It has been reported that radiation has various effects on the heart [8]. For patients with esophageal cancer, it is no longer uncommon for patients who have undergone radiotherapy to survive for a long time, but the adverse effects of radiation on the heart have become a problem and improvements are being hoped for. In the radiotherapy group, the cumulative heart-related death rate was significantly lower in patients treated in 2010–2015 than in patients treated in 2004–2009. This may be partially due to the short observation period for the group treated in 2010–2015. Although there was a significant difference in the cumulative heart-related death rate in 2004–2009 between patients with carcinoma in the middle thoracic to abdominal esophagus and patients with carcinoma in the cervical to upper thoracic esophagus, there was no longer a significant difference in 2010–2015. It is possible that recent advances in radiation techniques have resulted in the prevention of radiation-induced heart disease. Although heart-related deaths after surgery have not decreased in the new era, the fact that heart-related deaths after radiation therapy have decreased indicates that the effects of radiation on the heart have decreased. In fact, Sakar et al. reported that patients receiving IMRT had a 68% decreased risk of having a late cardiovascular event compared to the risk in patients receiving conventional radiation [9]. However, in our previous prospective cohort study in which patients were treated for esophageal cancer by 3D-CRT between 2013 and 2015, the 5-year and 7-year cumulative incidences of grade 3 or 4 cardiac events were 14.7% and 22.4%, respectively [10]. Serious radiation-induced heart diseases occurred in some patient in that study, and although IMRT has recently been used for treatment of esophageal cancer also at our institution, it has not yet been determined whether IMRT can reduce the incidence of heart disease. A past report suggested that a volume of the left ventricle receiving greater than or equal to 15 Gy (V15) of less than 1% appeared to confer a decreased risk of major adverse cardiac events among patients with coronary heart disease [11]. The Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC) recommended that V30 of the heart should be kept below 46% with a mean heart dose of less than 15 Gy [12]. Unfortunately, although grade 3 or higher cardiac events occurred in 19.8% of patients who were treated by IMRT or proton beam therapy in MD Anderson Cancer Center [13], it seems not to be significantly lower than the rate with 3D-CRT in our institution [10].
In the US, the standard strategy is preoperative chemoradiotherapy followed by surgery, so this analysis only included patients who did not receive the standard strategy. In the surgery group, in which patients who had undergone radiotherapy were excluded, it is expected that there would be many cases with some kind of complication. However, in patients with carcinoma in the middle thoracic to abdominal esophagus, which is thought to result in greater radiation exposure to the heart, the rate of cardiac death in the radiotherapy group was significantly higher than that in the surgery group, which seems to indicate effects of radiation on the heart. It is known that there are risks of cardiac complications, such as advanced age, obesity, drinking alcohol or smoking, in many patients with esophageal cancer, suggesting that special attention should be paid to radiation exposure to the heart. In fact, there are reports showing that reducing the radiation dose to the heart reduces the frequency of serious heart problems in patients in high-risk groups [13].
Our analysis of data in the SEER database showed differences in heart-related deaths among races in patients treated by postoperative radiotherapy for early breast cancer. That analysis showed that Asians or Pacific Islanders and Native Americans had fewer heart-related deaths after radiotherapy. However, there were no differences between races in the present study. It is unclear whether this is due to differences in the backgrounds of breast cancer and esophageal cancer patients, differences in radiation doses for the heart between breast cancer and esophageal cancer, or both [14].
As a limitation of this study, the SEER database does not have information on irradiation dose for the heart or complications, and the results of this study therefore provide only indirect evidence of radiation effects on the heart. Furthermore, disease of the heart as causes of death are not classified in the SEER database; therefore, the specific nature of the fatal heart disease is unknown.
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