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

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Definitive radiation therapy (RT) is the current standard of care for the treatment of solitary plasmacytoma (SP), as RT typically affords excellent local control with possible long-term remission or even cure. Given the low rates of local failure after RT, the optimal management of RT-refractory SPs is unknown. With IRB approval, we reviewed our institutional records of all SP patients (pts) treated with RT and evaluated at our institution from Jan 2000-Sept 2019. All pts met criteria for SP or SP with minimal marrow involvement. Refractory disease was defined as persistent disease on post-RT biopsy/surgery or persistent, residual avid disease on PET-CT. Of 102 total pts, 17 (17%) were considered refractory to RT and included in the study. Median age at diagnosis was 54 (range 33-75). Sixteen of 17 pts had solitary bone plasmacytoma (vs extramedullary). At diagnosis, the median maximal tumor dimension was 6.2 cm (range 2.1-11.9), 11 pts had positive blood paraprotein (median 0.8 gm/dL) and 5 pts had positive Bence Jones proteinuria (median 152 mg/24hrs). On SP biopsy, four pts had associated amorphous material or amyloid; 1 pt had amyloid on fat pad biopsy. Median RT dose was 45 Gy (range 39.6-56 Gy). Most pts underwent PET-CT pre- (15 pts, median maximum standardized uptake value [SUVmax] 9.3, range 2.3-32) and post- (15 pts, median SUVmax 9.3, range 2.9-23) RT; SUVs decreased for 35% of pts (n = 6) after RT. For eleven pts with positive proteins and data available, ten had persistent blood/urine protein levels after RT until next intervention. Ten pts had pathologic confirmation of residual plasmacytoma at median 7 months (range 1.5-32) post-RT. Median follow-up was 27 months. Thirteen pts had at least one additional treatment for refractory disease; the median time to next treatment was 9 months. Pts had the following interventions: surgical resection (n = 4), additional RT (n = 2), systemic therapy without evidence of MM (n = 4), systemic therapy for progression to MM (n = 3) and observation (n = 4, median observation follow-up 7.5 months). Of the 2 pts treated with additional RT (cumulative doses of 110 Gy and 70 Gy), neither responded to RT and both had pathologic confirmation of residual disease after the second course. All pts with follow up after systemic therapy or surgery had an imaging-based response at the SP site. Two pts died: 1 cause unknown, 1 from osteosarcoma developed at the site of the twice-radiated sacral plasmacytoma. Patients with RT-refractory SPs can achieve good outcomes with alternative therapies such as surgery or systemic therapy. Additional RT does not seem to be effective. Given the known high rates of progression from SP to multiple myeloma, treatment of SP should not cause significant morbidity and close observation of asymptomatic persistent disease after RT for a limited time is reasonable.