ABSTRACT
Two protocols of AAPM TG-51 and IAEA TRS-398 were compared followed by a measurement and Monte Carlo simulation of beam quality correction factor, KQ, AAPM TG-51 and IAEA TRS-398 protocols were compared for the absorbed dose to water for DW, and KQ parameters. Materials and Dose measurements by either protocols were performed with cylindrical and plane parallel chambers for 6 and 18 MV photons, and 6, 9, 12, 15, 18 MeV electron clinical beams were traced to the calibration factor of Iranian secondary standard dosimetry laboratory. MCNP-4C simulation of depth doses, beam profiles and KQ factors were validated typically for 18 MV and 12 MeV beams by experimental measurements. The differences between simulation and measurements were 0.07% for beam profile, -2.60% and 1.19% for 12 MeV build up and linear portion of the depth dose curve, respectively. The figures of merit for 18 MV were about -4.17%, - 1.62% and 0.38%. The differences of KQ's between simulation and measurement of 12 MeV, and 18 MV beams for TG-51 were -0.194% and 0.169%, and for TRS-398, they were about -0.465% and 0.097%, respectively. These differences between the two dosimetry protocols [IAEA TRS-398 and AAPM TG-51] from the point of absolute dosimetry were not significant at least when they were used under the same calibration procedure. The good agreement between Monte Carlo and measurement may also be even more important regarding the contribution into the development of radiotherapy treatment planning system, based on Monte Carlo procedures
ABSTRACT
The electron contamination may reduce or even diminish the skin sparing property of the megavoltage beam. The detailed characteristics of contaminant electrons are presented for different field sizes and cases. The Monte Carlo code, MCNPX, has been used to simulate 18 MV photon beam from a Varian Linac-2300 accelerator. All dose measurements were carried out using a PTW-MP2 scanner with an ionization chamber [0.6 CC] at the water phantom. The maximum electron contaminant dose at the surface ranged from 6.1% for 5 x 5 cm[2]to 38.8% for 40 x 40 cm[2] and at the depth of maximum dose was 0.9% up to 5.77% for the 5 x 5 cm[2] to the 40 x 40 cm[2] field sizes, respectively. The additional contaminant electron dose at the surface for the field with tray increased 2.3% for 10 x 10 cm[2], 7.3% for 20 x 20 cm[2], and 21.4% for 40 x 40 cm[2] field size comparing to the standard field without any accessories. This increase for field with tray and shaping block was 5.3% and 13.3% for 10 x 10 and 20 x 20 cm[2], respectively, while, the electron contamination decreased for the fields with wedge, i.e. 2.2% for the 10 x 10 cm[2] field. The results have provided more comprehensive knowledge of the high-energy clinical beams and may be useful to develop the accurate treatment planning systems capable of taking the electron contamination in to account