Volumetric-modulated arc therapy versus intensity-modulated radiotherapy for large volume retroperitoneal sarcomas: A comparative analysis of dosimetric and treatment delivery parameters.

Purpose: To compare dosimetric and treatment delivery parameter differences between volumetric-modulated arc radiotherapy (VMAT) and intensity-modulated radiotherapy (IMRT) for large volume retroperitoneal sarcomas (RPS). Materials and Methods: Both VMAT and IMRT planning were performed on CT datasets of 10 patients with RPS who had been previously treated with preoperative radiotherapy. Plans were optimized to deliver >= 95% dose to the PTV and were evaluated for conformity and homogeneity. Dose to the organs at risk (OARs) (kidney, liver, spinal cord, and bowel space), unspecified tissue, and dose evaluation volumes (DEVs) at 1, 2, and 5 cm from PTV were calculated and compared. Monitor units (MUs) and treatment delivery times were recorded and compared between the two techniques. The deliverability of the large volume RPS VMAT plans was verified by portal dosimetry on a Truebeam (TM) linac. Results: VMAT and IMRT plans were equivalent for PTV coverage and homogeneity (P > 0.05); however, VMAT plans had slightly better conformity index, CI (P < 0.001). Doses to the OARs were not significantly different between VMAT and IMRT plans (P > 0.05). Mean doses to the unspecified tissue as well as at 1, 2, and 5 cm DEVs were lower with VMAT compared with IMRT, P = 0.04 and P < 0.01, respectively. MUs and average beam-on times were both significantly lower in the VMAT vs IMRT plans, P < 0.001 and P = 0.001, respectively. All VMAT plans passed portal dosimetry delivery verification with an average gamma passing rate of 99.6 +/- 0.4%. Conclusions: VMAT planning for large volume RPS improved CI, and achieved comparable OAR sparing, as compared with IMRT. As treatment delivery time was lower, the use of VMAT for RPS may translate into improved treatment delivery efficiency.