This pathology case review focuses on a peri-ungual papule in a 40-year-old male, diagnosed as Superficial Acral Fibromyxoma (SAF), also known as Digital Fibromyxoma. Histologically, the lesion features a poorly circumscribed, moderately cellular nodule of bland spindle cells arranged in sheets and fascicles, showing infiltration into subcutaneous fat. The spindle cells demonstrate striking positivity for CD34. The primary differential diagnosis considered was Dermatofibrosarcoma Protuberans (DFSP) due to the swirling morphology and fat infiltration, a critical distinction given the differing surgical requirements. Molecular testing (FISH) confirmed the diagnosis by ruling out DFSP’s characteristic fusion and, importantly, revealing RB1 loss, a finding that links SAF to the spectrum of spindle cell lipoma-family tumors. The video notes that SAF is benign but has a high rate of local recurrence due to the conservative surgical margins often employed on acral surfaces.
Summarizing Case 4: Superficial Acral Fibromyxoma
0:13 Case Presentation: The video presents Case 4: a 40-year-old man with a peri-ungual papule (near the nail) on his finger.
0:45 Histological Description: The lesion is a poorly circumscribed, moderately cellular nodule composed of bland spindle and stellate cells, arranged in sheets and small fascicles, invading the dermis and subcutaneous fat.
1:36 Primary Diagnosis: Based on the location and histology, the diagnosis is Superficial Acral Fibromyxoma (SAF).
1:46 Immunoprofile: The lesion is strikingly positive for CD34. The speaker notes that the swirling and whirling pattern of the spindle cells is clearly visible with the stain.
2:47 Differential Diagnosis (DFSP): The most significant differential considered was Dermatofibrosarcoma Protuberans (DFSP) due to the bland, swirling spindle cells and the apparent entrapment/infiltration of fat.
3:44 Clinical Significance: Distinguishing SAF (benign) from DFSP (requires wide excision/amputation on a digit) is critical, although SAF is prone to local recurrence because surgeons often take small, conservative margins on hands and feet to minimize morbidity.
4:26 Molecular Findings: Molecular testing ruled out DFSP (COL1A1-PDGFB fusion negative) and revealed loss of RB1 by FISH.
4:41 Key Takeaway (RB1 Loss): Superficial Acral Fibromyxoma is now recognized as an entity associated with RB1 loss, placing it within the spindle cell lipoma family of tumors (including spindle cell lipoma, cellular angiofibroma, and mammary-type myofibroblastoma) which share this mutation and overlapping histologic features.
6:37 Histological Clue: As a general rule, any bland spindle cell lesion with mixoid change on the hands and feet should place SAF immediately high on the differential list.
7:04 Associated Features: The lesion often features a prominent vascular network. The terms Digital Fibromyxoma (Fletcher) and Acral Fibromyxoma are used interchangeably with Superficial Acral Fibromyxoma.