Dear Editor,
1.
Melanomas of unknown primary origin (MUP) means melanomas present metastatically without an identified primary lesion. In addition, some melanomas are “amelanotic,” in which melanin pigment is not detectable histopathologically. The features often make the diagnosis of MUP difficult. We herein report a case of multiple bone metastasis from an amelanotic MUP and describe the diagnosis of this disease.
A 49‐year‐old man was referred to our hospital due to low back pain off and on for years. The patient denied systemic disease. On a physical examination, the patient exhibited decreased muscle power of lower limbs. After admission, image studies, including bone scan, X‐ray, CT show multiple bone lesions (Figure 1A,B). The laboratory data showed leukocytosis (WBC: 12.59 × 103/mm3) and hypercalcemia (12.1 mg/dL). Metastatic carcinoma or multiple myeloma was impressed by the clinician. The markers, CEA, PSA, IgG, IgA, IgM, Beta‐microglobulin, Bence‐Jone protein were performed and they were all within normal limits. The humerus biopsy showed diffuse infiltration of bizarre spindle cells, epithelioid, and rhabdoid cells. Many multinucleated giant cells were also noted (Figure 1C). No evidence of melanin pigmentation was found. An immunohistochemical study demonstrated tumor cells positive for S100, SOX‐10, TTF‐1 and negative for Melanin‐A, HMB‐45, CK, SMA, desmin, and CD31 (Figure 1D,E). These results were consistent with the diagnosis of metastatic melanoma. However, there were no skin lesions or mucosal lesions. After diagnosis, the patient received chemotherapy of cisplatin with dacarbazine and tamoxifen. The patient died 1 year later.
Figure 1.
A, The bone scan showed increased radioactivity in the skull, cervical, thoracic and lumbar spines, sacrum, rib cage, sternum, clavicles, scapulae, humeri, sacroiliac joints, pelvic bones, and femora. B, Left humeral shaft X‐ray showed osteolytic lesion. C, Pathologic features were diffuse infiltration of bizarre spindle cells, epithelioid, and rhabdoid cells without melanin pigmentation. D and E, S‐100 and SOX‐10 immunostains were positive
In Asia, MUP with multiple bone metastases but without other organ metastasis is uncommon.1, 2 This case provided the important information: (a) When facing a patient with multiple bone metastases but without a solid tumor mass, a melanoma with spontaneous regression should be listed for differential diagnosis. In the absence of skin or mucosal lesions and the lack of melanin pigment, melanoma can easily be overlooked. (b) Melanomas could exhibit varying cellular morphology, mimicking poorly differentiated carcinoma or sarcoma. SOX‐10 can be used for confirmation, because it is a more specific melanoma marker when detecting metastatic melanoma. Furthermore, TTF‐1 could be diffusely positive for melanoma, pathologists should be careful to mislead.3
CONFLICTS OF INTEREST
All authors declare no conflicts of interests.
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