Research has shown that drinking water can effectively diagnose the depth of gastric cancer invasion after filling the gastric cavity(10, 11). In our study, the echogenic visualization aid of the grain mixture was used to fill the gastric cavity, achieving full contact with the gastric mucosa, staying in the gastric cavity for an extended time, and providing sufficient observation time to achieve clear visualization of the gastric wall. OCTU can evaluate the depth of tumor invasion by examining the disorder and the interruption to the gastric wall(7, 9, 12). In this study, the accuracy of OCTU's overall T staging rate was 72.2%, and the accuracy rates of T1 to T4 stages were 84.2%, 81.8%, 69.4%, and 65.5%, respectively. In a similar study, Liu et al. showed that the accuracy of OCTU in the overall staging of gastric cancer was 77.3%(13). In addition, studies by He et al. and Wang et al. showed that OCTU combined with venography (double contrast-enhanced ultrasound) can effectively supplement endoscopic ultrasound (EUS) and enhance the accuracy of preoperative gastric cancer T staging by CT(8, 9). EUS is the main method used for T staging in early gastric cancer. Large sample studies reported an accuracy rate of approximately 67.4%(14). Enhanced CT is currently the preferred imaging method for preoperative staging of gastric cancer. The overall accuracy of T staging is 73%-91%(15–21). The range of accuracy is first related to the sample size of each T stage included in the study and technological progress. The latest research shows that under the condition of the gastric window of enhanced CT, the T1 and T2 stages of gastric cancer can be diagnosed more accurately(22).
In this study, OCTU inaccurately determined the T stages of 30 gastric cancer cases. These included nine cases of T3 which were overstaged as T4 and seven cases of T4 which were understaged as T3. The difference between T3 and T4a in pathological staging is that in T3 the tumor has only invaded the subserosal connective tissue and not the visceral peritoneum. The resolution of ultrasound imaging depends on the difference in tissue echo between the interfaces. The five-layer structure of the normal gastric wall shows three high echo light bands and two low echo light bands in OCTU. From inside to outside, are high (superficial mucosa), low (muscular mucosa), high (submucosa), low (muscular propria), high (serosa and extraserosa adipose tissue) strip echo. Subserosal connective tissue and visceral peritoneal echo are hyperechoic, especially when combined with micro-invasion of cancer foci, it is difficult to judge by ultrasound whether the visceral peritoneum has been invaded. While enhanced CT can be more sensitive in judging the invasion of the serous membrane, diaphragm, omentum, abdominal wall, and other tissues(23, 24).
In addition to the above 16 cases with inaccurate staging, we also had 10 cases that were inaccurately staged. Of these, the cancer foci of seven cases were located in the greater curvature of the gastric body. The possible reasons for inaccurate staging could be that the depth exceeds the best observation distance range of the line array high-frequency probe, and the reduced resolution makes it difficult to distinguish the middle structure of the stomach wall. Both OCTU and enhanced CT in early gastric cancer identified one case of T1 as T3. The pathology of this case suggested that the cancer foci were limited to the submucosa, and the accompanying ulcer reached the subserosal layer. Ulcerative cancer tissue and normal gastric tissue penetrated by ulcers interfere with OCTU and enhanced CT to judge the depth of tumor invasion. So, both OCTU and contrast-enhanced CT may stage too deep, and the depth of ulcer penetration is misjudged as the depth of tumor invasion. Two cases of T1 were overstaged as T2, they were gastric antrum poorly differentiated carcinoma, which indicates that the inaccurate stage may be related to tumor differentiation, stronger gastric motility at the gastric antrum, and more gastric wall folds. One case of T2 was overstaged as T3 by OCTU, and postoperative pathology showed that both nerves and vessels were invaded. It was analyzed that the deep staging of this case may be related to the existence of tumor microinfiltration.
In this study, seven cases of T1 were overstaged as T2 by enhanced CT (six cases of poor differentiation and one case of moderate-low differentiation), compared with differentiated gastric cancer, undifferentiated gastric cancer lacks blood vessels and manifests as diffuse infiltration, and the use of angiographic contrast agents has an impact on the accuracy of CT staging(25, 26). Studies have pointed out that histological differences and perivascular infiltration have a certain influence on the accuracy of enhanced CT staging, and undifferentiated and inflammatory gastric cancer tends to be overstaged(27). In addition, enhanced CT is excellent in imaging the blood supply of tumors, but tomographic imaging has a partial volume effect, which may lead to overstaging, and insufficient gastric wall opening will also affect the accuracy of staging.
Currently, preoperative T staging comparison between OCTU and enhanced CT focuses on advanced gastric cancer. In our study, all tumor stages of gastric cancer were included for a more comprehensive comparison. There was no significant significance (P = 0.644) in the overall staging accuracy between OCTU [72.2% (78/108)] and enhanced CT [75.9% (82/108) ].
The accuracy of OCTU staging was higher than those of enhanced CT for T1 and lower for T3 stage (P<0.05). While the accuracy of OCTU staging was higher than those of enhanced CT for T2, and lower for T4 stage (P>0.05)
OCTU is better than enhanced CT in determining the depth of tumor invasion in the submucosa and lamina propria. This may be because ultrasound has a high resolution for soft tissues and can observe the detailed local conditions of small lesions in multiple directions. Early gastric cancer often has small lesions and no obvious enhancement of the mucosa, especially when there is edema, which will affect the 50% judgment index of the total thickness of the gastric wall on enhanced CT(28). Spatial resolution in enhanced CT is not affected by tumor location. Tumor blood supply imaging outlines the depth of invasion and is better than OCTU in identifying occult invasions such as the peritoneum and diaphragm(23).
In this study, firstly, the sample size of T2 stage accounted for a small proportion, and it is necessary to increase the sample size of T2 stage by expanding the sample size to improve the reliability and the diagnostic accuracy of OCTU staging. Secondly, patients with preoperative CT staging of T4b often lose the opportunity for radical surgery and received adjuvant chemotherapy first. Such patients could not obtain accurate pathological staging results and were not included in this study, which resulted in inclusion bias for the study population.
Among the preoperative staging methods for gastric cancer, EUS is invasive, and enhanced CT has the disadvantages of ionizing radiation, contrast agent allergy, and relatively high cost. The advantages of OCTU, such as low cost, no radiation, and convenience, provide clinicians and patients with more options.
In summary, enhanced CT is better than OCTU in evaluating T3 and T4 stages; that is, it is more accurate in judging tumor invasion of the visceral peritoneum and surrounding organs. Its excellent spatial resolution can visually display a wide range of lesions, which is an irreplaceable preoperative staging imaging method for gastric cancer. OCTU was more effective for T1 and T2 staging. It can assist enhanced CT for accurate preoperative T staging of gastric cancer, especially in patients with enhanced CT contraindications, such as patients with low renal excretion and allergy to contrast agents.