In order to correct for tissue heterogeneities on a voxel-by-voxel basis during CT-based treatment planning, the relationship between the correction factor (CF) and the CT number in Hounsfield units (HU) for the scanner in use must be established. Since the relationship between CF and electron density (rho e) of various materials is well documented, the rho e vs HU is required for direct computation of the correction factors by treatment planning computers. A CT phantom with 18 different tissue substitutes has been used to establish the rho e vs HU relationship. A description of the phantom and its contents is given and the calibration of the CT function of the planning computer is discussed.
Electron beam dosimetry at low monitor unit (MU) settings is important for dosimetric applications. Dose linearity, beam flatness, and beam energies were studied at low MU settings with various dose rates for different types of linear accelerators. It is observed that for the scattering foil units, the dose/MU is a smooth function of MU for all beam energies. Discrepancies in dose/MU are highest at the lowest MU. Significant variation (5%-245%) in dose linearity is observed among various linear accelerators at low MU settings. Dose rate has no effect on the dose linearity for all energies for the scattering foil units tested. On the contrary, for the scanned beam, there is no predictable pattern as dose/MU is random in nature and varies with time and beam energy. The maximum dosimetric error is observed for the highest energy beam where the beam width is most narrow. Using film, the beam uniformity was noticed to be very poor at low MU and high energy for scanned beams. The beam uniformity and dose linearity are random at low MU due to the random nature of the scan cycle. Under the adverse conditions, the deviation in dosimetric parameters was observed up to 200 MU.
Purpose: There has been great interest recently in replacing conventional film with computed radiography (CR) systems. However, the study of its dosimetric applications has been limited. This work presents a detailed evaluation of its performance in quality assurance (QA) for intensity modulated radiation therapy (IMRT). Method and Materials: A CR system comprised of a Kodak ACR‐2000i laser scanner, an Agfa CR plate, and a PC was used for IMRT verification. Both the scanner and the CR plate were calibrated by following the Kodak‐provided instructions. A dose calibration curve for the CR plate was generated in the dose range of 0.5cGy to 400cGy. In the IMRT verification experiment, the CR plate with 6cm solid water build‐up was irradiated to a clinical 6‐field IMRT treatment plan (Varian Eclipse) of 6MV photons. Gantry angles were set to 0° for the dose delivery. The CR plate was scanned right after irradiation. An independent EDR2 film measurement of the delivered dose was also conducted. A 3‐way comparison of dose information between the Eclipse calculations, film measurements, and CR measurements was performed. Dose maps, profiles, isodose lines, and Gamma maps were utilized for analysis. Results: The CR results show excellent agreement with both Eclipse calculations and film measurements within the 3% dose difference and 3mm distance to agreement criteria except in penumbra regions or just outside field edges. CR and film measurements agree with each other better than with Eclipse calculations in the regions outside field edges, indicating possible limitations of calculation algorithms in those regions. Conclusion: The CR system can provide comparable accuracy in relative dose measurements to conventional film. The CR system, having the advantages of digital nature, environmental cleanliness, reusability, and high convenience, is a feasible and reliable tool for routine IMRT QA.
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