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

Melanoma is one of the deadliest skin cancers, with metastatic cases carrying a 5-year survival rate of just 17%. What’s more, over half of these metastases show up in the skin—sometimes looking so harmless that they’re mistaken for common, benign lumps like lipomas. A recent case from China highlights just how tricky these diagnoses can be—and why medical history and vigilance matter more than ever.

A 71-year-old woman came to doctors at the Department of Dermatology, The First Hospital of China Medical University (Shenyang, Liaoning) with a soft, non-tender, egg-sized lump on the lateral side of her left upper arm. An ultrasound revealed a hypoechoic (darker on imaging) nodule in the subcutaneous fat layer, measuring 4.5 cm by 2.5 cm. A initial biopsy suggested the lump was a lipoma—a non-cancerous tumor made of fat—so doctors proceeded to surgically remove it.

But when they examined the entire tumor, things didn’t add up. The mass was ellipsoid with an uneven surface and incomplete outer envelope, measuring 5 cm by 3 cm. When sliced open, it showed a mix of grayish-white, grayish-yellow, and grayish-brown colors—far from the uniform, pale yellow of a typical lipoma. Then, doctors learned a critical detail: three years earlier, the woman had her left thumb amputated for malignant melanoma. Suddenly, the “lipoma” looked a lot more dangerous.

Further testing confirmed their suspicion. Under a microscope, the tumor cells were spindle-shaped with large oval nuclei, clear nucleoli, numerous mitotic figures (signs of rapid cell division), and abundant melanin deposits—classic features of melanoma. Immunohistochemical (IHC) staining, which uses antibodies to detect proteins specific to cancer types, came back positive for Melan A, human melanoma black 45 (HMB-45), S-100 protein, and sry-related HMg-box gene 10 (SOX-10)—all markers that confirm melanoma.

A positron emission tomography-computed tomography (PET-CT) scan revealed more widespread disease: metastatic lesions in her left axillary lymph nodes, the proximal muscles of her left arm, and her lungs. An ultrasound-guided biopsy of an enlarged lymph node confirmed melanoma cells there too—arranged in patches with large, dark nuclei, visible atypia (abnormal cell shape), melanin in the cytoplasm, and destroyed lymph node structure. The lymph node also tested positive for the same IHC markers as the arm tumor.

The patient started treatment with subcutaneous alpha-2b interferon (3 million units every other day) and began regular follow-ups every three months. She experienced no adverse reactions to the interferon.

This case illustrates “in-transit” melanoma metastasis—cancer that spreads to skin or soft tissue between the primary tumor (her thumb) and regional lymph nodes. The initial misdiagnosis likely stemmed from two factors: the pre-surgery biopsy sampled only superficial fat (not the actual tumor tissue) and the mass may have contained normal fat mixed with cancer cells, masking the malignancy.

The biggest takeaways from this case are clear:

  1. Medical history is non-negotiable: A past melanoma diagnosis should make doctors suspect even benign-looking lumps.
  2. Vigilance saves lives: Melanoma metastases can look like common conditions—never assume a subcutaneous lump is harmless without thorough testing.
  3. Follow-up is critical: Even early-stage melanoma patients have a high risk of metastasis. Strict, long-term monitoring (at least two years) is essential.

For patients with in-transit metastases, treatments like interferon or the bacillus Calmette-Guerin (BCG) vaccine are recommended to extend survival. In this case, the woman had refused systemic treatment and follow-up for three years after her thumb amputation—a choice that likely allowed the metastasis to grow undetected.

This case serves as a stark reminder: melanoma is a master of disguise. When it comes to skin lumps—especially in someone with a history of skin cancer—“wait and see” is never a safe approach.

Qian An, Jiu-Hong Li, Li Zhang, Hong-Duo Chen, Xing-Hua Gao
Department of Dermatology, The First Hospital of China Medical University, Shenyang, Liaoning 110001, China

References

  1. Siegel RL, Miller KD, Jemal A. Cancer statistics, 2016. CA Cancer J Clin 2016;66:7–30. doi: 10.3322/caac.21332.
  2. Tas F. Metastatic behavior in melanoma: timing, pattern, survival, and influencing factors. J Oncol 2012;2012:647–684. doi: 10.1155/2012/647684.
  3. Lookingbill DP, Spangler N, Helm KF. Cutaneous metastases in patients with metastasis carcinoma: a retrospective study of 4020 patients. J Am Acad Dermatol 1993;29:228–236.
  4. Tas F, Erturk K. Recurrence behavior in early-stage cutaneous melanoma: pattern, timing, survival, and influencing factors. Melanoma Res 2017;27:134–139. doi: 10.1097/CMR.0000000000000332.
  5. Guo J, Qin S, Liang J, Lin T, Si L, Chen X, et al. Chinese guidelines on the diagnosis and treatment of melanoma [in Chinese]. Chin Clin Oncol 2016;5:57. doi: 10.21037/cco.2015.12.02.

doi.org/10.1097/CM9.0000000000000283

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