2389 - Coronary Arterial Dose in Right Sided Breast Cancer Radiotherapy: A Dosimetric Analysis of Techniques

M. Pasli¹, M. C. Larkins¹, T. Stamey¹, M. Green², W. Belcher², and A. M. Burke²; ¹Brody School of Medicine, East Carolina University, Greenville, NC, ²Department of Radiation Oncology, Brody School of Medicine, East Carolina University, Greenville, NC

Purpose/Objective(s): Heart dose and pre-existing heart disease are known risk factors for late cardiac toxicity from radiation therapy. While this has been heavily studied in left-sided breast cancer and thoracic malignancies, there is less data on right-sided breast cancer treatment as historically the dose to the heart from 3D conformal radiotherapy (3D CRT) is very low. With advanced modalities such as intensity modulated radiation therapy (IMRT) and deep inspiratory breath hold (DIBH) becoming more commonly used, we sought to perform a dosimetric analysis incorporating these modalities to evaluate the relative benefits of these advanced technologies compared to standard delivery with free-breathing (FB) and 3D CRT. Materials/

Methods: Four consecutive patients with right-sided breast cancers were included in this analysis. Two were treated to whole breast and regional lymphatics, two were treated to whole breast alone. Organs at risk (OAR’s) included lungs, heart, left and right ventricles, left anterior descending artery (LAD) and right coronary artery (RCA). Structures were contoured on FB and DIBH scans and 3D CRT and IMRT plans were generated on both scans as well. Dose volume histogram (DVH) metrics were compared between the plans.
Results: The use of DIBH resulted in decreased mean heart dose compared to all FB when IMRT was used, however, there was less difference when 3D CRT was used. All dose metrics (D0.03, mean and V5%) to the RCA and right ventricle were higher when using IMRT compared to 3D CRT. Dose to the LAD was particularly higher when treating with IMRT with mean V5 doses between patients over 50%. Total heart mean, D0.03 and V5 were consistently higher when using IMRT vs. 3D CRT. For all heart metrics, there was still a benefit to using DIBH comparing 3D CRT to 3D CRT and IMRT to IMRT. Lung metrics including right lung mean, total lung mean and total lung V20 were lowest when using 3D CRT and DIBH. Right lung V20 was not substantially improved with DIBH.
Conclusion: DIBH improved heart and lung metrics when using 3D CRT or IMRT for radiotherapy to the right breast. When comparing 3D CRT to IMRT, the heart metrics including D0.03, V5 and mean dose to the RCA, LAD, left and right ventricles, and total heart were all improved with 3D CRT planning. Contouring and DVH analysis of heart substructures, included left-sided structures, should be included as standard OAR’s for right sided breast treatments particularly when treating with IMRT as low-dose spillage could be overlooked if only mean heart dose is analyzed.