Determination of field output correction factors in elongated small field photon beams

Abstract

IAEA along with AAPM have published a code of practice TRS 483 for small field dosimetry. The field correction output factors of various detectors were introduced to correct detector response changes in relative dosimetry. However, the publication of TRS 483 has focused on symmetry field sizes, the correction factors for elongated small fields are not included in the protocol. The overall objective of this work is to determine the field output correction factor in elongated small field. The correction factors of IBA CC01, Sun nuclear EDGE, and Exradin W2 PSD for 6 MV photon beam were determined using Monte Carlo (MC) simulation. The ratio of absorbed dose of water and of detectors was calculated using egs_chamber code, both at a depth of 10 cm with 90 cm SSD. The elongated field sizes were 3x2, 2x3, 1.5x4, 4x1.5, 1x6, 6x1, 0.6x10, and 10x0.6 cm2. The phase-space files from a Varian TrueBeamTM were used as a particle source. The calculated correction factors from this study were verified against the experiment. The field output factors (FOF) were measured using the same detectors, field sizes, and linear accelerator machine that were simulated with MC method. The comparison between FOF corrected with our correction factor and from TRS 483 was analyzed using percent standard deviation (%SD). For 3x2, 2x3, 1.5x4, and 4x1.5 cm2, the maximum %SD of uncorrected FOF was 1.1%. The FOF corrected with our correction factors and TRS 483 give the comparable %SD of 0.6%. For extremely elongated field sizes (1x6, 6x1, 0.6x10, and 10x0.6 cm2), the maximum %SD was 7.3% for uncorrected FOF. When our correction factors were applied, this value dropped to 3.8%. In contrast, the corrected FOF with TRS 483 gives the %SD of 6.9%. For IBA CC01 ionization chamber the correction factors were up to 7% for the smallest side of the fields, the most important contribution to the correction factor is volume averaging effect. The correction factors for Edge detector were of the order of -3% for the smallest side of the fields. The W2 PSD the correction factors are within 1% even for extremely elongated fields due to water equivalence characteristics. TRS 483 recommended to determine the for selecting the correction factor when the field is not too elongated with the ratio of Y/X FWHM in the range between 0.7-1.4. Our results show that the range may be extended up to 0.3-2.5 due to the %SD was less than 1% when the FOF was corrected with either our correction factor or TRS 483 for 1.5x4 and 4x1.5 cm2. However, when the field size becomes extremely elongated, the definition of equivalent square small field size ( ) is no longer hold. Then, the correction factor should be determined follow TRS 483.