Intensity-modulated radiotherapy with mri simulation to reduce doses received by erectile tissue during prostate cancer treatment

Purpose: The radiation doses received by erectile tissue may contribute to erectile dysfunction after treatment of prostate cancer. This is the first description of the ability to limit the dose received by the penile bulb (PB) and corporal bodies (CB) using intensity-modulated radiotherapy (IMRT). Methods and Materials: Twenty-three patients with palpation Stage T1c-T2bN0M0 prostate cancer received IMRT alone. The dose prescribed to the planning target volume was 74-78 Gy. All patients underwent CT and MRI simulation to define the target and normal structures. Three plans with identical beam arrangements and energy were generated for each patient, with varying dose constraints for the PB and CB: no dose constraint, intermediate-dose constraint (20 Gy and 15 Gy, respectively) and low-dose constraint (15 Gy and 7 Gy, respectively). All plans were normalized, such that 95% of the planning target volume received at least 100% of the prescribed dose. For each plan, the ability to meet prostate dose homogeneity criteria (PHC; prostate maximal dose ≤120% prescribed dose) and rectal tolerance dose-volume histogram criteria (RTC; ≤35% and ≤17% of rectal volume received 40 Gy and 65 Gy, respectively) was determined. The D ₉₀, V₅₀, and V₇₅ were determined for both PB and the CB, where D_i was the dose received by i% of the target volume and V_i was the target volume receiving i% of the prescribed dose. Results: The median PB D₉₀, V₅₀, and V₇₅ for the plans with no dose, intermediate-dose, and low-dose constraints was 20. 8 Gy, 33.8%, and 9.9%; 8.0 Gy, 1.7%, and 0%; and 7.1 Gy, 0.1%, and 0%, respectively. The median CB D₉₀, V₅₀, and V₇₅ for plans with no dose, intermediate-dose, and low-dose constraints was 10.2 Gy, 3.8%, and 0%; 6.0 Gy, 0%, and 0%; and 4.9 Gy, 0%, and 0%, respectively. Overall differences in the D₉₀, V₅₀, and V₇₅ among the groups were significant for both the PB and the CB (p <0.0001). All plans with no dose constraint met the PHC and RTC. Twenty plans with an intermediate-dose constraint met the PHC and 21 met the RTC. Eighteen plans with a low-dose constraint met the PHC and 19 met the RTC. No statistically significant difference was found in the number of beam segments for the three groups (median of 51, 55, and 53; p = 0.8). Conclusion: In the vast majority of cases, it is possible to limit the dose to erectile tissue with IMRT, usually by ≥50% without significantly compromising the PHC, RTC, or treatment duration. A Phase III randomized trial has been designed to test the clinical significance of the erectile tissue-sparing technique described here.