Validation of T Stage According to Depth of Invasion and N Stage Subclassification Based on Number of Metastatic Lymph Nodes for Distal Extrahepatic Bile Duct (EBD) Carcinoma

According to the current AJCC staging system, the T stage of distal extrahepatic bile duct carcinoma (EBD) is classified according to the extent of the tumor within or beyond the bile duct wall. However many invasive carcinoma accompany stromal desmoplasia that obscure lower boundary of bile duct wall; it is frequently difficult to clearly define the extent of tumors using the current T classification system. In this study, we validated an alternative T classification system by depth of invasion (DoI; T1: < 5 mm, T2: 5 to 12 mm, and T3: ≥ 12 mm). Specifically, we evaluated DoI in 114 cases of distal EBD carcinoma using digital scan images to achieve more objective measurements of tumor DoI. In addition, we evaluated the effect of the number of metastatic lymph nodes (LNs) as well as the number of total examined LNs on the survival rate in the same patient group, and performed a comparative analysis of these data to assess patient survival. We also analyzed 114 cases of distal EBD carcinoma using the current T and N classification of the AJCC staging system (7th edition). The T stage of the current AJCC staging system was not associated with significant differences in patient survival, especially between T2 and T3. However, T staging by DoI was associated with statistically significant differences in patient survival (P < 0.001 in DoI-1, P = 0.002 in DoI-2). With respect to N stage, we divided patients into 3 tiers comprising class 1 (no nodal metastasis), class 2 (1–3 nodal metastases), and class 3 (4 or more nodal metastases). In 3-tier classification analysis, the median survival times for classes 1, 2, and 3 were 79.2, 28.8, and 10.9 months, respectively. The difference in survival among the 3 classes was statistically significant (P < 0.001). We found the cut-off value of 11 LNs (1 to 10 vs ≥ 11) for N0 stage showed most significant difference (P = 0.007). We think at least 11 LNs should be examined for more accurate evaluation of N stage in distal EBD carcinoma. We propose an alternative T classification using DoI and 3-tier sub-classification of N stage for distal EBD carcinoma.