What is the learning curve of the “outside the cage” robotic approach?—a prospective single-center study

Background: Minimally invasive thoracic surgery, including robot-assisted thoracoscopic surgery (RATS), has improved outcomes by reducing postoperative pain. However, intercostal nerve trauma from transthoracic approaches remains a concern. The "outside the cage" (OTC) subcostal robotic approach may reduce pain while maintaining safety. This study aimed to evaluate the learning curve of this technique in major lung and anterior mediastinal resections. Methods: We conducted a prospective single-center study from January 2024 to April 2025. All procedures were performed by a single experienced robotic surgeon using the Da Vinci Xi system. Cumulative sum (CUSUM) analysis evaluated learning curves for docking and console times in two groups: major lung resections and thymectomy/anterior mediastinal mass resections. Results: Of 131 patients, 107 were operated on by one surgeon: 67 underwent major lung resection and 40 thymectomy or anterior mediastinal mass resection. CUSUM analysis of docking time identified three phases-training, improvement (cases 17-33 for thymectomy/anterior mediastinal mass resection, cases 31-38 for major lung resection), and mastery (case 34 onwards for thymectomy/anterior mediastinal mass resection, case 39 onwards for major lung resection)-in both groups. In contrast, no distinct learning curve was observed for console time, suggesting proficiency from the outset. No conversions occurred except one due to vascular invasion. At 30 days, no patients reported neuropathic pain or needed analgesics. Conclusions: The OTC subcostal approach appears safe and rapidly adoptable for experienced robotic surgeons, with a brief learning curve limited to docking maneuvers. These findings support its reproducibility and potential wider adoption of this pain-reducing technique.