Technical Note: Experimental verification of EGSnrc calculated depth dose within a parallel magnetic field in a lung phantom.

Purpose: The calculation of depth doses from a 6 MV photon beam in polystyrene using EGSnrc Monte Carlo, within a parallel magnetic field, has been previously verified against measured data. The current work experimentally investigates the accuracy of EGSnrc calculated depth doses in lung within the same parallel magnetic field.

Methods: Two cylindrical bore electromagnets produced a magnetic field parallel to the central axis of a Varian Silhouette beam. A Gammex lung phantom was used, along with a parallel plate ion chamber, for the depth dose measurements. Two experimental setups were investigated: top of phantom coinciding with the top of the magnet's bore, and top of phantom coinciding with the center of the bore. EGSnrc was modified to read the 3D magnetic field distribution and then used to simulate the depth dose in lung.

Results: The parallel magnetic field caused measurable increases in dose at the surface and in the buildup region for both setups. For the setup where the top of the lung phantom coincides with the top of the magnet, the surface dose increased by ~11% compared to the no magnetic field case but the depth of maximum dose remained unchanged. When the phantom's top surface coincided with the center of the magnet, the surface dose increased by 32% and dose maximum occurred at a shallower depth. EGSnrc was able to calculate these dose increases due to the magnetic field accurately for both setups. All the simulated depth dose values were within 2% (with respect to D ) of the measured ones, and most of the investigated points were within 1.5%.

Conclusions: Surface and dose increases due to a parallel magnetic field have been measured in a lung phantom at two separate locations within the magnetic field. EGSnrc has been shown to match these measurements to within 2%.