Mixed metabolic response on PET/CT in patients with metastatic breast cancer as an early predictor of disease progression.

e11522 Background: It has been observed that patients undergoing systemic therapy for metastatic breast cancer (MBC) may demonstrate a mixed metabolic treatment response on FDG PET (MMTR) while maintaining stable disease on CT (SDCT). The purpose of this study is to determine (1) the clinical outcome of MBC patients demonstrating MMTR+SDCT who were maintained on the same treatment regimen and (2) the behavior of oncologists when faced with a report of MMTR+SDCT. Methods: All FDG PET/CT scan reports performed at one institution between 2006-2011 were searched and studies describing a MMTR also containing the term “breast cancer” were identified. MBC patients without other active malignancies were extracted from the search and scans were re-analyzed. MMTR was defined as a PET/CT scan demonstrating a minimum of 2 lesions with a SUVmax change of +25% or more combined with at least 2 lesions demonstrating a SUVmax change of -25% or more. The subset of patients with MMTR+SDCT (by anatomical RECIST criteria) who were maintained on their current treatment regimen was further examined to identify time to progression (TTP). Results: Thirty-two cases were identified that demonstrated MMTR+SDCT in metastatic breast cancer patients. No change in therapy occurred within 3 months of the scan in 9/32 patients. Of the 9 patients who were maintained on their current treatment regimen, 5 patients demonstrated subsequent disease progression at 3 (n=2), 5 (n=2), and 11 (n=1) months (mean TTP = 5.4 months, range 3-11 months). One patient remains stable at 16 months and 3 patients have insufficient follow-up. Therapy was changed immediately after the scan in the remaining 22 patients for various reasons including preference of the treating oncologists, treatment toxicities, increase in tumor markers, or clinical progression. Conclusions: MMTR+SDCT in patients with MBC predicted subsequent disease progression within 6 months in the majority of individuals. Oncologists commonly alter treatment regimen when faced with a report of MMTR+SDCT. Based on the observation of outcomes in patients who did not change therapy, this practice seems justified.