Diagnostic accuracy of CT-staging of advanced gastric cancer following neoadjuvant chemotherapy.

Abstract

4551 Background: Neoadjuvant or perioperative chemotherapy has been accepted as a standard treatment globally in patients (pts) with locally advanced gastric cancer (LAGC). In PRODIGY phase III study (n = 530), we have demonstrated that neoadjuvant chemotherapy with DOS regimen (docetaxel, oxaliplatin, S-1) led to significant tumor downstaging and improved PFS in Korean LAGC pts (Kang, et al. ESMO 2019). Although CT has been performed to re-stage the tumor after neoadjuvant chemotherapy, there has been a relative paucity of diagnostic accuracy data. This study is to evaluate the diagnostic performance of restaging of LAGC after neoadjuvant chemotherapy using CT in PRODIGY study population. Methods: Of 266 pts, who had been diagnosed LAGC of T2-4 or N+ stage as assessed with CT and randomized to neoadjuvant chemotherapy arm (CSC) in PRODIGY study, 214 pts underwent gastrectomy were included in this analysis. The post-chemotherapy T- and N- stage was determined based on CT scan taken just prior to surgery and compared with the pathologic stage (AJCC 7th edition). Two experienced radiologists independently evaluated depth of primary tumor and reached consensus if any discrepancy between two readers. Diameter of short axis of the largest regional lymph node was measured to predict metastatic lymph node. Result of histopathologic T- and N-staging using surgical specimen was used as reference standard. Results: The study cohort consisted of pathologic T0 (n = 22), T1(n = 39), T2(n = 31), T3(n = 79), and T4(n = 43). The overall diagnostic accuracy of CT was 45%. For each T-stage, accuracy of T0,T1,T2,T3, and T4 was 0%, 26%, 29%, 55% and 79%, respectively. Rate of over- and under- staging was 47% and 8%, respectively. Accuracy for prediction of downstaging to early gastric cancer (T0-T1) was 83%. Interobserver agreement of T-staging was moderate (k = 0.41). There were 98 patients of N+ and 116 patients of N- at histopathology. Area under the curve of receiver operating characteristics to differentiate lymph node metastasis was 0.63. Sensitivity and specificity of size criteria of the largest lymph node (cut off value: > 6mm, > 7mm, and > 8mm) to predict pathologic N+ were 90% and 17%, 78% and 34%, and 68% and 51%, respectively. Conclusions: Re-staging using CT after neoadjuvant chemotherapy showed suboptimal accuracy and over-staged residual tumor. However, it predicted downstaging of gastric cancer with high accuracy.