Immunoreactive Acellular Keratin in Sentinel Lymph Nodes From a Patient With Endometrioid Carcinoma of the Endometrium With Squamous Differentiation: A Case Report of a Potential Diagnostic Pitfall

Sentinel lymph node biopsy is gaining increasing acceptance as a less morbid way to assess lymph node status in patients with endometrial carcinoma, compared with full pelvic node dissection. The sentinel nodes are usually subjected to ultrastaging, with sections taken at multiple levels from each block and immunstaining for keratin performed, in order to detect micrometastses. We report a case of an 80-yr-old woman who underwent a right sentinel lymph node biopsy at the time of surgery for clinically and radiologically apparent stage I endometrial endometrioid adenocarcinoma. The immunostains for AE1/AE3 performed on the 2 right pelvic sentinel lymph nodes were positive, corresponding to subcapsular acellular keratin on hematoxylin and eosin; however, carcinoma cells could not be identified on the hematoxylin and eosin–stained slides. Immunomarkers for Ber-EP4 and EMA, both of which were strongly expressed in the endometrial carcinoma cells, were negative on the nodal tissue, and we concluded that the sentinel lymph nodes were negative for metastatic carcinoma, despite the positive keratin immunostains. To our knowledge, this unusual finding is not described in the literature; recognition of this phenomenon and study of additional cases is warranted.