The present study demonstrated that the prognoses of patients with cN1 NSCLC were associated with age, the CEA value, and the primary tumor’s SUVmax value. The primary tumor’s SUVmax value was also an independent prognostic factor for both OS and RFS, which is consistent with previous reports on the associations between FDG uptake and tumor malignancy.13–16 Clinical and experimental studies have indicated that FDG accumulation during PET examination was correlated with tumor growth rate, cell density, and cell differentiation.17–19 These results support our findings showing that a high value for the primary tumor’s SUVmax may predict high-grade disease and a poor prognosis. Nevertheless, there was some variation in the FDG uptake values based on inter-institution or inter-model differences between the PET instruments, and it is possible that its true effectiveness as a prognostic marker cannot be accurately assessed without considering these differences. For example, it would be appropriate to consider the SUVmax value for a separate group when a phantom is used, and in addition to consider the PET model or a facility.20–23 Furthermore, SUVmax values can also vary according to the instrument model, size of the patient’s body, and the presence or absence of diabetic complications.24–26 Moreover, different facilities use different PET protocols (e.g., image acquisition timing and the use of one or two scans), which also complicates the analysis of data from multiple facilities. Therefore, we examined the original SUVmax values without correction for each instrument model. It would be ideal to conduct a study using the same PET instruments, although it would be difficult to accumulate an appropriate number of cN1 cases. Thus, we believe that a single-center analysis using only two PET instrument models may be a useful starting point. Finally, because FDG integration was significantly different between the instrument models, we categorized the SUVmax values as high or low using the median values that corresponded to the time periods when each respective instrument was used (2000–2008 and 2009–present).
While analyzing the cN1 NSCLC cases, we encountered two problems that should be considered. The first one was the accuracy of the preoperative diagnosis, and the second one was the recommended treatment strategy. An accurate preoperative assessment is critical for selecting the most suitable treatment for NSCLC patients.27 However, despite advances in diagnostic CT and FDG/PET-CT, over-diagnosis and under-diagnosis of nodal metastasis can occur easily because cN1 disease is a marginal stage for surgery. It has been reported that 19–30% of patients with preoperative diagnosis of cN1 were diagnosed with pN2 disease after the surgery,10,11,28,29 and our findings revealed a similar result (15 patients [25%] with preoperative cN1 disease were found to have pN2 disease). A meta-analysis has indicated that endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA) is a potentially useful technique that can provide a sensitivity of 88–94% for mediastinal staging of lung cancer.30 However, the use of EBUS-TBNA to diagnose lymph node metastasis is relatively new, and would likely be used in only a small proportion of cases, so this factor was not included in the present study.
Several randomized trials and meta-analyses have shown survival benefits for adjuvant chemotherapy after surgery in stage II–III NSCLC patients.31–33 However, some reports have indicated that induction chemotherapy was not associated with improved survival in cN1 NSCLC patients.34,35 Therefore, the current guidelines for patients with cN1 NSCLC recommend a surgery-first strategy, followed by adjuvant chemotherapy for patients who have pathologically confirmed nodal metastasis.36 However, cN1 NSCLC patients are a heterogeneous population and it does not make sense to apply a uniform treatment strategy to this population. Therefore, we hypothesized that cN1 NSCLC patients could be stratified based on preoperative factors, other than the standard clinical staging factors, which might guide the modification of treatment strategies.
The present study has several limitations that should be considered. First, the small sample size and retrospective nature of the study are prone to bias. However, we hope to collect additional cases and potentially incorporate more accurate preoperative diagnoses using EBUS-TBNA approach. Second, the cut-off value for SUVmax varied according to PET instrument model, although previous studies have also used values from multiple facilities/models, which might have obscured the association between SUVmax and prognosis.