Although median sternotomy is still accepted as the standard treatment for thymomas, minimally invasive resections, mainly including VAT and RAT, are becoming increasingly popular in clinical practice due to their great advantages in terms of a significantly reduced blood loss, less postoperative pain and rapid recovery. Larger thymomas (≥5 cm) or tumors involving major vascular infiltration with a substantially increased operative risk were once considered an absolute limiting factor for minimally invasive approaches [12, 13]. However, with the widespread adoption of the minimally invasive approach, several studies have reported that both VAT and RAT are safe and effective approaches for large thymomas with comparable surgical and oncological results to open surgery [14, 15]. Furthermore, robotic surgery, which not only offers similar advantages but also overcomes the technical limitations of VAT, has been regarded as a preferred approach in some studies [16]. However, due to the lack of comparison between the two approaches in the treatment of large thymomas, the choices of surgical approach have usually been based on the experience and preference of the surgeon and the patient's willingness thus far. Therefore, we designed the present study to explore the more appropriate approach for larger thymomas. To our knowledge, this study is the first to compare the perioperative outcomes of RAT and VAT for the treatment of large thymomas based on a propensity-matched analysis.
In the current study, we show that robotic thymectomy can achieve a similar complete surgical resection with free margins compared with a video-assisted procedure with comparable low postoperative complications and zero mortality. Although the robot set-up and docking time were included, a definite trend toward a reduction in the operative time was still easily observed in the robotic thymectomy group. Moreover, intra-operative blood loss volume shows more favorable result in the RAT group as well.
Consistent with previous studies, we defined a large thymoma as a tumor ≥ 5 cm in the current study [12, 14, 17]. The indicators of minimally invasive thymectomy (MIT) for a relatively large thymoma remain controversial. Most of studies indicate that tumors with a diameter < 5cm are oncologically safe and MIT is technically feasible [18]. Nonetheless, several investigators have performed MIT to treat large thymomas and documented that radical resection can be performed safely and effectively [15, 19]. In the present study, radical resection was performed on all patients, and intraoperative injury and perioperative mortality were not observed. Therefore, from our perspective, tumor size is not an absolute contraindication for MIT. In contrast, MIT, particularly the RAT, may provide excellent visualization for exposing the boundary between the tumor and healthy tissue, which is convenient for the resection of the invaded structures, even the great vessels. This result is similar to the study by Weng et al. which indicated that conversion to an open approach seems to mainly depend on tumor invasion to the great vessels regardless of the tumor size [14], and is consistent with our earlier study as well [20].
Although several studies have demonstrated that the VAT approach can achieve complete resection of large and advanced thymomas without conversion, the comparatively higher conversion rates in the current study might indicate that some technical limits of conventional thoracoscopy exist during the dissection of larger anterior mediastinal tumors. In the two-dimensional view of the operative field and under the physiologic hand tremor, the upper mediastinum with vulnerable large vessels and nerves becomes a delicate and difficult-to-dissect area [21]. Despite relatively proficient operation experience in thoracoscopic thymoma resection, surgeons can scarcely produce a fundamental change in these inherent limitations. By providing magnified 3D vision, flexible and finer instrument control and a stable operating system, the robotic surgical system overcomes several technical limitations of conventional thoracoscopy and is advantageous in dissection of larger thymomas requiring combined resection of adjacent structures. In fact, these advantages, which enable a better evaluation of the boundary between the tumor and healthy tissue and facilitate a more precise and low-risk dissection, have significantly increased the safety and expanded the indications of thymectomy for thymoma. Importantly, we preferred attempting RAT first for almost every patient with large thymomas; however, an open approach would be decisively performed if complete resection could not be achieved during the operation, especially for patients with invasion of the aorta, pulmonary artery trunk, and atrium. Certainly, the initial robotic approach which helps surgeons dissect the thymoma and visualize areas of tumor invasion to facilitate subsequent open surgery should not be considered useless.
According to previous studies, the three main surgical approaches for thymoma include the unilateral thoracic cavity (left or right), bilateral thoracic cavity, and subxiphoid process. In the present study, the right-side thoracic approach was preferred in both groups, except for tumors located predominantly in the left mediastinum. Compared to the left-sided approach, the right-sided thoracic approach can be quite beneficial in avoiding interference from the heart and the aortic arch, and the left innominate vein may be identified and followed easily based on the visualization of the superior vena cava (SVC). Recently, several studies have documented the safety and feasibility of subxiphoid thymectomy for surgical resection of advanced thymomas, which allows the surgeon to visualize both phrenic nerves and reach high into the anterior mediastinum under direct vision [10, 22–24]. Since 2015, subxiphoid thymectomy has been gradually performed in our center, achieving comparable surgical and oncological results to transthoracic surgery. Sufficiently complete removal of the upper poles of the thymus in subxiphoid thymectomy has made it one of the routinely selected procedures for large thymomas at present.
The potential merit of RAT in terms of enhanced recovery after surgery has already been noted in patients with early-stage thymomas [25]. Ye et al. analyzed the perioperative outcomes of patients with Masaoka stage I thymoma and found that robotic thymectomy was associated with a significantly shorter duration of chest drainage; and a shorter length of hospital stay [26]. Similar results were reported by Qian et al. in their study comparing three approaches for the treatment of early-stage thymomas [7]. However, our study showed that no difference in perioperative outcomes between patients who underwent RAT and those who underwent VAT. Even the total amount of postoperative drainage in the VAT group was less than that in the RAT group. We speculated that the primary cause of the discrepancy was a consequence of the relatively high proportion of patients requiring concomitant resection in the RAT group in the current study, implicating a potential need for additional manipulations and intraoperative dissections. In addition, omitting chest tube placement after surgery, which was more common in the VAT group, might be another contributing factor to this situation. Considering the higher rate of concomitant resection and relatively complex procedures performed in the RAT group, the similar postoperative parameters indicate that robotic thymectomy might have an advantage over VAT in postoperative rehabilitation, although further research is needed to confirm that.
In the present study, the retrospective nature, limited number of patients and single institution constituted major limitations. Especially, the decisions to perform excision with the use of VAT or the robotic system were not random and might lead to the selection biases. Although PSM was performed to improve comparability between the two groups, the results might be affected by the smaller sample sizes and potential biases. In addition, the follow-up was confined to the perioperative period, it was still inadequate to allow a definitive conclusion on oncologic outcome. Therefore, multicenter and randomized controlled studies comparing RAT and VAT in patients with large thymomas are urgently warranted.