Morphologically In Situ Solid Papillary Carcinoma of the Breast With Lymph Node and Lung Metastasis: A Rare Case Report and Literature Review.

Solid papillary carcinoma (SPC) is rare, accounting for less than 1% of all breast cancers. According to the 5th edition of the WHO Classification of Breast Tumors (2019), SPC is divided into invasive (ISPC) and in situ (SPC in situ) subtypes. ISPC is characterized by an irregular or jigsaw pattern lacking myoepithelial cells, while SPC in situ is well-circumscribed, regardless of myoepithelial cell presence. Metastasis is infrequently reported, with most patients involving lymph nodes and arising from ISPC. Metastases from SPC in situ without myoepithelial cells have rarely been reported. We present a 66-year-old woman with a 28 × 13 mm hypoechoic breast mass detected on ultrasound. Microscopic examination revealed well-circumscribed nests of SPC, positive for ER, PR, and synaptophysin, with a Ki-67 proliferation index of 10%. HER2 was negative. Myoepithelial markers, including p63, smooth muscle actin, keratin 5/6, p40, and CD10, were all negative, confirming the absence of a myoepithelial layer. Axillary lymph node dissection revealed metastasis in three of eighteen lymph nodes, with the metastatic foci demonstrating the same well-circumscribed SPC architecture as the primary tumor in the breast. Additionally, a lung biopsy exhibited metastatic SPC with identical histomorphology and immunohistochemical profile to the primary tumor. Although rare, SPC presenting a well-circumscribed morphology, and classified as in situ according to the WHO, should be approached with caution when myoepithelial cells are absent, as the in situ histomorphology does not necessarily exclude the potential for metastasis.