**4. Discussion**

During the long-term follow-up of 6503 patients undergoing lung cancer surgery, NTM-PD and NTM-positive patients occurred as 2.8% and 5.9% of the 10-year cumulative incidence, respectively. Among the total 59 patients who developed NTM-PD, MAC was the most common pathogen, and the incidence rate of the cavitary disease was 50%. Risk factors related to the development of NTM-PD were an older age, a lower BMI, underlying ILD, bronchiectasis and centrilobular bronchiolitis upon CT imaging, PPCs, and treatment with chemotherapy and radiotherapy. In addition, ever-smokers, a history of pulmonary TB, and thoracotomies were found to be factors that influenced whether a patient was NTM culture-positive.

Although the incidence of NTM-PD varies by region, it has been increasing worldwide, over the past few decades. In a tertiary referral hospital setting in South Korea, one study reported incidence rates of NTM-PD and NTM-positive to be 4.8 and 19.6 per 100,000 person-years in 2016, respectively [26]. In our study, the incidence rates of NTM-PD and NTM-positive results after lung cancer surgery were 1.9 and 5.1 per 1000 person-years, respectively. These incidence rates were approximately 40-fold and 26-fold higher, respectively, than for the general patient population that visits tertiary hospitals in South Korea.

The association between lung cancer surgery and the development of NTM-PD is not well established. There is only one previous study that reported 23 patients with NTM growth in at least one respiratory specimen culture of about 400 patients undergoing lung cancer surgery, and 12 patients met the NTM-PD diagnostic criteria [27]. They analyzed a small number of patients and focused on survival rather than incidence rate, whereas our study provided unprecedented evidence of the NTM-PD incidence after lung cancer surgery by presenting long-term follow-up results using the large-scale cohort data of approximately 7000 patients.

Similar to previous findings that the majority of NTM-PD is caused by MAC [28,29], MAC was the most common NTM species in our study. According to a large study from South Korea, the non-cavitary NB form accounts for about 70% of cases [29]. Compared to this study, we reported that half of the NTM-PD patients had cavity lesions. Considering that cavity lesions in NTM-PD were strongly associated with a poor outcome [29], we sugges<sup>t</sup> that NTM-PD should be intensively monitored in patients after lung cancer surgery.

In our study, an age > 65 years and a BMI ≤ 18.5 kg/m<sup>2</sup> predisposed patients to the development of NTM-PD. These results are similar to those reported by previous studies in the general population [10,30]. Comorbidities with structural lung disease, such as a previous history of pulmonary TB, bronchiectasis, COPD, and ILD, were also wellknown risk factors for developing NTM-PD [15,31,32]. In our study, a previous history of pulmonary TB, ILD, and bronchiectasis were also related to the development of NTM-PD, except COPD. The use of inhaled corticosteroids in COPD patients was a strong risk factor for developing NTM-PD [33]. In our study, most patients diagnosed with COPD were found through preoperative lung function tests, and their lung function was guaranteed to be adequate for surgery. These patients were distinct from those with advanced COPD, who had structural defects and used inhaled corticosteroids. Consequently, COPD should not have emerged as a risk factor in our analysis.

The presence of bronchiectasis or centrilobular bronchiolitis on CT imaging at the time of a lung cancer diagnosis was a related factor for developing NTM-PD in this study. However, some patients with bronchiectasis or centrilobular bronchiolitis on CT images might already have had NTM-PD at that time. A diagnosis of NTM-PD requires repeat tests using respiratory specimens, and the results of these tests need to meet the diagnostic criteria; therefore, NTM-PD detection might be underestimated at the time of a lung cancer diagnosis. Due to our concerns about this very situation, we excluded not only patients who had already been diagnosed with NTM-PD, but also those confirmed to be culture-positive for NTM even once based on respiratory specimens and those with suspicious pathologic results on surgical specimens.

We demonstrated that a PPC was a factor that influenced the development of NTM-PD. This finding suggested that structural defects caused by PPCs might contribute to the development of NTM-PD. Additionally, patients who underwent an open thoracotomy were at a greater risk of developing NTM-PD compared with those who received videoassisted thoracoscopic surgery. We sugges<sup>t</sup> that patients who undergo a thoracotomy are likely to have structural defects present, such as pleural adhesion, which may contribute to the development of NTM-PD after lung resection surgery. Meanwhile, treatment with chemotherapy and radiotherapy increased the risk of NTM-PD, which means that the immunocompromised state that results from chemotherapy and the lung injuries caused by radiotherapy contributed to the development of the NTM-PD [34,35]. To our knowledge, the present study is the first to demonstrate that lung cancer surgery-related factors and PPCs were related to the development of NTM-PD.

This study had several limitations. First, this was a retrospective cohort study of a single institution, which can be a source of selection bias. Second, it is possible that the results were underestimated because a diagnosis of NTM-PD requires repeated AFB cultures and NTM species identifications and usually takes several months to diagnose [22]. Because of this possibility, we included cases suspicious for NTM-PD in the analysis that did not satisfy the microbiological criteria but still reported NTM growth. Previous studies also found that a single NTM growth isolate was associated with the future occurrence of NTM-PD [36]. Despite these limitations, we found that previous risk factors of NTM-PD could be risk factors even in patients who are followed-up for a long time after lung cancer surgery. We also sugges<sup>t</sup> that surgical-related factors and neoadjuvant/adjuvant therapy might influence the development of NTM-PD.
