Toward Accurate Small Animal Dosimetry and Irradiator Quality Assurance

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.

Materials and Methods: (1) Machine-specific QA: (a) Large-field irradiator: Dose measurements were performed with an ion chamber and include: beam profile measurements at 50 cm SSD, linearity of output, in-air output for various irradiation settings, and light and radiation field coincidence measurement. A kVp meter was used to measure kVp and HVL for different irradiation settings. (b) Small-field irradiator: Dose measurements were completed using an ion chamber and MOSFET dosimeters. For the diagnostic mode measurements, the ion chamber was placed on the irradiation table and various diagnostic protocols were measured including table attenuation. MOSFETs were used to measure the backscatter factors (BSF) for various collimator sizes under therapy mode.

(2) Target-specific QA: (a) Large-field irradiator: A tissue-equivalent mouse phantom (2 cm diameter, 8 cm length) was used. MOSFET dosimeters were calibrated in air with an ion chamber and f-factor was applied to derive the dose to tissue. The MOSFET detectors were then placed in the phantom at center of the body and irradiated under the following settings: 320 kVp, 12.5 mA, for 30s for four runs. (b) Small-field irradiator: Accuracy of mouse dose between TG-61 based look-up table was verified with the MOSFET technology. The look-up table was obtained by TG-61 based commissioning data and used a tissue-equivalent block and radiochromic film. A tissue-equivalent mouse phantom was used with MOSFETs placed at the center of the body. MOSFETs were calibrated in air with an ion chamber and f-factor was applied to derive the dose to tissue. In CBCT mode, the phantom was positioned such that the system isocenter coincided with the center of the MOSFET with the active volume perpendicular to the beam. The absorbed dose was measured three times for seven different collimators, respectively. The exposure parameters were 225 kVp, 13 mA, and an exposure time of 20s. (c) 137Cs irradiator: Tissue-equivalent mouse phantoms were tested in target-specific set-ups. TLD calibration was performed on site. (3) The nano-scale phosphor detector was tested in both the small-field irradiator and the 137Cs irradiator. Calibration was performed equivalent to MOSFET/TLD calibration for the small-field irradiator and 137Cs irradiator. Other measurements included angular dependence measurements in-air and in-phantom, with and without the table.

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.