Purpose: The radiation field size is traditionally defined by the geometric field opening of the beam defining system (MLC/Jaw) which is normally consistent with measured dosimetric field size (50% isodose, full-width half maximum, FWHM). For very small fields (< 1 cm2) due to source occlusion, the measured dosimetric field size has been shown to be larger than geometric field size. The field output factors (FOF) become very sensitive to definition of the field size. The dosimetry protocol TRS-483 recommends the use of the field size defined by measured FWHM for the small fields. Using Monte Carlo simulations this study calculated both the output factors as a function of the field sizes defined by the geometric field opening as well as calculated FWHM.
Methods: The EGSnrc system is used to simulate 6 MV beam for Varian TrueBeam to generate square and rectangular fields from 5-50 mm with every possible permutation (keeping one jaw fixed and varying other jaw from 5 mm to 50 mm). The calculated output factors and FWHM are compared with the measured data with a microSilicon detector in water
Results: The measured and calculated outputs agree within 1-2% for square or rectangular fields with minimum jaw settings of 8 mm. At extremely small x or y-jaw openings of 5 mm measured output factors are underestimated by 7%. The calculated field sizes defined by FWHM are consistent with geometric field opening. Large uncertainties occur in measuring FWHM with a detector for extreme small fields in the presence of a sharper dose fall-off of >20%/mm.
Conclusion: The traditional definition of field size, i.e., the field size defined by geometric opening of the beam defining system, is still valid for small field. It is feasible to accurately tabulate the output factors as a function of geometric field opening.
Monte Carlo, Dosimetry Protocols, Radiosurgery
TH- External Beam- Photons: Small field computational dosimetry