Genitourinary Cancer

SS 30 - GU 4 - SBRT for Prostate and Renal Cancers

218 - Multi-institutional Analysis of Prostate-Specific Antigen Kinetics Following Stereotactic Body Radiation Therapy

Tuesday, October 23
5:15 PM - 5:25 PM
Location: Room 214 C/D

Multi-institutional Analysis of Prostate-Specific Antigen Kinetics Following Stereotactic Body Radiation Therapy
N. Jiang1, C. R. King1, A. T. Dang1, Y. Yuan1, S. P. Collins2, S. Suy2, C. A. Mantz3, L. Miszczyk4, A. Napieralska4, A. Namysl-Kaletka5, N. G. Nickols1,6, D. Shabsovich7, M. L. Steinberg8, P. A. Kupelian1, and A. U. Kishan1; 1Department of Radiation Oncology, University of California, Los Angeles, Los Angeles, CA, 2Department of Radiation Medicine, Georgetown University Hospital, Washington, DC, 321st Century Oncology, Fort Myers, FL, 4Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology Gliwice Branch, Gliwice, Poland, 5Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology Gliwice Branch, Gilwice, Poland, 6VA Greater Los Angeles Health System, Los Angeles, CA, 7David Geffen School of Medicine at UCLA, Los Angeles, CA, 8University of California, Los Angeles, Los Angeles, CA

Purpose/Objective(s): Understanding prostate-specific antigen (PSA) kinetics after radiation therapy plays a large role in the management and counseling of prostate cancer patients. This is particularly true with regards to the establishment of expectations regarding PSA nadir (nPSA) and the bounce phenomenon, which can be disconcerting. As an increasing number of patients are being treated with stereotactic body radiation therapy (SBRT) for low- and intermediate-risk prostate cancer, it is imperative to understand the PSA response to SBRT.

Materials/Methods: PSA data from 5 different institutions were retrospectively analyzed for patients with localized prostate cancer treated definitively with SBRT alone from 2008 to 2016. All patients were treated to a dose of 35-40 Gy in 5 fractions per institutional standards. Patients who had less than 12 months of PSA data or received androgen deprivation therapy (ADT) were excluded from this study. PSA nadir was defined as the lowest PSA value after treatment and time to nPSA was defined as the time between nPSA and the radiation start date. Biochemical failure was defined as a ≥2 ng/ml rise above nPSA per ASTRO guidelines. Benign PSA bounce was defined as any increase in PSA ≥0.2 ng/ml that then returned to previous nadir. Time to PSA bounce was defined as the time between first PSA bounce and the radiation start date. Unpaired two-tailed t-tests were used for statistical analyses.

Results: In total, 1020 patients were included in this study with 523 low-risk and 497 intermediate-risk prostate cancer patients as defined by the D’Amico risk stratification. Median PSA follow-up was 66 months (range 12-120 months). Biochemical failure occurred in 4% of patients. Median age at treatment was 68 years (range 41-87 years). Median nPSA was 0.2 ng/ml, median time to nPSA was 30.8 months, 82% of patients had a nPSA <0.5 ng/ml, and 45% of patients had a nPSA <0.2 ng/ml. Patients who were more likely to have a lower mean nPSA had low-risk disease (p<0.001), did not have biochemical failure (p<0.001), or experienced a benign PSA bounce (p=0.02). Benign PSA bounce was noted in 24% of patients and 22% of those that had a bounce experienced more than one PSA bounce. The mean PSA bounce size was 0.76 ng/ml (SD=0.65). Patients who experienced a benign PSA bounce did not have a statistically significant difference in initial PSA value (p=0.70). Median time to PSA bounce was 20 months.

Conclusion: In a multi-institutional cohort of patients with long-term follow-up, SBRT is confirmed to have low nPSAs, consistent with previous studies. It is important to note that benign PSA bounces occurred in almost a quarter of patients and can occur as late as several years after treatment. Further studies are needed to directly compare the PSA response of patients who receive prostate SBRT versus other treatment modalities.

Author Disclosure: N. Jiang: None. C.R. King: None. Y. Yuan: None. S.P. Collins: None. C.A. Mantz: None. L. Miszczyk: Honoraria; Accuray, Inc. A. Napieralska: None. A. Namysl-Kaletka: None. D. Shabsovich: None. M.L. Steinberg: Honoraria; Accuray. Department Chair; UCLA. P.A. Kupelian: Research Grant; Varian Medical Systems, Inc. Honoraria; Accuray. Consultant; ViewRay. Advisory Board; Accuray, ViewRay. Travel Expenses; Accuray, ViewRay. VP of Clinical Affairs; Varian Medical Systems. A.U. Kishan: None.

Naomi Jiang, MD

UCLA Radiation Oncology

Disclosure:
Employment
UCLA: Resident Physician: Employee

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