This video is a dermatopathology case review focusing on the diagnosis of a skin biopsy exhibiting an unusual proliferation of bland fibroblastic or myofibroblastic cells arranged in parallel fascicles. The presenter identifies the lesion as a Dermatomyofibroma, emphasizing the key diagnostic feature being the parallel alignment of these spindle cell bundles (0:36) running parallel to the epidermis (0:38). The presenter notes that while the name suggests "myo" (muscle), these cells are fibroblasts/myofibroblasts, and muscle markers like actin are often negative (0:42). The characteristic arrangement helps distinguish it from true smooth muscle or neurogenic lesions, often presenting with a "ramen noodle" sign (2:04). The differential diagnosis discussed includes the CD34-positive fibroblastic connective tissue nevus, which can also feature bland spindle cells, sometimes extending into fat. However, the discrete, hypocellular fascicular pattern seen here is considered classic for Dermatomyofibroma, distinct from Dermatofibrosarcoma Protuberans (DFSP), which typically shows more cellular, intersecting fascicles or a more cellular/blue appearance in higher grades (3:37).
Dermatomyofibroma Case Review: Distinguishing Benign Spindle Cell Proliferations
0:00 Specimen Presentation: Examination of a punch biopsy specimen showing a pink, band-like area in the center, suggestive of a superficial lesion.
0:31 Key Morphological Feature: The lesion is characterized by bland fibroblastic/myofibroblastic cells forming nice, organized fascicles running parallel to the epidermis (0:38-0:44).
1:14 Diagnosis: Dermatomyofibroma: This characteristic parallel fascicular pattern is defined as a Dermatomyofibroma.
1:17 Nomenclature Note: Despite the "myo" in the name, the cells are fibroblasts/myofibroblasts, and immunostaining for muscle markers like actin is often only positive about half the time (0:42).
2:04 The "Ramen Noodle Sign": Fibroblastic/myofibroblastic lesions often exhibit a characteristic wrinkly or rippled pattern resembling dry ramen noodles, which helps differentiate them from neural or smooth muscle lesions.
2:37 Differential Diagnosis (Fibroblastic Connective Tissue Nevus): A similar-appearing benign entity, the CD34-positive fibroblastic connective tissue nevus, is considered, especially if fat is trapped within the lesion.
3:34 Distinction from DFSP: The discrete, parallel fascicles of hypocellular cells seen here are not typical for Dermatofibrosarcoma Protuberans (DFSP); DFSP usually presents with intersecting fascicles, higher cellularity, and a "blue" appearance in higher grades.
4:11 Diagnostic Confidence: The distinct parallel fascicular pattern strongly supports a diagnosis of Dermatomyofibroma over the DDx entities in this case.
4:50 Conclusion: The specimen is a classic example of a benign Dermatomyofibroma.