The principal finding of the current study was that there were no differences in all-cause mortality risk between the PSP surgery group, PSP non-surgery group, and non-PSP group. To our knowledge, this is the first study to investigate the all-cause mortality risk of patients with PSP based on the treatment undergone compared with those without PSP. To date, the all-cause mortality risk of patients with PSP has not been reported, although mortality is the most important factor in treatment outcomes and prognosis in patients with tumors. In addition, no studies have compared outcomes based on undergoing surgical resection by the patients with PSP. Although surgical resection has been considered a treatment option [6, 14], the results of the present study questions the validity of surgical resection in all cases of PSP. The PSP surgery group did not show better survival than the PSP non-surgery group in our study. The two groups had similar baseline characteristics, including age, sex, smoking status, comorbidities, tumor size, and lymph node enlargement. Even the PSP non-surgery group showed a comparable all-cause mortality risk to age-, sex-, and smoking status-matched non-PSP control group.
The small impact of PSP on all-cause mortality might be explained by the benign nature of the tumor. There were no PSP-related deaths among the participants of our study. But one case of PSP-related death was identified through an extensive literature review [20]. It was the first case of death from respiratory and circulatory failure due to a large, multiple PSP tumor and lymph node and extrapulmonary metastases (liver, abdominal cavity, bone) compressing mediastinal tissue. In addition, we could identify only one serious PSP patient from the literature reviews, who had experienced respiratory arrest due to airway obstruction by endobronchial PSP, which improved after pneumonectomy [22].
The main reason for surgical resection of PSP is to curb its metastases and recurrence. In 1986, Tanaka et al. reported the first case of PSP with lymph node metastasis [8]. In a relatively large series of published PSP cases, only 1 out of 100 and 3 out of 239 PSP patients showed lymph node metastases [3, 13]. These reports suggested that PSP may be potentially malignant. When we extensively searched all references regarding PSP, we identified a total of 38 cases of PSP with lymph node metastases [3, 8–14], 6 with distant and pleural metastases [15–17], 4 with recurrence [18], 1 with massive necrosis and vascular invasion [23] and 1 with death [20]. However, considering that 3,469 PSP cases were reported in the past 65 years after the reporting of the first PSP case, only 1.33% of PSP cases can be regarded as malignant. Interestingly, there was only one death due to PSP in these patients presenting with metastasis or recurrence, suggesting that PSP does not have a significant impact on prognosis even if metastasis or recurrence occurs. Recently, a case of malignant transformation in both cuboidal surface cells and stromal round cells was confirmed for the first time, but it was reported as an outpatient follow-up state without recurrence or metastasis after surgery [19]. Rather, lung resection surgery might lead to significant complications, including prolonged air leak, bronchopleural fistula, pneumonia, acute respiratory failure, hemorrhage, atelectasis, pneumothorax, bronchospasm, pulmonary embolism, acute respiratory distress syndrome, and cardiovascular complications [24] affecting prognosis, although there were no deaths immediately after surgery in our study. Therefore, it could be helpful to identify the risk group requiring surgery among patients with PSP. Among the prior studies related to this, it was reported that PSP patients with spindle cells or male patients may be more prone to metastasis [13]. Another study reported that young male patients are prone to lymph node metastasis, and the tumor size is larger [25]. Moreover, resection has been performed to confirm the diagnosis in lesions that were otherwise not possible by endoscopic or percutaneous biopsy [26], to relieve local compression of adjacent structures [27, 28], or in the setting of mixed histology of PSP combined with another more aggressive tumor [29]. However, in the present study, we could not determine surgery candidates because there was no patient who presented with metastasis, recurrence, or death from PSP. Further studies related to the surgical indications are required.
The strengths of the current study are as follows. This is the first study to investigate the all-cause mortality risk of patients with PSP. A long-term follow-up duration (87.4 ± 52.0 months) is another advantage of our study. Comorbidities were included as confounders of mortality risk in the multivariate analysis. We also reviewed reports on PSP cases, lymph nodes, distant metastases, recurrence, and deaths published worldwide in 65 years since the first PSP case was reported.