Dosimetric comparisons of helical tomotherapy and step-and-shoot intensity-modulated radiotherapy in nasopharyngeal carcinoma

Purpose: The study evaluates and quantifies the potential dosimetric gains of helical tomotherapy (HT) versus step-and-shoot intensity-modulated radiotherapy (SaS-IMRT) for nasopharyngeal carcinoma (NPC). Materials and methods: Twenty consecutive NPC patients curatively treated by HT were examined. Each case was planned by HT and SaS-IMRT (ADAC Pinnacle3) planning system, respectively. Dose plans were compared using dose volume histograms (DVH), conformity index (CI), homogeneity index (HI), and minimal dose to 1 cc (Dmin1cc) of the planned target volume (PTV) and a comprehensive quality index (CQI) of ten organs at risk (OARs). The prescribed dose/fractionation was 72 Gy to the PTV, 64.8 Gy to the elective PTV, and 54 Gy to the clinically negative neck region. The plan of 54 Gy to the PTV (PTV54) was used to evaluate the CI and HI in the target. The cumulative doses of the three PTV plans to the OARs were calculated. Results: We observed the HT plans significantly improved the CI (improvement ratio: 11.9 ± 5.5%) and HI (improvement ratio: 8.8 ± 1.5%) of the PTV54 compared with SaS-IMRT plans. In addition, the mean/maximal dose of most of the OARs except chiasm was significantly reduced in HT plans, with the CQI of 0.92 ± 0.08. A negative result of HT in chiasm was observed but only significantly revealed in cases without skull base infiltration. Conclusions: A dosimetric gain in CI and HI of PTV and sparing of OARs was significantly obtained in HT versus SaS-IMRT plans in NPC patients. Whether such dosimetric superiority in HT could transfer into clinical advantages needs further investigation