Can 8 week 18F-FDG PET/CT predict clinical response in evaluation of bone metastases in breast cancer?

e12539 Background: 18F-FDG (fluorodeoxyglucose) PET/CT scan is widely used for staging and response assessment of metastatic breast cancer. However, the role of early treatment response assessment of bone metastases remains undefined and the optimal method not yet determined. Our hypothesis was that early 18F-FDG PET/CT can predict subsequent clinical response and our aim was to compare early 8 week 18F-FDG PET/CT with clinical response assessment up to 12 weeks in bone metastases following endocrine treatment. Methods: Eighteen patients starting endocrine treatment for de novo or progressive bone metastases were prospectively recruited. 18F-FDG PET/CT scans were performed before and 8 weeks after treatment. Percentage change in maximum SUV (SUVmax) from the same ≤ 5 index lesions was measured. Clinical response up to 12 weeks, (combination of CT/bone scintigraphy, patient symptoms, Ca-15.3), assessed by an oncologist blinded to PET imaging findings was used as a reference standard. Results: In the 4 patients with progressive disease (PD); SUVmax increased ( > 25%) in 2/20 (10%) and was stable in 15/20 (75%) lesions. Clinically, 2/4 (50%) patients had stable symptoms and 2/4 (50%) worsening bone pain at 8 and 12 weeks. Ca-153 increased ( > 40%) in 3/4 (75%) patients. Conventional imaging at 12 weeks showed PD in all 4 patients. In the 7 patients with clinical partial response (PR); SUVmax decreased ( < 25%) in 23/35 (66%) and remained stable in 10/35 (29%) lesions. Ca-153 decreased ( > 80%) in 4/7 (57%) patients. Clinically, symptoms remained stable or improved in 6/7 (86%) patients at 8 and 12 weeks. Conventional scanning at 3 months showed either PR (n = 3) or SD (n = 4). In the 7 patients with stable disease (SD); SUVmax remained unchanged in 15/27 (56%) and decreased in 12/27 (44%) lesions. Ca-153 showed minimal changes in 6/7 (86%) patients. Conventional scans showed SD in all 7 patients. Conclusions: Our data show that although 18F-FDG PET/CT is reliable at predicting PR or SD, it has poorer predictive ability for clinical PD in bone metastases at 8 weeks. Intra-patient heterogeneity of response between lesions is a common observation. Clinical trial information: 12/LO/1801.