High Dose "HDR-Like" Prostate SBRT: PSA 10-Year Results From a Mature, Multi-Institutional Clinical Trial

Donald B Fuller; Tami Crabtree; Brent L Kane; Clinton A Medbery; Robert Pfeffer; James R Gray; Anuj Peddada; Trevor J Royce; Ronald C Chen

doi:10.3389/fonc.2022.935310

High Dose "HDR-Like" Prostate SBRT: PSA 10-Year Results From a Mature, Multi-Institutional Clinical Trial

Front Oncol. 2022 Jul 29:12:935310. doi: 10.3389/fonc.2022.935310. eCollection 2022.

Authors

Donald B Fuller¹, Tami Crabtree², Brent L Kane³, Clinton A Medbery⁴, Robert Pfeffer⁵, James R Gray⁶, Anuj Peddada⁷, Trevor J Royce⁸, Ronald C Chen⁹

Affiliations

¹ CyberKnife Centers of San Diego, San Diego, CA, United States.
² Advance Research Associates, Santa Clara, CA, United States.
³ Community Cancer Center, Clovis, CA, United States.
⁴ Southwest Radiation Oncology, Oklahoma City, OK, United States.
⁵ Benefis Sletten Cancer Institute, Great Falls, MT, United States.
⁶ Sarah Cannon Research Institute, Nashville, TN, United States.
⁷ Penrose-St. Francis Health Services, Colorado Springs, CO, United States.
⁸ University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.
⁹ University of Kansas, Kansas City, KS, United States.

Abstract

Purpose/objectives: Although ample intermediate-term prostate stereotactic body radiotherapy (SBRT) outcomes have been reported, 10-year results remain relatively sparse.

Materials/methods: Eighteen institutions enrolled 259 low- and intermediate-risk patients. Median follow-up is 5.5 years, with 66 patients followed ≥ 10 years. This SBRT regimen specifically emulated an existing HDR brachytherapy dose schedule and isodose morphology, prescribed to 38 Gy/4 fractions, delivered daily by robotic SBRT, mandating > 150% dose escalation in the peripheral zone. Androgen deprivation therapy was not allowed, and a hydrogel spacer was not available at that time.

Results: Median pre-SBRT PSA 5.12 ng/mL decreased to 0.1 ng/mL by 3.5 years, with further decrease to a nadir of < 0.1 ng/mL by 7 years, maintained through 10 years. Ten-year freedom from biochemical recurrence measured 100% for low-risk, 84.3% for favorable intermediate risk (FIR), and 68.4% for unfavorable intermediate (UIR) cases. Multivariable analysis revealed that the UIR group bifurcated into two distinct prognostic subgroups. Those so classified by having Gleason score 4 + 3 and/or clinical stage T2 (versus T1b/T1c) had a significantly poorer 10 year freedom from biochemical recurrence rate, 54.8% if either or both factors were present, while UIR patients without these specific factors had a 94.4% 10-year freedom from biochemical recurrence rate. The cumulative incidence of grade 2 GU toxicity modestly increased over time - 16.3% at 5 years increased to 19.2% at 10 years-- while the incidence of grade 3+ GU and GI toxicity remained low and stable to 10 years - 2.6% and 0%, respectively. The grade 2 GI toxicity incidence also remained low and stable to 10 years - 4.1% with no further events after year 5.

Conclusion: This HDR-like SBRT regimen prescribing 38 Gy/4 fractions but delivering much higher intraprostatic doses on a daily basis is safe and effective. This treatment achieves a median PSA nadir of <0.1 ng/mL and provides high long-term disease control rates without ADT except for a subgroup of unfavorable intermediate-risk patients.

Keywords: CyberKnife; HDR; PSA nadir; SBRT; prostate cancer; relapse free rate.