6-MV small field output factors: intra-/intermachine comparison and implementation of TRS-483 using various detectors and several linear accelerators.

Abstract

To investigate the applicability of output correction factors reported in TRS-483 on 6-MV small-field detector-reading ratios using four solid-state detectors. Also, to investigate variations in 6-MV small-field output factors (OF) among nominally matched linear accelerators (linacs). The TRS-483 Code of Practice (CoP) introduced and provided output correction factors to be applied to measured detector-reading ratios to obtain OFs for several small-field detectors. Detector readings for 0.5cm×0.5cm to 8cm×8cm fields were measured and normalized to that of 10cm×10cm field giving the detector-reading ratios. Three silicon diodes, IBA PFD, IBA EFD (IBA, Schwarzenbruck, Germany), PTW T60017, and one microdiamond, PTW T60019 (PTW, Freiburg, Germany), were used. Output correction factors from the CoP were applied to measured detector-reading ratios. Measurements were performed on six Clinac and six TrueBeam linacs (Varian Medical Systems, Palo Alto, USA). An investigation of the relationship between the size of small fields and corresponding detector-reading ratio among the linacs was performed by measuring lateral dose profiles for 0.5cm×0.5cm fields to determine the full width half maximum (FWHM). The relationship between the linacs' focal spot size and the small-field detector-reading ratio was investigated by measuring 10cm×10cm lateral dose profiles and determining the penumbra width reflecting the focal spot size. Measurement geometry was as follows: gantry angle=0°, collimator angle=0°, source-to surface distance (SSD)=90cm, and depth in water=10cm. For a given linac and 0.5cm×0.5cm field, the deviations in detector-reading ratios among the detectors were 9%-15% for the Clinacs and 4%-5% for the TrueBeams. Use of output correction factors reduced these deviations to 6%-12% and 3%-4%, respectively. For field sizes equal to or larger than 0.8cm×0.8cm, the deviations were corrected to 1% using output correction factors for both Clinacs and TrueBeams. For a given detector and 0.5cm×0.5cm field, the deviations in detector-reading ratios among the linacs were 11%-17% for the Clinacs and 5-6% for the TrueBeams. For 1cm×1cm the deviations were 1%-2% for Clinacs and 1% for TrueBeams. For field sizes larger than 1cm×1cm the deviations were within 1% for both Clinacs and TrueBeams. No relationship between FWHMs and detector-reading ratios for 0.5cm×0.5cm was observed. For Clinacs, larger 10cm×10cm penumbra width yielded lower 0.5cm×0.5cm detector-reading ratio indicating an effect of the focal spot size. For TrueBeams, the spread of penumbra widths was lower compared to Clinacs and no similar relationship was observed. Output correction factors from the TRS-483 CoP are not sufficient for accurate determination of OF for 0.5cm×0.5cm fields but are applicable for 0.8cm×0.8cm to 8cm×8cm fields. Nominally matched Clinacs and TrueBeams show large differences in detector-reading ratios for fields smaller than 1cm×1cm.