1052 - Results of a Phase 2 Trial of Prostate SBRT with Integrated Tumor Boost and Dose Reduction at Urethra, Bladder and Rectal Interfaces

03:20pm - 03:25pm ET

Room 253

Presenter(s)

Trevor Wilson, MD - University of Wisconsin, Madison, WI

T. Wilson¹, M. M. Basree¹, B. A. Morris¹, R. J. Chappell², G. M. Cooley Jr¹, Z. S. Morris³, M. A. Ritter¹, and J. M. Floberg⁴; ¹Department of Human Oncology, University of Wisconsin Hospitals and Clinics, Madison, WI, ²University of Wisconsin, Madison, WI, ³Department of Human Oncology, University of Wisconsin, Madison, WI, ⁴Department of Human Oncology, University of Wisconsin-Madison, Madison, WI

Purpose/Objective(s):

In this single-institution phase II trial, we evaluated the incidence of GI and GU toxicity and quality of life (QOL) outcomes of patients undergoing prostate stereotactic body radiation therapy (SBRT) with simultaneous integrated boost (SIB) to intraprostatic lesion(s) and reduced dosing at the interface of critical organs-at-risk (OAR).

Materials/Methods:

Between 2015 and 2022, 115 patients were enrolled. Eligible patients had low- or intermediate-risk prostate cancer (Gleason score = 7, clinical stage = T2b, PSA = 20 ng/mL, AUA = 18). Treatment was prescribed in 5 fractions of SBRT to 40 Gy with a SIB to 45 Gy to intraprostatic lesion(s) identified by magnetic resonance imaging (MRI) while limiting the urethra, anterior rectal wall, and bladder to 36.25 Gy. Patients without MRI were treated to a uniform dose of 7.5 Gy x 5 fractions. Acute and late GI and GU toxicity was scored using modified RTOG grading. Initiation or increase in dose of an alpha blocker post-enrollment was considered as G2 GU toxicity. Patients completed QOL surveys including the American Urological Association Symptom Score – International Prostate Symptom Score (AUASS-IPSS), International Index of Erectile Function (IIEF-5), and Expanded Prostate Cancer Index Composite Short Form (EPIC-26) starting with the 6-month follow-up visit. Minimally important difference was calculated as 0.5 standard deviation from baseline. Biochemical-recurrence-free survival (BRFS) was estimated using the Kaplan-Meier method.

Results:

No acute G3+ GU or GI toxicity was observed. G2 acute GU and GI toxicity while on treatment was seen in 28.7% and 2.6% of patients respectively. Prevalence of late G2 GU toxicity was 26 (23.4%) at 12 months, 29 (28.2%) at 36 months, and 15 (34.1%) at 60 months. Among 54 (47.8%) patients who experienced G2 late GU toxicity, 16 (29.6%) were prescribed an alpha blocker after the start of radiation. Two (1.8%) patients experienced G3 late GU toxicity which occurred at 18 and 24 months. Six (5.3%) patients experienced G2 late GI toxicity and there were no G3+ acute or late GI events. With a median follow-up time of 51.7 months, patient-reported incontinence, hormonal, and sexual QOL scores showed no significant long-term change from baseline. Urinary irritation, bowel QOL, AUASS, and IIEF-5 scores declined significantly from baseline, exceeding the minimally important difference threshold over long-term follow-up (F-test p-values 0.0084, 0, 0.0113, and 0.0114 respectively). BRFS was 99.1% at 3 years and 95.9% at 5 years.

Conclusion:

Delivery of prostate SBRT with focal boost to MRI-defined tumor regions and dose reduction at OAR interfaces resulted in excellent biochemical control and acceptable rates of GU and GI toxicity at roughly 4-year follow-up.