Purpose: Surface dose (SD) is critical for understanding radiation outcome especially the cosmetic aspect of the treatment in megavoltage beams which is a complex phenomenon governed by the electron transport. The parameters that affect SD are beam energy, source to surface distance, thickness and atomic number of materials in beam path, field size and beam angle. With the introduction of MR-linac that uses magnetic fields for the imaging, the SD is becomes more critical due to Lorentz force affecting electron transport which is investigated in the study
Methods: Using MRIdian 0.35 T magnetic field with 6 MV photon beam, buildup dose was investigated post machine installation without magnetic field and same data were repeated with magnetic field on using Markus parallel plate ionization chamber embedded in a solid water phantom. The SDs were measured with and without magnetic field for small, intermediate, reference and maximum field sizes. Readings were corrected for the oblique thickness as well as correction for the parallel plate ion chamber.
Results: As expected, SD increases with increasing FS and gantry angles. The magnitude is 10%-34%, and 12.6-16.2% for small to large fields without and with magnetic fields respectively in zero-degree gantry angle. The magnetic field has a significant impact on the surface and build up region, but obliquity factor is not pronounced in magnetic field compared in presence of magnetic field. The magnitude of the SD is reversed with magnetic field as field size increases.
Conclusion: Surface dose is a complex function of the field size and gantry angle; however, the magnitude is significantly lower compared to the traditional accelerator in the 0.35T field.
Not Applicable / None Entered.