148 - Stereotactic Intensity Modulated Radiotherapy after Radical Prostatectomy (SCIMITAR): Four-Year Outcomes of a Phase II Clinical Trial
Presenter(s)
J. E. Juarez Casillas1, P. Sargos2, T. Romero3, S. Chabaud4, M. Brihoum5, A. Sachdeva1, J. Nikitas1, T. M. M. Ma6, L. K. Ballas7, L. Valle1, R. E. Reiter8, C. Saigal9, K. Chamie8, M. Litwin9, M. Rettig10, N. G. Nickols11, M. Cao12, P. Pommier13, M. L. Steinberg1, and A. U. Kishan1; 1Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, CA, 2CRLCC Institut Bergonie, Bordeaux, Aquitaine, France, 3Department of Medicine, University of California, Los Angeles, Los Angeles, CA, 4Centre Leon-Berard, Lyon, France, 5Unicancer, Paris, France, 6Department of Radiation Oncology, University of Washington - Fred Hutchinson Cancer Center, Seattle, WA, 7Department of Radiation Oncology, Cedars-Sinai Medical Center, Los Angeles, CA, 8Department of Urology, University of California, Los Angeles, Los Angeles, CA, 9UCLA, Los Angeles, CA, 10Department of Medical Oncology, University of California, Los Angeles, Los Angeles, CA, 11University of California Los Angeles, Department of Radiation Oncology, Los Angeles, CA, 12Department of Radiation Oncology, University of California San Francisco, San Francisco, CA, 13Centre Leon Berard, Lyon, France
Purpose/Objective(s): The efficacy of stereotactic body radiation therapy (SBRT) targeting the prostate fossa ± pelvic nodes after radical prostatectomy (RP) remains unclear. This phase II trial was designed to prospectively evaluate the oncologic efficacy of this approach.
Materials/Methods: Patients with a rising PSA or adverse pathologic features after RP were enrolled on a dual-center phase II trial (NCT03541850). Patients received 30 to 34 Gy in 5 fractions to the prostate bed ± bed boost ± pelvic nodes, with the use of androgen deprivation therapy (ADT) and nodal therapy at the discretion of the treating physician. The primary endpoint was 4-year biochemical recurrence-free survival (BCRFS), with biochemical recurrence (BCR) defined as a rise in the post-treatment nadir by =0.2 ng/mL. To achieve 83.9% power to detect an increase in 4-year BCRFS calculated by the Kaplan-Meier method from 60% (literature-based historical control) to 72% with a one-sided, one-sample logrank test at 0.05 significance required a target accrual of 100 patients. A competing risk framework was used to estimate 4-year cumulative incidence of BCR, accounting for death as a competing risk. Multivariable Fine-Gray models were used to compare time to BCR between patients on the SCIMITAR trial and 743 patients enrolled on the phase III GETUG-AFU 16 trial of conventionally-fractionated salvage radiotherapy (CFRT) with or without 6 months of ADT, using individual patient data obtained via the MARCAP consortium. Fine-Gray models were adjusted for age, pre-radiotherapy PSA, extracapsular extension, seminal vesicle invasion, surgical margin status, and pathological Gleason score, and stratified by ADT use.
Results: 100 patients were enrolled from February 2018-March 2021, receiving a median prostate bed dose of 32Gy (interquartile range [IQR] 30-34). Median follow-up was 52 months (IQR 45-59). 27 patients (27%) received elective pelvic nodal RT and 41 (41%) received ADT with a median duration of 6 months. Median pre-SBRT PSA was 0.3 ng/mL (IQR 0-9.3). Four patients (4%) experienced local recurrence, 14 (14%) had nodal recurrence, and 18 (18%) developed distant metastasis. 4-year BCRFS was 62% (95% CI, 0.52-0.72), which was not significantly different from the literature-based historical control (p=0.34). In the cross-trial comparison with individual patient data, among patients not receiving ADT, CFRT was associated with higher BCR (subdistribution HR [sHR]1.80, 95%CI 1.02-3.15, p=0.04). There was no significant difference between SBRT and CFRT in patients receiving ADT (sHR 0.62, 95%CI 0.22-1.75, p=0.4).
Conclusion: In this phase II study, post-RP SBRT demonstrated favorable 4-year BCRFS, both compared with a historical control threshold and in an individual-patient data comparison to patients treated on a large phase III trial with CFRT. Local recurrences were rare.