Porocarcinoma with YAP1-NUTM1 fusion presenting as a NUT immunohistochemistry-positive lymph node metastasis

IntroductionPorocarcinoma is a rare, malignant adnexal tumor that recently has been shown to contain YAP1-NUTM1 and YAP1-MAML2 fusion transcripts, with nuclear protein in testis (NUT) immunohistochemistry (IHC) positivity in a subset of these tumors. Consequently, NUT IHC may either aid in the differential diagnosis, or represent a confounding factor depending on the clinical scenario. Here, we present a case of NUTM1-rearranged sarcomatoid porocarcinoma of the scalp presenting as a NUT IHC-positive lymph node metastasis. Case ReportA mass was excised from the right neck level 2 region with a lymph node initially diagnosed as metastatic NUT carcinoma with unknown primary site. An enlarging scalp mass was identified 4 months later, excised and diagnosed as NUT-positive carcinoma. Additional molecular testing was performed to detect the fusion partner in the NUTM1 rearrangement, confirming a YAP1-NUTM1 fusion. Given this molecular data along with the histopathologic characteristics, the clinicopathologic picture was retrospectively determined to be most consistent with a primary sarcomatoid porocarcinoma of the scalp with metastasis to a right neck lymph node and the right parotid. DiscussionPorocarcinoma is a rare entity, and typically only enters the differential diagnosis when the clinical consideration is a cutaneous neoplasm. In an alternative clinical scenario such as the approach to tumors of the head and neck, porocarcinoma is not typically a consideration. In the latter scenario, as seen in our case, positivity with NUT IHC led to the initial misdiagnosis of NUT carcinoma. This case represents an important presentation of porocarcinoma that will occur not infrequently, and pathologists must be aware of this presentation to avoid this pitfall.