Lung cancer is the leading cause of cancer-related death worldwide and despite advances in diagnostic tools and treatments, a high percentage of patients are still diagnosed in advanced stages when the disease is inoperable and therapeutic options such as chemotherapy and radiotherapy do not serve for curative purposes (30).
Screening studies with radiography and sputum cytology have not been useful in reducing mortality from this disease, which is the ultimate goal of a screening program (30). The concept of biomarkers is based on the biological properties of cancer as a systemic disease in which, as the disease progresses, it secretes proteins necessary for its growth and enhance its metastatic capacity (30). Biomarkers have been widely used in lung cancer, mainly to monitor the efficacy of therapy and for early detection of recurrences (31). It has been previously described how the low detection rate of true positives and the inability of any single biomarker to diagnose lung cancer due to the heterogeneity among individuals, differences in biochemical pathways and in the tumor biology, among other characteristics, make biomarkers mostly useful in the context of differential diagnosis rather than in screening programs (32,33). Even though the use of serum markers is still controversial specifically to distinguish between histological types of the disease given their low sensitivity (34).
In our study, patients with SCLC had median values of NSE and Pro-GRP at least 2 times and 20 times higher, respectively, compared to patients with NSCLC or NMLD. This is consistent with the literature, in which median pro-GRP and NSE values have been reported around 28 and 4.5 times higher in the SCLC histology (35). Based on this trend, the probability of SCLC increases as the levels of pro-GRP and/or NSE elevates; in fact, pro-GRP levels higher than 300 ng/L are said to be 99% specific for detecting SCLC (33). Although in our study, all patients had creatinine levels in the normal range, it is important to consider that serum biomarkers, particularly pro-GRP and CYFRA 21 − 1 elevate in patients with chronic renal impairment, being a potential confounding factor (26)
The predictive capacity of serum biomarkers analyzed in this study for diagnosing lung cancer was varied, being the most accurate CEA, with the limitation that it rises in many benign and malignant medical conditions (32,36,37). Additionally, and despite their low sensibility, serum biomarkers such as CEA, CYFRA 21 − 1 and NSE when analyzed alone, showed a high specificity (87.9% − 100%), ideal for ruling out cancer in patients with benign lesions. Histological subtyping, and specifically differentiating between SCLC and NSCLC is crucial in terms of prognosis and therapeutic targets. We observed that CEA and CYFRA 21.1 levels were higher in patients with malign pathology more specifically in patients with NSCLC suggesting its potential as a serum biomarker this histological type, however, we did find multiple outliers. Consistently different studies have used different combination of biomarkers to distinguish lung cancer patients. It has been reviewed the potential use of CYFRA 21.1, CEA, SCCA, tissue polypeptide antigen (TPA), and cancer antigen-125 (CA-125) as biomarkers for NSCLC and NSE for SCLC (27,38,39). Some of these biomarkers have also been associated with outcomes. For example, a recent meta-analysis reported a significant correlation between positive tests for CEA and nodal involvement and mortality, even in patients with stage I NSCLC (40,41). Correspondingly, NSE has been proven as a useful prognosis biomarker for survival, monitoring of treatment and relapse prediction (32,42)
Due to the limitations that individual serum biomarkers might have to support lung cancer diagnosis, it has been proposed that assessing a combined panel of biomarkers delivers more accurate results. Pro-GRP has been studied in patients with lung cancer and has been suggested as a good biomarker for the differential histological diagnosis of lung cancer patients (30). In our study, the combination of Pro-GRP, CYFRA 21.1 and CEA showed a high diagnostic value of AUC of 80.4% with a sensitivity of 70.6% and specificity of 81.8% for NSCLC. Similarly, the combination of Pro-GRP, CYFRA 21.1 and NSE showed the highest diagnostic value of AUC of 97.3% with a sensitivity of 88.8% and specificity of 98.9% for SCLC. A recent publication that combined the same biomarkers of the present study, showed an average diagnostic performance of the individual biomarkers, with a significant increase in accuracy using a combined approach, reaching a sensibility of 88.5% and specificity of 82% (37).
Our study has several limitations, although we included twice as many patients with malignant pathology compared to our sample size calculation, we were only able to find 9 patients with SCLC during the study period. Although this is consistent with the distribution of histological subtypes among the population, it probably affected the power of our study to detect a higher accuracy of biomarkers for discriminating between SCLC and NSCLC, as it has been reported previously in the literature. Furthermore, we did not design this study to evaluate different cutoff values for the biomarkers, nor did we realized serial testing of several biomarkers to increase the biomarkers specificity.