Purpose: The purpose of this article is to report dose-volume histograms of coronary vessels from irradiation of the intact left breast.
Patients and methods: Fifteen women with cancer of the left breast underwent computed tomographic treatment planning for radiation treatments of an intact left breast. Images through the heart were reconstructed at 1-mm increments to permit contouring of the coronary vessels. Five treatment plans were created for each patient; one plan from the simulated treatment fields and four additional plans that were generated from virtual treatment fields created by shifting the isocenter 5 mm and 10 mm both superficially and deep. The radiation dose was calculated using a three-dimensional treatment planning system that incorporated heterogeneity correction factors.
Results: With no adjustment to the perpendicular lung distance, a mean volume of 12% of the left anterior descending coronary artery received 20 Gy, 6% received 30 Gy, and 3% received 40 Gy. The dose to the left anterior descending coronary artery varied significantly with changes in the perpendicular lung distance. From the mean perpendicular lung distance of 1.87 for the simulated fields, a 5-mm increase in the perpendicular lung distance resulted in an increase of 20%, 15%, and 12% in the percentage of the left anterior descending coronary artery treated to 20 Gy, 30 Gy, and 40 Gy, respectively. With a 10-mm increase, the respective volumes were increased to 49%, 41%, and 34%, respectively. A 5-mm reduction of lung distance in the original plan resulted in a decrease of 10%, 5%, and 3% in the percentage treated to 20 Gy, 30 Gy, and 40 Gy, respectively. The dose to the left main coronary artery, the right main coronary artery, and the left circumflex coronary artery was limited to scatter and was less than 7 Gy. Changes in the perpendicular lung distance did not significantly affect the dose administered to these vessels.
Discussion: The left anterior descending coronary artery is anatomically located at the edge of the cardiac silhouette on traditional treatment films. Small changes in the perpendicular lung distance can significantly change the dose delivered to this vessel. A fundamental change in the shape of the dose-volume histogram occurs at a perpendicular lung distance of 2.3 cm, whereas the dose is very low when the perpendicular lung distance is less than 1.3 cm. These points may serve as clinically important values in the treatment planning for cancer of an intact breast.