Optimizing Image Guidance Strategies for Breast Boost Radiation Therapy

Breast boost following whole breast radiotherapy (RT) in breast-conserving therapy can reduce local recurrence. At our institution, breast boost treatments are delivered using a 3 field conformal technique following daily image guidance with cone beam computed tomography (CBCT). The current image registration protocol involves manually matching the ipsilateral breast/chest wall and lung interface of the reference treatment planning contours to the daily acquired localization CBCT as a surrogate of cavity location. This protocol may lead to variability and inconsistency in image registration due to breast swelling and edema. The aim of this study is to identify appropriate and reliable surrogates to optimize CBCT image registration for breast boost patients to reduce inter-user variability. Thirty patients that completed 5 fractions of breast boost RT between October and December 2014 were identified for this ethics approved initiative. Patients were categorized into 3 groups based on their cavity visibility on CT: high (highly visible cavity); moderate (moderately visible cavity with indistinct borders); and low (indistinct or no visible cavity), with ten patients represented in each group. Three independent observers manually registered the daily CBCTs for each patient utilizing 2 methods to assess inter-observer variability. Method 1 followed standard protocol by matching the ipsilateral breast/chest wall and lung interface, followed by positional assessment of the cavity within the planning target volume contour. Method 2 involved direct registration to the cavity. Additionally, automatic image registration on the ipsilateral breast/chest wall and lung interface, facilitated by a shaped region of interest defined within the imaging software, was performed for all patients over all treatment fractions. Automatic registration was explored as an alternative approach to manual image matching as a strategy to reduce inter-observer variability. This investigation will potentially identify image registration surrogates and methods that produce the least amount of inter-observer variability for breast boost RT. Opportunities exist to adapt the image registration strategy based on cavity visualization scores, and increase the utilization of automatic registration tools within the imaging software. The evidence-based findings from this study will inform best practice procedures, and change education guidelines for breast boost image registration. The results from this investigation will determine which image matching technique is most consistent and reliable, resulting in the least amount of inter-observer variability for breast boost RT. This study will also potentially identify and inform future evidence-based guidelines and surrogates used for image registration based on cavity visualization scores.