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A Comparison of the Histopathologic Growth Patterns Between Non–Merkel Cell Small Round Blue Cell Tumors and Merkel Cell Carcinoma

Bandino, Justin P., MD*; Purvis, Caitlin G., BS; Shaffer, Blake R., MD; Gad, AbdAllah, MD§,¶; Elston, Dirk M., MD

The American Journal of Dermatopathology: November 2018 - Volume 40 - Issue 11 - p 815–818
doi: 10.1097/DAD.0000000000001232
Original Study

Background: Merkel cell carcinoma (MCC) is a rare neuroendocrine cutaneous malignancy that shares cytologic, histopathologic, and immunohistochemical features with other small round blue cell (SRBC) tumors. Although the trabecular pattern is anecdotally associated with MCC, objective data are lacking.

Methods: This was a retrospective institutional review board–approved observational study conducted on microscopic images of 79 MCCs and 74 other SRBC tumors (desmoplastic small round cell tumor, primitive neuroectodermal tumor, neuroblastoma, embryonal rhabdomyosarcoma, alveolar rhabdomyosarcoma, synovial sarcoma, carcinoid, metastatic small cell lung cancer, non-Hodgkin small cell lymphoma, retinoblastoma, medulloblastoma, nephroblastoma, small cell osteosarcoma, and round cell liposarcoma). An expert dermatopathologist evaluated blinded and randomized microscopic specimens and recorded histologic patterns (diffuse, infiltrative, large anastomosing nests, small islands, any trabecular, focal trabecular, mixed trabecular, and predominately trabecular).

Results: Trabecular features were identified in over 72% of MCCs but only rarely in non-MCC SRBC tumors. The presence of any amount of a trabecular pattern favored a diagnosis of MCC over SRBC tumors with a sensitivity of 72.2% and a specificity of 87.8%. If “any” and “focal” trabecular patterns were discounted, specificity rose to 93.2%.

Conclusion: The presence of a trabecular pattern helps to differentiate MCC from other SRBC tumors, and specificity approaches that achieved with immunostaining.

*Department of Dermatology, San Antonio Military Medical Center, San Antonio, TX;

School of Medicine, University of South Carolina, Greenville, Greenville, SC;

Pentagon Flight Medicine Annex, Joint Base Andrews, MD;

§Department of Internal Medicine, Texas Tech University Health Sciences Center, Odessa, TX;

Department of Biostatistics and Cancer Epidemiology, National Cancer Institute, Cairo University, Cairo, Egypt; and

Department of Dermatology, Medical University of South Carolina, Charleston, SC.

Correspondence: Justin P. Bandino, MD, Department of Dermatology, San Antonio Military Medical Center, 1100 Wilford Hall Loop, JBSA-Lackland AFB, TX 78236 (e-mail: tantomed@gmail.com).

The authors declare no conflicts of interest.

Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.