Author ORCID Identifier
0000-0003-1066-1999
Date of Graduation
5-2023
Document Type
Thesis (MS)
Program Affiliation
Medical Physics
Degree Name
Masters of Science (MS)
Advisor/Committee Chair
Dr. Stephen Kry, PhD
Committee Member
Paola Alvarez, MS
Committee Member
Dr. Rebecca Howell, PhD
Committee Member
Dr. Adam Riegel, PhD
Committee Member
Dr. Ryan Sun, PhD
Abstract
Purpose: Optically Stimulated Luminescent Dosimeters (OSLDs) are a prominent form of in-vivo dosimeter used both in clinics as well as for the audits of radiological
equipment at the Imaging and Radiation Oncology Core (IROC)-Houston. These
dosimeters have a recommended dose limit of 10 Gy due to a change in signal response with dose. To assist with the OSLD operation at IROC-Houston, evaluating the signal response of these dosimeters with IROC’s methodologies offers the potential to extend the dose limit past 10 Gy, improve the efficiency of handling OSLDs, and reduce the cost and time spent on commissioning OSLDs.
Methods: The signal response of OSLDs were evaluated using the American Association of Physicists in Medicine (AAPM) Task Group (TG)- 191 recommendations. Evaluations of sensitivity and linearity characteristics were performed as accumulated dose increased. To re-use the OSLDs, the dosimeters were bleached. Four different monochromatic and one polychromatic light source was compared to the IROC light source to determine the impact that bleaching wavelength had on signal response. In addition, the OSLDs were evaluated for how the choice of bleaching light and accumulated dose affected signal regeneration. Finally, the response of OSLDs as a function of accumulated dose were evaluated as a function of different fractions of dose. Every irradiation was performed ona Co-60 beam at the same SSD of 80cm and field size of 24.5 x 24.5
Results: For the IROC system, we found that the signal response of OSLDs are stable within 1% up to 23 Gy. After this point, the sensitivity beings to decrease. The sensitivity of each OSLD relative to each other, ks,i did not change up to 50 Gy showing that the sensitivity change amongst all the OSLDs was applied universally amongst the group. There is a well characterized change in the slope of the linearity correction factor as accumulated history increases. For chromatic effects, we find that lower wavelengths remove signal the fastest, but polychromatic sources preserve the signal response to a greater accumulated dose history. For charge repopulation, we find that the degree of charge repopulation is related to dose, time, and bleaching light though this effect is nonsignificant with the IROC the newly analyzed dose limit of ~15-20 Gy. We find that fractionations at 5 Gy and higher yield a greater signal response compared to reference dosimeters, with larger fractions leading to a greater signal response. The greatest effect measured was with 30 Gy fractions at a value of 6.4% greater signal compared to reference. The greater fractions also exhibited a steeper increase in slope of the linearity correction factor.
Conclusion: IROC can extend the dose limit of 15-20 Gy of accumulated dose. The
amount of charge repopulation at this dose level is insignificant, so the OSLDs in storage do not need to be rebleached prior to reintroducing them back into operation. For the application of correction factors, ����,�� can re-use it’s commissioned value whereas a value of ���� needs to be evaluated based on the dose history and fractions used.
Keywords
OSL, Output Checks, Calibration