Endometrial serous carcinoma with extensive squamous differentiation mimicking primary endometrial squamous cell carcinoma: Clinicopathological and molecular analysis of a case with literature review

In the histological assessment of endometrial carcinoma, the presence of squamous differentiation is generally considered inconsistent with a serous histotype. Herein, we report a case of endometrial serous carcinoma with extensive squamous differentiation mimicking squamous cell carcinoma, with literature review. A 67-years-old-woman underwent hysterectomy, bilateral salpingo-oophorectomy, sentinel lymph node resection, omental and peritoneal biopsies, due to a 8 cm endometrial mass. Histologically, the tumor showed squamous carcinoma features with positivity for squamous cell markers (p40, p63, cytokeratin-5/6, cytokeratin-34βE12) and negativity for Müllerian markers (PAX8, estrogen receptor, progesterone receptor). No immunohistochemical/molecular alterations typical of endometrioid carcinoma (i.e., PTEN loss, mismatch repair deficiency, nuclear β-catenin expression, POLE mutation) were observed. P53 showed aberrant pattern and p16 showed basal positivity. The tumor infiltrated the myometrium full-thickness with extensive lymphovascular space invasion. Surprisingly, the sentinel lymph node metastasis showed overt serous carcinoma features. Additional sampling of the endometrial mass revealed a minor. In conclusion, serous carcinoma may show diffuse squamous differentiation. In the case of suspected primary endometrial squamous cell carcinoma, an extensive sampling is warranted.