Realistic Respiratory Motion and its Impact on Partial Breast Intensity Modulated Radiotherapy Treatment Planning
Creators: name: Quirk, Sarah
Publisher: name: Graduate Studies
Year: 2013
Subjects: subject: General
subject: Radiation
subject: Respiratory Motion
subject: Partial breast irradiation

description: Respiratory motion degrades plan quality in partial breast intensity modulated radiation therapy resulting in the requirement for some patients to have respiratory management during treatment. In this thesis we identify which patients require respiratory management through the evaluation of patient-specific anatomy and extent of respiratory motion. The first step in this work was to realistically model respiratory motion for this patient population. We first examined an extensive patient database of external respiratory motion, then also conducted a volunteer study of healthy females in order to better represent the breast cancer population. For both of these populations peak-to-peak amplitude, period, and end exhale were found to quantify the extent of motion. Inter- and intra-fraction variability were quantified, as well as baseline drift for both populations. In order to extend the simplified sinusoidal models typically found in the literature, a shape analysis was completed employing Akaike’s Information Criterion to evaluate candidate models. A four-parameter sigmoid fit was found to be optimal. With this fit we found an improvement on sin^2(x) for 98% of patient exhale and 70% of inhale traces and better than sin(x) for 100% of both inhale and exhale traces. This respiratory extent of motion, variability, and shape analysis were combined to build a realistic respiratory trace generator (RTG). This provides a method of generating custom respiratory data that can be used for initial implementation and testing of new technologies. The knowledge gained from the respiratory modelling part of this project was implemented in determining the impact of respiratory motion on partial breast intensity modulated radiation therapy. The volunteer population data was used with the entire database of partial breast intensity modulated radiation therapy plans from the RAPID clinical trial to determine which patient anatomies were more susceptible to the effects of respiratory motion. We examined two patient selection metrics found in the literature, ipsilateral breast volume (IBV) and PTV-to-IBV ratio, as well as proposing our own metric: DEV-to-PTV ratio. We found that the DEV-to-PTV ratio is a better patient selection metric to predict which patient plans will experience more extensive dose degradation due to respiratory motion and patient anatomy. This metric is also independent of the IBV definition. Current inconsistencies in breast contouring protocols render breast volume and PTV-to-breast volume ratios subject to intra-observer and intra-study variability for quantifying patient suitability for partial breast radiotherapy. We recommend respiratory management for patients with a DEV-to-PTV ratio of less than 55% as these plans experienced a larger degradation in plan quality. For patients with DEV-to-PTV ratio of greater than 55%, population-based respiratory motion has little impact on plan quality. However, there will be a maximum amplitude of motion that will cause these plans to degrade to unacceptable levels of plan quality. We examined ten patients with DEV-to-PTV ratios of greater than 55% and escalated the amplitude of respiratory motion from 2 - 20 mm. We found that dose homogeneity, namely hotspot and homogeneity index, were the limiting factor in plan quality and not target coverage. We recommend respiratory management for patients with respiratory amplitude greater than 10 mm. Due to the propensity of hotspots and regions of inhomogeneity to become exacerbated with respiratory motion, we recommend caution during the planning process. Our results show that if the plan does not meet planning criteria, or if it only barely meets planning criteria, respiratory management should be considered.