Gastric cancer has a worldwide distribution and is considered as the fourth common cancer worldwide especially in Asians. It has a high mortality rate notably in late and advanced cases; thus, an early detection and tumor staging with proper intervention performance can carry a potential curability [2,3,4].
Due to its availability and its relatively low cost, CT had been widely used in the evaluation of recurrences in gastric cancers; however, its role is limited to detect the suspected gastric recurrences and is often unable to differentiate a true tumor recurrence from the morphological changes that may follow the treatment interventions like operative bed scarring [3].
It worth mentioning that CT depends mainly on the size assessment for evaluation of the tumor response to therapy, and from a practical point of view, this is not accurate and is not feasible in many gastric tumors as most of them are not measurable [3].
Positron emission tomography (FDG PET) had been proven as a useful diagnostic tool for the precise detection of tumor respectability and in the detection of distant hematogenous deposits, but unfortunately, it is lacking the anatomical details that made its use limited in the locoregional staging [6].
FDG PET/CT had offered an accurate diagnostic tool in the primary diagnosis of gastrointestinal malignancies and in the recurrent status as compared to the conventional diagnostic tools [2].
In the current study, we were concerned with the detection of recurrences in the gastric neoplasm in the treated cases in addition to the nodal and distant metastases through a comparative correlation between the image findings of both PET/CT and CECT.
Gastric residual/recurrence as detected by PET/CT in our study was found in 30 patients (44%) of all cases, based on the metabolic activity that was seen in the lesions; this was statistically significant (P value = 0.006) as compared to the results of the CECT where the gastric residual/recurrence was found in 46 patients (67.5%) (Tables 5 and 6) (Figs. 1, 2, 3, and 5), hereby, a discordance with Gedik et al. and Sim et al. studies, where they reported a comparable diagnostic value for the PET/CT with the standard CECT in patients with gastric carcinoma and in evaluation of the gastric recurrences [2, 4]; this difference could be explained by the difference in the sample size as our study was enrolling a relatively larger number of patients (68 patients) compared to their studies that were enrolling 20 and 52 patients respectively; however, Sim et al. had warranted further studies to validate the role of PET/CT in gastric cancer recurrences [4].
Our study was greatly matching Bilici et al. study who had reported superiority of PET/CT compared to the diagnostic CT for the follow-up of the treated gastric cancers and in the detection of tumor recurrences [7]. A high sensitivity reaching 80% had been described in the literature for detection of the tumor recurrences by PET/CT in gastric cancers as compared to the standard post-contrast CT examinations [2, 8, 9].
Despite having a bad prognosis; tumor recurrences should be carefully evaluated as the early identification of small recurrent masses in the presence of minimal adenopathy could have a better response to chemotherapy or radiotherapy [3].
A good point that could explain why is the PET/CT considered as a superior surveillance diagnostic modality for evaluation of post-therapy gastric malignancies when compared to the CECT is that the former has the ability of detecting the metabolic activity inside the true lesions where the glucose metabolism is elevated but is significantly lower in the postoperative scarring; thus, the use of the PET/CT could be of value in differentiating the two conditions and helping for the clinical decision-making process to be accomplished [3, 7, 10].
Tirumani et al. had reported excellent results for PET/CT in the initial assessment of lymphoma and is considered as a more accurate modality for detection of recurrence/residual lesions than the standard anatomical imaging modalities [11].
Nodal metastasis is another important factor in the prognosis of gastric cancers. PET/CT can detect the metabolic activity of the lymph glands and not only the anatomical details that are given by the conventional CT [2].
In our study, the nodal metastasis was found by PET/CT in 18 patients (26.5%) compared to 26 patients (38%) that were detected by CECT; however, this was statistically insignificant (P value = 0.143) (Tables 5 and 6) (Figs. 3, 5, and 6). This was matching with many studies in the literature and eventually some considered that CECT is more sensitive in detection of metastatic lymph nodes than the PET/CT; however, the role of the PET/CT in the detection of the metabolic activity of normal-sized and mildly enlarged lymph glands has to be considered in this comparison. Moreover, the absence of the metabolic activity in enlarged but tumor-free lymph glands may beneficial in the evaluation of the therapeutic response to chemotherapy and to radiotherapy and sometimes, the detection of metabolically active distant nodal metastasis (like cervical or mediastinal nodes) may preclude unnecessary surgical interventions [2,3,4]; in other words, CECT can give better anatomical details and perfect localization of the nodal metastasis in addition to the pattern of enhancement and the presence of necrosis with high spatial resolution, but the metabolic activity of the malignant nodes could be better assessed by PET/CT, especially the nodes that are not or mildly enlarged. However, the high cost of the PET/CT in comparison to its benefit in the assessment of the nodal metastasis should be carefully weighed when a diagnostic modality is to be chosen.
