Variability of the location of internal mammary vessels and glandular breast tissue in breast cancer patients undergoing routine CT-based treatment planning

Abstract

Purpose: To determine the variability of position of internal mammary vessels (IMV) and glandular breast tissue (GBT) in patients undergoing breast-conserving radiation therapy. To assess the frequency and magnitude of tangential field border shifts based on preradiation therapy (RT) computed tomography (CT) imaging in breast cancer patients. Methods and Materials: Five hundred and ninety breast cancer patients irradiated between 9/94 and 3/98 underwent routine CT-based treatment planning. Two analyses were performed. First, the position of IMV and GBT, outlined on the central axis CT image, was determined relative to the midsternum in 111 patients irradiated during a 12-month period. In the second analysis, the difference between anticipated (pre-CT) and actual (CT-based) tangential field borders was assessed in 254 patients irradiated during a 2-year period. Results: In the first analysis, the depth of the IMVs varied from 1 to 6 cm (median 2.4 cm). The lateral distance from the midsternum also varied widely (range 1.7 to 3.7 cm, median 2.5 cm). Similar variability was found in the position of the GBT. In the second analysis, CT information led to changes of anticipated field borders in 65% of patients. The lateral border was shifted in 56% of patients (anteriorly 18%, posteriorly 38%). When the patients were segregated based on internal mammary node (IMN) treatment, the medial border was shifted in 49% of patients when the IMNs were treated in the tangential fields and in 24% when the GBT only was treated. The frequency of lateral field border shifts was similar in both groups. Conclusions: The position of IMVs and GBT varies widely in breast cancer patients. Tangential field borders based on surface anatomy may not be ideal. Among 254 breast cancer patients, the field borders were shifted in 65% of patients when CT information was available. Thus, in most breast cancer patients, field borders are shifted when CT-based treatment planning is used.