3% at V70). For the ARW, the ERB did not significantly change the volume radiated at any dose level. For the AR-CTV, the ERB significantly decreased the volume radiated for dose levels V20-V60 (p<0.05). Conclusions: The ERB provided a significant decrease in R volume radiated at all dose levels, and significantly decreased the AR-CTV volume, which represents a high risk volume, radiated at intermediate dose levels. Rectal doses with SPT, however, were low whether using an ERB or not. In the setting of the low rectal doses delivered, further study regarding the clinical benefit is warranted.