Dosimetric feasibility of the hybrid Magnetic Resonance Imaging (MRI)-linac System (MRL) for brain metastases: The impact of the magnetic field.

Abstract

We aimed to investigate the suitability of treating patients with single brain metastases using stereotactic radiosurgery (SRS) with the MRL and to characterize the dosimetric impact at tissue-air interfaces resulting primarily from the electron return effect (ERE). 24 patients treated for intact single brain metastases were analyzed. Three radiotherapy plans with the same prescribed dose were generated for each case: (1) noncoplanar volumetric modulated arc therapy (VMAT), (2) coplanar step-and-shoot intensity modulated radiotherapy (IMRT) on the MRL in the absence (MRLB=0), and (3) in the presence of the transverse magnetic field (MRLB=1.5). The plans were evaluated using cumulative dose-volume histograms and by calculation of Paddick conformity index (CI), V100%, V12Gy minus gross tumor volume (V12Gy - GTV), and V2Gy. At tissue-air boundaries, the dosimetric impact of the magnetic field was quantified using a 5 mm rim of tissue. All plans met the target coverage and organs-at-risk planning objectives. Differences between all investigated dosimetric parameters significantly favored the VMAT plans as compared to the MRLB=0 and MRLB=1.5 plans, except for V2Gy. The mean V12Gy - GTV and V2Gy marginally favored the MRLB=0 plans compared to the MRLB=1.5 plans (mean difference: 0.45 cm³, p = 0.0019 and 16.46 cm³, p < 0.0001, respectively). The presence of the magnetic field resulted in a statistically significant but small increase in mean dose and D_2cc in the skin (0.08 Gy, p < 0.0001 and 0.6 Gy, p < 0.0001, respectively) and around air cavities (0.07 Gy, p = 0.0092 and 0.3 Gy, p = 0.0004, respectively). It is feasible to generate stereotactic radiation plans that satisfy clinical requirements using the MRL in the setting of single brain metastases. The dosimetric impact of the magnetic field including the ERE at tissue-air boundaries is minor and does not negatively impact target conformity or dose gradient.