Metastatic Melanoma Misdiagnosed as Lipoma Manifesting as a Subcutaneous Soft-Tissue Mass

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Metastatic Melanoma Misdiagnosed as Lipoma Manifesting as a Subcutaneous Soft-Tissue Mass

Metastatic melanoma remains one of the most aggressive malignancies, characterized by rapid systemic dissemination and a dismal prognosis. Despite advances in diagnostic techniques, misdiagnosis of metastatic lesions as benign tumors continues to pose significant clinical challenges. This case report highlights the diagnostic pitfalls and clinical implications of a subcutaneous metastatic melanoma initially misdiagnosed as a lipoma, underscoring the critical role of integrating clinical history, imaging, histopathology, and immunohistochemistry in achieving an accurate diagnosis.

Clinical Presentation and Initial Evaluation

A 71-year-old woman presented with a painless, egg-sized subcutaneous mass on the lateral aspect of her left upper arm. The lesion measured 4.5 cm × 2.5 cm on ultrasonographic examination, appearing as a hypoechoic nodule localized within the subcutaneous adipose layer (Figure 1A). The mass exhibited a soft consistency and lacked surface pigmentation or ulceration, leading to an initial clinical suspicion of a lipoma. A preoperative biopsy was performed, and histopathologic analysis of the sampled tissue revealed features consistent with a lipoma, including mature adipocytes without evidence of malignancy.

Surgical Excision and Gross Findings

Under local anesthesia, the mass was surgically excised via an incision 0.1 cm from its periphery. Intraoperative dissection confirmed the lesion was entirely confined to the subcutaneous layer, with no involvement of underlying muscle or fascia. Gross examination of the resected specimen revealed an ellipsoid mass measuring 5 cm × 3 cm, characterized by an uneven surface and an incomplete pseudocapsule. Sectioning demonstrated heterogeneous coloration, with areas of grayish-white, grayish-yellow, and grayish-brown tissue, along with a firm, irregular texture (Figure 1B, 1C).

Histopathologic Re-evaluation and Immunohistochemistry

Despite the initial benign diagnosis, the patient’s medical history prompted further investigation. Three years prior, she had undergone amputation of her left thumb for primary malignant melanoma. This critical detail raised suspicion of metastatic disease, necessitating re-examination of the excised mass. Histopathologic analysis revealed spindle-shaped tumor cells with large, hyperchromatic nuclei, prominent nucleoli, and frequent mitotic figures. Abundant cytoplasmic melanin deposits were observed, alongside a diffuse infiltrative growth pattern (Figure 1D).

Immunohistochemical staining confirmed the malignant melanocytic origin of the tumor. The cells exhibited strong positivity for melanocyte-specific markers, including Melan-A, HMB-45, S-100 protein, and SOX-10 (Figure 1E1–1E4). These findings unequivocally established the diagnosis of metastatic melanoma.

Systemic Staging and Additional Metastases

Comprehensive staging with positron emission tomography-computed tomography (PET-CT) revealed disseminated metastatic lesions in the left axillary lymph nodes, proximal muscles of the left upper extremity, and bilateral lungs (Figure 1F). An ultrasound-guided biopsy of an enlarged left axillary lymph node confirmed metastatic involvement, demonstrating tumor cells with melanin pigmentation, nuclear atypia, and architectural effacement of nodal tissue (Figure 1F). Immunohistochemical staining of the lymph node mirrored the primary subcutaneous lesion, showing positivity for Melan-A, HMB-45, S-100, and SOX-10 (Figure 1G1–1G4).

Treatment and Follow-Up

The patient was initiated on subcutaneous interferon α-2b therapy at a dose of 3 million units administered every other day. She tolerated the treatment without adverse effects and was scheduled for regular follow-up evaluations every three months to monitor disease progression and therapeutic response.

Discussion

Diagnostic Challenges and Pitfalls

This case illustrates the potential for metastatic melanoma to mimic benign subcutaneous tumors, such as lipomas, both clinically and radiologically. The initial misdiagnosis stemmed from two key factors:

  1. Sampling Error: The preoperative biopsy likely captured only superficial adipose tissue, missing the underlying malignant component.
  2. Histologic Heterogeneity: The presence of intermixed benign adipose tissue within the tumor may have contributed to diagnostic confusion.

Metastatic melanoma to the skin can manifest with considerable morphologic variability, including nodular, cystic, or lipoma-like presentations. In-transit metastases, defined as cutaneous or subcutaneous deposits occurring between the primary site and regional lymph nodes, are particularly prone to misdiagnosis due to their nonspecific appearance.

Importance of Clinical History

The patient’s history of primary melanoma three years earlier was pivotal in redirecting the diagnostic approach. Despite undergoing curative surgery for the primary lesion, she had discontinued routine follow-up and declined adjuvant therapy, highlighting the consequences of inadequate post-treatment surveillance. Early-stage melanoma carries a substantial risk of metastasis, even years after initial resection, necessitating long-term vigilance.

Pathologic and Immunohistochemical Correlates

The histopathologic findings in this case align with known features of metastatic melanoma:

  • Cytomorphology: Spindle or epithelioid cells with nuclear pleomorphism, prominent nucleoli, and mitotic activity.
  • Melanin Production: Variable cytoplasmic pigmentation, though amelanotic variants may lack visible melanin.
  • Immunoprofile: Consistent expression of S-100 (sensitivity >95%), SOX-10 (nuclear staining), and melanocytic markers Melan-A and HMB-45.

The combined use of these markers enhances diagnostic accuracy, particularly in poorly differentiated or amelanotic cases.

Therapeutic Considerations

Management of metastatic melanoma requires a multidisciplinary approach. For in-transit metastases, options include surgical excision, intralesional therapies (e.g., bacillus Calmette-Guérin vaccine), and systemic immunotherapies. Interferon-α, as utilized in this patient, has immunomodulatory and antiangiogenic effects but is limited by toxicity and modest efficacy in advanced disease. Contemporary regimens incorporating immune checkpoint inhibitors (e.g., anti-PD-1 agents) or targeted therapies (e.g., BRAF/MEK inhibitors) have significantly improved outcomes but were not available in this case.

Prognostic Implications

The 5-year survival rate for metastatic melanoma remains approximately 17%, emphasizing the need for early detection and aggressive management. Subcutaneous metastases, while less common than nodal or visceral involvement, are associated with reduced survival compared to localized disease.

Conclusion

This case underscores several critical lessons in the management of melanoma:

  1. Clinical Vigilance: Any new subcutaneous mass in a patient with a history of melanoma warrants exhaustive evaluation to exclude metastasis.
  2. Diagnostic Rigor: Deep or repeat biopsies may be necessary when clinical suspicion persists despite initial benign findings.
  3. Long-Term Follow-Up: Adherence to surveillance protocols is essential for early detection of recurrence or metastasis.
  4. Multimodal Therapy: Integration of surgery, histopathologic analysis, and systemic treatment optimizes outcomes in disseminated disease.

The misdiagnosis of metastatic melanoma as a lipoma highlights the deceptive potential of cutaneous metastases and reinforces the importance of correlating clinical history, imaging, and advanced pathologic techniques in achieving diagnostic accuracy.

doi.org/10.1097/CM9.0000000000000283

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