Experimental verification of dose enhancement effects in a lung phantom from inline magnetic fields

Background and purpose To present experimental evidence of lung dose enhancement effects caused by strong inline magnetic fields. Materials and methods A permanent magnet device was utilised to generate 0.95 T–1.2 T magnetic fields that encompassed two small lung-equivalent phantoms of density 0.3 g/cm3. Small 6MV and 10MV photon beams were incident parallel with the magnetic field direction and Gafchromic EBT3 film was placed inside the lung phantoms, perpendicular to the beam (experiment 1) and parallel to the beam (experiment 2). Monte Carlo simulations of experiment 1 were also performed. Results Experiment 1: The 1.2 T inline magnetic field induced a 12% (6MV) and 14% (10MV) increase in the dose at the phantom centre. The Monte Carlo modelling matched well (±2%) to the experimentally observed results. Experiment 2: A 0.95 T field peaked at the phantom centroid (but not at the phantom entry/exit regions) details a clear dose increase due to the magnetic field of up to 25%. Conclusions This experimental work has demonstrated how strong inline magnetic fields act to enhance the dose to lower density mediums such as lung tissue. Clinically, such scenarios will arise in inline MRI-linac systems for treatment of small lung tumours.