3184 - Long-Term Results of Prostate Stereotactic Body Radiation Therapy (SBRT) with Real-Time Tracking and Novel Urethral- and Rectal-Sparing Techniques

12:45pm - 02:00pm PT

Hall F

Screen: 8

POSTER

Presenter(s)

Katherine Amarell, MD, BS - Cleveland Clinic Foundation, Cleveland, OH

K. Amarell¹, C. A. Reddy², K. L. Stephans³, O. Y. Mian⁴, P. Qi², and R. D. Tendulkar²; ¹Cleveland Clinic Foundation, Cleveland, OH, ²Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH, ³Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH, ⁴Department of Radiation Oncology, University of Washington/ Fred Hutchinson Cancer Center, Seattle, WA

Purpose/Objective(s): Our institution has standardly used a novel urethral-sparing SBRT technique to mitigate genitourinary (GU) toxicity since 2011. Rectal spacers are increasingly adopted, but little long-term data exist among patients treated with SBRT. We report our long-term outcomes of urethral- and rectal-sparing SBRT in patients with a minimum 5-year follow up.

Materials/Methods: An IRB-approved database was queried for patients with treated with SBRT for prostate cancer using a standardized organ-sparing protocol. A 3 mm expansion of the prostate and proximal seminal vesicles was prescribed 36.25 Gy in 5 fractions. The urethra, bladder, and rectum were expanded by 3 mm to create an avoidance structure which was subtracted from the prostate volume to define a high-dose planning target volume (PTV) that was prescribed 40 to 50 Gy.

Results: 175 patients were treated per institutional protocol between 2011 and 2019, with a median follow up of 5 years. This cohort included low-risk (15%), intermediate-risk (56%), and high-risk (29%) patients, with 34% receiving ADT. A rectal balloon was utilized in 74% of cases (n=130), while 26% (45) had a rectal spacer. Real-time tracking was utilized in 77% (134) of patients, with tracking modalities that varied over time and included a real-time tracking system (7%), ultrasound-based tracking (10%), and kV triggered imaging (59%). At five years, biochemical recurrence-free survival (bRFS) was 84% (95% CI: 77%–90%), clinical failure-free survival was 91% (95% CI: 86%–96%), distant metastasis-free survival was 94% (95% CI: 90%–98%), and overall survival was 89% (95% CI 84%-94%). There was no difference in biochemical failure between patients treated with rectal balloon versus spacer (HR 1.12, 95% CI: 0.46–2.74, p=0.8). On univariate analysis, use of real-time tracking, risk group, and dose to the high-dose target were associated with biochemical and clinical failure, but only risk group remained significantly associated on multivariate analysis.

The 5-year incidence of late Grade =2 GU toxicity was 15% (mainly due to the use of alpha blockers), and late Grade =3 GU toxicity was 0.7% (one stricture requiring dilation 2 years post-treatment). The 5-year incidence of Grade =2 gastrointestinal (GI) toxicity was 3.8% overall, occurring in 5% with rectal balloon and 0% with rectal spacer (p=0.16). The 5-year incidence of Grade =3 GI toxicity was 1.8% overall, occurring in 2.4% with a rectal ballon (two cases of rectal bleeding requiring argon plasma coagulation at 12 and 21 months post-treatment), and 0% with rectal spacer.

Conclusion: Urethral-sparing SBRT with a rectal spacer and real-time tracking offers low rates of late GU/GI toxicity and favorable oncologic outcomes in this population of mainly intermediate- and high-risk disease. This novel technique using a standardized workflow remains our institutional standard to minimize late toxicities.