Abstract: 【Abstract】 Objective To evaluate the clinical efficacy and safety of electron beam radiation therapy for Kaposi sarcoma, and to analyze factors associated with treatment outcomes. Methods This was a single-center retrospective cohort study. Clinical data were retrospectively collected from patients with Kaposi sarcoma who received electron beam radiation therapy at the People′s Hospital of Xinjiang Uygur Autonomous Region between January 2006 and February 2025. All patients received perpendicular irradiation of the lesion area using 6 - 12 MeV electron beams, with individualized total radiation doses and fractionation schedules. Treatment response was evaluated according to the Response Evaluation Criteria in Solid Tumors, and categorized as complete response, partial response, stable disease, or progressive disease. Adverse reactions were graded according to the Radiation Therapy Oncology Group radiation morbidity scoring criteria. Multivariable logistic regression analysis was performed to identify factors influencing treatment response, and odds ratios （ORs） were calculated. Cox proportional hazards regression analysis was used to identify factors associated with overall survival, and hazard ratios （HRs） were calculated. Results A total of 48 patients with Kaposi sarcoma were included, comprising 44 males （91.7%） and 4 females （8.3%）, with the disease duration being 21 （7.3, 60.0） months. A total of 137 lesions were analyzed. The total radiation dose was 34 （24, 44） Gy, and the number of fractions was 17 （12, 22）. The treatment depths were mainly 0.5 cm （61 lesions, 44.5%） and 1.0 cm （50 lesions, 36.5%）. Objective response was observed in 88 lesions （64.2%）, and the condition was controlled in 126 lesions （92.0%）. According to the total radiation dose, lesions were categorized into the low-dose group （< 30 Gy, 53 lesions）, intermediate-dose group （30 to < 40 Gy, 20 lesions）, and high-dose group （≥ 40 Gy, 64 lesions）. The objective response rate was significantly lower in the low-dose group （26 lesions, 49.06%） than in the intermediate-dose group （15 lesions, 75.00%） and high-dose group （47 lesions, 73.44%） （both P < 0.05）. Multivariable analysis showed that higher total radiation dose, greater treatment depth, and nodular-stage lesions were independent favorable factors associated with objective response （all P < 0.05）. During a follow-up period of 3 - 108 months, 69 lesions （50.4%） recurred. The recurrence rate was significantly higher in the low-dose group （35 lesions, 66.04%） than in the high-dose group （25 lesions, 39.06%; P = 0.009）. A higher total radiation dose was an independent protective factor against recurrence （HR = 0.96, 95% CI: 0.94 - 0.98, P < 0.001）. Multivariable Cox regression analysis showed that recurrence was the most important factor associated with progression-free survival, with out-of-field recurrence conferring the highest risk of disease progression or death（HR = 21.14, 95% CI: 5.58 - 80.04, P < 0.001）. Regarding safety, treatment-related adverse reactions occurred in 41 lesions （29.93%）, with acute radiation injury observed in 39 lesions and late-onset chronic adverse reactions in 33. No grade 4 or 5 late-onset chronic adverse reactions or treatment-related deaths were observed. Conclusions Electron beam radiation therapy is a safe and effective treatment for Kaposi sarcoma. A total radiation dose of ≥ 30 Gy could significantly improve tumor response rates, reduce the risk of recurrence, and improve patient outcomes.

Key words: Sarcoma, Kaposi, Electron beam radiation, Efficacy, Safety, Root cause analysis, Survival analysis