Breast-shape changes during radiation therapy after breast-conserving surgery

Background & purpose: With the introduction of more conformal techniques for breast cancer radiation therapy (RT), motion management is becoming increasingly important. We studied the breast-shape variability during RT after breast-conserving surgery (BCS). Materials & Methods: Planning computed tomography (CT) and follow-up cone-beam CT (CBCT) scans were available for 71 fractions of 17 patients undergoing RT after BCS. First, the CT and the CBCT scans were registered on bones. Subsequently, breast-contour data were generated. The CBCT contours were analyzed in 3D in terms of deviations (mean and standard deviation) relative to the contour of the CT scan for the upper medial, lower medial, upper lateral, and lower lateral breast quadrants, and the axilla. Results: Regional systematic and random standard deviations of the breast quadrants varied between 1.5 and 2.1mm and 1.0-1.6 mm, respectively, and were larger for the axilla (3.0 mm). An absolute average shape change of >= 4.0mm in at least one region was present in 21/71 fractions (30%), predominantly in breast volumes > 800 cc (p= < 0.01). Furthermore, seroma was associated with larger shape changes (p= 0.04). Conclusions: Breast-shape variability varies between anatomic locations. Changes in the order of 4mm are frequently observed during RT, especially for large breasts. This should be taken into account in the development of protocols for partial breast irradiation and boost treatment.