COVID-19, mainly transmitted through the respiratory tract, and the lungs are one of its main attack organs. On the other hand, lung cancer is known worldwide for its high morbidity and mortality, and is the number one killer of malignant tumors. Fortunately, clinical studies have demonstrated that early screening for specific populations with low-dose spiral CT (LDCT) technology can significantly reduce the mortality rate of lung cancer, and this effective method has been recommended by professional guidelines in several countries2. However, the emergence of COVID-19 has brought new challenges to the medical field. The CT imaging characteristics of some patients with COVID-19 were similar to those of early lung cancer, presenting as ground glass or solid nodules.This undoubtedly increases the difficulty of their differential diagnosis and the difficulty of treatment. Studies have shown that the late stage of COVID-19 infection may lead to an increase in the number of pulmonary nodules.3 However, it is not clear whether COVID-19 will lead to increased lung nodules and whether it will lead to lung cancer. This field still needs to be studied in depth so that we can more accurately understand the long-term effects of COVID-19 on lung health.
This study showed that the detection rate of pulmonary nodules in the post-COVID-19 epidemic group was higher than that in the pre-epidemic group.This indicates that the prevalence of COVID-19 leads to the increased detection rate of pulmonary nodules. Our study also found that the increased nodules were mainly solid nodules,and the subjects of these increased pulmonary nodules were mainly concentrated in the subjects over 51 years of age. Studies have shown that in the early stage of COVID-19, the lesions are mainly located in the lung periphery or under the pleural area, and the ground glass nodules are more common, which is related to the formation of transparent membrane in the alveolar wall4. In the progressive stage of the disease, the distribution of the lesions becomes wider, the ground glass nodules can coexist with consolidation opacity and linear opacification, and may be accompanied by lobular septal thickening, The pathological mechanism may be related to the early novel coronavirus involvement of the alveolar wall5.Severe patients usually progress to diffuse lesions in multiple lobes and segments of both lungs, mainly consolidation combined with ground glass nodules and linear opacities. Entering the remission period, the lesions in the two lungs gradually dissipated and relieved. CT showed that the ground glass lesions and their scope decreased, but some of the lesions did not dissipate and remained in the form of solid nodules, which may be related to age and immune function, and it is in line with our findings. A study of 1953 patients with COVID-19 found that patients had elevated levels of IL-6, IL-8, TNF-a, and IL-1B. They show that the novel coronavirus can induce the activation of the immune response, often referred to as a "cytokine storm"6. Another study demonstrated increased numbers of Th 17 and highly pro-inflammatory CCR 6 + in CD4 T cells in COVID-19 patients was associated with the pathogenicity of the COVID-19 cytokine storm7. These mechanisms may help to explain how COVID-19 induced pulmonary nodules. The reason for the increased detection rate of pulmonary nodules in subjects over 51 years may be related to the decreased immunity of increased age. Therefore, we believe that the increase of pulmonary nodules after the COVID-19 epidemic is mainly solid pulmonary nodules, and mainly concentrated in the elderly population with low immunity.
Our study found that there was no increase in suspected lung cancer nodules in the pre-epidemic versus post-epidemic group. Through literature search, we found that there are no relevant reports of lung cancer caused by COVID-19, which is in line with our study results. However, some studies have shown that lung cancer is a risk factor for COVID-19-related pneumonia. IL-6 levels are significantly higher in lung cancer patients with COVID-19, and these factors induce a cytokine storm and lead to acute respiratory distress syndrome, leading to the shorter mean survival time of COVID-19 patients with lung cancer8. Therefore, we believe that novel coronavirus pneumonia does not lead to lung cancer occurrence.
Before the COVID-19 epidemic, the Chinese consensus on pulmonary nodule screening recommended conducting chest CT examination in people aged from 50 to 74 years old. Our study found that the post-epidemic group had more ≤ 40 year young people involved in screening for chest CT9. After the COVID-19 epidemic, most people have cough, fever and other symptoms, so more young people choose to do chest CT to search for the cause of cough. Chinese COVID-19 prevention guidelines take chest CT examination as one of the indicators for the diagnosis of COVID-19 infection, which also leads to more young people willing to do chest CT examination10. These factors may lead to significantly more chest CT screening rate among people younger than 40 years after the COVID-19 outbreak than before the outbreak.
The results of this study revealed that the detection rate of pulmonary fibrosis in the post-epidemic group was higher than that in the pre-epidemic group. The vast majority of severe COVID-19 survivors harbor some sequelae, including pulmonary, neurological, psychological, and cardiovascular complications. The lungs are the main damaged organ of the Novel coronavirus, so the major long-term complication after COVID-19 is pulmonary fibrosis. The study showed that chest CT examination of patients who recovered from COVID-19 infection found that 38% of patients had persistent interstitial pulmonary abnormalities, mainly manifested as ground glass shadow (GGO) and spinal cord consolidation and physiological injury, and some lesions gradually evolved into pulmonary fibrosis, which persisted during follow-up11. The cause of pulmonary fibrosis formation maybe related to the physiological injury12.
Our study has several limitations. First, this study is observational and requires further prospective studies to confirm, but it may not have this opportunity later. Secondly, our dataset is based on hospital physical examination data and is not fully representative for the entire population. Third, although we were a multi-center study, but we only included data from three hospitals, and all the three hospitals were in Chengdu, China, which can not representative for other regions. Fourth, the main results of the study are evaluated by CT examination, and some of the results are judged by doctors, without the pathological support, so there may be deviation.