Solitary Plasmacytoma (SP) is a rare malignant plasma cell tumor, including Solitary Bone Plasmacytoma (SBP) and Solitary Extramedullary Plasmacytoma (SEP). Radiotherapy is the preferred treatment for SP. Although the local response rate of SP after radiotherapy is as high as 86%, 55% of patients will experience disease progression within 5 years. Stereotactic Body Radiation Therapy (SBRT), as a new technology for precision radiotherapy, delivers a higher dose of radiation to the tumor site through a few short treatment sessions while maintaining low-dose exposure to surrounding normal tissues. This approach achieves good local tumor control and effectively reduces radiotherapy-related side effects, making it valuable for application in SP patients. Additionally, numerous preclinical studies have confirmed that SBRT has positive immunomodulatory effects. Based on data published in the New England Journal of Medicine in 2013, lenalidomide-based immunomodulatory therapy significantly delays the progression of symptomatic myeloma with minimal toxicity in patients with smoldering multiple myeloma. This study aims to assess the efficacy and safety of combining SBRT with lenalidomide in patients with SP, compared to conventional intensity-modulated radiotherapy. The goal is to extend progression-free survival (PFS) in newly diagnosed SP patients, reduce adverse reactions, and improve quality of life.