Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Medical Biophysics

Supervisor

Dr. Jerry J. Battista and Dr. Stewart Gaede

Abstract

Stereotactic Body Radiation Therapy (SBRT) is an effective treatment option for patients with inoperable early-stage lung cancer. SBRT uses online image-guidance technology [e.g. cone-beam CT (CBCT)] to focus small-fields of high energy x-rays onto a tumour to deliver ablative levels of radiation dose (e.g. 54 Gy) in a few treatment fractions (e.g. 3). For the combination of these treatment parameters and a low density lung, lateral electron disequilibrium (LED) can potentially occur, reducing lung and tumour doses. The goal of this thesis was to determine the impact of LED on stereotactic body radiation therapy for lung cancer.

The effect of LED on lung dose distribution was studied using Monte Carlo simulations of a lung slab phantom. The magnitude of lung dose reduction due to LED, and the specific conditions (beam energy, field size, and lung density) that cause the phenomenon, were quantified and could be predicted using a relative depth dose factor (RDDF).

The RDDF concept was then used to develop a novel SBRT technique, called LED-optimized SBRT (LED-SBRT), which creates steep dose gradients, caused by intentional LED, to elevate tumour dose, while reducing/maintaining dose levels in healthy lung. Further, the RDDF aided in assessing the accuracy required in CBCT-derived lung density, when applied to adaptive SBRT dose calculations. In this regard, we determined that CBCT image artefacts produced erroneously low lung density, artificially triggering LED, and incorrectly predicting lower lung/tumour dose levels. As a result, CBCT number corrective techniques were developed in order to improve dose calculation accuracy.

The results of this thesis provide physicians and physicists with a much better prediction of the radiation dosimetry under disequilibrium conditions, and allow exploration of irradiation conditions that can cause LED. With this knowledge in-mind, competent decisions can be made regarding the choice of dose calculation algorithm, and aid in the design and interpretation of SBRT clinical trials. Furthermore, the outcomes of this work can help launch a new generation of SBRT techniques that exploit LED effects that may offer a dosimetric benefits for selected patients.


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