Evaluation of dose to cardiac structures during breast irradiation

Abstract

Objective: Adjuvant radiotherapy for breast cancer can lead to late cardiac complications. The highest radiation doses are likely to be to the anterior portion of the heart, including the left anterior descending coronary artery (LAD). The purpose of this work was to assess the radiation doses delivered to the heart and the LAD in respiration-adapted radiotherapy of patients with left-sided breast cancer.

Methods: 24 patients referred for adjuvant radiotherapy after breast-conserving surgery for left-sided lymph node positive breast cancer were evaluated. The whole heart, the arch of the LAD and the whole LAD were contoured. The radiation doses to all three cardiac structures were evaluated.

Results: For 13 patients, the plans were acceptable based on the criteria set for all 3 contours. For seven patients, the volume of heart irradiated was well below the set clinical threshold whereas a high dose was still being delivered to the LAD. In 1 case, the dose to the LAD was low while 19% of the contoured heart volume received over 20 Gy. In five patients, the dose to the arch LAD was relatively low while the dose to the whole LAD was considerably higher.

Conclusion: This study indicates that it is necessary to assess the dose delivered to the whole heart as well as to the whole LAD when investigating the acceptability of a breast irradiation treatment. Assessing the dose to only one of these structures could lead to excessive heart irradiation and thereby increased risk of cardiac complications for breast cancer radiotherapy patients.

Figure 1

(a) Axial view of a CT-based breast radiation treatment plan. Two tangential beams (white arrows) are used to cover the breast tissue (in some cases, the treatment field will be extended to include lymph nodes). The area in red colour-wash represents the anatomy that will receive a dose of radiation of 20 Gy or more. (b) Projected view of a tangential radiation field onto a breast cancer patient anatomy. A multileaf collimator (displayed as blue lines) is used to exclude as much of the heart as possible from the radiation field without compromising dose to the breast. However, one can clearly see that the anterior heart (yellow) and a small portion of the left anterior descending coronary artery (LAD) (pink) are still included in the treated area.

Figure 2

Dose–volume histograms for a typical breast irradiation plan. For each dose level on the horizontal axis the per cent volume of each organ receiving that dose level or more is plotted. The dose delivered to several structures is assessed. The breast should be covered by 90–110% of the prescription dose (here 48 Gy). The dose to critical healthy organs should be limited. In this case, the left lung receives a dose of 20 Gy or more to 30% of its total volume. The volume of arch left anterior descending coronary artery (LAD) irradiated to 20 Gy is virtually zero (as illustrated by the dotted arrow). This fulfils the dose constraints described in the “methods” section. Hence, the plan was judged satisfactory for treatment.

Figure 3

Dose–volume histograms for all patients for (a) the heart, (b) the arch of the left anterior descending coronary artery (LAD) and (c) the whole LAD. Each patient has a separate colour code. (a) The dose thresholds recommended by the Danish Breast Cancer Cooperative Group are illustrated as dotted black lines.