Heterogeneity of Metabolic Response to Systemic Therapy in Metastatic Breast Cancer Patients

The aim of this retrospective study was to describe the intra-individual heterogeneity of the ¹⁸F-labelled fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) response among lesions in bone-dominant metastatic breast cancer patients treated with systemic therapies.The metabolic response was analysed by comparing PET/CT scans carried out before and during a new treatment phase (n=46) in 25 bone-dominant metastatic breast cancer patients. Patients presented both bone and extra-bone metastases in 48% treatment phases. The metabolic response was analysed according to European Organization for Research and Treatment of Cancer (EORTC) criteria. A heterogeneous response was defined as the coexistence of responding and non-responding lesions within the same patient.The lesion-based response analysis showed a heterogeneous metabolic response in 48% of treatment phases. In the subset with both bone and extra-bone metastases (n=20), PET/CT showed discordant responses between bone and extra-bone metastases in 6/20 (30%) treatment phases. Considering all the cases included in the study, the time to progression (TTP) was longer in cases with a metabolic response compared with the cases with a metabolic non-response (P=0.02). In cases with a PET/CT non-response, TTP seemed to be lower in those with a homogeneous non-response compared with those with a heterogeneous metabolic response (P=0.07).Whole-body FDG-PET allows frequent heterogeneous responses after systemic therapy to be identified in bone-dominant metastatic breast cancer patients.