For both Stage II and III colon cancer patients, increased numbers of LNs retrieved were associated with reduced risk of recurrence and improved cancer-specific survival. Identification of ≥20 LNs correlated significantly with reduced risk of recurrence and mortality of Stage II but not Stage III patients. For Stage III patients, collection of six additional LNs resulted in identification of one additional positive LN, and the probability of finding patients with pN2 nodal stage increased with increasing numbers of LNs examined.
LN involvement determines the pathologic stage and forms the basis for selection of patients for adjuvant therapy . Although Stage II disease, which has a relatively good prognosis, is characterized by the absence of LN involvement, about one third of these patients experience recurrences, due either to missed micro-metastases or to aberrant drainage of LNs beyond the field of resection, leading to under-staging [3, 9, 12]. Furthermore, for Stage III patients, inadequate LN recognition is associated with a poorer prognosis [15, 22], and increased LN collection has a favorable effect on prognosis [29–31]. Thus, diligent searches for LNs are needed for accurate assessment of nodal status and for correct assignment of stage.
The AJCC, ASCO, CAP, ACoS-CoC, and NCCN have recommended that a minimum of 12 LNs be examined in order to rule out metastases via the lymphatic system to nodal tissues [8, 20, 21]. This recommendation, however, is not widely practiced. Only 58% of those in the SEER database had ≥12 LNs harvested , and the NCCN database review documented a 60% failure rate in achieving resection of 12 LNs among various USA hospitals . Various minimum numbers of LNs harvested (range: 6 to 40) from colon resections, have been suggested for adequate staging of colon cancer patients [15, 22, 26, 32].
An increased LN examination confers a survival benefit, especially for stage II disease [14, 27, 32–34]. In the current investigation, however, examination of 12 LNs showed no significant survival benefit. By disease stage, there was a 55% and 31% reduced risk of cancer-specific mortality for Stage II and III patients, respectively, for those with ≥ 20 LNs examined. The 5-year survival of Stage II cases was 54.9%, whereas the survival of those who had ≥ 9 LNs examined after surgery was 79.9% . The 5-year survival of Stage II patients who had ≤ 8 LNs examined was similar to that for Stage III patients (51.8%). Sixteen of 17 studies of Stage II and 4 of 6 studies of Stage III showed improved patient survival with increased number of LNs examined . In contrast, for Stage III patients, the number of LNs examined did not serve as a prognosticator . The demonstration of increased mortality associated with the examination of ≤ 6 LNs, compared to > 6, especially in Stage II patients, is in concordance with other reports [16, 24, 27].
With tumor recurrence as the outcome for Stage II patients, all higher categories of LN collections showed a decreased risk, but only the ≥ 20 category approached significance, with a 67% decreased risk of recurrence within 2 years after surgery. In contrast, for Stage III patients, there was no relationship between increasing numbers of LNs examined with colon cancer recurrence. A low risk of recurrence for patients with ≥ 14 LNs examined compared to smaller numbers was reported earlier . There was a significant difference in recurrence with the number of LNs examined. Further, for pN1 and pN2 patients, disease-free survival improved as more LNs were removed, but there was no such association for node-negative patients. The impact of LN ratio (ratio of tumor-infiltrated nodes to total number of harvested LNs) on 3-year, disease-free survival was more prominent for patients with > 12 LNs examined .
In general, examination of an increased number of LNs results in greater chances of identifying LN metastases, thus minimizing under-staging [14, 27, 32–34]. In our investigation of patients with Stage III disease, for each additional LN collected, there was 19% increased probability of finding a positive LN. Thus, collection of six additional LNs resulted in finding one additional positive LN. In a mathematical model, the predictive probability of identifying single LN metastases was 0.25 if 12 LNs were examined and 0.46 if 18 LNs were examined . Since the probability of LN positivity increases as the number examined increases, there is no minimum number that reliably stages all patients [27, 33]. Higher LN counts, however, do not always correlate with increased rates of nodal positivity .
The accuracy of staging depends on multiple factors, including those that are modifiable (e.g., surgeon and pathologist) and un-modifiable (e.g., age, obesity, and socioeconomic status of the patient and anatomic location of the tumor) [16, 38, 39]. Pathologists encounter challenges in adequate LN retrieval. In Stage II colon cancer, the age of the patient, tumor size, specimen length, use of a structured pathology template, and academic status of the hospital are predictors of LN collection . Up to 70% of metastases are found in LNs that are < 5 mm in diameter and hence likely to be missed on routine visualization or palpation . Another challenge for pathologists relates to micro-metastases or isolated tumor cells that are missed in routine histological examinations. Although immunohistochemistry and polymerase chain reactions to identify cytokeratin and carcinoembryonic antigen  have been used to highlight malignant cells, the prognostic significance of LNs containing such micro-metastases is uncertain . Targeted LN examination by mapping of the most proximal LN (sentinel LN) improves the staging accuracy for colon cancer [43, 44]. In LN mapping, however, there are inconsistencies [45–47] that may be attributable to inadequate standardization, training, and interpretation of micro-metastases and to skip metastases [43, 48].
Survival in colon cancer is influenced by the presence of positive LNs and by the total number of positive LNs . For Stage III tumors, the AJCC sub-classifies nodal staging into pN1 and pN2, based on the presence of ≥4 positive LNs . In our analysis, the probability of having pN2 patients increased from 18% to 43% as the number of LNs examined increased from < 7 to ≥ 20. Similarly, there was increased disease-free survival as more LNs were examined from pN1 and pN2 patients . The probability of missing a positive LN was 29.7%, 20.0%, and 13.6% when five, eight, and twelve LNs, respectively, were examined . For node-positive patients, increased numbers of LN examination correlated with a lower LN ratio, which was associated with a better prognosis [37, 50]. Although most of these investigations involved large sample sizes, the cutoff values differed; thus, further investigations were warranted.