Room: Exhibit Hall | Forum 1
Purpose: To investigate 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.
Methods: Six plans with an increasing number of irradiation fields were created by a treatment planning system (TPS) for a thorax phantom with a lung tumor. The six plans were composed of irradiation with 1, 2, 5, and 8 fields, half-arc (0Â°~179Â°) and 1-arc fields. The prescription dose was 48 Gy for Dâ‚‰â‚… in the planning target volume (PTV). The dose distributions without and with uniform transverse magnetic fields of 0.35 T and 1.5 T were calculated by the Monte Carlo method. The calculation voxel size was 2.5 Ã— 2.5 Ã— 2.5 mmÂ³. The statistical uncertainty of calculated doses was less than 1% at the isocenter. The dose distributions were analyzed by dose volume histograms (DVHs), dose difference (DD) maps (%), and dose indices.
Results: For the 1-field, the dose distributions were more affected at 1.5 T rather than 0.35 T. Reversely, the DVHs in PTV at 1.5 T almost agreed with those at 0 T for more than 5-fields. In contrast, the Dâ‚‰â‚ˆ in the PTV at 0.35 T reduced constantly by 6.0% with more than 5-fields. However, the dose distributions in the PTV for the half-field were disturbed at 0.35 T and 1.5 T owing to biased irradiation directions. The DDs in PTV between 1.5 T and 0 T became smaller according to the number of irradiation fields than that for 0.35 T and 0 T, expecting an asymmetric half-arc field.
Conclusion: 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.