Heart Substructures Delineation in Lung SBRT: Central and Ultra-Central Lesions

et al., Heart Substructures Delineation in Lung SBRT: Central and Ul-tra-Central Lesions. Biomed J Sci & Tech Res 40(1)-2021. BJSTR. MS.ID.006408.

publications try to elucidate whether the doses received by the different substructures of the heart impact on non-specific cancer mortality or increase cardiovascular events [8][9][10][11]. This study aims to report the doses to the different substructures of the heart and the cardiac toxicity associated with SBRT treatment in patients with central lung lesions treated in our centre.

Materials and Methods
From 1 April 2018 to 31 December 2020, 28 patients with 30 central lesions were treated with the SBRT technique with radical intent. Central and ultra-central lesions were defined according to RTOG 0813 criteria. Lesions located 2 cm from the mediastinum or main bronchus were defined as "central" [12].
Different immobilization systems (abdominal compression, vacuum cushion or head-neck-shoulders mask were used for treatment. ITV was defined by the 4D RPM-Varian™ system. PTV was obtained by creating an isotropic (uniform) expansion of the ITV. Two fractions were used: 60Gy in 8 fractions or 50-55Gy in 5 fractions. UK consensus constraints were applied. For our study, 17 cardiac substructures were retrospectively contoured following the atlas of Feng, et al. [13] The atlas of Loap et al was also used for the contouring of the atrioventricular (AV) node [14]. Cardiac substructures were manually contoured by the same radiation oncologist (MRP) to avoid inter-observer variability.
Contouring was performed in a CT without contrast. The following substructures were contoured: the cardiac chambers (right and left atrium, right and left ventricle), the superior and inferior vena cava, the pulmonary artery, the aortic arch, the ascending and descending aorta, the four cardiac valves (mitral, tricuspid, pulmonary and aortic), the left main coronary artery (LMCA), the left anterior descending coronary artery (LADCA), the circumflex artery, the right coronary artery (RCA) and the AV node. Subsequently, the dosimetry in these substructures and the cardiac toxicity of the patients were reviewed retrospectively according to CTCAE v5.0 grading.

Results
Ten patients with early-stage non-small cell lung cancer, 15 metastatic lesions and 5 non-biopsied lesions suspected of PET-CT malignancy were analysed. Of these lesions, 26 were central and 4 ultra-central. The mean follow-up was 16 months (3-33).
The mean age of the patients was 70 years (52-89). The mean GTV/PTV volume was 26/57.6cc (0.8-100/6.1-219). The baseline characteristics of the patients are described in Table 1. Once the substructures were contoured, the doses were reported at 0.5cc (D0.5cc) of each of them. D0.5cc and mean dose in heart were also reported (e.g. Figures 1 & 2). Table 2 and      (Table 3).

Discussion
Different atlases have been developed for several disease sites in order to standardize target delineation. It is known that organat-risk delineation is also important. The heart, along with the main bronchus, is one of the main risk organs to consider in pulmonary SBRT of central and ultra-central tumours. For this reason, different studies have aimed to standardize contouring this organ and analyse the possible cardiac toxicity associated with this treatment [8][9][10][11].
When analysing the results obtained in the studies, we must take . This group also concluded that inter-observer differences are greater in small-volume structures such as the subsegments of coronary arteries. Other authors have also reported the same findings [13,15].
In our study, the atlas of Feng et al. was used for two main reasons: to reduce inter-observer variability associated with the segmentation of small volumes (such as the subsegmentation of coronary arteries) and to report the doses of the 4 cardiac valves due to their possible involvement in cardiovascular events. The growing interest in knowing the possible cardiac toxicity associated with radiation therapy treatment has resulted in the development of software that allows for automatic segmentation of the heart (self-segmentation/auto-contouring). In some studies this software has already been used to contour cardiac substructures [8,10]. Some authors already report data on its use.

Conclusion
There were no deaths secondary to cardiovascular events with the doses received in the cardiac substructures Constraints employed in the heart were safe. The contouring of the cardiac substructures do not have a routine clinical task. Furthermore, no consensus for each heart substructure constraints has been met yet.
Knowing the dose that each substructure of the heart can tolerate may be useful for the treatment of central and ultra-central lesions in certain clinical situations (e.g. multiple lesions or re-irradiation).
Prospective trials and long follow-up are necessary to elucidate possible cardiac toxicity taking each substructure of the heart into account.

Conflicts of Interest
The authors declare no conflict of interest.