Comparison of External Beam and Combination Therapy for Prostate Cancer: Patient-Reported Outcomes of Sexual Function With 5-Year Follow-up

Abstract

prostate and proximal seminal vesicles as defined on non-contrast CT and fused T2 MRI with a 3 mm posterior margin and a 5 mm margin in all other directions. Patients were instructed to empty their bladder prior to simulation and treatment, and the entire bladder and its contents were contoured as a single structure. Inverse plans were generated with a prescription dose (PD) of 35 to 36.25 Gy in 5 fractions to the PTVusing 6 MV photons. QOL surveys including the American Urological Association (AUA) Symptom Index were conducted before and after treatment every 3 months for the first year and then every 6 months. Late urinary flare was defined as an AUA score > 15 with an increase 5 above baseline with subsequent return to baseline within 2 years. Phenomenologic NTCP models were fit to bladder DVHs and late urinary flare outcome data using maximum likelihood estimation. Results: Twenty-nine patients experienced late urinary flare within 2 years of completion of prostate SBRT. Fitting of urinary bladder DVH data to a Lyman NTCP model resulted in parameter estimates of m, TD50, and n of 0.19 (-0.09 to 0.47), 38.7 Gy (31.1 to 46.4), and 0.13 (-0.14 to 0.41), respectively. Subsequent fits to threshold dose and hottest volume probit models revealed a significant association of late urinary flare with volume of bladder receiving doses greater than 36.6 Gy as well as dose to the hottest 12.7% of bladder volume. Similar results were obtained using a dose-volume cutoff model that additionally demonstrated a significant association of volume of bladder receiving at least 50% of the PD with late toxicity. The hottest volume probit model demonstrated the best fit of the data and suggests dose-volume cutoffs for D12.7% of 30 and 33.5 Gy in five fractions for a predicted complication rate of 5% and 10% at 2 years, respectively. Conclusions: NTCP modeling of late urinary flare after prostate SBRT demonstrates a relatively small volume effect for dose to the urinary bladder, suggesting that reduction of volume receiving elevated dose will result in decreased incidence of late urinary toxicity. Future studies will be needed to examine the impact of dose to the prostatic urethra. Author Disclosure: T.P. Kole: None. B. Wu: None. M. Tong: None. S. Lei: None. L.N. Chen: None. S. Suy: None. A. Dritschilo: None. E.D. Yorke: None. S.P. Collins: G. Consultant; Accuray, Inc.