The evaluation of SUV is intuitive and convenient, but whether SUVmax provides additional prognostic value in addition to the TNM stage remains to be explored[12, 13, 21]. Previous studies showed that higher SUVmax was associated with worse overall survival, but these studies were mostly based on early stage lung cancer[4, 6, 18, 22],and results have been conflicting. Furthermore, some studie[12, 21, 23]reported that SUV has no prognostic value independent of the TNM stage in advanced lung cancer. Hoang et al[23]. suggested that SUV cannot predict prognosis in patients with advanced lung cancer (stage IIIB and stage IV) because a continuous increase in SUV does not increase the risk of mortality. In contrast, Paesmans et al[21]. reported that SUV is an independent predictor for patients with stage I–III NSCLC but not for those with stage IV disease. The possible reason for this is that the median survival for stage IV patients is only 12 months. The short survival period resulted in failure to identify any independent prognostic value for SUVmax. Our study confirmed that SUV is a prognostic factor for overall survival in advanced lung cancer patients. Our analysis showed that patients with EGFR mutations accounted for 32.5% of the patients in our study, and the development of targeted therapy has resulted in significantly prolonged survival in patients with advanced NSCLC[24, 25]. A high SUVmax indicates a worse severity of malignancy and more aggressive tumor proliferation[6, 26]. Because of these aggressive biological behavioral characteristics, patients with a high SUVmax lose the chance to receive second-line treatment. Thus, the benefit from a variety of treatments and the overall survival in these patients is limited[27].Patients who received first-line treatment had a significantly greater SUV than did those who received second-line treatment or more. The results appear to support this view. Studies on the optimal SUV threshold have also reported inconsistent findings[2, 4, 6, 13, 15, 19, 28, 29]. In this study, the median survival time and survival curves showed that an SUV of 6 was the optimal cut-off value. Meanwhile, Detterbeck et al. suggested that the correlation between SUV and prognosis is a gradual association without an absolute threshold[30].
With respect to the association between SUVmax and clinical factors, our study found that the highest SUVmax of all lesions significantly increased among patients with loss of weight, primary lung cancer lesion size > 3 cm, and number of metastases ≥ 2. The higher SUV may indicate more severe malignancy, shorter doubling time[6],and more aggressive invasion[17, 26]. This would lead to a larger malignant lesion[2, 4, 8, 11, 14, 26],more extensive organ involvement and more obvious weight loss[2, 3, 31].
In addition, we also found that the SUVmax of all lesions was markedly higher in patients with squamous cell carcinoma than in patients with non-squamous-cell carcinoma. Previous studies showed that SUV was correlated with histology, with the SUV of squamous cell carcinoma being greater than that of adenocarcinoma[2, 8, 10, 11, 21, 32]. In contrast, we found no significant difference in the histology and SUV of primary lung cancer lesions. This may be because squamous carcinoma is a central lung cancer and thus lymph node enlargement is common and more likely to form in the cavity, mixed with obstructive pneumonia and atelectasis. Previous studies also showed that the SUV of poorly differentiated malignant lesions was higher than that of well-differentiated tumors[8, 29]. We could not perform a more in-depth analysis in this study because the number of cases of squamous cell carcinoma was less than that of adenocarcinoma. Further studies with a well-balanced histology distribution are needed to evaluate the correlation between tumor histological types and SUV.
Our study has some limitations that should be considered. First, it is a retrospective study conducted only in a single center. Therefore, further studies with a larger sample size and conducted in a multicenter setting are needed to confirm our findings. Second, the estimated SUV of primary lung cancer lesions was influenced by various factors; including the patient’s blood glucose levels, imaging time, reconstruction method, maximum absorption value, and analysis by the same observer. However, it is impossible to guarantee that all patients will receive PET/CT imaging under the same conditions in the real world. Third, the inclusion of only patients who received treatment may have led to selection bias.