Surgical clips in planning the electron boost in breast cancer: a qualitative and quantitative evaluation.

PURPOSE: To evaluate, qualitatively and quantitatively, the role of surgical clips in planning the tumor bed electron boost in patients undergoing breast conserving surgery and radiotherapy.

METHODS AND MATERIALS: In 50 patients, the excision cavity boundaries were marked by clips at surgery. The electron boost field was first planned using clinical information, aiming to achieve a margin of 2 cm, and its accuracy evaluated by screening the surgical clips and, if necessary, adjusting the field to encompass all clips with 2 cm margins. Orthogonal radiographs were take with solder wire delineating the clinical and screened fields and the scar. Hypothetical clinical and radiological fields, with 1 and 3 cm margins, were reconstructed on the radiographs.

RESULTS: The clinical field was inadequate in 34 patients (68%). The precision of each clinical setup was quantified by two indices. The Normal Tissue Index defined the percentage of the clinical field comprised of tissue, beyond the tumor bed, not at high risk of local recurrence, and gave an estimate of potential spring of normal tissue: median 14.6% (range 0-83.0), 17 out of 50 > 25%; median 13% (range 0-70.7), 12 out of 50 > 25%; median 9.7% (range 0-59.8), 10 out of 50 > 25%, for 1, 2, and 3 cm margins, respectively. The Geographical Miss Index defined the percentage of the radiologically defined field, at high risk of local recurrence, not predicted by the clinical field, and gave an estimate of the extent of geographical miss: median 32.9% (range 0-83.5), 28 out of 50 > 25%; median 26.1% (range 0-69.8%), 26 out of 50 > 25%; median 18.6% (range 0-60.3), 20 out of 50 > 25%, for 1, 2, and 3 cm margins, respectively. The median distance from the scar midpoint to the furthest clip was 3.8 (range 1.2-8.1) cm. The median maximal clip depth was 3.1 (range 1.4-5.2) cm.

CONCLUSION: (a) Electron boost field planning by clinical landmarks alone was inaccurate in 68% of cases. (b) Quantitative measures, based on margins of 1, 2, and 3 cm, revealed that in 20-34% of patients more than one-quarter of the clinical field covered tissue at low risk of local recurrence, and in 40-56% of patients less than three-quarters of the final radiological field was predicted clinically. (c) The relative positions of the scar and clips may be widely disparate. (d) Clip depth measurements reveal a significant risk of underdosing at depth.