Fused PET/CT is an important imaging modality for detection of gastric tumoral recurrence as it gives detailed morphologic and metabolic activity compared to single contrast-enhanced CT .
Park et al. reported that the sensitivity, specificity, PPV, NPV, and accuracy of FDG PET/CT for the diagnosis of true recurrence on a per-person basis were 75, 77, 89, 55, and 75%, respectively. So, they concluded that PET/CT has a role in detection of tumoral recurrence in gastric cancer .
Sun et al. evaluated the value of PET/CT in postoperative follow-up of gastric cancer patients and reported that the overall accuracy, PPV, and NPV of FDG PET/CT were 82.6, 77.7, and 85.7%, respectively . On the other hand, De Potter et al. reported that FDG-PET has a limitation as a primary tool for follow-up due to its moderate accuracy .
In recent studies, Sim et al., Bilici et al., and Baiocchi et al. showed importance of using FDG PET/CT for the diagnosis of tumor recurrence in patients with previously treated gastric cancer and equivocal imaging findings. Also, they made comparative studies between PET/CT and contrast-enhanced CT for detection of tumoral recurrence. They reported that the overall sensitivity, specificity, accuracy, positive predictive value, and negative predictive value for FDG PET/CT were significantly higher to those of diagnostic CT [5, 16, 17].
In our study, according to organ-based analysis in the five organ-based groups (locoregional recurrence, regional lymph node recurrence, liver metastasis, peritoneal metastasis, and distant metastasis), we found that all values of diagnostic performance were greater than 80%. The diagnostic accuracy rates were very high reaching up to 100% at regional lymph nodes recurrence and liver metastasis detection. Our results seemed better than those of the fore-mentioned studies as well as the results of Cayvarli et al.; their diagnostic accuracy rates reached up to 80% with low specificity in locoregional recurrence 47.8% and distant metastasis 71%. They explained that the decease of diagnostic specificity in their study is especially due to low specificity value of locoregional recurrence, causing of false-positive FDG uptake in anastomotic site as result of postoperative inflammatory changes. Another reason for that was the low specificity value of distant metastasis. However, in our study, we found seven false-positive patients with locoregional FDG activity. We reached the final diagnosis by biopsy and follow-up as postoperative inflammatory changes. In addition, our study conducted comparison between PET/CT diagnosis and final diagnosis in each organ-based group; the results were statistically significant where p values were 0.001, < 0.001, < 0.001, < 0.001, and < 0.001 for locoregional recurrence, regional lymph node recurrence, liver metastasis, peritoneal metastasis, and distant metastasis, respectively.
Similar to Cayvarli et al., we evaluated the most common distant metastatic groups separately; we showed high diagnostic accuracy values reached up to 100%. These results were better than those of the above-mentioned report. Although, we found low-positive predictive value of adrenal gland metastasis < 50%; this result is almost explained that we found one true-positive and two false-positive patients. Additionally, we found statistically significant difference in diagnostic performance of PET/CT compared to final diagnosis on per-lesion basis in most of the distant metastatic groups. P value was statistically significant in lung, bone, mediastinal lymph nodes, para-aortic lymph nodes, and subcutaneous/intramuscular nodules groups (p values were < 0.001, 0.001, < 0.001, < 0.001, and 0.001). No statistical significance was found in the adrenal gland metastasis.
PET/CT has some limitations affecting the sensitivity and specificity of this modality for detection of recurrence such as low-grade metabolic activity of a malignant lesion or small-sized peritoneal deposit can give false-negative results [15, 18, 19]. Some types of gastric cancer such as SRC and mucinous adenocarcinoma also show low-grade FDG activity, and thus they can give false-negative results [20, 21]. We disagreed with these findings in our results, although the number of patients in the SRC/mucinous adenocarcinoma subgroup was small (n = 8). Also, John M. Findlay at el. emphasized the role of PET/CT in finding metastases at which its risk differentiate between false positives and false negatives which justifies its potential benefit .
Our study had several limitations; the major limitation of this study was the small sample size. In addition, a short-term follow-up period may not sufficient to confirm the absence or presence of recurrence. Not all the recurred cases were confirmed by histopathological diagnosis. Therefore, there was the possibility of including cases in which false-positive lesions were treated as true-positive lesions by anticancer drugs, or true-positive lesions were not identified in the clinical setting. Further studies with larger number of patients to examine other correlations and to set standard methods of measurement and ranges of normality are recommended.