| Lung cancer is the second most common cancer in both men and women, estimated to account for 158,080 deaths in 2016 according to the American Cancer Society(1). Approximately 85% of all lung cancers are non-small cell lung cancer (NSCLC). While surgical resection remains the standard therapy for operable stage I lung cancer patients(2), stereotactic body radiation therapy (SBRT) is a noninvasive alternative for the inoperable population(2--4).;SBRT is a technique that delivers high doses of radiation to tumors, generally in one to five treatments which are known as "fractions". Since SBRT demands a high degree of confidence in tumor definition and localization, it requires the assessment and management of tumor motion (i.e. the respiratory excursion of the tumor)(3--7). Respiratory motion is of particular concern for TomoTherapy (Accuray, Inc, Sunnyvale, CA), which utilizes a continuously rotating gantry (maximum speed of 12 seconds per rotation) synchronized with a linearly moving couch to irradiate a target volume in a helical manner(8,9). The interplay between the motion of the radiation and the motion of the tumor can cause a discrepancy between the desired dose and the dose actually delivered. This is commonly referred to as the "interplay effect." The goal of this work was to investigate the interplay effect using a cohort of patient respiratory waveforms and a motion phantom. Respiratory waveforms were characterized using principal component analysis of tumor displacement sampled via four-dimensional computed tomography (4DCT). These waveforms, programmed into the motion phantom were used to evaluate both the imaging and treatment accuracy of TomoTherapy. Ion chamber and film measurements were collected for the static condition, motion at the native breath frequency of the patient, and a modified motion averaging 5 breaths per minute (bpm).;TomoTherapy megavoltage CT (MVCT) scans did not show marked interplay effect. Scanning in fine mode resulted in smaller positional errors, though larger imaging artifacts were visible. Lower breath frequencies were significantly associated with larger variance in dose, and larger amplitude of tumor displacement correlated with larger penumbral blurring. Banding of hot and cold were observed on film for several cases, although ion chamber measurements were mostly within the recommended 2% criteria.;Strategies for minimizing the interplay effect involve maintaining an elevated breath frequency, improving regularity of the respiratory waveform, and avoiding the use of TomoTherapy plans with a fast gantry period of 12 seconds per rotation. The results of this dissertation provide a clinically based evaluation of the interplay effect on TomoTherapy for SBRT treatments. |
