Bone scan “flare” in patients receiving abiraterone acetate (AA) for metastatic castration resistant prostate cancer (mCRPC): Analysis of data from a phase II study of the Department of Defense Prostate Cancer Clinical Trials Consortium.

Abstract

4545 Background: The utility of bone scintigraphy in metastatic prostate cancer is limited by the risk of false positive findings. False positives can occur for many reasons, including a “flare” phenomenon due to the osteoblastic response that occurs as a result of bone healing after successful anti-neoplastic therapy. The incidence of bone flare in metastatic prostate cancer is unknown clinically but is important because it can result in patients (pts) being taken off active agents too soon on the basis of a potentially erroneous bone scans. Methods: 33 pts with progressive metastatic castration resistant prostate cancer (mCRPC), normal organ function, and no prior chemotherapy were enrolled on a phase II trial of Abiraterone acetate (AA) plus prednisone. PSA was obtained monthly and bone scan (BS) results, as reported locally, were evaluated along with PSA changes at baseline, just prior to starting cycle (c) 4 and c7. BS flare was defined as BS findings interpreted by a local radiologist as “disease progression” in the face of a ≥50% decline in PSA, which on follow up scan revealed an improved scan (i.e. disease regression). Results: A ≥50% decline in PSA at the time of c4 day 1 imaging was seen in 25/33 (76%) pts. Of these 25 pts, 23 were evaluable for flare (i.e. bone scans were evaluable at baseline, c4, and c7). Of 23 evaluable pts (each had a PSA response), 12 had discordant (worse) bone scans at c4 (52%). Of these 12 pts, 10 (83%) had subsequent improvements in BS at c7. Thus, overall BS flare was observed in 10/23 (43.5%) evaluable pts or 10/33 (30%) enrolled pts. Conclusions: Discordant bone scans occurred in over half of the mCRPC pts responding to AA by PSA criteria (12/23 patients). Over 80% of these scans were subsequently found to be flare phenomena, as demonstrated by improved follow up bone scans. Thus, overall, the flare phenomenon was a common event (43% of responding patients and 30% of all pts enrolled). BS “flare” and potentially false declarations of disease progression are common in this setting and should not be misconstrued as a treatment failure. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Ortho Biotech Oncology (Cougar Biotechnology)