Dosimetric impact of port number in TomoDirect‐based lower‐body total body irradiation

Background The feasibility of total body irradiation (TBI) delivery using the TomoDirect mode of a TomoTherapy system has been demonstrated. In clinical practice, TomoTherapy‐based TBI is typically performed in two parts: upper and lower body. Purpose This study aimed to evaluate the dosimetric impact of varying port numbers on dose evaluation indices for lower body irradiation using TomoDirect and to compare these results with those of TomoHelical in a simulation setting. Methods Sixteen patients who underwent myeloablative TBI using TomoHelical between October 2017 and March 2021 were retrospectively analyzed. TomoDirect plans with 2 to 12 ports at approximately equal beam angles were generated (modulation factor = 1.5, field width = 5.0 cm, pitch = 0.500). TomoHelical plans used identical parameters except for a pitch of 0.397. The prescribed dose was 12 Gy in six fractions. Dose indices (D2, D98, D50, homogeneity index [HI]) and beam‐on time were compared. Results In TomoDirect plans, all dose evaluation indices worsened as the number of ports increased up to five, but changes became minimal beyond eight ports. D2 was significantly improved in all TomoDirect plans compared with those of TomoHelical plan. D98 was significantly lower for the three‐ and five‐port TomoDirect plans, and no TomoDirect plan achieved higher D98 than TomoHelical. The D50 and HI were significantly improved in all TomoDirect plans except the five‐port configuration. The two‐port TomoDirect plan achieved the most favorable dose indices and the shortest beam‐on time. Conclusions The two‐port TomoDirect approach with anterior‐posterior beam configuration provides an efficient and clinically reasonable option for lower‐body irradiation, offering improved dose homogeneity and reduced treatment time compared with TomoHelical.