Low predictive value of mean heart dose for coronary artery dosimetry in breast cancer radiotherapy
Résumé
Purpose or Objective
In many studies that investigated radiation-induced
cardiac toxicity of breast cancer radiotherapy, doses are
described as those received by the entire heart and the
mean heart dose is used as the reference dose for
analyzing dose-response relationship. However the
specific relationships between doses to cardiac substructures, in particular coronary arteries, and subsequent
toxicity have not been well defined. Detailed individual
dosimetry information for the heart and its sub-structures
is required to better understand cardiac damage from
radiation exposure. The aim of this dosimetric study was
to analyze the distribution of individually-determined
radiation exposure, in a population of breast cancer
patients treated with three dimensional conformal
radiation therapy (3D-CRT), and clarify whether mean
heart dose is a good surrogate parameter for the dose to
coronary arteries, in particular the left anterior
descending artery.
Material and Methods
Patients with left or right unilateral breast cancer (BC)
treated with 3D-CRT between 2015 and 2017 were
included (BACCARAT clinical study). Before RT, a coronary
computed tomography angiography (CCTA) was
performed. Registration of the planning CT and CCTA
images allowed precise delineation of the coronary
arteries on the planning CT images. Using the 3D dose
matrix generated during treatment planning and the
added coronary contours, dose distributions were
generated for the following cardiac structures: whole
heart, left main coronary artery (LMCA), left anterior
descending artery (LAD), left circumflex artery (LCX) and
right coronary artery (RCA). A descriptive analysis of the
physical doses in Gray (Gy) was performed.
Results
Dose distributions were generated for 89 left-sided BC and
15 right-sided BC patients. The treatment schedule with
tangential beams was either 50 Gy delivered in 25
fractions of 2 Gy or 47 Gy in 20 fractions of 2.35 Gy, with
or without irradiation of regional lymph nodes. Additional
beams to tumor bed (boost) were used, if clinically
indicated. The mean heart dose (Dmean Heart) was 2.9 ±
1.5 Gy for left-sided BC and 0.5 ± 0.1 Gy for right-sided
BC. For left-sided BC patients, the mean ratio Dmean
LAD/Dmean Heart was around 5. All other ratios were
below 1 except for RCA in right-sided BC patients
(ratio=2.7). However, the coefficients of determination R²
indicated that the proportion of the variance in Dmean
LAD or Dmean RCA predictable from Dmean Heart was low
(R²=0.45 and 0.36 respectively). For left-sided BC patients
with lower exposure (Dmean Heart<3Gy), 56% of patients
received doses > 40Gy to 20% of the LAD volume on
average (V40Gy).
Conclusion
Our study illustrates that the predictive value of the mean
heart dose was not good enough for coronary arteries, in
particular for LAD, illustrating the importance of
considering the distribution of doses within these cardiac
substructures rather than just the mean heart dose to
enhance knowledge on the risk of radiation-induced
cardiotoxicity in breast radiotherapy.