Comparison of dose distributions between transverse magnetic fields of 0.35 T and 1.5 T for radiotherapy in lung tumor using Monte Carlo calculation.

We investigated the impact of the transverse magnetic fields of 0.35 T and 1.5 T on the dose distributions for a 6 MV beam, by using a thorax phantom with a lung tumor. First, the dose distributions in the magnetic flux densities of 0 T, 0.35 T, and 1.5 T were compared by increasing the number of irradiation fields. Next, the dose distributions for stereotactic body radiotherapy (SBRT) with 5-fields for an isolated lung tumor was compared in transverse magnetic fields. All dose distributions were calculated by the Monte Carlo method. The prescription doses for SBRT with 5-fields was 48 Gy for D95 (dose covering 95% volume) in the planning target volume (PTV). The dose distributions were analyzed by the dose difference map (DD map), dose volume histogram (DVH), and dose indices. For the 1-field, the dose distributions were more affected at 1.5 T rather than 0.35 T. The DVHs for PTV at 1.5 T almost agreed with those at 0 T for more than 5-fields. In contrast, the D98 in the PTV at 0.35 T reduced constantly by 6.0% with more than 5-fields. The D95 in PTV for SBRT with 5-fields was 9.0% lower at 0.35 T and 2.5% higher at 1.5 T, in comparison with that at 0 T. For dispersed irradiation angles of more than 5-fields, it is more desirable to use the magnetic flux density of 1.5 T than 0.35 T for the radiotherapy in the lung tumor.