Objectives: Solitary plasmacytoma (SP) is characterized by a single mass of clonal plasma cells. Definitive RT can result in long-term local control of the SP. Due to the small number of patients and narrow range of doses, phase III randomized trials are lacking. The aim of this study is to further support the potential use of RT for the treatment of SP. Methods: Clinical data of all patients treated for SP at our Institution between 1992 and 2018 were reviewed. A total of 42 consecutive patients were analyzed. Results: The median follow-up was 84.8 months. Radiation dose did not differ significantly as a function of sex, type of SP (solitary bone plasmacytoma or as extramedullary plasmacytoma), tumor size; conversely differs significantly as a function of age (p = 0.04). The 5y-OS and 10y-OS were, respectively, 96 and 91%. Local recurrences developed in 21.4% of patients (9/42). 16 patients progressed to MM (38.1%). The 5y-progression to MM free survival (PMFS) and the 10y-PMFS were, respectively, 68.6 and 61.9%. Conclusions: Our data confirm that good results are achievable with RT to treat SP, but they don’t allow defining a dose–effect correlation; therefore, it remains uncertain which is the most effective dose and whether lower doses can guarantee adequate disease control.