Breast Carcinoma with a Special Histological Pattern of Sebaceous Differentiation and High-Frequency Microsatellite Instability: A Case Report and Review of the Literature.

The World Health Organization in its 2019 Classification of Breast Tumors termed breast sebaceous carcinoma as invasive breast carcinoma of no special type (IBC-NST), with a sebaceous pattern. Approximately 30 cases of IBC-NST with a sebaceous pattern have been reported in the literature, and in all cases the expression of mismatch repair proteins in tumors was normal. Here, we report a case of IBC-NST with a sebaceous pattern and high-frequency microsatellite instability (MSI-H). This case was a sporadic sebaceous pattern of IBC-NST with MSI-H and was unrelated to Muir-Torre syndrome. Its histopathological characteristics were similar to those of MSI-H-associated triple-negative breast carcinoma (TNBC) with a high histological grade but were without tumor-infiltrating lymphocytes (TILs). The tumor did not recur after 20 months of follow-up.