Definition of postlumpectomy tumor bed for radiotherapy boost field planning: CT versus surgical clips

Purpose: To compare the location and extent of the tumor bed as defined by surgical clips and computed tomography (CT) scans, after lumpectomy, for electron boost planning as part of breast radiotherapy. Methods and Materials: Planning CT images of 31 operated breasts in 30 patients who underwent lumpectomy were reviewed. One or more clips were placed in the lumpectomy cavity. Serial CT images were used to measure the depth and transverse and longitudinal dimensions. The area and geometric center of the tumor bed were defined by the clips and CT. Results: The CT and clip measurements were identical for the maximal tumor depth in 27 of 30 patients. The CT bed extended beyond the clips by 0-7 mm medially in the transverse/longitudinal extent (multiclip patients). The median distance between the geometric centers in the coronal plane for the tumor bed center was larger for patients with single clips than for those with multiple clips (p < 0.025). Tumor bed areas in the coronal plane defined by both methods correlated strongly. However, the CT-defined area was larger by 13.9 mm². The CT bed was more readily visible in patients with a shorter interval between surgery and radiotherapy. Conclusion: The maximal depth of the tumor bed was similar using the two methods. The extent and centers of the clip-and CT-determined beds differed significantly. This may indicate an underestimation of the tumor bed as defined by clips only and justifies integration of CT information in boost field planning.