Gastric malignant tumour, its morbidity and mortality have been ranked high all the time in the world. According to statistics, as the most common malignant tumour of the digestive system, the five-year survival rate of gastric cancer is between 20% and 30% [15]. In China, the mortality rate of gastric cancer ranks second [16]. With the change of the living environment and diet structure of individuals, the incidence rate of gastric cancer is increasing every year. Therefore, early diagnosis and accurate staging of gastric cancer are particularly important, which can directly affect the treatment and prognosis of patients [17]. Accurate staging (including the scope of local invasion, lymph node metastasis and distant metastasis) can optimize the treatment plan and guide surgery [18]. Due to its high spatial and density resolution, enhanced CT has become a positive and extensive preoperative staging method for gastric cancer [19]. Admittedly, gastroscopy is one of the most effective methods for early gastric cancer examination, and its status as the gold standard cannot be shaken. However, the gastroscope can only observe the local lesions on the surface of gastric mucosa, which still lacks the analysis of the depth of tumour invasion, lymph node metastasis and invasion and metastasis of adjacent organs. Moreover, the gastroscope, as an invasive examination, causes pain to patients, and its results will also be affected more or less by the accuracy of sampling materials. At present, the treatment methods of cancer tend to be individualized and precise medical treatment and multidisciplinary comprehensive treatment [20].
With the continuous improvement of medical level and updating of medical equipment, PET/CT has been widely used to evaluate the metastasis and staging of malignant tumours [21–22]. In gastric cancer, the degree of glucose transporter 1 (GLUT1) expression is also different in different pathological types. The expression level of GLUT1 in highly differentiated gastric cancer cells is higher than that in gastric signet-ring cell carcinoma, therefore, the latter has lower results for 18F-FDG uptake [23]. In addition, compared with other histological types of gastric cancer, pyruvate kinase M2 (PKM2) subtype in signet-ring cell carcinoma is at a low expression level [24]. All these factors lead to the poor prognosis of signet-ring cell carcinoma, but the uptake of 18F-FDG is less, which makes it express low SUV results. Therefore, many clinicians and researchers are sceptical about the diagnostic value of PET/CT in gastric cancer. This research is based on the results of a retrospective study to illustrate this issue.
The results of this retrospective study showed that the sensitivity and accuracy of gastric CT (or abdominal enhanced CT) in distinguishing benign and malignant gastric lesions are lower than that of PET/CT. In this study, there were two false-negative results on PET/CT. Pathological results confirmed that the proportion of mucinous adenocarcinoma was high or it was signet ring cell carcinoma, which was considered to be related to the low expression of GLUT-1 and PKM2, so that the intake was low and the SUVmax was small, therefore, a false negative result appeared. Considering that the proportion of gastric signet-ring cell carcinoma was relatively small after all, the diagnostic advantages of PET/CT in this aspect will be more obvious when the sample size is large.
A total of 108 patients were confirmed to have gastric malignant tumours by pathological results, including 78 cases of gastric adenocarcinoma and 30 cases of primary gastric lymphoma. The difference between the primary gastric lymphoma and gastric adenocarcinoma groups was statistically significant (t = 4.13, P < 0.05). Sun Gaofeng et al. [25] studied the PET/CT results of 43 patients with gastric cancer and 13 patients with primary gastric lymphoma and found that there was a difference in SUVmax between them, and the P < 0.05. Similarly, Li XF [26] et al. retrospectively analysed 73 cases of gastric cancer and 52 cases of gastric lymphoma, and found that compared with gastric cancer, the results of PET/CT examination of gastric lymphoma were different, and higher SUVmax suggested that gastric lymphoma was more likely. The above results are consistent with the results of the present study. Clinically, for patients with possible lymphoma, when evaluating extra-nodal lymph nodes prior to treatment, apart from the invasive operation of tissue biopsy, the most common inspection method is computer imaging, among them, PET/CT plays a vital role because of its dual functional characteristics in metabolism and anatomy. Most studies believe that the application of PET/CT in the staging, treatment, and prognosis of different histological types of gastric lymphoma is feasible, such as gastric mucosa associated lymphoid tissue and diffuse large B cell lymphoma [27]. To distinguish gastric cancer and primary gastric lymphoma, it is undeniable that gastroscopy is the best method [28]. However, PET/CT shows its advantages for patients whose physical condition cannot tolerate examination or when the depth of lesions is large. Non-Hodgkin's lymphoma is the most common primary gastric lymphoma, among them, the proportion of high-grade lymphoma is higher than that of low-grade lymphoma. The pathological type of the former is mainly diffuse large B-cell lymphoma, and the latter is gastric. Mucosa-associated lymphoid tissue lymphoma is predominant [29]. Primary gastric lymphoma mainly infiltrates in the mucosa. Although its ulcer area is generally shallow, its growth range is wide, and its primary focus is large. Moreover, most lymphoma subtypes have a strong affinity for 18F-FDG, so the SUVmax of primary gastric lymphoma is higher. This fully demonstrates that SUVmax in PET/CT examination results provides an important diagnostic value for distinguishing gastric cancer and primary gastric lymphoma. In summary, PET/CT can identify stomach diseases in a more comprehensive way, especially in distinguishing gastric adenocarcinoma from primary gastric lymphoma, and can provide reliable semi-quantitative basis, so it has a wider application and higher clinical value.
