An investigation of the depth dose in the build-up region, and surface dose for a 6-MV therapeutic photon beam: Monte Carlo simulation and measurements

摘要

The MC simulations were based on the EGSnrc code system, developed by the National Research Council of Canada (NRC) [25, 26]. The 6-MV photon beam generated from a Varian Clinac 23EX medical linear accelerator was simulated using the BEAMnrc user code. The dose distribution in a phantom was obtained by the use of the DOSXYZnrc user code. The Varian Clinac 23EX linear accelerator, equipped with a Millennium 120-leaf MLC and on-board imaging system, located at the Department of Radiation Oncology, Siriraj Hospital, Thailand, was used in this study. All measurements were performed on the 6-MV photon beam along the central axis with square open field sizes of 5 × 5, 10 × 10, 15 × 15 and 20 × 20 cm2at a constant source-to-surface distance of 100 cm. The complete percentage depth doses were measured in a Blue water phantom (Wellhofer Scanditronix, Germany) at a depth ranging from 0 to 30 cm with a scanning resolution of 2 mm. The detectors used with this water phantom were a compact cylindrical ionization chamber of type CC13 (Wellhofer Scanditronix, Germany) and a silicon p-type photon semiconductor dosimeter of type PFD (Wellhofer Scanditronix, Germany). The CC13 dosimeter has an active diameter of 6 mm, a cavity volume of 0.13 cm3, and a wall thickness of 0.07 g/cm2. The PFD dosimeter has an active diameter of 2 mm and an effective thickness of 0.06 mm from the detector's front surface. These two dosimeters are routinely used to acquire the common beam data, such as the percentage depth dose, the beam profiles and the output factor. The depth doses along the central axis of the beam were obtained for different square field sizes of our 6-MV photon beam simulation using the DOSXYZnrc code. They were normalized as a percentage of the maximum dose. Here, the percentage dose near or at the phantom surface was estimated by the third-order polynomial extrapolation of the simulated data. The observed percentage doses at the surface and at a depth of 0.0007 and 0.005 mm were compared with the published results of Devic et al. [13] and Parsai et al. [19], since they had performed reliable measurements of the surface dose and the dose in the build-up region at the same beam energy from similar medical linear accerelators (the Varian Clinac 1800 and the Varian 2300 (C/D), respectively). Accordingly, a substantial discrepancy in the dose in the build-up region from our MC simulation-based calculations and their previously reported empirical measurements was not anticipated. Table 1summarizes the dose comparison using the MC simulation results obtained here and the previously measured values [13, 19] for the square fields with lengths of 5, 10 and 15 cm. Consistent results were generally observed in which all of the differences were less than 2%. Therefore, we conclude that our calculated surface doses using the MC simulation were justified. The percentage depth doses along the beam central axis for our 6-MV photon beams were acquired experimentally with four detectors for the four square field sizes of 5 × 5, 10 × 10, 15 × 15 and 20 × 20 cm2. The readings from each detector were assigned to the effective point of measurement for each individual detector. For the CC13 chamber, the effective point was determined automatically by the computerized scanning system. The effective point of measurement for the PFD dosimeter and Markus chamber was assumed to be at the front surface and at the bottom of the entrance window electrode, respectively. For the TLD chip, the effective point was assigned to the middle of its thickness and scaled by its density of 0.0496 g/cm2. From the extrapolation of the measured dose in the build-up region, the percentage doses at zero depth (surface doses) for the 6-MV photon beam with different field sizes are shown in Fig. 3. The measured surface dose clearly increases with increasing field size, regardless of the detector used in the measurement, and this is also observed with the MC simulation. This is mainly due to the increasing number of scattered electrons in the air and collimator. The measurements obtained from the TLD chip and Markus chamber gave surface dose values close to that of the MC simulation, while a very large discrepancy was found when using the PFD dosimeter and, especially, the CC13 chamber. Before scaling of the effective depth of measurement, the over-response of the TLD chip and Markus chamber for the surface dose were about 10%, which is consistent with the reports of Devic et al. [13] and Gerbi and Khan [16]. After taking into account the effective point of measurement, the over-response from the TLD chip was reduced to approximately 4%, while that for the Markus chamber remained almost unchanged due to its larger effective volume.