Primary cutaneous plasmacytoma is a rare B-cell lymphoma consisting of a monoclonal proliferation of mature plasma cells in the skin without underlying multiple myeloma. The disease may be regarded as a subtype of extramedullary plasmacytoma arising primarily in the skin. It must be distinguished from metastatic (secondary) cutaneous plasmacytoma in the context of multiple myeloma. There have been only about 30 cases reported. It is somewhat controversial as to whether it is a distinct entity or represents a form of primary cutaneous marginal zone B-cell lymphoma with a predominant population of plasma cells. It is considered a form of primary cutaneous marginal zone B-cell lymphoma under the World Health Organization and the European Organization for Research and Treatment of Cancer (WHO-EORTC) classification. Cases classified in the past as primary cutaneous plasmacytoma most likely represented an example of primary cutaneous marginal zone B-cell lymphoma with prominent plasmacytic differentiation. The term plasmacytoma is no longer used in the WHO-EORTC classification.

Secondary cutaneous plasmacytoma occurs in the setting of multiple myeloma or extramedullary plasmacytoma. Multiple myeloma is a generalized malignant proliferation of plasma cells. Under the microscope, plasmacytomas originating from the skin and cutaneous metastases from systemic disease are identical. Bone marrow, skeletal x-ray, peripheral blood, and urine investigations should be included in the work-up of the patient with skin lesions consistent with plasmacytoma. Features indicative of multiple myeloma include bone marrow plasmacytosis of more than 10%, osteolytic lesions, and detection of a plasma or urine paraprotein.

Localized forms of malignant proliferations of plasma cells include solitary plasmacytoma of bone and extramedullary plasmacytoma. A rare manifestation of a solitary plasmacytoma of bone is an overlying erythematous skin patch. Lymphadenopathy is present, and some of the patients have or develop POEMS syndrome (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, and skin changes). This syndrome has been called AESOP (adenopathy and extensive skin patch overlying a plasmacytoma). The most common origin of extramedullary plasmacytoma is the upper respiratory tract, including the naso- and oropharynx, nasal cavities, larynx, and nasal sinuses. Extramedullary plasmacytoma has a tendency to remain localized and metastasize only to the draining lymph nodes. Rarely, it disseminates with secondary involvement of the skin. Also rarely, soft tissue extramedullary plasmacytomas may be associated with post-transplantation plasma cell dyscrasia. Systemic disease may evolve in either form of solitary plasmacytoma.

Skin lesions are usually asymptomatic. Rarely, lymphadenopathy and systemic symptoms of fever, malaise, weight loss, night sweats, or dyspnea (from interstitial pneumonia) may be present. The prognosis for primary cutaneous plasmacytoma is greater than 90% survival at 5 years. The main prognostic factor seems to be the clinical presentation; patients with solitary lesions have a better prognosis than those with multiple lesions.