Multicentric characterisation of lateral beam profiles generated by 6FFF beam of three 0.35 T MR-linac systems

Background: The physical characterisation of FFF-beam profiles in the presence of a magnetic field requires a new standardization procedure and formulation. Purpose: The aim of this multicentric experience is to propose new normalisation factors to allow for the calculation of standard parameters typical of flattened beams, such as dosimetric field size and penumbra, for a 6 MV FFF beam from an MR-linac. Methods: The measurements were carried out on three ViewRay-MRIdian systems. An equal set of measurements was acquired using the same equipment. Transverse beam profiles were acquired at seven different depths and for six beam dimensions. The inflection point was estimated as the position of the maximum of a Gaussian fit obtained from the first derivative of the profiles. The position of the minimum and maximum points of the second derivative of the above Gaussian described the fall-off region, and the external peaks of the third derivative were considered as the in-field and out-field points. The profile normalisation was determined by imposing a 55% dose level at the inflection point and the renormalisation factors were calculated. Results: The position of the inflection point, and the second and third derivatives peaks were analysed, and the renormalisation factors as a function of field size and depth were determined. The values of the unflatness and the slope have been calculated for different depths and field sizes. Conclusion: This study represents the first multi-centric evaluation of the profiles on different low-field MR-Linac systems and the set of renormalisation parameters to analyse the FFF-beam on that system was effectively proposed.