Abstract P3-08-30: Low levels of metabolic tumor volume at baseline significantly associated with longer progression-free survival in patients with locally advanced or metastatic breast cancer treated with endocrine therapies

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Abstract Background Endocrine therapy is an essential treatment choice for patients with estrogen receptor (ER)-positive and human epidermal growth factor receptor 2 (HER2)-negative locally advanced and metastatic breast cancers. Although patients with non-visceral metastasis or with favorable efficacy induced by prior endocrine therapy, are reported to be associated with a longer duration of endocrine therapy, predictive markers have yet to be established. In general, breast cancers with low proliferative activity and small tumor burden seem to obtain favorable efficacy induced by endocrine therapy. Metabolic tumor volume (MTV) on fluorodeoxyglucose-positron emission tomography (FDG-PET) examination measures uptake activity of glucose in tumor cells, which represents tumor aggressiveness, and tumor volume. Thus, using MTV as a metabolic parameter for both proliferative activity and tumor burden, we evaluated the predictive usefulness of MTV for treatment efficacy of endocrine therapy.MethodA total of 83 patients treated with endocrine therapy for locally advanced and metastatic breast cancers, treated with an aromatase inhibitor (AI; n=39) or fulvestrant (Ful; n=44), examined by FDG-PET before start of treatment, were constitutively recruited in this retrospective study. The treatment lines were as follows; 1st, n=46; 2nd, n=22; 3rdor more, n=15. The maximum value of the standard uptake value (SUV, regional radioactivity concentration (Bq/mL)/injected dose (Bq)/patient weight (g)) in the volume of interest (VOI) was defined as the SUVmax. Total volume of voxels of ≥40% of the SUVmax in the VOI was defined as the MTV. The cut-off value of MTV was defined at 45 and the relationships between MTV and progression-free survival (PFS) was calculated.ResultsPFS of patients treated with Ful (median, 19.0 months) was marginally favorable than those with AI (median, 9.8 months; p=0.084). In all participants, PFS of patients with low levels of MTV (n=29; median, 23.6 months) was significantly longer than those with high levels (n=54; median, 10.0 months; p=0.0124). The association between low levels of MTV and longer PFS was significantly (p=0.0369) and marginally (p=0.0715) recognized in Ful and AI, respectively. Similarly, association between MTV and PFS seems to be consistent in subgroups in visceral or non-visceral metastasis, de novo or recurrence, and treatment lines.Discussion and conclusionIn the present study, we demonstrated that low levels of MTV at baseline are significantly associated with longer PFS for patients treated with AI or Ful. MTV seems to be a predictor for the efficacy of endocrine therapy by evaluating both tumor aggressiveness and tumor volume. This data may be useful in selecting patients who could benefit from endocrine therapy with non-invasive method. Citation Format: Reiko Fukui, Tomoko Higuchi, Yukie Fujimoto, Atsushi Sata, Ayako Bun, Hiromi Ozawa, Hiromi Ozawa, Yoshimasa Miyagawa, Michiko Imamura, Kazuhiro Kitajima, Yasuo Miyoshi. Low levels of metabolic tumor volume at baseline significantly associated with longer progression-free survival in patients with locally advanced or metastatic breast cancer treated with endocrine therapies [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-08-30.