SHM Converge Scientific Abstract ePoster Gallery-The Lymph Node That Spoke First: Unraveling a Case of Primary-Unknown Merkel Cell Carcinoma

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Merkel Cell Carcinoma (MCC) is a rare, aggressive neuroendocrine skin cancer with rapid growth and early spread. When no skin primary is found, it is called Merkel Cell Carcinoma of Unknown Primary (MCC-UP), which may have different biology and prognosis.<br /><br />This case describes a 59-year-old woman with a 25-pack-year smoking history who presented with a rapidly enlarging, painful left groin mass. CT imaging showed isolated left inguinal lymphadenopathy. Biopsy revealed a poorly differentiated neuroendocrine carcinoma with a very high Ki-67 index (about 90%). Immunohistochemistry supported MCC, with positivity for AE1/3, CK20, synaptophysin, chromogranin, and CD56, and negativity for CK7 and calretinin.<br /><br />Despite multiple skin examinations and a biopsy of a subtle foot lesion, no primary tumor was identified. PET/CT showed strong uptake only in the left femoral/inguinal region, and brain MRI, upper endoscopy, and colonoscopy were negative for other disease.<br /><br />A multidisciplinary tumor board diagnosed MCC-UP with isolated nodal involvement. The patient underwent left inguinal lymph node dissection and Port-A-Cath placement, followed by adjuvant radiation and pembrolizumab every three weeks for one year. Treatment was well tolerated, with only a mild rash after the third dose that resolved on its own. Surveillance imaging every three months showed no recurrence.<br /><br />The case highlights that MCC-UP can present as isolated lymph node disease, possibly due to regression of the primary lesion or unique tumor behavior. Accurate diagnosis depends on pathology, immunohistochemistry, and thorough imaging, while treatment typically involves surgery, radiation, and immunotherapy. Multidisciplinary coordination is essential for effective management.

Asset Subtitle

Gunjan Malhotra

Meta Tag

Author List Ankur Singla, Gunjan Malhotra, Hargun Malhotra, Jawad Ahmed, Jun Hu, Simrat Kaur Parmar, Tareq Braik

Category Clinical Vignettes

Concept Merkel Cell Carcinoma of Unknown Primary

Concept Lymphadenopathy

Concept Immunohistochemistry

Concept Merkel Cell Carcinoma

Concept Neuroendocrine Tumor

Distinguished Non-Finalist

Presenter Organization Northwest Health-Porter, Valparaiso, IN, USA

Presenting Author Gunjan Malhotra

Track Adult

Keywords

Merkel cell carcinoma

unknown primary

neuroendocrine skin cancer

inguinal lymphadenopathy

poorly differentiated carcinoma

Ki-67 index

immunohistochemistry

pembrolizumab

adjuvant radiation

multidisciplinary management

Merkel Cell Carcinoma of Unknown Primary

Lymphadenopathy

Immunohistochemistry

Merkel Cell Carcinoma

Neuroendocrine Tumor