Extramedullary plasmacytoma of the thyroid, refractory to radiation therapy and treated with bortezomib

Discussion

Plasma cells are well-differentiated B-lymphocytes that secrete antibodies and provide protective immunity to the human body.⁹ Plasma cell neoplasms are clonal proliferation of plasma cells, producing monoclonal immunoglobulins. They are of the following different types: plasma cell myeloma, monoclonal gammopathy of unknown significance, immunoglobulin deposition disease, POEMS (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, skin changes) syndrome, and plasmacytomas, which are divided into 2 types – solitary plasmacytoma of the bone, and extramedullary plasmacytoma (EMP).¹⁰ EMP is a rare condition and encompasses 3% to 5% of all plasma cell neoplasms, depending on the study.^1,2,5 It is more common in men than in women (2.6:1, respectively), with equal incidence among black and white patients. Median age at diagnosis is 62 years, and it is more common among those aged 40 to 70 years.^2,11 The most common sites of occurrence are the respiratory tract, the mouth, and the pharynx, but other sites such as the eyes, brain, skin, and lymph nodes may also be involved.²

EMP involving the thyroid gland is a very rare occurrence, but plasma cell myeloma has been shown to secondarily involve the thyroid.⁴ Similar to our report, EMP of the thyroid in the setting of thyroiditis has been reported by other authors.^3,4 The incidence of EMP occurring in the thyroid varies according to different authors. Wiltshaw found 7 cases involving the thyroid out of 272 cases of EMP.¹ Galieni and colleagues reported only 1 case that involved the thyroid out of 46 described cases of EMP.¹²

El- Siemińska and colleagues showed that levels of interleukin (IL)-6 are elevated in thyroiditis.¹³ IL-6 promotes monoclonal as well as polyclonal proliferation of plasma cells. Kovalchuk and colleagues showed an increase in EMP in IL-6 transgenic mice, suggesting a pathophysiologic explanation.¹⁴

The diagnostic requirements of EMP include the following: histology showing monoclonal plasma cell infiltration in tissue; bone marrow biopsy with normal plasma cell aspirate and biopsy (plasma cells, <5%); no lytic lesions on skeletal survey; no anemia, renal impairment, or hypercalcemia; and absent or low serum M protein.¹²

Our case fulfilled those criteria.

The treatment options for EMP include surgery, radiotherapy, or a combined approach including both. Usually, EMPs are very sensitive to radiotherapy, and complete remission can be achieved by radiotherapy alone in 80% to 100% of cases.^6,11,15 Surgery is considered if the tumor is diffuse or is causing symptoms secondary to pressure on surrounding structures. A combined approach is recommended in cases with incomplete surgical margin or lymph node involvement.^5,6

There is limited evidence about and experience with the use of chemotherapy in the treatment of EMP. It has been recommended that chemotherapy be considered in patients with refractory or relapsed disease using the same regimen used in plasma cell myeloma.⁵ Katodritou and colleagues have reported using bortezomib and dexamethasone without surgery in a solitary gastric plasmacytoma to avoid the toxicity of gastrointestinal irradiation.⁷ Wei and colleagues treated a patient with EMP in the pancreas with bortezomib and achieved a near-complete remission.⁸ To our knowledge, there is no documented literature about the treatment of EMP of the thyroid with chemotherapy. In our patient, despite the treatment with surgery and radiation, there was persistent uptake on the PET scan, so we treated her with bortezomib and dexamethasone. Because there is an 11% to 30% risk of progression to multiple myeloma, long-term follow-up is recommended in EMP.¹¹