This pathology case presentation details the diagnostic challenge posed by a skin nodule in an 80-year-old female, initially presenting as a small, painful nodule above the knee. The initial differential diagnosis, based on histology showing a round blue cell lesion with scant epidermis, leaned towards Merkel cell carcinoma (MCC) due to focal positivity for low molecular weight keratin, CK7, and neuroendocrine markers (CD56, Chromogranin). However, subsequent crucial immunostains for MCC—Synaptophysin and CK20—were negative, as were pan-hematopoietic markers (CD43/CD45) and TTF-1, effectively ruling out lymphoma and metastatic small cell lung cancer. The key diagnostic pivot occurred when a Desmin stain, typically reserved for sarcomas, was performed, revealing strong, level-11 expression. This led to the final, rare diagnosis: Alveolar Rhabdomyosarcoma (ARMS) presenting primarily in the skin of an elderly patient (a "triple rare" occurrence). The case underscores the critical importance of avoiding diagnostic "shortcuts" based on initial impressions (like jumping straight to MCC) and maintaining a comprehensive workup, as demonstrated by the missed Desmin stain that would have immediately pointed toward the correct diagnosis had it been included earlier in the panel. The patient had a rapidly aggressive course, dying within 18 months.
Diagnostic Challenge of an Elderly Patient's Dermal Nodule
0:00:08 Initial Presentation: Case of an 80-year-old female with a 3.5 cm painful skin nodule above the knee, initially worked up elsewhere.
0:00:51 Importance of Independent Review: The presenter emphasizes reviewing H&E slides first without clinical bias to allow for open-minded morphological assessment.
0:01:46 Initial Differential Diagnosis (MCC/Lymphoma): Based on round blue cells, molding, and some scattered mitoses, the initial thought process centered on Merkel Cell Carcinoma (MCC) or potentially lymphoma, despite some clear cell change.
0:02:20 Initial Immunostains: The case arrived with focal positivity for low molecular weight keratin, CK7, Pan-Keratin, CD56, and granular Chromogranin, supporting a neuroendocrine/MCC differential.
0:04:46 Ruling Out MCC/Metastatic Lung Cancer: Subsequent stains showed negative Synaptophysin and negative CK20, effectively ruling out typical MCC. Hematopoietic markers (CD43/CD45) and TTF-1 were also negative, excluding hematopoietic malignancies and metastatic small cell lung cancer.
0:05:36 The Pivot Point: After initially considering a CK20-negative MCC, the presenter re-evaluated the protocol, noting the absence of a Desmin stain.
0:07:51 The Desmin Revelation: A Desmin stain showed "level 11" (screaming, blazing) positive expression, strongly suggesting a sarcoma.
0:09:00 Confirmation of Rhabdomyosarcoma: Myogenin (Myf4), a key marker for muscle lineage, was strongly positive (99% of nuclei).
0:09:25 Rare Diagnosis: The final diagnosis was Alveolar Rhabdomyosarcoma (ARMS) presenting as a primary skin tumor in an elderly patient—a "triple rare" occurrence (Rhabdo in skin, elderly patient, alveolar subtype).
0:10:10 Lesson Learned: The presenter stresses the danger of diagnostic shortcuts (e.g., skipping steps based on initial MCC suspicion) that cause clinicians to skip necessary steps in the diagnostic tree, missing crucial markers like Desmin.
0:11:04 Molecular Confirmation and Prognosis: The tumor was confirmed by PAX/FOXO1 fusion, characteristic of ARMS. The case had a rapidly aggressive course, with the patient dying about 18 months post-diagnosis, despite treatment differing from what would be used for MCC.
0:12:59 Histological Feature of ARMS: The structures initially interpreted as processing artifact were identified as the characteristic alveolar spaces formed by nests of tumor cells breaking apart.