Identifier

etd-06292015-213159

Degree

Master of Science (MS)

Department

Physics and Astronomy

Document Type

Thesis

Abstract

Purpose: To improve the dosimetric accuracy of intensity modulated radiation therapy (IMRT) dose distributions as calculated by the treatment planning system (TPS) by optimizing the parameters that govern multileaf collimator (MLC) transmission and rounded leaf offset. Methods: The MLC leaf transmission was optimized based on measurements made with ionization chambers and radiographic film. The rounded leaf offset table was optimized by measuring the radiation field edge as a function of leaf bank position with an ionization chamber in a water scanning tank and comparing the location to TPS equivalent dose calculations. Optimizations were validated by performing IMRT quality assurance (QA) tests on 19 gantry-static IMRT plans. Planar dose measurements were performed with film and a planar diode array and compared to TPS calculated dose distributions with default and optimized parameters. Results: Based on measurements, the leaf transmission factor was changed from a default value of 0.001 to 0.005. This optimization resulted in a statistically significant worsening of IMRT QA gamma index passing rate, because the currently used model is already slightly higher than the measured data originally used to commission the machine. The rounded leaf offset table had little room for improvement, with the average difference between the default and optimized offset values being -0.2 ± 0.7 mm. This reflects the excellent leaf position calibration protocol of physics staff. Conclusion: The hypothesis that TPS dosimetric accuracy of IMRT fields could be improved by optimizing the rounded leaf offset table and MLC transmission parameters was not supported by the results of this work.

Date

2015

Document Availability at the Time of Submission

Secure the entire work for patent and/or proprietary purposes for a period of one year. Student has submitted appropriate documentation which states: During this period the copyright owner also agrees not to exercise her/his ownership rights, including public use in works, without prior authorization from LSU. At the end of the one year period, either we or LSU may request an automatic extension for one additional year. At the end of the one year secure period (or its extension, if such is requested), the work will be released for access worldwide.

Committee Chair

Zhang, Rui

DOI

10.31390/gradschool_theses.1080

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