We examined the prognostic significance of the number of negative lymph nodes in population of stages 1–4 colorectal cancer patients who were concurrently assessed for other clinical and molecular predictors of patient outcome. We observed a significant relation between negative lymph node count and survival, independent of patient characteristics and other related molecular variables including p53, KRAS, BRAF, MSI, the CIMP, and LINE-1 hypomethylation. The effect of the negative lymph node count was apparent in all stages of disease, although the benefit was significantly greater among patients with earlier pathologic stage.
Examining molecular features or clinical outcome is important in colon cancer research (32
). In previous studies (1
), the number of recovered lymph nodes or the number of negative lymph nodes has consistently been associated with longer survival in colorectal cancer. Nonetheless, the benefit associated with a higher number of negative lymph nodes may simply reflect the host lymphocytic reaction to tumor (which is associated with lymph node count (10
)), as lymphocytic reaction to tumor has been associated with longer survival in colorectal cancer (40
). Moreover, greater lymphocytic reaction has been associated with MSI high (42
), which, in turn, is associated with longer patient survival (44
). Recent studies have further shown that MSI is associated with the CIMP, BRAF
), and LINE-1 methylation level (28
), and all of these factors (MSI, CIMP, BRAF
mutation, and LINE-1 methylation) have been independently related with survival of colon cancerpatients(46
).Therefore, numerous pathologic and molecular features (the lymph node count, lymphocytic reactions, MSI, CIMP, BRAF
mutation, and LINE-1 methylation) could account for the effect of the negative lymph node count. However, none of the previous studies of the number of lymph nodes and patient survival examined the aforementioned molecular features (MSI, CIMP, BRAF
mutation, and LINE-1 methylation) in colorectal cancer. In our analysis, the benefit associated with higher negative lymph node count remained significant after adjusting the various pathologic and molecular features.
The mechanism underlying the survival advantage associated with the negative lymph node count remains uncertain. Examining more lymph nodes may more accurately identify a metastatic focus and thus, avoiding misclassification of pathologic stage. Although this hypothesis can explain the effect of the negative lymph node count in patients with stages 1–2 cancers, it does not adequately account for the beneficial effect observed in stage 3 or 4 patients. In addition, a greater number of recovered lymph nodes may be an indicator of quality of surgical care or pathology (11
). However, a recent study has shown that hospitals that examine more lymph nodes in colon resection specimens are not associated with superior patient survival, raising a question on the value of lymph node counts as measure of quality of care (50
Alternatively, the number of recovered lymph nodes may be an indicator of host immune response to tumor cells (12
) or reflect some other specific tumoral molecular alteration associated with indolent tumor behavior. In this study, the negative lymph node count was associated with MSI high and CIMP high, both of which have been shown to be independently associated with longer patient survival (44
). Nonetheless, the effect of negative lymph node number on survival was not substantially altered by adjusting for lymphocytic reactions to tumor or tumoral molecular alterations including MSI and CIMP, although other unidentified tumoral feature may still account for the negative lymph node effect.
A location of a lymph node that shows a metastatic tumor may be important in predicting patient outcome. Metastasis to apical lymph node may imply worse outcome. Metastasis to non-regional lymph nodes, such as external iliac and para-aortic nodes, should be classified as distant metastasis (stage 4). However, a study with detailed record of locations of all recovered lymph nodes is difficult to conduct, and needs a close collaboration between surgeons and pathologists. Such collaborative large-scale studies are necessary in this area.
There are limitations in this study. For example, data on cancer treatment were limited. Nonetheless, it is unlikely that chemotherapy use differed substantially according to the number of negative lymph nodes, as such data were not typically used for treatment decision making during the conduct of the study. In addition, beyond cause of mortality, data on cancer recurrences were not available in these cohorts. Nonetheless, given the median survival for metastatic colorectal cancer was approximately 10–12 months during much of the time period of this study (46
), colorectal cancer–specific survival should be a reasonable surrogate for cancer-specific outcomes.
Most patients in this study developed colorectal cancers in 1990s and late 1980s, when the recommendation to obtain at least 12 lymph nodes was not present or widespread. Th e number of recovered lymph nodes in some stage 2 patients was indeed low; 30 % of stage 2 cases had less than seven lymph nodes examined. Nonetheless, we were indeed able to show that those stage 2 cases with fewer number of nodes experienced a significantly reduced survival, and this relationship between the node count and survival was independent of year of diagnosis, and other clinical, pathologic, and molecular features examined.
There are advantages in using the database of the two independent prospective cohort studies, the Nurses’ Health Study and Health Professionals Follow-Up Study to examine prognostic significance of the lymph node count and its interactions with tumoral and host factors. Anthropometric measurements, family history of cancer, other clinical information, pathologic and tumor staging data, and tumoral molecular features were collected prospectively, and entered into the database blinded to patient outcome. Cohort participants who developed colorectal cancer were treated at hospitals throughout the United States, and thus more representative of colorectal cancers in the general population, than studies based on a single to few hospitals. Tumor specimen procurement rate has been 60–70 %, and there were no demographic difference between cases with tumor tissue analyzed and those without tumor tissue analyzed (20
). In addition, our rich tumor database enabled us to simultaneously assess pathologic and molecular features of tumor and control for confounding by tumoral variables to some extent. None of the previous studies on lymph node count and patient outcome has examined as many molecular variables as we did in this study.
In summary, our large cohort study suggests that the number of negative lymph nodes is associated with longer survival of colorectal cancer patients, independent of patient, pathologic and molecular characteristics. Future studies are needed to elucidate exact mechanisms by which the lymph node count affects clinical outcome of colorectal cancer.