Study design and population
The protocol of the study has been described elsewhere (21). Briefly, between October 2015 and December 2017, this prospective monocentric study included 113 BC female patients aged 40 to 75 years, with left or right unilateral BC, who underwent RT after breast-conserving surgery, without chemotherapy, at the Clinique Pasteur (Toulouse, France). Patients with metastatic BC, prior thoracic RT, history of severe cardiovascular disease such as coronary artery disease including acute myocardial ischemia or infarction, renal failure, allergies to iodinated contrast injection, and pregnancy were excluded.
Patients were included at baseline before RT with follow-up visits at 6 and 24 months after RT, and the follow-up period of the last patient ended in January 2020. The patient's medical history was collected at baseline, and cardiac examinations, particularly echocardiography examinations, were performed by cardiologists during the programmed visit at baseline (i.e before RT), 6 and 24 months after RT. After the exclusion of the patients with incomplete echocardiographic follow-up data, the study population presented here consisted of 72 patients with complete echography measurements (both LVEF, and GLS measurements) at baseline, six months, and 24 months after RT, as well as cardiac radiation dosimetry data.
Radiotherapy
All patients were treated with three Dimensional Conformal Radiation Therapy (3D-CRT) with 6 and 25 MV photon beams by tangential fields second to the surgical treatment. Two modalities of the planning target volume dose were applied. The first modality was to administer 50 Gray (Gy) delivered in 25 daily fractions of 2 Gy over five weeks or 47 Gy delivered in 20 daily fractions of 2.35 Gy over five weeks. This second fractionation choice was only driven by the need to slightly limit the number of sessions per patient due to the technical problems that arose in one 3D-CRT machine from January 2016 to May 2016. Six MV photons were used for most patients, except for a few patients with big breast sizes, where 25 MV additional photons were used. An extra 9–15 Gy boost might be applied to the tumor site using photon/electron beams with energies ranging from 6 MeV to 18 MeV. The treatment planning system (TPS) applied to conduct dose calculations was Eclipse™ with the Analytical Anisotropic Algorithm (AAA version 13.6) (Varian Medical System, Palo Alto, CA, USA). Each patient's RT was planned in a manner in which the dose distribution was optimized and normalized to the International Commission on Radiation Units and Measurements (ICRU) reference point of the breast cancer and to achieve QUANTEC dose constraints to organs at risk, including the whole heart(22).
Cardiac dosimetry
The methods of evaluating cardiac radiation doses in the BACCARAT cohort were already described elsewhere (21, 23). Clinic Pasteur RT department provided the Dose-Volume Histograms (DVHs) for the whole heart. The delineation of the other cardiac structures (left ventricle (LV) and left coronary artery, including left anterior decreasing coronary artery (LAD), circumflex coronary artery (Cx), and right coronary artery (RCA)) were performed manually. Using the 3D dose matrix made during planning treatment and the delineated substructure, DVH for cardiac structures was produced with ISOGray TPS by the dosimetry department of IRSN in partnership with the Clinic Pasteur RT department. This study focussed on left ventricular cardiac dysfunction (CTRCD), and we thus focussed on the following heart substructures: the whole heart, left ventricle, and the left coronary artery that carries blood to the left part of the heart (LAD and CX). The following absorbed dose metrics for the cardiac substructures were collected: Dmean (in Gy), the volume-weighted mean dose; D2 (in Gy), the minimal dose absorbed by the most irradiated 2% of the structure volume, which can be considered as a substitute for maximum dose; V2 (in %) the relative volumes exposed to at least to 2 Gy.
Echocardiography
An exhaustive 2-dimensional (2D) echocardiography examination was done at baseline before RT, at six months, and 24 months after RT using a commercially available ultrasound Acuson S2000 (Siemens Medical Solutions USA, Inc. Malvern, USA), using a 3 MHz cardiologic probe transducer. The echographic data were independently analyzed by a single-blinded cardiologist unaware of the patient's medical history and clinical examination. Biplane Simpson's method from apical two- and four-chamber windows was used to measure LV ejection fraction (LVEF). Strains were calculated using 2D speckle tracking echocardiography (2D-STE) and the automated function imaging technique for tracking acoustic markers (speckles) (24). We used the 2DSTE vendor offline Syngo Velocity Vector Imaging 2.0 software (Siemens Medical Solutions USA, Inc., Moutain View, Calif, USA). Based on the American Society of Echocardiography guidelines, images were analyzed in a 16-segment model (25). All segmental values were averaged GLS, in %.
CTRCD events definition
Based on ESC guidelines in cardio-oncology (20), symptomatic CTRCD was defined by the presence of heart failure (incapacity of the heart to pomp enough blood to the tissues) and asymptomatic CTRCD was defined as follows:
- Severe CTRCD: New LVEF reduction to < 40%
- Moderate CTRCD: New LVEF reduction by ≥ 10% points to an LVEF of 40-49%
OR
A new LVEF reduction by < 10% points to an LVEF of 40-49% AND a new relative decline in GLS by > 15% from baseline
- Mild CTRCD: LVEF ≥ 50% AND new relative decline in GLS by > 15% from baseline
In the present study, "CTRCD" was defined for any grade asymptomatic CTRCD observed at 6 or 24 months after RT; "early CTRCD" for any grade asymptomatic CTRCD observed at six months and "mid-term CTRCD" for any grade asymptomatic CTRCD observed at 24 months. Grade of CTRCD ("mild CTRCD"; "moderate CTRCD"; "severe CTRCD") was defined as the highest grade of asymptomatic CTRCD observed during the follow-up period.
Statistical analysis
Continuous variables are presented with mean and standard deviation or median and range values. Student's t-test or Wilcoxon-Mann-Whitney non-parametric test was used to compare continuous variables when appropriate. Categorical variables are reported as counts and percentages. The χ2 and Fisher's exact tests were used to compare categorical variables, as applicable. To analyze the associations between CTRCD and radiation and non-radiation factors in univariable analysis, we used binary logistic regressions (odds ratios (ORs), 95% confidence intervals (CI), p-values). Cardiac radiation exposure factors included the laterality of BC, Dmean, D2, and V2 for the whole heart, left ventricle, left anterior decreasing coronary artery (LAD), and circumflex coronary artery (Cx). Non-radiation factors included age, body mass index (BMI), smoking status, hypertension, diabetes mellitus, hypercholesterolemia, menopausal status, and endocrine therapy. For multivariable analysis, we only considered for adjustment the non-radiation variables with p-value < 0.20 in univariable analysis. For dosimetry parameters significantly associated with the risk of CTRCD, receiver operating characteristic (ROC) curves were constructed, and the maximum Youden Index was utilized to determine the optimal cut-off values and thresholds for predicting CTRCD. A p-value <0.05 was considered statistically significant in all two-sided statistical tests. All analyses were performed using SAS 9.4 and STATA 14.2 software.