Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Correction factors for reference dosimetry in magnetic resonance (MR) imaging-guided radiation therapy ([Formula: see text]) are often determined in setups that combine a conventional 6 MV linac with an electromagnet. This study investigated whether results based on these measurements were applicable for a 7 MV MR-linac using Monte Carlo simulations. For a Farmer-type ionization chamber, [Formula: see text] was assessed for different tissue-phantom ratios ([Formula: see text]). [Formula: see text] differed by [Formula: see text] between [Formula: see text] (6 MV linac) and [Formula: see text] (7 MV MR-linac) at [Formula: see text]. The agreement was best in an orientation in which the secondary electrons were deflected to the stem of the ionization chamber.