Pilomatrixoma Misdiagnosed as Dermatofibrosarcoma Protuberans

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Pilomatrixoma Misdiagnosed as Dermatofibrosarcoma Protuberans: A Case Analysis and Diagnostic Considerations

Pilomatrixoma, a benign tumor originating from hair follicle matrix cells, and dermatofibrosarcoma protuberans (DFSP), a low-grade fibroblastic sarcoma, represent two distinct entities in dermatopathology with overlapping clinical features that may lead to diagnostic confusion. This article explores a case initially suspected as DFSP but later confirmed as pilomatrixoma through surgical excision, emphasizing the challenges in clinical and histopathological differentiation.

Clinical Presentation and Initial Diagnostic Challenges

A 29-year-old woman presented with a 1-year history of a progressively enlarging, asymptomatic mass on the posterior neck. The lesion began as a bean-sized, reddish nodule but transformed into a painful, firm, ulcerated mass measuring several centimeters in diameter after external friction. Initial treatment with topical mupirocin ointment provided partial pain relief and minor size reduction. Clinical suspicion for DFSP arose due to the lesion’s rapid growth, ulceration, and firm consistency—features often associated with aggressive fibroproliferative tumors.

A punch biopsy revealed spindle cell proliferation in the dermis with indistinct borders (Figure 1B). Immunohistochemical staining showed positivity for factor XIIIa, vimentin, and CD68, while markers such as CD34, CD1a, and S100 were negative. The Ki67 proliferation index was 10%. These findings initially suggested DFSP; however, the absence of CD34 expression—a hallmark of DFSP—created diagnostic ambiguity. This discrepancy prompted further investigation through complete surgical excision.

Definitive Diagnosis Through Surgical Pathology

Postoperative histopathological examination revealed a well-demarcated tumor in the lower dermis and subcutaneous fat, composed of basophilic cells, shadow cells, and calcifications (Figures 1C and 1D). These features are pathognomonic for pilomatrixoma, confirming the diagnosis. Margins were free of residual tumor, and no recurrence was observed during the 1-year follow-up.

Comparative Analysis of Pilomatrixoma and DFSP

Epidemiology and Clinical Features
Pilomatrixoma typically presents as a solitary, firm, blue-gray nodule on the head, neck, or upper extremities. It predominantly affects children and adults over 50 years, with most lesions <3 cm. In contrast, DFSP commonly manifests as an indurated plaque on the trunk or proximal extremities in middle-aged adults, evolving into raised, ulcerated nodules over years. Accelerated growth and bleeding, as seen in this case, are atypical for pilomatrixoma but align with DFSP progression.

Histopathological and Immunohistochemical Distinctions
Pilomatrixoma is characterized by basophilic cells with eosinophilic cytoplasm, transitional “shadow” cells, and calcifications. DFSP exhibits monomorphic spindle cells arranged in a storiform (“straw mat”) pattern, infiltrating subcutaneous tissue. Immunohistochemically, DFSP cells express CD34 and vimentin but lack factor XIIIa, whereas pilomatrixoma may show variable CD68 and factor XIIIa positivity due to histiocytic infiltration. The absence of CD34 in this case ruled out DFSP, highlighting the importance of comprehensive marker panels.

Diagnostic Pitfalls and Biopsy Limitations
The initial misdiagnosis underscores the limitations of punch biopsies for deep-seated tumors. Inadequate sampling may miss diagnostic features, as seen here where the biopsy captured spindle cells but not the basaloid or shadow cells pathognomonic for pilomatrixoma. Studies report only 60.9% concordance between punch biopsy and excision specimens in skin tumors, emphasizing the need for clinical-pathological correlation and repeat biopsies for ambiguous cases.

Therapeutic Approaches and Prognosis

Both tumors require surgical excision, but management strategies differ. Pilomatrixoma has a low recurrence rate (<3%) after complete excision. DFSP, however, demands wider margins (2–4 cm) or Mohs micrographic surgery to address its infiltrative growth and high local recurrence rate (15–50%). Radiotherapy may adjuvant therapy for DFSP with positive margins.

Key Lessons for Clinical Practice

  1. Clinicopathological Correlation: Discordance between clinical presentation and biopsy findings necessitates reevaluation. In this case, the neck location and young age were atypical for DFSP, prompting further scrutiny.
  2. Immunohistochemical Pitfalls: Factor XIIIa positivity in pilomatrixoma reflects reactive histiocytes, not tumor lineage. Reliance on CD34 negativity helped exclude DFSP.
  3. Biopsy Sampling: Deeper or repeat biopsies are critical for tumors with ambiguous initial results.

Conclusion

This case illustrates the diagnostic challenges posed by overlapping features of pilomatrixoma and DFSP. Accurate diagnosis requires integration of clinical context, histopathology, and immunohistochemistry, particularly in lesions with atypical behavior. Surgical excision remains the gold standard for definitive diagnosis and treatment. Clinicians must remain vigilant for sampling errors and interpret immunohistochemical results within the broader diagnostic framework.

doi.org/10.1097/CM9.0000000000001457

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