Monte Carlo Characterization of skin doses in 6 MV transverse field MRI-linac systems: effect of field size, surface orientation, magnetic field strength, and exit bolus

Purpose: The main focus of this work is to continue investigations into the Monte Carlo predicted skin doses seen in MRI-guided radiotherapy. In particular, the authors aim to characterize the 70 μm skin doses over a larger range of magnetic field strength and x-ray field size than in the current literature. The effect of surface orientation on both the entry and exit sides is also studied. Finally, the use of exit bolus is also investigated for minimizing the negative effects of the electron return effect (ERE) on the exit skin dose. Methods: High resolution GEANT4 Monte Carlo simulations of a water phantom exposed to a 6 MV x-ray beam (Varian 2100C) have been performed. Transverse magnetic fields of strengths between 0 and 3 T have been applied to a 30×30×20 cm3 phantom. This phantom is also altered to have variable entry and exit surfaces with respect to the beam central axis and they range from −75° to +75°. The exit bolus simulated is a 1 cm thick (water equivalent) slab located on the beam exit side. Results: On the entry side, significant skin doses at the beam central axis are reported for large positive surface angles and strong magnetic fields. However, over the entry surface angle range of −30° to −60°, the entry skin dose is comparable to or less than the zero magnetic field skin dose, regardless of magnetic field strength and field size. On the exit side, moderate to high central axis skin dose increases are expected except at large positive surface angles. For exit bolus of 1 cm thickness, the central axis exit skin dose becomes an almost consistent value regardless of magnetic field strength or exit surface angle. This is due to the almost complete absorption of the ERE electrons by the bolus. Conclusions: There is an ideal entry angle range of −30° to −60° where entry skin dose is comparable to or less than the zero magnetic field skin dose. Other than this, the entry skin dose increases are significant, especially at higher magnetic fields. On the exit side there is mostly moderate to high skin dose increases for 0.2–3 T with the only exception being large positive angles. Exit bolus of 1 cm thickness will have a significant impact on lowering such exit skin dose increases that occur as a result of the ERE