br Corresponding author St Luke s Radiation Oncology Network
* Corresponding author. St. Luke’s Radiation Oncology Network, St. James’s Hospital, Ushers Quay, Dublin 8, Ireland. E-mail address: [email protected] (F.K. Duane).
1879-8500/ 2019 The Authors. Published by Elsevier Inc. on behalf of American Society for Radiation Oncology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Practical Radiation Oncology: May-June 2019 Cardiac segment radiation doses breast cancer 159
Methods and Materials: The radiation therapy charts of 470 women with cardiac segment injury after breast cancer radiation therapy were examined, and 41 regimens were identified. Regimens were reconstructed on a typical CT scan. Doses were estimated for 5 left ventricle (LV) and 10 coronary artery segments. Correlations between cardiac segments were estimated. Interpatient dose variation was assessed in 10 randomly selected CT scans for left regimens and in 5 for right regimens.
Results: For the typical CT scan, interregimen segment dose variation was substantial (range, LV segments <1-39 Gy; coronary artery segments <1-48 Gy). In 10 CT scans, interpatient segment dose variation was higher for segments near field borders (range, 3-47 Gy) than other segments (range, <2 Gy). Doses to different left-anterior descending coronary artery (LADCA) segments were highly correlated with each other, as were doses to different LV segments. Also, LADCA segment doses were highly correlated with doses to LV segments usually supplied by the LADCA. For individual regimens there was consistency in hotspot location and segment ranking of higher-versus-lower dose.
Conclusions: The scope for developing quantitative cardiac segment dose-response relationships in patients who had 2-dimensional planning is limited because different segment doses are often highly correlated, and segment-specific dose uncertainties are not independent of each other. However, segment-specific doses may be reliably used to rank segments according to higher-versus-lower doses.
2019 The Authors. Published by Elsevier Inc. on behalf of American Society for Radiation Oncology. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
Breast cancer radiation therapy reduces breast cancer mortality1,2 but may increase the risk of ischaemic 454-29-5 disease (IHD)3,4 by causing macrovascular coronary artery disease or microvascular myocardial disease.5 Most evi-dence that links breast cancer radiation therapy with heart disease is based on women treated in previous decades with outdated techniques. A number of contemporary studies suggest that modern regimens pose a much-reduced risk of radiation-induced heart disease owing to improvements in radiation therapy techniques,6e9 and in some cases a reduction in the prescribed doses.10e12 Nonetheless, radiation-induced heart disease is still likely to be relevant to subgroups of women such as those who cannot tolerate breathing adaptation, have an atypical anatomy, or are undergoing internal mammary chain irradiation.
Currently, doses to small regions, such as cardiac structures, in 3-dimensional computed tomography (CT)-based radiation therapy planning can be modified by changing beam angles or using a different technique. As a result, oncologists often have a choice as to which structures are exposed. The coronary arteries and myocardium have different structures and functions and may respond differently to radiation. Knowing if the dose-response relationship was steeper for radiation-related coronary artery disease or myocardial disease, or whether they were equally sensitive, would be useful to know. However, few studies to date have related coronary artery or left ventricle (LV) segment radiation doses to detailed cardiology information.13,14
In a recent case-control study of 963 women who developed IHD after breast cancer radiation therapy, the best available predictor of IHD was mean heart dose,3 and coronary artery doses were not significantly associated with the rate of IHD events after the mean whole heart dose was taken into account. This may be because the coronary arteries and myocardium are equally sensitive to radiation. Alternatively, it may be due to the strong cor-relations between coronary artery and myocardial doses in breast cancer radiation therapy and the greater un-
certainties in estimated coronary artery doses compared with whole heart doses.15,16
Clinical cardiac disease often occurs years after exposure; thus, studies that relate cardiac doses to radiation-related injury inevitably need to be carried out in patients treated before the era of CT-based radiation therapy planning. Information on cardiac segment injury for these patients can be abstracted from angiogram and echocardiogram reports in their cardiology medical notes. Individual radiation therapy charts can be abstracted from oncology notes, but because these women did not receive CT-planning, segment doses need to be estimated retro-spectively by reconstructing the regimens on a typical CT scan. How reliable these typical segment doses are, and whether they can be used in segment dose-response re-lationships, is unclear.