Low-grade endometrial endometrioid carcinoma of the p53-abnormal group: case presentation and diagnostic issues.

P53-abnormal endometrial carcinomas are high-grade and aggressive tumors which should be treated with chemo-/radiotherapy. In low-grade endometrioid carcinoma (LGEC), abnormal expression of p53 is an exceptional finding and is typically accompanied by patchy p16 positivity and diffuse hormone receptor expression. Herein, we report a case of LGEC exhibiting both p53 and p16 overexpression, highlighting the diagnostic pitfalls related to such phenotype. A 60-year-old woman underwent hysterectomy and bilateral salpingo-oophorectomy with pelvic lymphadenectomy due to a deeply myoinvasive endometrial mass. The tumor showed glandular architecture, low-grade nuclei and glandular differentiation. Focal lymphovascular space invasion and no lymph node metastases were observed. Immunohistochemically, the tumor showed p53 overexpression, p16 block-type positivity, diffuse hormone receptors positivity and retained mismatch repair proteins expression. No POLE mutations were identified. A diagnosis of p53-abnormal LGEC was eventually made. A glandular neoplasm with p53 and/or p16-overexpression on endometrial biopsy specimens may raise the concern of other entities such as serous carcinoma, HPV-related endocervical adenocarcinoma, and gastric-type adenocarcinoma. An immunohistochemical panel including hormone receptors, p53, p16 and mismatch repair proteins appears necessary for an accurate diagnosis of uterine adenocarcinomas.