Session Item

Breast cancer (BC) is the most common female cancer worldwide and the incidence is increasing. Due to improved treatment and the implementation of mammography screening, the prognosis has increased during the last decades, thus the 10-year survival is more the 90% in the developed countries. Radiation therapy (RT) plays an important role in the treatment of early BC to reduce the risk of recurrence and improve overall survival. However, BC RT may cause some incidental RT dose to the heart with a subsequent risk of heart disease. These studies were based on patients treated before the implementation of CT-based RT planning. Each patient's RT plan was reconstructed on a CT scan representing a woman with standard anatomy and the mean heart dose (MHD) was estimated. The relative risk of heart disease was 7.4% for each 1Gy increase in the MHD.

In recent years, several changes in the treatment of early BC patients occurred. First; CT-based RT planning gradually replaced 2D-based RT, second; RT fields became less extensive and today, tangential fields are widely used. Third; the heart was delineated as an organ a risk, giving higher attention to cardiac exposure. Fourth; the implementation of mammography screening resulted in more women being diagnosed with node-negative disease, decreasing the need for regional node irradiation. Finally; respiratory-gated RT was introduced aiming to reduce lung and heart dose without compromising target dose.

Along with the changes in RT, the MHD has decreased substantially for both left and right-sided irradiated patients. However, RT dose delivered with tangential fields is highly heterogeneous and focus on RT dose to the cardiac substructures has increased and questioned the ability of MHD as the best predictor for heart disease. In a Dutch study containing 910 BC patients treated with CT-based RT techniques, 30 patients developed heart disease during a median follow-up of 7.6 years. For MHD, the relative increase per Gy was 6.4%. However, the volume of left ventricle receiving 5Gy or more was considered as a better predictor for heart disease than MHD, as it reflects the inhomogeneous dose distribution in tangential RT and showed a 16.5% increase risk per Gy. In a cohort study of 30.000 Danish BC patients irradiated between 1999 and 2016, an increased risk of heart disease could not be detected within the first 10 years after irradiation for patients treated with CT-based RT planning.

Radiation-induced heart disease is a long-term side effect and what happened beyond 10 years after irradiation is still uncertain. Pre-existing cardiac morbidity is an important factor of subsequent heart disease. Furthermore, calcium deposition in an atherosclerotic plaque estimated before RT has been associated with an increased risk of heart disease. Due to the long latency between RT and symptomatic heart disease, early markers predicting the risk of heart disease would be useful. Several imaging modalities such as single-photon emission computed tomography scans, cardiac magnetic resonance imaging or echocardiography and biomarkers as pro-B-type-natriuretic peptide and troponin have been suggested. However, the most optimal cardiac imaging and biomarkers are not fully established and may depend on new knowledge regarding RT dose to the cardiac substructures and the risk of heart disease associated with contemporary RT.