BACKGROUND: and Purpose: Incidental cardiac exposure during radiotherapy may cause heart disease. Dose-response relationships for cardiac structures (segments) may show which are most sensitive to radiation. As radiation-related cardiac injury can take years to develop, such studies inevitably need involve women treated using 2-dimensional planning, with segment doses estimated using a "typical CT-scan". We assessed whether such segment doses are accurate enough for use in dose-response relationships using the radiotherapy charts of women with known segment injury. We estimated inter-regimen and inter-patient segment dose variability, and segment dose correlations. MATERIALS AND METHODS: Radiotherapy charts of 470 women with cardiac segment injury after breast cancer radiotherapy were examined and 41 regimens identified. Regimens were reconstructed on a "typical CT-scan". Doses were estimated for five left ventricle (LV) and ten coronary artery segments. Correlations between cardiac segments were estimated. Inter-patient dose variation was assessed in ten randomly-selected CT-scans for left-regimens and in five for right-regimens. RESULTS: For the "typical CT-scan", inter-regimen segment dose variation was substantial (range: LV segments <1-39 Gy, coronary artery segments <1-48 Gy). In 10 CT-scans, inter-patient segment dose variation was higher for segments near field borders (range: 3-47 Gy) than other segments (range: <2 Gy). Doses to different 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. CONCLUSION: The scope for developing quantitative cardiac segment dose-response relationships in patients who had 2D-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 dose.
Pract Radiat Oncol