In this study, we tried to differentiate between AC and SCC of the lung using PET/CT parameters. We found a statistically significant difference between the SCC and AC regarding the tumor size, SUVmax of the tumor, and the TLR with these parameters are higher among patients with SCC.
We studied 60 patients who were pathologically proven to have either lung SCC or AC which are considered the most common pathological types. The number of patients with AC was larger than patients with SCC at 60% and 40%, respectively. This is consistent with the international epidemiology of lung cancer and the switch that happened after 1990 with lung AC becoming the first type of lung cancer representing 60% of all types followed by SCC representing 20% . Karam et al. , Kim et al. , and Wang et al.  also included patients with only pathologically proven AC and SCC during their research and found that the AC patients are more than SCC patients; AC patients in their sample represented 60.2%, 56.3%, and 66.4%, respectively, which is very close to our patient sample. However, Lu et al.  and Sunnetcioglu et al.  included patients with other pathological types such as bronchoalveolar carcinoma and small cell lung cancer.
A significant correlation was found between the size of the tumor measured on contrast-enhanced CT images and the pathological type of the tumor with SCC masses showed higher sizes compared to the AC masses with P value = 0.008. This is also in keeping with previous studies that found a larger size of SCC tumor [12, 14, 17].
Multiple previous studies found a significant correlation between the pathological type of the NSCLC and SUVmax of the tumoral lesions with SCC lesions showed higher SUVmax compared to AC [12,13,14,15,16,17]. This is in agreement with our result and this can be explained by the higher size of the SCC tumors as shown in our study and subsequently containing a larger number of malignant cells leading to increased metabolic activity compared to AC tumors which reflects the FDG uptake and SUVmax.
De Geus et al.  found a significant difference between the SUVmax of SCC compared to AC and large cell carcinoma yet there was no significant difference found between AC and large cell carcinoma. Lu et al.  found a statistically significant difference between the SUVmax of SCC, AC, and bronchoalveolar carcinoma. Multiple studies also found a correlation between the SUVmax and the degree of differentiation of the tumor [9, 17].
No statistically significant correlation was found between the SUVmax of the tumor and the sex or gender of our patients and this is consistent with Karam et al.  and Lin et al. .
Regarding the SUVmax of the lymph nodes, no significant difference was found between the SUVmax of the lymph nodes of AC patients and SCC patients and no significant correlation between the SUVmax of the lymph nodes and the size of the tumor; however, we found a highly significant correlation between the SUVmax of the LN and SUVmax of the tumor. Wang et al.  also found no difference between AC and SCC as regards the metastatic lymph nodes SUVmax. Nambu et al.  and Li et al.  reported that tumor with higher SUVmax has a higher risk of lymph nodes metastases.
In the current study, we tried to make a normalization for the SUVmax of the tumor by dividing it by the SUVmax of the liver trying to eliminate the effects of other parameters that may affect the accuracy of the SUVmax such as the dose of FDG, the weight of the patient, the time gap between the injection and the acquisition and lastly the patients’ glucose level. Multiple previous types of research used the liver as a parameter for normalization [17, 22]. TLR showed a significant difference between patients with AC and patients with SCC being higher in patients with SCC and this is consistent with Duan et al.  who concluded that TLR is one of the parameters which can be used to differentiate between SCC and AC and also showed a significant correlation with the tumor differentiation.
To our knowledge, no previous studies tried to calculate the cut-off values of SUVmax of the tumor, size of the tumor, and TLR which can be used to differentiate between the SCC and AC. However, all these parameters showed the same sensitivity (75%), and TLR showed the highest specificity 77.78% compared to 76.47% for tumor size and 72.22% for SUVmax.
Shao et al.  tried to use PET/CT to predict the different pathological subtypes and growth patterns of early adenocarcinoma. They found higher SUVmax in cases with invasive adenocarcinoma compared to adenocarcinoma in situ and minimally invasive adenocarcinoma with median SUVmax = 2.0 which was the optimal cutoff value with P value = 0.008. Also, they found a SUVmax of 1.4 was the optimal cutoff value for differentiating the growth pattern of adenocarcinoma.
Liu et al.  tried to use SUVmax to differentiate between the synchronous multiple primary lung tumors and the lung metastases and they found SUVmax of 1.7 the best cut-off value with 62.7% sensitivity and 82.6% specificity.
Limitations The current study tried to differentiate between the two commonest pathological subtypes of lung cancers. So, further studies can be conducted on more pathological types. Also, bigger sample size and multicentric studies are needed to obtain more accurate results. Finally, the degree of tumor differentiation is better to be added in comparison.