Tumor size and T stage may influence the sensitivity of PET imaging in the detection of the primary gastric lesion. Sensitivity has been reported as low as 21% for detecting tumors <30 mm in size, and increased to 76% for lesions over 30 mm [7]. Moreover, gastric cancer limited to the mucosa or submucosa (T1 lesions) is less likely to be detected by PET than more advanced T2–T4 lesions. Early gastric cancers (T1) sensitivity for detection ranges from 26% to 63%, whereas that for more advanced disease (T2–T4) ranges from 83% to 98% [3, 6, 7, 10]. Figure 14.2 graphically depicts the range of sensitivity to diagnose early and advanced gastric cancer.

Histological subtype variants also influence glucose uptake and therefore the ability of PET to detect the primary lesion. The ability of [¹⁸F]FDG-PET to detect non-intestinal gastric primary tumors can range from 0% for T1 non-intestinal primaries to 77% for advanced non-intestinal disease. For intestinal-type tumors, sensitivity ranges from 44% for T1 tumors to 92% T2 or greater disease [7, 8, 13]. This may relate to the fact that the GLUT-1 transporter has been shown to be preferentially expressed on intestinal type gastric carcinoma cell subtype, with decreased expression on mucous secreting and signet ring type cells [14, 15]. GLUT-1 expression has been shown in multivariate analysis to be the most influential factor relating to [¹⁸F]FDG uptake in gastric carcinoma [16]. There is also a strong correlation between intra- or extracellular mucin production and SUV uptake, and many non-intestinal type tumors are represented by signet ring or mucinous gastric carcinomas [8]. However, a number of studies have not shown correlation between histological subtype and SUV uptake and sensitivity of [¹⁸F]FDG-PET in detection of primary gastric cancer [5, 6, 17]. It is not clear whether the location of the tumor (proximal/middle/distal) in the stomach influences the results of [¹⁸F]FDG-PET scanning, as results in this area are conflicting [6–8].

Simple measures such as distention of the stomach by water or less commonly food have been shown to improve the accuracy of detection of gastric lesions both preoperatively and in the postoperative remnant stomach [10, 18, 19]. In an effort to improve detection of gastric cancer by PET, the pyrimidine analog 3′-deoxy-3′-¹⁸F-fluorothymidine (¹⁸F-FLT) has been used as an alternative radiotracer. One study demonstrated increased sensitivity of ¹⁸F-FLT-PET for detection of gastric tumors, especially if those tumors were not [¹⁸F]FDG avid [4]. This may improve detection of previously difficult to detect tumor types such as mucin producing and signet ring cell tumors. A second smaller study showed comparable efficacy between the two moieties [5]. In both studies, mean SUV uptake was lower for ¹⁸F-FLT-PET than for [¹⁸F]FDG-PET. Additional improvements may be made possible by improving spatial resolution of the imaging equipment [20].

[¹⁸F]FDG-PET has not been shown to be an effective screening tool for the diagnosis of gastric cancer. In one study, combined with endoscopy in asymptomatic individuals, PET/CT detected 2/20 cancers from 2,861 patients screened giving a sensitivity of 10% only and a positive predictive value of 8.3%. 18/20 cancers were early gastric cancers (T1). There were 22 false positives on this study. There was no significant difference between the SUV values of the false positives and the true positives [21]. A second study of 1,336 asymptomatic patients detected two gastric cancers in addition to nine other malignancies. The rate of false positive in this study was 3 times the rate of true-positive findings [22]. Therefore, the screening sensitivity of [¹⁸F]FDG-PET in an asymptomatic population is less again than that in a diseased population. Furthermore, in a study of 1,006 subjects without a history of cancer who underwent screening PET and CT scan and were divided into two groups: those with a significantly increased SUV in the gastric wall and those without (SUV 4.1 and 2.8, respectively). There were no CT correlates of disease at the time of the PET/CT scan. The subjects were then followed for 1 year. None in the higher SUV group developed a gastric malignancy, and one in the lower SUV group developed a carcinoma in the stomach wall [23]. Thus, even increased SUV without a CT correlate in an otherwise healthy person may not be indicative of malignancy.

Survival in gastric cancer patients decreases with lymph node involvement, and with the number of lymph nodes involved. Five-year survival for patients with N0, N1, N2, and N3 gastric cancer is 86.1%, 58.1%, 23.3%, and 5.9%, respectively [24]. Knowledge of lymph node status therefore is not only of importance with respect to prognosis but may also guide in surgical treatment planning and when deciding which patients may benefit from neoadjuvant chemotherapy.

[¹⁸F]FDG-PET has been examined both alone, in comparison with CT imaging, and combined as CT-PET, in the preoperative assessment of nodal status for gastric cancer (Table 14.3). The sensitivity of PET is generally low for the detection of lymph node metastases ranging from 22% to 60% for normal resolution scans [3, 6, 7, 9, 17, 20, 25–27]. It is possible that this may reflect the low spatial resolution of PET which leads to difficulty discerning perigastric lymph nodes from the gastric primary tumor, as sensitivity has been shown to increase to up to 73% with a higher resolution scan [20]. This compares poorly to the sensitivity of CT which ranges from 52% to 77% in the same series. By contrast, the specificity of PET is higher than that of CT, ranging from 62% to 100%, compared to CT (range 62–94%).