Background/Aim: We investigated the effectiveness and safety of using a tissue expander (TE) for adjuvant helical tomotherapy (HT) for curatively resected retroperitoneal sarcoma (RPS). Materials and Methods: This study was conducted with 60 RPS patients who underwent curative resection with or without TE insertion followed by HT from June 2009 to December 2016. Among the patients, TE was inserted in 37 (61.7%). The quality of TE insertion was evaluated. Median follow-up after surgery was 19.4 months (range=4.5-93.2 months). Results: A higher biologically-equivalent dose (α/β=10) was used with patients who had TE insertion (median, 72.0 gray [Gy] vs. 67.1 Gy, p=0.02). The quality of TE insertion was excellent in 18 patients (48.6%), good in 10 (27.0%), fair in seven (18.9%), and poor in two (5.4%). Toxicity was not significantly different according to TE insertion. Local recurrence-free survival (LRFS) was 78.2%, and overall survival was 95.6% at 3 years. TE insertion was not a significant factor for LRFS (91.1% in TE vs. 62.9% in non-TE group at 3 years, p=0.62). In the subgroup of patients with R1 or unknown tumor margin status, however, LRFS was marginally higher in the TE insertion group (100.0% in TE vs. 62.9% non-TE group at 3 years, p=0.05). Conclusion: HT with TE insertion as adjuvant RT for curatively resected RPS was feasible with acceptable toxicity. In the patients with R1 or unknown tumor margin status, LRFS was marginally higher in the TE insertion group after HT.