Management and Outcomes of Patients with Refractory Solitary Plasmacytoma after Treatment with Definitive Radiation Therapy

Purpose: Radiation therapy (RT) is the standard treatment for solitary plasmacytoma (SP); however, the optimal management of RT-refractory SPs is unknown. We examined outcomes after early systemic therapy, surgical resection, or observation for patients with RT-refractory disease and assessed the potential impact of treatment selection on disease outcomes. Methods and Materials: We retrospectively reviewed patients with SP treated with definitive radiation and evaluated at a single institution with persistent disease on imaging or biopsy. Descriptive statistics were used to characterize patient and disease characteristics and treatment outcomes. Results: Of 102 total SP patients, 17 (17%) were RT-refractory. The median RT dose was 45 Gy, and median follow-up was 71 months from end of RT. Fifteen patients had additional treatment for refractory disease at a median time of 9.5 months after RT, with the following subsequent interventions: surgical resection (n = 4), additional RT (n = 2), systemic therapy without evidence of multiple myeloma (MM; n = 4), systemic therapy for progression to MM (n = 5), and observation (n = 2). Of 4 patients treated with surgical resection, 3 progressed to MM 22 to 43 months after diagnosis. Of 2 patients treated with additional RT, neither responded, and both had pathologic confirmation of residual disease after the second course. Four patients treated with systemic therapy without MM all had complete responses on positron emission tomography and no subsequent MM progression. Eight patients were initially observed after RT for ≥12 months (n = 8) or ≥24 months (n = 6). Of the 2 patients in continued observation, both had stable/unchanged avidity after radiation treatment for 12 and 22 months and ultimately had a slow decrease of disease avidity over multiple years. Conclusions: Patients with RT-refractory SPs can achieve good local control with alternative therapies, such as surgery or systemic therapy, if needed. Additional RT does not seem to be effective. Given the known high rates of progression from SP to MM, close observation of asymptomatic persistent disease until disease progression is likely sufficient in most cases.