Dr. Jared Gardner presents a series of challenging dermatopathology unknown cases to Emory University dermatology residents, focusing on diagnostic pitfalls and differentials for various skin nodules and tumors. The session covers common granulomatous patterns, including Rheumatoid Nodule (distinguished by red fibrinoid necrosis) and Granuloma Annulare (characterized by blue mucin), and introduces the dangerous mimicker, Metastatic Breast Carcinoma (which can present with an interstitial pattern resembling Granuloma Annulare, highlighting the need for clinical history). The discussion then addresses rare mesenchymal tumors, including a highly aggressive Epithelioid Rhabdomyosarcoma that initially mimicked Atypical Fibroxanthoma (AFX) due to its rhabdoid cytology and CD10 expression, emphasizing the utility of Desmin and Myogenin stains. Other highlights include a rare collision tumor (Basal Cell Carcinoma with Dermatofibroma) that broke typical diagnostic rules, a classic presentation of Pyogenic Granuloma with the unusual bonus finding of Extramedullary Hematopoiesis, and the clinical/pathological distinction between Gardner Fibroma (in children, linked to Gardner Syndrome) and Nuchal Type Fibroma (in adults).
Dermatopathology Unknowns Session with Dr. Jared Gardner
0:34 - Case 1: Rheumatoid Nodule (RN). A nodule on the elbow of a 60-year-old woman.
Key Pathologic Feature: Palisaded necrobiotic granuloma pattern with central red/brick red fibrinoid necrosis (dead collagen and fibrin). Mitoses are commonly seen and benign.
Pitfall: Must differentiate from Epithelioid Sarcoma (ES), which shows true tumor cell necrosis and overtly malignant cytology. Keratin, EMA, and loss of INI1/SMARCB1 help distinguish ES.
7:02 - Case 2: Deep Granuloma Annulare (GA). A nodule on the dorsal finger of a 12-year-old girl.
Key Pathologic Feature: Palisaded necrobiotic granuloma pattern with a core rich in blue mucin (glycosaminoglycans). Also exhibits the interstitial pattern at the periphery.
Takeaway: If unsure between RN and GA, report "palisaded necrobiotic granuloma" and recommend correlation with the patient's history (e.g., rheumatoid arthritis). Deep GA often presents as nodules in children.
13:35 - Case 3: Metastatic Breast Carcinoma. An annular plaque on the left side of a 65-year-old woman.
Pitfall: This carcinoma strongly mimicked the interstitial pattern of GA, but key clues were suspicious cytology and the lesion spanning a large area only with the interstitial pattern.
Diagnostic Point: The mass proved positive for keratin, confirming metastatic cancer (the patient had a known history of stage four breast cancer near a prior mastectomy site). Clinical history and photos are crucial.
20:20 - Case 4: Epithelioid Rhabdomyosarcoma. An ulcerated nodule on the scalp of a 75-year-old man.
Cytology: Characterized by "rhabdoid/plasmacytoid" cells—eccentric nuclei pushed aside by a dense blob of pink, intermediate-filament-filled cytoplasm.
Differential/Misdiagnosis: Initially considered Atypical Fibroxanthoma (AFX) due to CD10 positivity and negative standard markers.
Final Diagnosis: Confirmed by strong Desmin positivity (the intermediate filament) and nuclear Myogenin expression (skeletal muscle marker). Highly aggressive tumor with poor prognosis (patient developed lung metastases within 6 months).
32:08 - Case 5: Basal Cell Carcinoma (BCC) Colliding with Dermatofibroma (DF). An ulcerated nodule on the shin.
Key Pathologic Feature: The deeper component was a typical DF. The superficial component was a basaloid proliferation (follicular/adnexal induction) that, contrary to the general rule of thumb, proved to be a true BCC.
Takeaway: This case is a rare exception to the rule that basaloid growth over a DF is always benign induction.
39:56 - Case 6: Pyogenic Granuloma (PG)/Lobular Capillary Hemangioma. A large, ulcerated nodule on the fourth toe.
Key Pathologic Feature: Distinct lobular growth of small capillaries, plump endothelial cells, and pericytes (actin positive) at the periphery. Mitoses are common and benign.
Pitfall: Must consider Bacillary Angiomatosis if excessive neutrophils are present.
49:46 Bonus Finding: Rare presence of Extramedullary Hematopoiesis (EMH), identified as clusters of perfectly round, hyperchromatic erythrocyte and myeloid precursors near the base of the lesion.
53:11 - Case 7: Gardner Fibroma vs. Nuchal Type Fibroma. Comparison of two similar hypocellular sclerotic nodules of collagen.
Differentiation: Age is the primary factor: Gardner Fibroma (child, risk of Gardner Syndrome/FAP) versus Nuchal Type Fibroma (adult). The Gardner Fibroma requires workup for underlying familial polyposis.
57:44 - Final Discussion: Dr. Gardner notes that Actin staining, used to identify pericytes and differentiate benign from malignant vascular lesions (like angiosarcoma), is often less reliable and consistent than hoped for.