This pathology case review focuses on a rare presentation of Dermatofibrosarcoma Protuberans (DFSP) in a 68-year-old female, presenting as a circumscribed nodule on the dorsal foot. Morphologically, the lesion displayed highly cellular, monotonous spindle cells arranged in intersecting, sharp-angled fascicles, characteristic of the "herringbone" pattern, which the speaker associates with a "fibrosarcomatous DFSP" subtype. Initial immunohistochemistry was largely unrevealing, showing strong, diffuse positivity for CD34 but negative results for S100 and keratin, confusing the initial diagnosis given the lack of classic storiform fat-trapping areas typical of conventional DFSP. The case was resolved through molecular testing, following a suggestion to investigate the less common $PDGFD$ fusion, which was positive. This confirms the diagnosis as DFSP, highlighting that the absence of classic features (like fat-trapping) or specific immunohistochemical markers (like loss of CD34) should not preclude the diagnosis, especially when guided by molecular findings.
Dermatopathology Case Study: Fibrosarcomatous Dermatofibrosarcoma Protuberans (DFSP)
0:00 Case Introduction: A case of a 68-year-old female with an excised dorsal foot nodule, an unusual age and location for DFSP.
0:18 "Ramen Noodle Sign": Describes an area characterized by bland spindle cells interspersed with wavy collagen fibers, indicative of a fibroblastic/myofibroblastic process.
1:26 Herringbone Pattern: Notes areas with larger, hyperchromatic nuclei where fascicles intersect at sharp angles, a pattern historically linked to fibrosarcoma but now seen in various entities.
2:24 CD34 Positivity: Immunohistochemistry was strongly and diffusely positive for CD34, but negative for S100 and keratin, complicating the morphological assessment.
3:07 Atypical Morphology: The lesion lacked classic conventional DFSP features like storiform pattern or fat-trapping, leading the speaker to suspect a "fibrosarcomatous DFSP" pattern, despite low mitotic activity.
4:02 Molecular Guidance: The case was sent for molecular testing, initially yielding negative results for the typical $COL1A1/PDGFB$ fusion.
4:55 Diagnosis Confirmation (PDGFD Fusion): Following consultation, testing for the alternative $PDGFD$ fusion was performed, which resulted in rearrangement, confirming the diagnosis as DFSP.
5:45 Publication: The speaker mentions publishing this case alongside a conventional DFSP in 2020 in the Journal of Cutaneous Pathology.
6:05 Key Takeaways on DFSP: The case illustrates that DFSP can occur in older patients, in unusual sites (foot/digits), and may present without classic fat-trapping or loss of CD34 expression.
6:26 CD34 Expression Nuance: Cellular, herringbone DFSP subtypes can retain CD34 positivity; loss of CD34 often correlates with more cellular or myxoid variants.
7:27 Diagnostic Comfort Level: The speaker is typically comfortable diagnosing classic DFSP based on H&E and basic stains (CD34, S100).
7:45 Role of FISH/Molecular Testing: Molecular testing is reserved for ambiguous cases (e.g., bland CD34+ spindle cell tumors) or to confirm diagnosis before proceeding with aggressive surgery (e.g., Mohs).
8:25 Reflexive Testing: When performing DFSP FISH, the speaker recommends reflexively testing for $PDGFD$ if the $COL1A1/PDGFB$ test is negative, anticipating more such cases in the literature.