Purpose: The definition of radiation field size has been traditionally interpreted using machine digital readout and in elongated field by equivalent square. For small fields a new definition, Sclin, is proposed that is related to the square root of the full width at half maximum (FWHM) in X and Y axes. This concept has been questioned and is even more inquired in the magnetic field used in MR-linac which is investigated in this study.
Methods: The validity of Sclin is investigated on a 0.35T MR-linac system. The field size is defined by two sets of double-focused and double-stacked multileaf collimator (MLC) with the top and bottom stacks offset by half a leaf width. MLC produces field sizes from 3.14 to 24.1 cm2 with a precision of 0.1 mm geometrically and linked to digital readout. Using multiple MR safe detectors (ionization chamber, microSilicon, microdiamond, Edge, Plastic scintillator, EBT film) the field size concept is investigated using a scanning MR-safe water tank at 10 cm depth keeping machine SAD (90 cm). Additionally, field output factors (FOF) were measured.
Results: The measured profiles in X and Y directions were analyzed to provide Sclin which is plotted as machine geometrical field size for square fields. The data were linear with regression (r2) of 0.999 for all detectors. However, as expected, FWHM in X and Y are different that provides different equivalent square field which depend on the selection of the detector. This ambiguity is exasperated in the FOF thus giving different values for a field size. For small fields this relates to a difference of 11.5% in FOF
Conclusion: It is concluded that Sclin is not a satisfactory parameter to measure the field size in MR-linac. Traditional field definition that is provided by the machine should be used to reduce FOF values within ±2%