Proton Beam Versus Photon Beam Dose to the Heart and Left Anterior Descending Artery for Left-Sided Breast Cancer

Abstract

Dose to the whole organ heart structure has been shown to be reduced for left-sided breast radiation by techniques including photon IMRT, deep inspiration breath hold (DIBH), and proton beam irradiation. However, dose to the whole heart may not correlate well with dose to the critical structure for late events–particularly coronary events involving the left anterior descending (LAD) artery. Our purpose was to compare the dose to heart, LAD, and lung between tangential proton and photon beam irradiation for left-sided early stage breast cancer patients receiving radiation therapy. Five women with early stage left-sided breast cancer were treated with breast-conserving surgery and radiation. Each gave permission to participate on an IRB-approved prospective study of DIBH and proton beam radiation planning. Whole breast radiation was delivered via a tangential technique utilizing DIBH, inverse planning and sliding window IMRT. The DIBH scan was used for this protocol in order to eliminate the potential effects of motion on the dose distributions. Each patient was replanned using tangential proton beam radiation. The heart and LAD were contoured by a single radiologist/radiation oncologist (SV). Prescription dose to the whole breast was 50 Gy in 2 Gy fractions with breast clinical target volume coverage of 95% >95%. The coverage of the breast clinical target volumes was comparable between photon and proton plans. The heart V20 was lower with protons vs photons 0.1% (0-3.2) v 0.6% (0-1.8). Additionally, there was a lower dose to 0.2 cc of LAD with protons vs photons median 4.7 (0-28) Gy vs 30 (0-46) Gy as well as a lower minimum and maximum doses to LAD with protons vs photons 0 vs 2.2% and 27.4% vs 66.6%, respectively. Lung dose was comparable between modalities. Lung V20 median was 12.9% (9.2-15.0) vs 10.4% (4.7-17%) for photons vs protons, respectively. The dose to whole heart was relatively low in this study of patients treated under conditions of DIBH. However, proton beam radiation was associated with lower minimum, maximum, and dose to 0.2 cc of the LAD which is the critical structure for late RT effects. Due to the variations in the magnitude of benefit within each patient, comparative planning may be needed for each patient prior to treatment to determine the benefit of protons. Further study in a larger group of patients will be performed.