Although the number of lymph nodes evaluated for colon cancer has increased over the past 20 years this has not resulted in an increase in the overall proportion of cancers that are node positive, reports a population-based study in JAMA. The results, say the authors, call into question the "upstaging mechanism" as the primary basis for improved survival in patients with more lymph nodes evaluated.

Among patients surgically treated for colon cancer, several studies have demonstrated better survival for patients with more lymph nodes evaluated.

The proposed mechanism behind this association suggests that a more extensive lymph node evaluation reduces the risk of understaging, in which inadequate assessment may incorrectly identify a patient with node-positive disease as node negative, thus failing to identify appropriate treatment.

Based on these studies, most consensus panels now advocate for the surgical evaluation of 12 or more lymph nodes for staging of newly diagnosed colon cancer. However, more recently some studies have questioned the benefit of increasing the number of lymph nodes evaluated.

In the current study Helen Parsons and colleagues from the National Cancer Institute (Bethesda, Maryland, US) analyzed 20 year trends in the degree of lymph node evaluation for 86 394 patients with colon cancer using the data collected between 1988 and 2008 from the Surveillance, Epidemiology, and End Results (SEER) program.

The results showed that the number of lymph nodes evaluated increased between 1988 and 2008, but did not result in a significant increase in lymph node positivity.

Between 1988 and 1990 34.6% of patients (3875/11200) had 12 or more lymph nodes evaluated, increasing to 73.6% (9798/13 310) between 2006 and 2008 (P<.001). The proportion of node positive cancers, however, was 40% between 1988 and 1990 and 42% between 2006 and 2008 (P=.53).

Although patients with high rates of lymph node evaluation were only slightly more likely to have node-positive disease, they experienced significantly lower relative hazard of five-year death compared with those with fewer lymph nodes evaluated. Results showed that the adjusted HR for mortality for patients with 30 to 30 lymph nodes evaluated versus patients with 1-8 lymph nodes evaluated was 0.66.

"Our findings do indicate that greater lymph node evaluation unlikely leads to improved survival primarily through the more accurate detection of node-positive disease. These findings suggest that other factors besides upstaging, such as improved surgical quality or postsurgical care, may be the driving mechanism behind the lymph node-survival relationship," write the authors, adding that implementing wide-ranging quality improvement initiatives to increase lymph node evaluation for colon cancer may have limited effect on improving survival in this population.

In an accompanying commentary Sandra Wong, writes: "It is likely that the relationship between lymph node counts and survival is confounded by unmeasured variables."

Improved lymph node counts, she adds, may be a proxy for improved cancer care overall, whether on the part of surgeons who perform a more thorough cancer operation or pathologists who are more diligent in examining operation specimens.(ie, factors that are related to both the exposure and outcome)."

Reference

H M Parsons, TM Tuttle, KM Kuntz et al. Association between lymph node evaluation for colon cancer and node positivity over the past 20 years. JAMA September 14, 2011, 206: 1089-1097. doi:10.1001/jama.2011.1285

SL Wong. Lymph node evaluation in colon cancer: assessing the link between quality indicators. Ibid, 1139-1140. doi: 10.1001/jama.2011.1318