Dosimetric Comparison Among HT, SIMRT And VMAT In The Treatment Of Nasopharyngeal Carcinoma

Objective:To compare helical tomotherapy (HT) with volumetric modulated arc therapy (VMAT) and step and shoot intensity modulated radiation therapy (sIMRT) in nasopharyngeal carcinoma (NPC) patients, and find out which stage patient gains the maximum benefit from which technique.Methods:Eighteen patients that randomly selected from 97 patients received HT at a single center were retrospectively replanned with VMAT and sIMRT. For a fair comparison, the coverage of the 3 plans was normalized to the same level. A standard planning constraint set was used; the constraints for the OARs were individually adapted. The calculated doses to the PTV and OARs were compared for HT and VMAT and sIMRT plans from Mim system, and the plans were generated using the tomotherapy planning station and Pinnacle treatment planning system.Results:HT and VMAT and s-IMRT plans had similar PTV coverage, but the former two had better sparing in most OARs within all stages. Regardless of the stage, HT achieves better homogeneity, conformity and sparing of the lens, brain stem, spinal cord, TMJ, oral cavity, trachea-esophagus, compared with VMAT and sIMRT (P<0.05), meanwhile, VMAT shows significant advantage for lowering the dose to the optic nerve, optic chiasm as well as improving the treatment efficiency, compared with HT and sIMRT (P< 0.05). In stratified analysis, VMAT plans lead to better sparing of the back neck (regarded as OAR-Skin in the study) compared with HT in stage IV patients and sIMRT in stage III patients. The delivery time per fraction for VMAT (181.25±4.99s) was shorter than HT (446.08±30.61s; P< 0.01) and sIMRT (500.75±54.49s; P<0.01).Conclusions:With the same PTV coverage, HT achieves better homogeneity, conformity and sparing of the lens, brain stem, spinal cord, TMJ, oral cavity, trachea-esophagus, compared with VMAT and sIMRT. VMAT is the best choice for those who cannot bear immobilization for a long time, or those who are diagnosed with stage Ⅲ or IV NPC and want to protect their back neck from edema, or those who have the requirement to protect the inner ears. sIMRT shows significant advantage for lowering the dose to the optic nerve, optic chiasm as well as improving the treatment efficiency, compared with HT and sIMRT. This study shows the tendency of the dose distribution of the three techniques, and more researches are required to evaluate the conclusions.