This pathology case review details the diagnostic process for a rare presentation of Alveolar Rhabdomyosarcoma (ARMS) occurring primarily in the skin of an 80-year-old female patient. Initially, the clinical and morphological features—a painful, poorly differentiated nodule composed of round blue cells with minimal epidermis—led the consulting pathologist to suspect Merkel Cell Carcinoma (MCC) or possibly a lymphoma. Initial immunohistochemical (IHC) stains supported the initial impression, showing focal positivity for low molecular weight keratins, CK7, pan-keratin, CD56, and Chromogranin, while ruling out hematopoietic malignancies (CD45/CD43 negative). However, the crucial markers for MCC, Synaptophysin and CK20, were negative, prompting a re-evaluation. A subsequent, essential IHC stain for Desmin revealed strong positivity (level 11/blazing positive expression), immediately shifting the differential diagnosis toward a sarcoma, specifically Rhabdomyosarcoma (RMS). Confirmation followed with Myogenin (MYOD1 relative) staining, which was strongly positive. The case highlights the critical danger of "skipping steps" in the diagnostic tree when an initial impression (MCC) seems plausible, leading to the omission of key, non-routine stains like Desmin, which ultimately revealed the correct, extremely rare diagnosis of cutaneous ARMS. The aggressive nature of ARMS was confirmed, as the patient progressed rapidly and died within 18 months.
Diagnostic Journey of a Rare Cutaneous Malignancy
0:00:08 Case Presentation: 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 looking at H&E slides cold (without initial clinical/stain context) to reduce bias.
0:01:16 Initial Impression (Based on Morphology): Round blue cell lesion in the dermis, suggesting Merkel Cell Carcinoma (MCC), small cell melanoma, or lymphoma.
0:02:20 Initial Stains Supporting MCC: Focal positivity for low molecular weight keratin, CK7, pan-keratin, CD56, and granular Chromogranin.
0:04:46 Ruling Out Initial Impression: Crucially, the stains for definitive MCC markers (Synaptophysin and CK20) were negative. Hematopoietic malignancies were also excluded (CD45/CD43 negative).
0:05:36 Shift in Diagnosis: Despite initial leaning toward CK20-negative MCC, the pathologist reviewed the case again before signing out.
0:07:06 Standard Soft Tissue Panel Review: The presenter notes he failed to run a standard panel for undifferentiated spindle cell tumors, specifically Desmin.
0:07:51 The "Level 11" Finding: A subsequent Desmin stain showed "blazing distortion" (level 11/screaming positive expression).
0:08:08 Correct Diagnosis: Strong Desmin positivity indicated a Rhabdomyosarcoma (RMS).
0:09:00 Confirmation: Myogenin (Myf4) staining was "blazing positive" (99% of nuclei), confirming Alveolar Rhabdomyosarcoma (ARMS).
0:09:30 Rarity of the Case: This diagnosis was a "triple Whammy": RMS in the skin (rare), in an elderly patient (rare), and specifically the Alveolar subtype (most common in children).
0:10:19 Key Takeaway on Diagnostic Process: The initial MCC suspicion caused the presenter to skip steps on the diagnostic tree, omitting the essential Desmin stain until a final review.
0:11:04 Molecular Confirmation: PAX-FOXO1 fusion was positive, confirming ARMS.
0:11:17 Aggressive Prognosis: Unlike some cutaneous sarcomas, ARMS remains aggressive; this patient developed nodal and peritoneal metastases and died approximately 18 months after diagnosis.
0:12:32 Morphological Insight: The spaces initially thought to be processing artifact are actually "alveolar spaces" characteristic of ARMS morphology.