Comparison of dosimetric characteristics between flattening-filter-free and flattening filter mode VMAT plans in nasopharyngeal carcinoma

Abstract

Objective To compare the dosimetric characteristics of the VMAT plans between the flattening-filter-free (FFF) and flattening filter (FF) modes for nasopharyngeal carcinoma and analyze the feasibility of VMAT plans in the FFF model applied in clinical practice. Methods Ten patients diagnosed with stage Ⅱ nasopharyngeal carcinoma between 2016 and 2017 were recruited in this investigation. For the CT image and clinical target volume (CTV) of the identical patient, FFF-and FF-mode VMAT plans were established with the identical prescription dose of 6 975 cGy and the identical modified parameters on the RayStation platform (6 MV X-ray). The dosimetric characteristics, the number of machine hops and the time of beam were recorded and statistically compared between the FFF and FF modes. Results The minimum dose D98% target, target coverage, conformal index (CI) did not significantly differ between the FF and FFF plans (P=0.262-0.975), whereas the maximum dose of D2%, the average dose of Dmean and target homogeneity index (HI) of the FF plans were slightly advantageous to those of the FFF plan (P=0.004-0.038). The acceptable dose of bilateral lenses, the volume of the left parotid gland with an acceptable dose of> 30 Gy and the average dose of the left temporal lobe in the FFF mode were declined compared with those in the FF mode (P=0.002-0.025). The volume of normal tissues (Body) within the dose range of 2-30 Gy in the FFF mode was less than that in the FF mode (P=0.000-0.003). In the FFF mode, the mean number of machine hops was 1.188 times (P=0.000) and the average time of beam was 91.2% of the FF plan (P=0.024). Conclusions Both the FF and FFF plans qualify the dosimetric requirements in clinical settings. FF plan is advantageous in terms of the dose distribution in the target area, whereas FFF plan yields better OAR protection and low-dose irradiation. In spite of the increasing quantity of machine hops, the time of beam is declined in the FFF plan. Consequently, FFF mode probably contributes to enhance the clinical efficacy. Key words: Nasopharyngeal neoplasm/volumetric modulated arc therapy; Flattening-filter-free; Dosimetry