High-grade Endometrial Endometrioid Carcinoma: A Case Report of Complete Transdifferentiation to Pilomatrix-like Carcinoma

Introduction Endometrial endometrioid carcinomas can show multiple lines of differentiation, including pilomatrix-like high-grade endometrioid carcinoma, a recently described tumor with similarity to cutaneous pilomatrix carcinoma and associated with very aggressive clinical behavior. Methods We present a 56-year-old woman with an endometrial tumor associated with secondary involvement of both ovaries, left tubo-ovarian ligament and obturator lymph nodes. The diagnosis of high-grade endometrioid carcinoma in a previously performed curettage was confirmed in the hysterectomy specimen. Results Microscopically, the tumor exhibited a solid, nested/insular pattern with basaloid cells, predominantly seen at the periphery, ghost cell keratinization towards the center of the nests, and extensive geographic necrosis. No low-grade endometrioid carcinoma component was identified throughout the primary tumor or metastases after extensive sampling. Immunohistochemical assessment showed aberrant cytoplasmic and nuclear expression of β-catenin, and focal CDX2 expression. Tumor cells were negative for PAX8, and estrogen and progesterone receptors (ER/PR). The next-generation sequencing (NGS) analysis found a CTNNB1 pathogenic mutation (p.Ser37Phe, c.110C > T; variant allele frequency: 18.6%). Based on these morphologic, immunohistochemistry and NGS analysis, a diagnosis of pilomatrix-like high-grade endometrioid carcinoma was established. Conclusion The absence of a low-grade endometrioid carcinoma component makes this pilomatrix-like high-grade endometrioid carcinoma, a very rare tumor, even more special. This and the absence of PAX8 and ER/PR expression in an unusual morphological context proved to be diagnostically challenging. This patient's presentation at high stage is concordant with the literature's description of this tumor as aggressive. It is not yet known whether standard adjuvant therapies for high-risk endometrial carcinomas are effective.