Objective: To investigate potential prognostic factors affecting patient outcomes and to evaluate the optimal methods and effects of radiotherapy (RT) in the management of extramedullary plasmacytoma (EMP). Methods: Data from 55 patients with EMP between November 1999 and August 2015 were collected. The median age was 51 (range, 22-77) years. The median tumor size was 3.5 (range, 1.0-15.0) cm. The median applied dose was 50.0 (range, 30.0-70.0) Gy. Thirty-nine patients (70.9%) presented with disease in the head or neck region. Twelve patients received RT alone, 9 received surgery (S) alone, 3 received chemotherapy (CT) alone, and 3 patients did not receive any treatment. Combination therapies were applied in 28 patients. Results: The median follow-up duration was 56 months. The 5-year local recurrence-free survival (LRFS), multiple myeloma-free survival (MMFS), progression-free survival (PFS) and overall survival (OS) rates were 79.8%, 78.6%, 65.2% and 76.0%, respectively. Univariate analysis revealed that RT was a favourable factor for all examined endpoints. Furthermore, head and neck EMPs were associated with superior LRFS, MMFS and PFS. Tumor size < 4 cm was associated with superior MMFS, PFS and OS; serum M protein negativity was associated with superior MMFS and PFS; age >= 50 years and local recurrence were associated with poor MMFS. The dose >= 45 Gy group exhibited superior 5-year LRFS, MMFS and PFS rates (94.7%, 94.4%, 90.0%, respectively), while the corresponding values for the dose < 45 Gy group were 62.5% (P=0.008), 53.3% (P=0.036) and 41.7% (P<0.001). Conclusions: Involved-site RT of at least 45 Gy should be considered for EMP. Furthermore, patients with head and neck EMP, tumor size < 4 cm, age < 50 years and serum M protein negativity had better outcomes.