Author ORCID Identifier

mitchell.s.carroll@uth.tmc.edu

Date of Graduation

12-2018

Document Type

Dissertation (PhD)

Program Affiliation

Medical Physics

Degree Name

Doctor of Philosophy (PhD)

Advisor/Committee Chair

Geoffrey Ibbott, Ph.D.

Committee Member

David Followill, Ph.D.

Committee Member

Michael Gillin, Ph.D.

Committee Member

Kenneth Hess, Ph.D.

Committee Member

Mark Oldham, Ph.D.

Abstract

Radiotherapy techniques have advanced and radiation dose plans have become much more complex over the last decade. This is especially true in proton therapy, which involves extremely steep dose gradients as a result of positioning the Bragg peak to cover the volumes to be treated. The Bragg peak can be shifted significantly in the patient as a result of nonuniformities in the tissue composition in its path, which can result in treatment complications. Some traditional dose verification tools used in proton beam commissioning and treatment plan verification are film, TLD, and ionization chambers. Such 0D and 2D dosimeters are incapable of fully registering a complete dose volume. This shortcoming has led to an interest in a robust 3D dosimeter with dosimetric accuracy matching that of traditional devices. PRESAGE®, a novel radiochromic dosimeter, has shown promise in dose verification of complex IMRT treatments. As such, there have been numerous studies of PRESAGE® as a 3D dosimetry tool in photon and electron radiotherapy, but little work towards adapting it to proton therapy. The primary limitation for using PRESAGE® in a proton beam has been the appearance of ‘quenching’, principally seen in the Bragg peak region of the delivered field, resulting in a pronounced dose under-response. Very little research has gone into investigating the cause of this quenching and no system is in place to account for it. Currently, PRESAGE® continues to find more of a place in photon radiation therapy research and clinical applications, and while interest in proton therapy increases around the world, Presage has not been demonstrated to be an accurate dose verification system for proton therapy.

I hypothesized that the mechanisms causing signal quenching in PRESAGE® when irradiated by a proton beam can be evaluated and corrected for resulting in a system that yields relative dose measurements agreeing with ionization chamber measurements to ±3% in the Bragg peak and with calculated doses to 5%/5mm. This hypothesis was tested using three aims. The first aim developed a PRESAGE® manufacturing protocol capable of producing dosimeters to meet the needs of a wide range of clinical applications. The second aim evaluated the effects of the concentrations of the active components in PRESAGE® formulation on specific dosimetric properties and optimized formulations minimizing signal quenching chemically. The final aim developed and applied a quenching correction factor to PRESAGE® as a way to reduce dose inaccuracies caused by quenching.

Keywords

PRESAGE, proton therapy, 3d dosimetry, radiotherapy, proton dosimetry

Available for download on Thursday, August 29, 2019

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