Four cases of unique thoracic duct anatomical characteristics identified by fluorescence thoracoscopy.

Introduction
Thoracic duct (TD) exhibits significant anatomical variations, and several collateral branches join the duct during TD’s course. These collateral branches can be injured when performing mediastinal lymph node (LN) dissection, leading to postoperative chylothorax [1, 2]. Since October 2023, the Department of Thoracic Surgery at Beijing Hospital has applied fluorescence thoracoscopy for TD visualization in right lung cancer surgery to identify and manage intraoperative injuries to the branches, thereby preventing postoperative chylothorax. During these procedures, we encountered a unique anatomical variation of the TD in the upper mediastinum.

Cases report
Four patients exhibited similar anatomical characteristics of TD in the upper mediastinum. Among them, three lung cancer patients underwent pulmonary resection with mediastinal LN dissection, while one patient with a benign nodule received wedge resection. The patient of Case 1 received two cycles of combination chemotherapy and immunotherapy prior to surgery. Detailed information was presented in Table 1. The fluorescence thoracoscopy system used in Case 1 was from the brand Optomedi, and DePuy system in Case 2–4.
Approximately 30 min preoperatively, 25 mg of indocyanine green was injected subcutaneously into the right inguinal region. Intraoperatively, fluorescence thoracoscopy clearly visualized the TD. The TD demonstrated the following anatomical features (Fig. 1A–D): The main trunk of TD ascended along the right posterior mediastinum to the root of the right neck. At the level of the azygos vein arch, a lymphatic vessel was observed originating from the posterior mediastinum, running closely along the upper edge of the azygos vein arch, and then turning upward along the anterior or posterior edge of the superior vena cava to enter the right root of the neck. During its course, collateral branches could be seen connecting to the 2R+4R LN region and the anterior mediastinum.
In Case 1, collateral branch injuries with contrast leakage occurred in stations 2R+4R and 7 during LN dissection (Fig. 2A, B). Intraoperative TD ligation was performed. In Case 2, intraoperative pathological assessment identified minimally invasive adenocarcinoma, prompting selective nodal sampling with avoidance of stations 2R/4R dissection to circumvent potential damage to the adjacent TD’s branch. In Case 3, LN dissection was performed under real-time guidance of fluorescence thoracoscopy, and no contrast leakage was observed. In Case 4, the frozen section of the lung nodule indicated a benign lesion, and no LN dissection was performed. All four patients recovered smoothly after surgery, with no complications.

Discussion
Nine variations of the TD trunk were proposed [3], and the most common type accounted for 63%, which was as follows: TD entered the right thoracic cavity, ascended along the posterior mediastinum to the level of the 4th or 5th thoracic vertebra, then crossed to the left thoracic cavity anterior to the vertebral body and ascended to the left root of the neck. In such patients, TD can only be visualized below the level of the azygos vein arch under fluorescence in right thoracic cavity. Our department has applied fluorescence thoracoscopy for TD visualization in over 200 right lung surgeries. The anatomical characteristics of TD described in this study were the most unique type we have identified, and no similar cases have been reported in previous literature.
Some researchers reported that the most common site of chylous fistula in patients with postoperative chylothorax after lung cancer surgery was the station 4R LN region [4]. In cases of this study, TD gave off a lymphatic branch at the level of the azygos vein arch, which ran anteriorly. Collateral branches could be present in the upper mediastinal LN region and the anterior mediastinum. These patients were at high risk of lymphatic branch injury during right thoracic surgery or even anterior mediastinal surgery. However, the anatomical course of TD in the left thoracic cavity of these patients were unknown, which limited our understanding of their overall TD anatomy. Knowledge about TD and its branches has progressed slowly these years. The cases described above remind thoracic surgeons to strengthen research on more detailed anatomy of TD in the future.

Conclusion
The TD’s collateral branches demonstrate unique anatomical complexity. Fluorescence imaging enables precise intraoperative identification, improving surgical safety and postoperative outcomes.