ABSTRACT
Objective To implement the finite discontinuity?volumetric modulated arc therapy ( FD?VMAT) in the Pinnacle planning system, and to investigate its clinical significance. Methods Eight patients with thoracic esophageal cancer in our hospital were enrolled as subjects. FD?VMAT was fulfilled in the Pinnacle planning system using a developed program. FD?VMAT, VMAT, and fixed?field intensity?modulated radiotherapy ( IMRT ) plans were designed for each patient. The conformity index ( CI ) and homogeneity index ( HI) of the planning target volume ( PTV) ,doses to organs at risk,passing rate for plan verification,number of monitor units,and treatment time were used to evaluate the plans. Comparison between different plans was made by paired t test. Results For the PTV,there was no significant difference in CI between FD?VMAT and VAMT ( P=0?186 );FD?VMAT had a significantly worse HI than VMAT ( P=0?001);however,both the CI and HI were significantly improved in FD?VMAT than in IMRT ( P=0?006, 0?002) . Compared with IMRT, FD?VMAT, retaining the advantage of VMAT, had pulmonary V20 and V30 significantly reduced by 19?79% and 20?32%,respectively (P=0?000,0?000).For the pulmonary low?dose regions (≤V5 ) ,FD?VMAT retained the advantage of IMRT and had lower doses than VMAT. Particularly, pulmonary V2 was significantly reduced by 16?79%(P=0?000).The mean lung dose was significantly lower in FD?VMAT than in VMAT or IMRT (P=0?001,0?000).There were no significant differences in D1cc to spinal cord PRV,heart V30,or passing rate for plan verification between the three therapies. The heart V40 and mean heart dose in FD?VMAT were similar to those in VMAT (P=0?175,0?468),but significantly lower than those in IMRT ( P=0?021,0?002) . FD?VMAT had a larger number of monitor units and longer treatment time than VMAT. Compared with IMRT, the number of monitor units and treatment time were reduced by 13?6% and 49?6% in FD?VMAT,respectively. Conclusions Compared with VMAT and IMRT, the application of the developed FD?VMAT in the treatment of thoracic esophageal cancer can further reduce the lung dose while keeping the PTV coverage,protection of the heart and spinal cord,and high efficacy. FD?VMAT is a new therapy available for thoracic esophageal cancer.