Designing and Evaluating a Simple Small Phantom for Dosimetry Intercomparison of Iinacs Photon Beams

This paper evaluate the functionality of a designed small phantom for determining the accuracy and performance of megavoltage radiotherapy machines through an external quality audit. The small phantom was designed to check the photon beams at the reference and some non-reference conditions on a linac at two radiotherapy centers in Tehran. LiF TLDs were used for experimental dosimetry. A standard phantom with an ionization chamber and the Monte Carlo code were used to calculate the conversion factor of the doses in the small phantom to the conventional condition. The result of this study showed that the dose conversion factors derived from the measurements made in the designed and standard phantoms are in good agreement with those calculated from the MCNPX simulations. At the reference condition¡ the amount of difference between the simulation and measurement were 0.5 at 6 and 18 MV. The differences for the 7cm×7cm field were 0.2% and 0.3% respectively¡ while for the 20cm×20cm field were 0.1% and 0.6%¡ respectively for the above energy. Using the designed phantom¡ two similar Varian linacs were checked at the reference conditions and amounts of their deviation between the planned and measured doses were estimated. The amount of variations of the phantom dose conversion factor with the field size indicates that the small designed phantom is not only appropriate for the small field sizes but also for the large ones. The results confirmed the functionality of the designed simple phantom with TLDs as an appropriate mailing system to carry out dosimetric intercomparison among different radiotherapy centers.