Dosimetry in an IMRT phantom designed for a remote monitoring program

An accurate delivery of prescribed dose is essential to ensure the most successful outcome from advanced radiation treatments such as intensity modulated radiation therapy (IMRT). An anthropomorphic phantom was designed and constructed to conduct a remote-audit program for IMRT treatments. The accuracy of the dosimetry in the phantom was assessed by comparing the results obtained from different detectors with those from Monte Carlo calculations. The developed phantom has a shape of a cylinder with one target and three organs at risk (OARs) inside the unit. The target and OARs were shaped similar to those of nasopharyngeal cancer patients, and manufactured for their identification during computed tomography imaging. The phantom was designed with thermoluminescent dosimeter (TLD) holders that were inserted inside the target and the OARs for the measurements of absolute dose. In addition, the phantom allowed measurements with ionization chambers placed at the TLD locations. As a result, an inter-comparison between the two types of dosimeters was possible. For the measurement of the relative dose distribution across the target and OARs, two film slots were orthogonally placed near the center of the phantom, which also enabled the insertion of inhomogeneities near the target. Measurements with TLDs, provided by Korea Food and Drug Administration and Radiological Physics Center, and measurements with an ionization chamber (IC) were performed in four cases. The first case was one anterior field of 6 MV x rays delivered to the phantom; the second case used the same anterior field, but with inhomogeneities inserted into the phantom. The third case was three fields of 6 MV beams at an equi-gantry angle for the homogeneous phantom, and the fourth case was IMRT delivery to the phantom without inhomogeneities. For each case, measurements with both TLDs and IC were performed. For cases 1-3, theoretical predictions were obtained by using the Monte Carlo (MC) codes (BEAMnrc and DOSXYZnrc6.0). The TLD measurements were larger than the IC readings by 2.2% (1.3-2.5%), 2.2% (1.2-2.9%), and 2.1% (0-3.3%) on average for case 1, case 2 and case 3, respectively. The average deviation between TLDs and MC results was 0.97% (-0.13-2.07%) for the first case, 1.27% (0.34-2.18%) for the second case, and 1.13% (0.31-1.94%) for the third case. The IC reading was less than the MC results; the average deviations were -1.2% (-2.44 - 0.43%), -0.96% (-1.74 - 0.54%) and -0.94% (-1.53-0.27%) for the first, second, and third cases, respectively. For the IMRT case, the average deviation between IC readings and TLD measurements was 0.5% (-7.0-3.9%). In conclusion, the TLD measurements in the developed phantom agreed with IC and MC results with less than 3% of an average difference. The developed phantom with TLD dosimeters should be useful for remote monitoring of IMRT.