Effect of Radiation Dose to Cardiac Substructures on the Acute Development of New Arrhythmias Following Conventionally Fractionated Radiation Treatment to the Lung

The impact of radiation dose on specific cardiac substructures and its relationship with the development of new cardiac arrhythmias in the acute setting (either during the radiation treatment course itself, or within the month immediately following completion of treatment) is not well-established in the existing literature. We aimed to identify patients who experienced acute cardiac arrhythmic toxicities and determine the dose received by the sinoatrial (SA) node and right atrium in these patients. We conducted a single-institution, retrospective review of patients who received at least 45 Gy of external-beam radiation treatment to the lung between October 2013 and June 2017. 193 patients were identified within this cohort, and these patients’ medical records were reviewed for incidence of new cardiac events during or after radiation treatment. Seven patients were identified as having suffered a Grade 3 acute cardiac arrhythmic toxicity requiring inpatient admission, all of whom had no prior history of arrhythmias. Contouring guidelines for regional cardiac anatomy were created, and cardiac substructures, including the SA node and all four main chambers of the heart, were contoured on these patients. An analysis of the dose received by the SA node and the right atrium, two substructures critical to the electrical conduction system of the heart, was subsequently completed on this patient subset. Of the seven patients with an acute cardiac arrhythmic toxicity, the median age was 65 and the median follow-up from conclusion of radiation treatment was 17 months. The median prescribed dose was 60 Gy in 30 fractions. Two of the patients were prescribed radiation for left-sided lung malignancies, and the other five were treated for right-sided cancers. Two of the seven patients experienced cardiac toxicity during the treatment course, while the other five experienced toxicities within the month following conclusion of radiation. Four of the seven patients experienced atrial fibrillation, and the remaining toxicities (one in each patient) were Mobitz Type 2 heart block, multifocal atrial tachycardia, and atrial flutter. Across all seven patients, the mean SA node dose ranged from 1702 cGy to 6778 cGy and the maximum SA node dose ranged from 3157 cGy to 6904 cGy; the mean right atrial dose ranged from 443 cGy to 4057 cGy and the maximum right atrial dose ranged from 3620 cGy to 6938 cGy. Three patients received over 60 Gy of mean SA node dose, maximum SA node dose, and maximum right atrium dose. While acute development of new cardiac arrhythmias following conventionally fractionated radiation treatment to the lung is an uncommon occurrence, increased radiation dose to the SA node and right atrium may predispose patients to this toxicity; further study is warranted to elucidate specific dose limits to cardiac substructures.