Among the 110 patients in this retrospective study, 36 patients underwent radical surgery for gastric cancer in our hospital. More than 90% of the remaining patients were treated with radiation or by chemotherapy because they had reached advanced stages when the gastric malignant tumour was discovered, and a small number of the remaining patients chose to seek medical treatment in areas with better medical conditions. This also reflects the cause of the high mortality rate of gastric cancer in China, which lies in the failure of early detection, early diagnosis, and early treatment. In addition, the research hospital is located in north-western China. There is not much medical publicity, the local lack of awareness of medical treatment of the inhabitants, and financial or material resources are limited, which are the reasons why the tumour reaches advanced stages by the time of first visit to the hospital. The number of patients who have an opportunity of surgery or timely and proper treatment could have been higher if they were located in a developed area. All 36 patients received radical surgery, and postoperative pathology showed that they were all gastric adenocarcinoma. According to the histological types by the World Health Organisation, the patients were divided into high, medium, and low differentiation groups. After statistical analysis, the difference between these three groups of SUVmax was not statistically significant. Theoretically speaking, gastric cancer should show different uptake rates of 18F-FDG due to different degrees of differentiation, thus obtaining different SUVmax. However, because of the pathological reasons, this study could not subdivide histological types, such as intestinal type, diffuse type, and mixed type in Lauren's classification, hence, we could not get theoretical results. In general, the results of this study are consistent with those of Liu Y et al. [30], who believe that the degree of invasion of the gastric lining by the primary gastric tumour is related to the uptake of 18F-FDG, which shows different SUVmax. Therefore, PET/CT plays a high clinical diagnostic role in evaluating the T staging and progression of gastric cancer. Yoon J K [31] and others found that the depth of tumour invasion was an independent predictive factor of the high uptake rate of 18F-FDG by PET/CT in 229 patients with gastric cancer, so, the affinity of 18F-FDG in advanced gastric cancer is much higher than that in early gastric cancer. In this study, because T stage is an index used to describe the infiltration depth of malignant tumours, different infiltration depths mean different uptake rates of 18F-FDG. Furthermore, with the increase of infiltration depth, the uptake rate of 18F-FDG in the gastric mucosa, muscle layer, and serosa layer is higher, and the semi-quantitative SUVmax is larger. Similarly, in the postoperative pathological TNM staging, a later stage indicates a higher degree of local tumour infiltration, lymph node metastasis and distant metastasis. Correspondingly, the higher the SUVmax value in the PET/CT examination results. In addition, this study failed to find a statistically significant difference between N staging and SUVmax. As is well-known, lymphatic metastasis is the most important method of gastric cancer metastasis, N staging is a stage describing the number of lymph node metastases in gastric cancer. In the early stage, lymph node metastasis is mostly manifested as the metastasis of lymph nodes around the stomach, such as the lymph nodes of the cardia and lesser curvature of the stomach. As for the later stage of the disease, it is manifested as metastasis to distant lymph nodes or jumping metastasis, such as the left supraclavicular lymph node. The SUVmax does not show different results in different N stages, which may be due to the relatively high uptake rate of 18F-FDG in the primary focus of gastric cancer, which masks the uptake rate of surrounding metastatic lymph nodes [32]. Moreover, generally speaking, the volume of tumour focus must be larger than the volume of peri-gastric lymph nodes, so there is a certain influence on the uptake of lymph nodes. Therefore, the diagnostic value of PET/CT for lymph node metastasis may lie in the uptake rate of 18F-FDG in the local lymph node, rather than in the uptake rate of the primary tumour. The above results of the present study fully illustrate that PET/CT can provide an effective diagnostic value in the pathological staging of gastric cancer, which can better guide clinical treatment.
In terms of prognosis, relevant research findings at home and abroad are summarized as follows: Kim [33] et al. found that the difference between SUVmax and PFS was statistically significant through the study of 97 patients with gastric cancer examined by PET/CT prior to treatment, suggesting that SUVmax can be used as a good independent predictor of PFS, thus evaluating the prognosis of gastric cancer. Similarly, Park [34] et al. retrospectively analysed 82 cases of advanced gastric cancer and concluded that the high SUVmax group often suggests lower overall survival time (Overall Survival [OS]) and PFS, indicating that clinically, SUVmax can effectively predict the prognosis of patients with advanced gastric cancer. In addition, Grabinska [35] et al. and Na [36] et al. also studied and analysed the prognosis of patients with untreated gastric cancer, and found that many semi-quantitative parameters of PET/CT, including SUVmax, are closely related to OS, which has a clinical application value in evaluating prognosis. In the present study, due to objective factors such as short follow-up time and small sample size, the relationship between tumour differentiation and prognosis could not be found, but the relationship between SUVmax and survival time can still be found from this small sample study. This can be explained by the fact that from the medical professional point of view, the higher the uptake of 18F-FDG by the tumour focus, the higher the malignant degree and invasive ability of tumour, thus having a worse prognosis [37]. The small sample size of the present study may be the main reason for the bias of research results in factors such as tumour differentiation degree. Admittedly, the sample size and types of lesions in this study are relatively small, but there is some persuasiveness in the application value of PET/CT in the diagnosis and prognosis of gastric cancer, and to some extent, a new research direction of PET/CT in malignant tumours is defined.
In conclusion, for patients newly diagnosed with gastric cancer, PET/CT examination prior to treatment has obvious advantages in diagnostic and prognostic evaluations. PET/CT does not only provide reliable semi-quantitative basis for differentiating benign from malignant, but also has a great value for staging diagnosis and even pathological diagnosis. In terms of prognosis, the level of SUVmax also has a certain reference value in the survival time of patients after surgery, and becomes an independent risk factor for predicting prognosis, which can help doctors make better treatment plans in clinical aspects and achieve the purpose of improving prognosis. Admittedly, compared with other examinations, PET/CT examination is expensive and requires higher technical conditions at present. However, in the future, it will be more widely used in clinical practice, giving full play to its value in the diagnosis and prognosis of diseases.