Toward Accurate Small Animal Dosimetry and Irradiator Quality Assurance.

摘要

Purpose: To demonstrate specific methods of small animal dosimetry and quality assurance through (1) machine-specific quality assurance and (2) target-specific quality assurance (QA) protocols for different types of biological irradiators: (a) a large-field orthovoltage irradiator, (b) a small-field orthovoltage irradiator, and (c) a 137Cs irradiator. Additionally, (3) a dosimetric characterization of a novel nano-scale phosphor detector for small animal dosimetry is performed.;Results: (1) Machine-specific QA: (a) Large-field irradiator: The output was shown to be linear. The kVp measurements were consistent for both data sets. The light and radiation field coincidence measurement yielded a shift in the left-right direction of 3 mm and the front-rear direction of 2 mm with respect to the radiation field. The in-air output measurements for the exposure settings of 320 kVp, 12.5 mA, and 165s for 4 filters were: 252.9 (no filter), 208.6 (F1), 76 (F2), and 176.3 (F4) cGy/min. (b) Small-field irradiator: A kVp check and HVL measurements were performed and dose or dose rate for the diagnostic protocols are as follows: 4.5 and 3.9 cGy/min AP and PA, respectively, for the 40 kVP protocol and 1.9 and 1.7 cGy/min AP and PA, respectively, for the 80 kVp protocol (fluoroscopy), 0.47 cGy (scout), and 8.6 +/- 0, 4.3 +/- 0.1, and 1.7 +/- 0.1 cGy/min for two 40 kVp protocols (first one has half the rotations per minute) and an 80 kVp protocol (CBCT). (2) Target-specific QA: (a) Large-field irradiator: The average DR for the head and body was calculated to be 228.6 +/- 3.1 cGy/min and 228.1 +/- 2.4 cGy/min, respectively, for a total average DR of 228.3 +/- 2.0 cGy/min. (b) Small-field irradiator: For a 10 mm, 15 mm, and 20 mm circular collimator, the dose measured by the phantom was 4.3%, 2.7%, and 6% lower than TG-61 based measurements, respectively. For a 10 x 10 mm, 20 x 20 mm, and 40 x 40 mm collimator, the dose difference was 4.7%, 7.7%, and 2.9%, respectively. (c) 137Cs irradiator: Lab 1: The average dose rates for the head DRhead 1-5 were between 138.7 +/- 10.5 cGy/min for level 1 to 167.8 +/- 10.5 cGy/min for level 5. The average dose rates for the body DRbody 1-5 was 156.4 +/- 7.4 cGy/min for level 1 to 179.5 +/- 4.6 cGy/min for level 5. Lab 2: The average dose rate for the head DRhead was 133.8 +/- 0.5 cGy/min and the average dose rate for the body DRbody was 140.4 +/- 3.8 cGy/min for an averaged DRavg of 137.1 +/- 1.9 cGy/min. (3) The nano-scale phosphor detector behaved strictly linear for a dose range of 2 - 350 cGy with a variation in sensitivity of about 0.3%. The limit of detection was observed to be about 0.44 cGy in air. The in-air angular response was shown to have a coefficient of variation of 4.3%, while the in-phantom measurement without the table had a coefficient of variation of only 1.2%.;Conclusion: (1) Machine-specific QA: (a) Large-field irradiator: Machine-specific quality assurance checks dosimetric and mechanical parameters of the irradiator. (b) Small-field irradiator: Baseline quality assurance data was accumulated for all diagnostic mode protocols. The BSF was determined for therapy mode and shown to agree with published data. (2) Target-specific QA: (a) Large-field irradiator: The target-specific quality assurance performed using a mouse phantom yield a dose rate 14% higher than that estimated by the investigator. (b) Small-field irradiator: The MOSFET data was systematically lower than the commissioning data. The dose difference is due to the increased scatter radiation in the solid water block versus the dimension of the mouse phantom leading to an overestimation of the actual dose in the former. The MOSFET method with the use of mouse phantom provides less labor intensive geometry-specific dosimetry and accuracy with better dose tolerances of up to +/- 2.7%. (c) 137Cs irradiator: Lab 1: Dose measurements from levels 3 and 4 were compared with the estimated dose rate. The average measured dose was found to be 19.8 +/- 2.6% and 13.8 +/- 2.0 % lower than the estimated dose. Lab 2: No comparison could be made due to user-error during irradiation. (3) The nano-scale phosphor detector displays equivalent or superior dosimeteric characteristics in comparison to commonly used TLD and MOSFET dosimeters for small animal dosimetry. (Abstract shortened by UMI.).