Subcutaneous or cutaneous ¹⁸F -FDG uptake in the breast co-existing with breast cancer

Objectives: In breast cancer patients, ¹⁸F -FDG accumulation just below or along the skin is occasionally seen in the affected breast. In this study, we investigated clinical and imaging findings associated with the subcutaneous/cutaneous uptake (SCU) and its predictive value of nodal and distant metastasis.

Methods: This retrospective study included a total of 114 patients with breast cancer (age 23-85 years), who underwent ¹⁸F- FDG PET/CT at diagnosis in our institution between January 2017 and December 2019. SCU was defined as abnormal ¹⁸F -FDG accumulation just below or along the skin or under the areola in the affected breast, apart from or spreading from tumors. Presence of nodal metastasis (N) was primarily determined based on pathological diagnosis, but for those without available specimen was judged by imaging examinations. Extensive nodal metastasis (N-extensive) was defined as pN2, cN2 or higher N-stage. Distant metastasis (M) was evaluated based on PET/CT. We examined the association of SCU to patient age, tumor size, tumor SUVmax, presence of massive direct skin invasion, N, N-extensive, and M using unpaired t-test or Fisher’s exact test. Multivariate logistic analysis was performed to identify independent factors predictive of metastasis using 5 imaging findings in the affected breast as variables (tumor size, tumor SUVmax, massive direct skin invasion, and SCU).

Results: Overall, SCU was observed in 33 patients, accounting for 29% of the whole subjects. SCU was significantly associated with older age (p < 0.001), larger tumor size (p < 0.001), higher tumor SUVmax (p < 0.001), and presence of direct skin invasion (p < 0.001). All of 15 patients with direct skin invasion had SCU. There were 77, 30 and 10 patients positive for N, N-extensive, and M, respectively. SCU was significantly associated with N (p = 0.015), N-extensive (p = 0.005), and M (p < 0.001). The significant relationship between SCU and M was maintained in a subgroup analysis for patients without massive direct skin invasion (n = 99, p = 0.018). Multivariate analysis showed that SUVmax was independently associated with N (p = 0.019, AUC = 0.715), and that SCU was an independent predictor of M (p = 0.014, AUC = 0.891).

Conclusions: Although further study is needed, our study suggests that SCU on ¹⁸F -FDG PET/CT is one of imaging features associated with advanced stage and can be a potential indicator of distant metastasis in patients with breast cancer.