Calculation of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) for several small detectors and for two linear accelerators using Monte Carlo simulations.

PURPOSE

The scope of this study was to determine a complete set of correction factors for several detectors in static small photon fields for two linear accelerators (linacs) and for several detectors.

METHODS

Measurements for Monte Carlo (MC) commissioning were performed for two linacs, Siemens Primus and Elekta Synergy. After having determined the source parameters that best fit the measurements of field specific output factors, profiles, and tissue-phantom ratio, the generalized version of the classical beam quality correction factor for static small fields, k(Q(clin),Q(msr) ) (f(clin),f(msr) ), were determined for several types of detectors by using the egs_chamber Monte Carlo user code which can accurately reproduce the geometry and the material composition of the detector. The influence of many parameters (energy and radial FWHM of the electron beam source, field dimensions, type of accelerator) on the value of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) was evaluated. Moreover, a MC analysis of the parameters that influence the change of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) as a function of field dimension was performed. A detailed analysis of uncertainties related to the measurements of the field specific output factor and to the Monte Carlo calculation of k(Q(clin),Q(msr) ) (f(clin),f(msr) ) was done.

RESULTS

The simulations demonstrated that the correction factor k(Q(clin),Q(msr) ) (f(clin),f(msr) ) can be considered independent from the quality beam factor Q in the range 0.68  ±  0.01 for all the detectors analyzed. The k(Q(clin),Q(msr) ) (f(clin),f(msr) ) of PTW 60012 and EDGE diodes can be assumed dependent only on the field size, for fields down to 0.5 × 0.5 cm². The microLion, and the microchambers, instead, must be used with some caution because they exhibit a slight dependence on the radial FWHM of the electron source, and therefore, a correction factor only dependent on field size can be used for fields ≥ 0.75 × 0.75 and ≥ 1.0 × 1.0 cm², respectively. The analysis of uncertainties gave an estimate of uncertainty for the 0.5 × 0.5 cm² field of about 0.7% (1σ) for k(Q(clin),Q(msr) ) (f(clin),f(msr) ) factor and of about 1.0% (1σ) for the field output factor, Ω(Q(clin),Q(msr) ) (f(clin),f(msr) ), of diodes, microchambers, and microLion.

CONCLUSIONS

Stereotactic diodes with the appropriate k(Q(clin),Q(msr) ) (f(clin),f(msr) ) are recommended for determining Ω(Q(clin),Q(msr) ) (f(clin),f(msr) ) of small photon beams.