Improving radiotherapy treatment for left-sided breast cancer

Abstract

Left-sided breast cancer patients with large irradiatedheart volumes are identified as patients at high risk for late cardiac mortality. This thesis describes the development of IMRT techniques where significant heart sparing is possible. First, dosimetric effects of contour changes and tissue inhomogeneities as a function of setup uncertainty were evaluated. The results suggest that these effects are negligible as long as the setup uncertainty probability function is symmetrical, and that rigid body dose distributions are sufficiently accurate. Furthermore, the optimal 2-beam orientation for heart sparing in left-sided breast cancer patients for uniform and nonuniform beam intensity techniques was investigated. An exhaustive search through all axial 2-beam combinations was performed. Optimal 2-beam IMRT orientations for left-sided breast cancers are bimodal, containing hinge angles around 160° and 210°. IMRT techniques are less sensitive to beam orientation compared to uniform beam techniques and result in significantly improved heart sparing, but at a cost of slightly compromised coverage of the PTV. The next study compared the reduction in cardiac and pulmonary complication rates between wedged and intensity-modulated beam plans for the left breast. Modifying the treatment technique from wedged rectangular to wedged conformal to intensity-modulated beams, reduced the cardiac complication rate, on average, from 5.9% to 4.0% to 2.0%, respectively. The results of that study lead to the design of a class solution to reduce late cardiac complications for left-sided breast cancer patients using simplified IMRT with predefined segments. Applying optimal IMRT beam orientations improves PTV coverage while maintaining significant heart sparing but increases the PTV dose heterogeneity. We also compared IMRT techniques against uniform beam (wide split tangents) and mixed modality (oblique electrons) treatment plans irradiating the left breast and internal mammary lymph node chain (IMC). The IMRT plan provides the best compromise in terms of heart sparing and target coverage. The wide split tangent plan tends to overdose the heart while the oblique electron plan tends to underdose the IMC. Based on these results clinical guidelines have been formulated for the treatment of a subgroup of left-sided breast cancer patients who present with a large amount of heart within conventional tangential fields.