Small-field output factor dependence on the field size definition in MR-Linac

Radiation beam characteristics are primarily evaluated based on field size. However, in small fields, especially with magnetic fields used in new technology (MR-Linac), the field size definition is altered. Typically, field size is defined by two methods: geometric and dosimetric, which are evaluated in this study.

Small field size definitions are distorted due to lateral electron disequilibrium and the presence of magnetic fields. MR-Linac systems, which combine an MR imaging system and a linear accelerator on a single gantry, require precise evaluations of field size definitions and beam parameters, particularly for small fields. which is investigated in this study.

A 0.35 T MRIdian Viewray system was evaluated using beam profiles and field output factors (FOF) with various MR-compatible microdetectors, such as ion chamber, microDiamond, microSilicon, and plastic scintillators. Validity of geometric field size (S) and dosimetric field size (S_clin) is investigated with measurements performed with MR compatible scanning water phantom at 85 cm source-to-surface distance (SSD) at a depth of 5 cm. Measured FOF data was compared with treatment planning systems (TPS) and independent Monte Carlo simulations.

The measured S_clin data is detector and machine-dependent, while S is machine-dependent only. The FOF was found to be a smooth function of S within experimental uncertainties, showing higher reproducibility compared to S_clin which exhibited erratic behavior.

It is concluded that geometric field size (S) provides accurate beam characterization data, whereas S_clin may not be a reliable parameter in MR-Linac systems.