Response evaluation with ¹⁸F FDG PET CT in hormone receptor (HR) positive, human epidermal growth factor receptor 2 (HER2) negative (HR+/HER2−) advanced metastatic breast cancer (mBC) treated with cyclin-dependent kinase 4 and 6 (CDK4/6)inhibitors plus endocrine treatment.

Background: Despite the availability of many therapies, HR+/HER2− mBC are rarely curable. The current treatment paradigm involves sequencing endocrine therapy, targeted therapy, and/or chemotherapy to prolong patient's lives, delay disease progression, minimize cancer-related symptoms and improve quality of life. The CDK4/6 inhibitors are rapidly transforming this treatment landscape and demonstrated meaningful improvement in PFS, although OS data are still immature.

Purpose: Data of 82 female patients (age 29 to 77 years) of 82 HR+/HER2− mBC female patients with treated with CDK4/6 inhibitors plus endocrine treatment between 2016 - 2020, was retrospectively analysed. The findings are very much positive and developing a hope. A visual analysis scoring from 1 to 5 was given by two nuclear medicine physician for response in follow up ¹⁸F - FDG PET - CT. Score 5 was given to near complete or complete response (more than 80% decrease in disease burden on visual analysis). Score 4 for significant partial metabolic response (~50 - 80%% decrease), score 3 for partial metabolic response (~25 - 75% decrease), score 2 for no significant change (~20% decrease or increase in disease burden) and score 1 for disease progression (~20% increase in disease burden on visual analysis). Score 3 or more were considered positive. SUV value at baseline PET is also correlated with severity and prognosis of disease. Findings: Score 5 - 9/81 patients (11.1%). Score 4- 47/81 patients(58%). Score 3 - 17/81 patients (21%). Score 2 - 6/81 patients (7.4%). Score 1 - 2/81 patients (2.5%).

Results: Our study shows very positive results 73/81 (~90.12%) in interim PET - CT and almost 100% correlation with baseline PET - CT SUVmax valve and disease severity and prognosis. Further study/trials are needed for approval of first line treatment with CDK4/6 inhibitors plus endocrine treatment and evaluation with PET/CT in HR+/HER2− mBC.