Date of Graduation
12-2014
Document Type
Dissertation (PhD)
Program Affiliation
Medical Physics
Degree Name
Doctor of Philosophy (PhD)
Advisor/Committee Chair
Sam Beddar, Ph.D.
Committee Member
Kenneth Hess, Ph.D.
Committee Member
Rajat Kudchadker, Ph.D.
Committee Member
Andrew Lee, Ph.D.
Committee Member
Dragan Mirkovic, Ph.D.
Abstract
In vivo dosimetry, the direct measurement of dose delivered to patients during radiation therapy, has significant potential in ensuring safe and effective treatment in radiation therapy. It can serve as point-of-delivery, patient specific quality assurance and direct verification of treatment. Despite evidence that in vivo dosimetry can detect errors in patient treatment that would otherwise go undetected, it is not commonly practiced. This is due in part to a lack of available detectors ideally suited to perform in vivo dosimetry. Plastic scintillation detectors (PSDs) possess a number of dosimetric characteristics advantageous for in vivo dosimetry including water equivalence, real-time capability, small size, and energy independence. However, PSDs have not been used for in vivo dosimetry of external beam radiation therapy to date. The overall purpose of this work is to apply PSDs to in vivo dosimetry of external beam radiation therapy, and demonstrate the utility and practicality of performing in vivo dosimetry with PSDs.
Three avenues of research were pursued in accordance with this purpose. First, the temperature dependence of PSDs was characterized. Prior to this work, PSDs were understood to be temperature independent detectors. However responses of PSDs constructed with BCF-60 and BCF-12, two common scintillating fibers, were demonstrated to decrease by 0.5% and 0.1% per °C increase relative to 22 °C, respectively. The spectral distribution of light was observed to change with temperature as well. This resulted in a non-negligible error in measured dose at human body temperature, requiring a temperature-specific correction factor.
Next, PSDs were used for in vivo dosimetry of the rectal wall in five patients undergoing intensity modulated radiation therapy for prostate cancer. This was done as part of an Institutional Review Board approved protocol. PSDs were attached to endorectal balloons used routinely during prostate radiotherapy, positioning the detectors in close proximity with the rectal wall. Two PSDs were used for two treatment fractions each week for the duration of each patient’s treatment. The difference between the measured dose and expected dose was used to evaluate the accuracy and precision of the system. The mean difference between the measured and expected dose for the five patient population was -0.4%, with a standard deviation of 2.8%. The mean differences for individual patients fell between -3.3% and 3.3%.
Finally, a thorough characterization of the response of PSDs used for absolute entrance dosimetry in proton beams was performed. Entrance dose measurements for a passively scattered proton beam performed with a PSD were compared to measurements made with an ion chamber and radiochromic film. Ionization quenching, an under-response due to densely ionizing radiation, was found to be responsible for a 7% loss of signal at the highest energy studied (250 MeV) and a 10% loss at the lowest (140 MeV). The under-response was found to be insensitive to other beam parameters, such as the width of the spread out Bragg peak.
Keywords
in vivo dosimetry, plastic scintillation detector, real-time, prostate cancer, external beam radiation therapy