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SU_31_2320 - PSA Bounce after Stereotactic Body Radiation Therapy for Prostate Cancer: Pooled Analysis from 4 SBRT Trials Evaluating Different Time-Dose-Fraction Schedules

Sunday, October 21
1:15 PM - 2:45 PM
Location: Innovation Hub, Exhibit Hall 3

PSA Bounce after Stereotactic Body Radiation Therapy for Prostate Cancer: Pooled Analysis from 4 SBRT Trials Evaluating Different Time-Dose-Fraction Schedules
S. Roy1, D. A. Loblaw2, P. Cheung3, W. Chu3, H. T. Chung4, D. Vesprini4, A. T. Ong5, A. Chowdhury5, D. Panjwani6, G. Pang7, R. Korol3, M. T. Davidson8, A. Ravi9, B. McCurdy10, J. Helou11, L. Zhang12, A. Mamedov7, A. Deabreu7, and H. C. Quon1; 1Department of Oncology, Division of Radiation Oncology, Tom Baker Cancer Center, University of Calgary, Calgary, AB, Canada, 2Sunnybrook Odette Cancer Centre, University of Toronto, Toronto, ON, Canada, 3Department of Radiation Oncology, Sunnybrook Odette Cancer Centre, University of Toronto, Toronto, ON, Canada, 4Department of Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, ON, Canada, 5CancerCare Manitoba, Winnipeg, MB, Canada, 6BC Cancer Agency, Abbotsford, BC, Canada, 7Sunnybrook Health Sciences Centre, Toronto, ON, Canada, 8University of Toronto, Toronto, ON, Canada, 9Department of Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, ON, Canada, 10University of Manitoba, Winnipeg, MB, Canada, 11Institute of Health Policy, Management and Evaluation, University of Toronto, Toronto, ON, Canada, 12MacroStat Inc., Toronto, ON, Canada

Purpose/Objective(s): Prostate serum antigen (PSA) bounce can mimic biochemical failure after stereotactic body radiotherapy (SBRT) for prostate cancer (P-Ca). We carried out a pooled analysis of 4 SBRT trials for low and intermediate P-Ca to evaluate the incidence of PSA bounce and its correlation with different time-dose-fraction schedules.

Materials/Methods: The study included 4 treatment groups (TG): 35 Gy/5 fractions (F) once per week (QW) (n=84); 40 Gy/5F QW (n=105), 40 Gy/5F every other day (EOD) (n=77); and 26 Gy/2F QW (n=30). Bounce was defined as an increase in PSA to nadir+0.2 ng/mL or nadir+2 ng/mL followed by a decrease back to nadir. Kruskal-Wallis test and Fisher exact test were used for continuous and categorical variables. P-value < 0.05 was considered statistically significant. Univariable and multivariable logistic regression were used to evaluate the influence of patient and treatment factors on bounce incidence.

Results: 287 patients were included with a median follow-up of 4.9 years (y) (IQR: 3.7-8.7y). Overall 2y and 5y cumulative incidences of PSA bounce by nadir+2 ng/mL were 6.3% and 8%, respectively where as overall cumulative incidences of bounce by nadir+0.2 ng/mL were 22.5% and 31% at 2y and 5y respectively. Results are reported by TG for 35 Gy/5F QW, 40 Gy/5F QW, 40 Gy/5F EOD, and 26 Gy/2F QW, respectively. The 2y cumulative incidences of bounce by nadir+0.2 ng/mL were 28.9%, 21%, 19.6% and 16.7% (p=0.12) and by nadir+2 ng/mL were 7.2%, 8%, 2.7% and 6.7% (p=0.32). Median times from start of radiotherapy to first bounce were not significantly different by nadir+0.2 ng/mL (18.2, 17.7, 17, and 11.5 months; p=0.16) or nadir+2 ng/mL (18.2, 15.7, 10, and 14 months; p=0.09). The duration of bounce among the TG was significantly different by nadir+0.2ng/mL (18.2, 15, 12.8, and 11.9 months; p=0.005) but not by nadir+2ng/mL (16.5, 20.4, 13.7, and 14.6 months; p=0.69). Multivariable analysis revealed that for nadir+0.2 ng/mL bounce, patients with older age (OR 0.95; 95% CI 0.91-0.98) and cT2 tumors (OR 0.43; 95% CI 0.21-0.84) had a lower probability of bounce while for nadir+2 ng/mL bounce, patients with higher baseline PSA were less likely to have bounce (OR 0.49; 95% CI 0.26-0.97).

Conclusion: The cumulative incidence of PSA bounce does not differ between the 4 prostate SBRT schedules used in this pooled analysis. However, 1 in 13 patients experiences a PSA bounce of nadir+2 ng/mL by 5 years and may be misdiagnosed as biochemical failures if not followed closely. The duration of PSA bounce does vary significantly when a low threshold of nadir+0.2 ng/mL is used. There is a higher incidence of PSA bounce associated with younger age, cT1 stage, and lower baseline PSA depending on the bounce threshold.

Author Disclosure: S. Roy: None. D. Loblaw: Honoraria; AbbVie, Astellas, Bayer, Janssen. Consultant; AbbVie. Advisory Board; Amgen, Astellas, Ferring, Janssen. Patent/License Fees/Copyright; Sunnybrook Research Institute. P. Cheung: Independent Contractor; Ontario Ministry of Health and Long-Term Care. Research Grant; Sanofi Aventis, Pfizer, Abbvie. H.T. Chung: None. D. Vesprini: None. A.T. Ong: None. A. Chowdhury: None. G. Pang: None. M.T. Davidson: None. H.C. Quon: Research Grant; CancerCare Manitoba Foundation.

Soumyajit Roy, MD

BC Cancer Agency-Abbotsford Centre

Biography:
Dr. Soumyajit Roy completed his residency in Radiation Oncology from All India Institute of Medical Sciences, New Delhi, India and joined BC Cancer Agency for an year of clinical research fellowship in 2016. After finishing his fellowship at BC Cancer, he worked as a brachytherapy fellow in Tom Baker Cancer Center, University of Calgary. His areas of interest are prostate cancer, lung cancer, gastro-intestinal and CNS malignancies. He is highly motivated in research and his research interests include patient reported outcome, health services research, integration of basic sciences like radiation biology and physics into mainstream clinical studies. He is a trained singer and loves to sing and write poems in his free time.

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