Mucinous Cystadenocarcinomas of the Breast Exhibit Heterogeneous Genomic Profile as Triple Negative Breast Cancer, Harbor Alterations in <i>PIK3CA, PTEN, AKT1,</i> and <i>GNAS</i> Genes, and May Not Be Indolent

Mucinous cystadenocarcinoma of the breast is an exceedingly rare entity with less than 40 tumors reported to date. It is classified as a special subtype of breast carcinoma which exhibits low-grade indolent behavior despite having a triple-negative phenotype. Awareness of this rare tumor is essential to avoid misinterpreting it as a routine mucinous (colloid) carcinoma of the breast or metastasis from a nonmammary mucinous carcinoma. Though the clinical, histological, and immunohistochemical features of breast mucinous cystadenocarcinoma have been described thoroughly, data on molecular features remains scarce. Herein, we present 3 patients with mucinous cystadenocarcinoma, highlighting the molecular features in 2 patients and discussing the literature in this regard. Additionally, we report that these tumors have the potential for short-term recurrence despite complete surgical excision, and they therefore should probably be treated as any other triple-negative breast cancer (TNBC) with careful consideration of other factors such as patient's age, tumor size, nodal stage, and tumor grade. Review of the literature on this rare tumor together with findings from the current report suggests that MCAs at the molecular level are similar to TNBC, with frequent recurring variants in PI3K pathway genes, including <i>PIK3CA, PTEN,</i> and <i>AKT1</i> genes. We report here for the first time the presence of <i>GNAS</i> variants in mucinous cystadenocarcinoma. This information could be useful to offer targeted treatment against <i>PIK3CA</i> and <i>AKT1</i> in patients with this rare tumor.