HVL evaluation of orthovoltage X-ray machine using EGSnrc code of simulation

Making use of the orthovoltage machines in Radiotherapy, is one of the routine methods for the treatment of the superficial lesions. In this study, an important determinant of X-ray quality, the HVL [Half Value Layer], has been evaluated. The HVLs of a orthovoltage X-ray machine in 120 and 180 kVp are measured, using an empirical method, in which the HVLs are derived from the absorption curves. The measured HVLs are compared with calculated [Monte Carlo simulation] HVLs. Using the BEAMDP code of simulation, the output spectra are obtained and employed for the measurement of the HVLs. Comparing the calculated and measured HVL values, the results show that the highest and lowest differences between the two are 4.96% and 2.27%, respectively, which are, in fairly good agreement with those obtained in the former studies. This study shows that the EGSnrc simulation code is capable of being used for the extraction of the quality indices for the superficial X-ray radiotherapy machines. It seems that, the mentioned code, with the mentioned experimental method, can be employed as a routine clinical test tool for every superficial radiotherapy department

Verifying the accuracy of dose distribution in gamma Knife unit in presence of inhomogeneities using PAGAT polymer gel dosimeter and MC simulation

Polymer gel dosimetry is still the only dosimetry method for direct measuring of threedimensional dose distributions. MRI Polymer gel dosimeters are tissue equivalent and can act as a phantom material. In this study the obtained isodose maps with PAGAT polymer gel dosimeter were compared to those calculated with EGSnrs for singleshot irradiations of 8 and 18 mm collimators of Gamma Knife [GK] unit in homogeneous and inhomogeneous phantoms. A custom-built, 16 cm diameter spherical Plexiglas head phantom was. Inside the phantom, there was one cubic cutout for insertion of gel phantoms, and another cutout for inserting the inhomogeneities. The phantoms were scanned with a Siemens clinical 1.5 T MRI scanner. The multiple spin-echo sequence with 32 echoes was used for the MRI scans. The results of measurement and simulation in homogeneous and inhomogeneous phantoms showed that the presence of inhomogeneities in head phantom could cause spatial uncertainty higher than +/- 2 mm and dose uncertainty higher than 7%. the presence of inhomogeneities could cause dose differences which were not in accordance with accuracy in treatment with GK radiosurgery. Moreover, the findings of Monte Carlo calculation revealed that the applied simulation code [EGSnrc] was a proper tool for evaluation of 3D dose distribution in GK unit