Wed Eve—45: Imaging and Radiation Delivery in Helical Tomotherapy: Phantom Study of a Moving Target
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Radiation is an effective method of treating lung cancer, however, the tumour moves while the patient breathes. To ensure radiation always covers the tumour, a volume larger than the target must be treated. For conventional treatments, gating can be employed, but helical tomotherapy treatments cannot be gated. We hypothesized that, through judicious choice of planning computed tomography (CT) imaging methods, radiation can be accurately planned and delivered to a moving tumour using tomotherapy. A breathing phantom was used which allowed one-dimensional motion of a lung-equivalent cylinder containing a target. By varying the imaging parameters, we obtained numerous different planning studies with target motion in both the superior-inferior and lateral directions. These studies included a static study, fast-CT studies, a maximum intensity projection (MIP), an average intensity projection (AveIP) and an untagged average study. Planning studies were acquired with the cylinder moving sinusoidally with a period of 4 s and amplitude of 1 cm. Treatment plans were created for each CT study and delivered using tomotherapy. Dose was measured using self-developing film. Dose-area histograms were used to compare the dose delivery to the central coronal plane of the target for all planning studies. A similar experiment was also performed using an irregular breathing pattern. Results indicated that planning using the AveIP study results in the most accurate treatment of a moving target. The amount of dose delivered to the normal tissue did not change significantly, due to the small increase in irradiated area compared to the entire area of the film.