This presentation details the diagnostic process for a rare case of a dermatofibrosarcoma protuberance (DFSP) in an unusual location (dorsal foot) and presentation in a 68-year-old female. Histologically, the lesion displayed cellular, monotonous spindle cells arranged in an intersecting herringbone pattern, along with areas showing the "ramen noodle sign" (wavy collagen between bland spindle cells), suggestive of a fibroblastic/myofibroblastic process. Initial immunohistochemistry was only positive for CD34. The lack of conventional DFSP features, such as storiform fat trapping, initially made the diagnosis difficult, leading the speaker to consider other entities like fibrosarcoma or MPNST. Molecular testing proved crucial: after the standard FISH panel for $COL1A1/PDGFB$ fusion was negative, testing was reflexively extended to include the recently described $PDGFD$ rearrangement, which confirmed the diagnosis as a fibrous sarcomatous variant of DFSP. The speaker uses this case to emphasize the diagnostic utility and decision-making framework for employing molecular testing in complex or atypical soft tissue tumor cases, particularly when classic morphological and immunohistochemical findings are absent or ambiguous.
Diagnostic Review of an Atypical Dermatofibrosarcoma Protuberance (DFSP)
0:00 Case Presentation: A case review of a dorsal foot nodule excised from a 68-year-old female.
0:18 Histomorphology: Lesion featured circumscribed nodular growth in the subcutis with bland spindle cells in elongated, parallel fascicles.
0:33 "Ramen Noodle Sign": Identified areas with wavy collagen fibers between spindle cells, suggesting a fibroblastic/myofibroblastic nature.
1:30 Herringbone Pattern: Cellular areas showed fascicles intersecting at sharp angles, historically associated with fibrosarcoma, but now known to occur in various tumors.
2:08 Immunohistochemistry Results: The lesion was strongly and diffusely positive for CD34 but negative for other markers like S100 and keratin.
2:30 Morphological Difficulty: The highly cellular herringbone pattern, lack of storiform fat trapping, and unusual age/location made standard DFSP diagnosis challenging.
3:07 Differential Diagnosis Consideration: The morphology suggested a Fibrosarcomatous DFSP (FS-DFSP) variant rather than conventional DFSP, despite the absence of typical fat trapping.
4:02 Referral for Molecular Testing: The case was sent to a molecular pathologist due to diagnostic uncertainty.
4:42 Initial FISH Results: FISH for the common $COL1A1/PDGFB$ fusion was negative.
4:55 Critical Molecular Finding: Reflexive testing for the alternative $PDGFD$ rearrangement was performed, which yielded a positive result, confirming the diagnosis as DFSP.
5:42 Publication Note: The case, representing a fibrous sarcomatous variant of DFSP, was published in 2020.
6:07 Key Takeaways on Atypical DFSP: The case highlights that DFSP can occur in older patients, in unusual sites (foot/digits), and may retain CD34 positivity even in the highly cellular, herringbone $FS-DFSP$ variant.
6:53 Diagnostic Strategy: The speaker is comfortable diagnosing classic DFSP with H&E and minimal staining (CD34/S100), but recommends FISH when suspicion is high but the sample is small, or when atypical features (like a bland CD34+ spindle cell tumor with some fat trapping) suggest it might not be DFSP.
8:25 Recommended FISH Panel: When testing for DFSP, the standard $PDGFB/COL1A1$ panel should be reflexively followed by testing for the $PDGFD$ rearrangement.