Lung dosimetry in a linac-MRI radiotherapy unit with a longitudinal magnetic field.

Dosimetry issues observed in a transverse linac-MR geometry such as changes to the depth dose distribution and tissue interface effects were significantly reduced or eliminated in a longitudinal geometry on a representative lung plan. Further, an increase in dose to the PTV, resulting from the magnetic field confining electrons to the forward direction, shows potential for a reduction in dose to the surrounding tissues.