This video presents a detailed pathology case study of a 4 cm foot nodule from a 30-year-old man, initially presumed to be a cyst. Histological examination reveals a multi-nodular tumor with a dual population of cells: small, round blue cells and larger cells with abundant eosinophilic, clear, or oncocytic cytoplasm. This appearance creates a significant diagnostic pitfall, as it closely mimics a sweat gland neoplasm like a hidradenoma. The final diagnosis is confirmed as a glomus tumor. Dr. Jared Gardner highlights key diagnostic clues to differentiate it from its mimics, including the characteristic tracking of glomus cells along peripheral blood vessels, the distinct cell borders created by individual basement membrane wrapping (demonstrated with a PAS stain), and the typically patchy, perivascular staining pattern of smooth muscle actin.
Case Study: A Glomus Tumor Mimicking a Sweat Gland Neoplasm
00:00:28 Case Presentation: The case involves a 30-year-old man with a 4 cm nodule on the plantar surface of his foot, clinically suspected to be a cyst.
00:00:58 Histological Features: Microscopically, the tumor is a large, multi-nodular lesion deep in the subcutis. It displays a biphasic pattern, with areas of small, round blue cells alternating with areas of cells that have abundant pink (eosinophilic), clear, or oncocytic cytoplasm.
00:03:50 The Diagnostic Dilemma: The primary teaching point is that the prominent eosinophilic and clear cell changes in this tumor make it a significant mimic of a sweat gland tumor, such as a hidradenoma or dermal duct tumor. It's crucial to include both in the differential diagnosis.
00:04:04 Final Diagnosis: The lesion is identified as a glomus tumor.
00:04:57 Key Clue #1: Peripheral Tracking: A critical diagnostic feature for a glomus tumor is finding small clusters of the tumor cells tracking away from the main mass along the surface of blood vessels at the periphery.
00:06:20 Key Clue #2: Basement Membrane Wrapping: Individual glomus cells are surrounded by a layer of basement membrane. This creates very discrete, well-defined cell borders and is highlighted by a PAS stain (or a collagen type IV stain).
00:07:05 Immunohistochemical Staining: Smooth muscle actin (SMA) is positive in glomus tumors but is often patchy and may be strongest around blood vessels, rather than diffusely positive throughout the tumor. The tumor is negative for keratins (ruling out a sweat gland origin) and endothelial markers.