Abstract
High‐precision radiotherapy planning and quality assurance require accurate dosimetric and geometric phantom measurements. Phantom design requires materials with mechanical strength and resilience, and dosimetric properties close to those of water over diagnostic and therapeutic ranges. Plastic Water Diagnostic Therapy (PWDT: CIRS, Norfolk, VA) is a phantom material designed for water equivalence in photon beams from 0.04 MeV to 100 MeV; the material has also good mechanical properties. The present article reports the results of computed tomography (CT) imaging and dosimetric studies of PWDT to evaluate the suitability of the material in CT and therapy energy ranges.We characterized the water equivalence of PWDT in a series of experiments in which the basic dosimetric properties of the material were determined for photon energies of 80 kVp, 100 kVp, 250 kVp, 4 MV, 6 MV, 10 MV, and 18 MV. Measured properties included the buildup and percentage depth dose curves for several field sizes, and relative dose factors as a function of field size. In addition, the PWDT phantom underwent CT imaging at beam qualities ranging from 80 kVp to 140 kVp to determine the water equivalence of the phantom in the diagnostic energy range. The dosimetric quantities measured with PWDT agreed within 1.5% of those determined in water and Solid Water (Gammex rmi, Middleton, WI). Computed tomography imaging of the phantom was found to generate Hounsfield numbers within 0.8% of those generated using water. The results suggest that PWDT material is suitable both for regular radiotherapy quality assurance measurements and for intensity‐modulated radiation therapy (IMRT) verification work. Sample IMRT verification results are presented.PACS number: 87.53Dq
Highlights
The increasing complexity in radiotherapy treatment planning and delivery warrants increased quality assurance (QA) to ensure accurate dose delivery to the patients
Dosimetric protocols recommend the use of water as the phantom for dose measurements.[1,2,3] because of the tedious setup required for performing water phantom measurements, other Solid Water (Gammex rmi, Middleton, WI)–equivalent materials have been used in performing some of these measurements
The phantom materials should be carefully checked to ensure that their densities, slab thickness, and radiologic characteristics are consistent
Summary
The increasing complexity in radiotherapy treatment planning and delivery warrants increased quality assurance (QA) to ensure accurate dose delivery to the patients. The International Commission on Radiation Units Report 44 states that, for any solid phantom to be considered water-equivalent, it should not introduce more than 1% uncertainty to the absorbed dose. To perform true QA, it would be ideal to simulate the entire treatment process Simulation of this kind would mean scanning a phantom using CT and using the same phantom for radiotherapy measurements. In performing patient-specific intensity modulated radiation therapy (IMRT) verification measurements, the patient fluence is transferred to the Solid Water phantom, and a forward dose calculation is performed. The calculated dose is compared to the point dose and to the dose distribution measured in the phantom In this process, it is desirable that the phantom characteristics match attenuation and absorption properties in the diagnostic and therapeutic ranges alike
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