MOSFET Sensitivity Variation With Integrated Dose History, Application of MOSFET in CT Dosimetry, Physics Characterization of Portal Monitor and Analysis of Portal Monitor Usage for Screening Contaminated People Based on Queueing Theory
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<bold>Purpose</bold>: (1) Evaluate use of portal monitor in a radiological emergency, (2) study the variation of MOSFET sensitivity with integrated dose history and (3) assess the accuracy of AAPM 96 formalism for estimating effective dose from pediatric CT scans.
<bold>Methods and Materials</bold>: (Project 1): The maximum distance between the portal monitor and a variety of test sources where radiation could be detected was measured as well as the exposure rate at the center of the portal monitor. This information was used in conjunction with queue theory to create a plan of use for the portal monitor as well as improvements to make it more suitable for a radiological emergency.
(Project 2): The variation of MOSFET sensitivity was investigated for two diagnostic and two therapeutic MOSFETs (high and standard bias, respectively). The MOSFETs were exposed to an orthovoltage beam with an ion chamber in order to obtain the calibration factor (mV/cGy) at several points over the MOSFET lifespan. The diagnostic MOSFETs were exposed to a 120 kVp beam and the therapeutic MOSFETs were exposed to a 250 kVp beam.
(Project 3): Organ doses were measured for a variety of pediatric CT protocols using diagnostic MOSFETs loaded into a pediatric anthropomorphic phantom. The protocols investigated were head, chest, abdomen-pelvis and chest-abdomen-pelvis. Organ doses were measured for these protocols using a GE and a Siemens 64 slice CT. The effective dose was obtained using the ICRP 103 formalism. These values of effective dose were compared to the effective dose estimation method described by AAPM report 96, which employs a DLP-to-effective dose conversion factor.
<bold>Results</bold>: (Project 1): The minimum exposure rate at the center of the portal monitor where radiation could be detected was measured to be 30 μR/hr. In order to prevent false positive readings when using the portal monitor in a radiological emergency, there must be some distance between the portal monitor and the group of people waiting to be scanned, which depends on the collective activity. Assuming a collective activity of 100 mCi of <super>137</super>Cs, there must be at least 33.1 m between the portal monitor and those waiting to be scanned. Additionally, multiple portal monitors should be at least 10 m apart in order to prevent false positive readings, which assumes an individual contamination of as much as 10 mCi of <super>137</super>Cs. If portable shielding is available, these distances can be dramatically decreased.
(Project 2): The diagnostic MOSFETs showed a fairly constant sensitivity from 0 mV to 12,000 mV, with a maximum variation of 4%. At this point, the calibration factor decreased by an average of 19% from 12,000 mV to 18,000 mV. Conversely, the therapeutic MOSFETs showed an approximately linear decrease in sensitivity. The calibration factor decreased by an average of 3% every 3,000 mV until 18,000 mV.
(Project 3): The effective dose for the GE scans was underestimated by 133.27%, 55.84%, 30.24% and 19.13% for the head, chest, abdomen-pelvis and chest-abdomen- pelvis scans, respectively. Conversely, the AAPM 96 formalism underestimated the effective dose from the Siemens head scan but overestimated the effective dose from the three other Siemens scans. The effective dose for the Siemens head scan was underestimated by 88.66% but the effective dose was overestimated by 102.81%, 114.52% and 96.19% for the chest, abdomen-pelvis and chest-abdomen-pelvis scans, respectively.
<bold>Conclusion</bold>: (Project 1): The portal monitor is not suitable for use in a radiological emergency unless an abundance of space is available. In order to improve the portal monitor, the sensitivity should be reduced and shielding should be added around the detectors.
(Project 2): Data suggests that the diagnostic MOSFETs can be used with their initial calibration factor until the age of 15,000 mV. At this time, a new calibration factor should be obtained or the MOSFETs should be discarded. On the other hand, a correction factor can be applied to the initial calibration factor for the therapeutic MOSFETs. This takes advantage of the approximately linear nature of the decrease in sensitivity with integrated dose.
(Project 3): A "one size fits all" conversion factor for estimating CT effective dose is not sufficient. This conversion factor should be expanded to specific CT manufacturers in addition to patient age and scan location. Additionally, these conversion factors should be updated with modern Monte Carlo simulations.
CitationFredrickson, Mats (2011). MOSFET Sensitivity Variation With Integrated Dose History, Application of MOSFET in CT Dosimetry, Physics Characterization of Portal Monitor and Analysis of Portal Monitor Usage for Screening Contaminated People Based on Queueing Theory. Master's thesis, Duke University. Retrieved from http://hdl.handle.net/10161/3777.
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