ABSTRACT
Physical wedges are still widely used as beam modifiers in external beam radiotherapy. However the presence of them in the beam trace may cause beam hardening which may not be considered in many treatment planning systems. The aim of this study is to investigate the beam hardening effect generated by physical wedges via different beam quality indexes as photon spectrum, half value layer, mean energy and tissue-phantom ratio. The effect of physical wedges on the photon beam quality of a 6-18MV Varian 2100C/D accelerator was studied with the BEAMnrc Monte Carlo code. Good agreements were obtained between measured and calculated depth doses and beam profiles for open and wedged photon beams at both energies. It was noticed that for 6 MV photon beams, physical wedges have more significant effects on beam quality than for 18 MV. Also it was obtained that at 18 MV photon beam as the wedge angle increased, the effect of wedge on beam quality becomes reversed and beam softening occurred. According to these results, it is recommended that beam hardening and softening of physical wedges should be considered in treatment planning systems in order to increase the accuracy in dose delivery
Subject(s)
Monte Carlo Method , PhotonsABSTRACT
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
In treating patients with radiation, the degree of accuracy for the delivery of tumor dose is recommended to be within +/- 5% by ICRU in report 24. The experimental studies have shown that the presence of low-density inhomogeneity in areas such as the lung can lead to a greater than 30% change in the water dose data. Therefore, inhomogeneity corrections should be used in treatment planning especially for lung cancer. The usual methods for inhomogeneity correction are the Tissue-Air Ratio [TAR] method, the power low tissue-air ratio [Batho] method, and the Equivalent Tissue-Air Ratio [ETAR] method. But they are not able to calculate the dose with required accuracy in all cases. New and more accurate methods are based on Monte Carlo methods. They are able to account for all aspects of photon and electron transport within a heterogeneous medium. The focus of this paper is the application of MCNP [Monte Carlo N-Particle] code in radiotherapy treatment planning. Materials and methods: Some special test phantoms were made of cork and Perspex instead of lung and normal tissue respectively [with electron densities relative to water equal to 0.2 and 1.137 respectively]. Measurements were obtained using cobalt-60 radiation for four different fields. Then the results of RTAR, Batho and MCNP methods were compared to the measurements. RTAR method has an error equal to 10% approximately. Also Batho method has an error especially in the low-density material. At least, MCNP method calculates correction factors very accurately. Its average error is less than 1% but it takes a long time to calculate the dose. Monte Carlo method is more accurate than other methods and it is currently used in the process of being implemented by various treatment planning vendors and will be available for clinical use in very near future