Small field dosimetric characterization of a new 160-leaf MLC

The goal of this work was to perform a 6 MV small field characterization of the new Agility 160-leaf multi-leaf collimator (MLC) from Elekta. This included profile measurement analysis and central axis relative output measurements using various diode detectors and an air-core fiber optic scintillation dosimeter (FOD). Data was acquired at a depth of 10.0 cm for field sizes of 1.0, 0.9, 0.8, 0.7, 0.6 and 0.5 cm. Three experimental data sets, comprised of five readings, were made for both the relative output and profile measurements. Average detector-specific output ratios (OR[overline](f(clin))(det))) were calculated with respect to a field size of 3.0 cm and small field replacement correction factors (k(f(clin),f(msr))(Q(clin),Q(msr))) derived for the diodes using the scintillation dosimeter readings as the baseline. The standard experimental uncertainty on OR[overline](f(clin))(det)) was calculated at a 90% confidence interval and the coefficient of variation (CV) used to characterize the detector-specific measurement precision. The positional accuracy of the collimation system was also investigated by analyzing the repeated profile measurements and field width constancy investigated as a function of collimator rotation. For comparison the output and profile measurements were repeated using the Elekta 80-leaf MLCi2 on a beam matched linac at 6 MV. The measured OR[overline](f(clin))(det)) varied as a function of detector and MLC design. At the smallest field size the standard experimental uncertainty on OR[overline](f(clin))(det)) was consistent across all detectors at approximately 0.5% and 1.0% for Agility and MLCi2 collimators respectively. The CV associated with the FOD measurements were greater than that of the diodes but did not translate into increased measurement uncertainty. At the smallest field size, the diode detector correction factors were approximately 2% greater for MLCi2 than that required for the Agility. Profile data revealed the Agility MLC to have a greater positional reproducibility than both the MLCi2 and the linac diaphragms (jaws), as also reflected in the experimental uncertainties on OR[overline](f(clin))(det)). The relative output, profile widths and associated uncertainties were all found to differ between the two MLC systems investigated, as were the field size specific diode detector replacement correction factors. The data also clearly showed that the Agility 160-leaf MLC performs to a tighter positional tolerance than the MLCi2.