Distant metastasis including the solid organ hematogenous metastasis and the peritoneal seeding was found in 18 patients (26.5%) by PET/CT in our study compared to 24 patients (35%) that were detected by CECT; this was also considered statistically insignificant (P value = 0.265) (Tables 5 and 6) (Figs. 1, 2, 3, 4, 5, and 6). In agreement with Gore and Miller et al.’s studies, the hepatic metastasis (portal vein born metastasis) [12, 13] was dominating in our study being found in 16 patients (23.5%) form a total of 18 patients with distant metastasis (Figs. 1, 2, and 3).
Chung et al., Kinkel et al., and Lim et al. had reported a high sensitivity of the PET/CT in the detection of distant metastasis in the solid organs, and this was considered as a major advantage of the PET/CT over other imaging modalities [3, 14, 15]. However, Gedik et al. had found a higher specificity for PET/CT in the detection of the distant metastasis than the CECT in patients with metastatic gastric carcinoma but in concordance with our study, and they reported that the difference could not be considered as statistically significant (P > 0.05, McNemar’s test) [2].
There is a trend in the literature of the low sensitivity of PET/CT in relation to the CECT in the detection of the peritoneal seeding [2,3,4]; however, in our study, the peritoneal seeding was seen in six patients (8.8%) (Fig. 5). Nevertheless, the presence of the peritoneal seeding may be considered as a sign of advanced disease and may imply incurability and often carry a bad prognosis; however, some studies had shown that the peritoneal seeding is usually detected intraoperative and may be overlooked or under detected by the CT [3].
CECT can show ascites in cases of peritoneal metastasis, and this was seen in two of our cases (2.9%) (Fig. 6). Other CECT findings include focal nodular thickening of the peritoneal fat, thickening and enhancing parietal peritoneum, and distorted small bowel walls. In PET/CT, two patterns are recognized, the first one is a diffuse even uptake delineating the visceral surfaces in the abdominal and pelvic cavities and the second one is scattered focal nodules that are randomly distributed in the abdominal and pelvic cavities [3], the latter was identified in our study (Fig. 6).
In our study, two patients had small pulmonary nodules as detected by CECT; however, their sizes were too small to demonstrate FDG uptake by PET/CT (beyond the PET resolution). On the other hand, one of our cases with lymphoma and had a gastric recurrence and mediastinal nodes metastases is detected by both CECT and PET/CT. There was an additional finding—not identified by the CECT—that was added by the PET/CT which was the osseous deposits affecting both iliac blades (Fig. 6), denoting the presence of blood-borne bone metastasis, and consequently, this may imply a different treatment decision.
A lower survival rate had been reported in the patients who had high SUV values (FDG avidity) either in their recurrent primary tumor or in the distant metastatic lesions as compared to those who had lower SUV values. However, the mucinous and the signet ring cell carcinomas (both are reported as aggressive types of tumors) may typically present a low FDG uptake; thus, a low FDG uptake does not necessarily mean a better prognosis [16]. It was also reported that a negative PET scan after a curative surgical treatment with curative intent was significantly associated with a longer survival rate [17].
There were some limitations in our study including the following: the first one was its retrospective nature with a relatively small sample size. The second one was the lacking data about the size of the primary tumor and the histopathological confirmation of the recurrent gastric lesions and nodal and distant metastatic deposits.
Despite these limitations, our study had provided a significant role of PET/CT as postoperative surveillance and in the decision-making regarding the gastric recurrence/residual lesions. However, further prospective studies with a larger sample size are warranted and a gold standard reference like the histopathology examination of the recurrent and the metastatic lesions should be included for the accuracy measures.