Risk Assessment in Stage II Colorectal Cancer
Risk Assessment in Stage II Colorectal Cancer
ABSTRACT: In the treatment of colon cancer today, the decision-making involved in the treatment of stage II disease is probably the most challenging aspect. The major question is whether or not these patients should receive postoperative adjuvant chemotherapy. Approximately 75% of stage II colon cancer is cured by surgery alone. For the remaining 25% of cases, there is great debate over whether adjuvant chemotherapy is sufficiently effective in enough patients to warrant the exposure to potentially toxic treatments. In the important QUASAR clinical trial, stage II patients were randomized to either fluorouracil (5-FU)-based therapy or observation. The results demonstrated an approximate 3% improvement in outcome for the 5-FU–treated patients. This leads to the assumption that treating all stage II patients with adjuvant chemotherapy is gross overtreatment, when essentially 97% of these patients will not benefit. Clearly the only way to approach this decision is through risk determination. In this article, I will describe the current state of defining high- and low-risk disease, which is mainly through histopathologic characteristics, as well as discuss emerging approaches such as molecular markers and genomic profiling.
Possibly the most difficult and confusing subject in colon cancer today is the decision-making involved in the treatment of stage II colon cancer. The pendulum has been swinging back and forth over the past several decades as to whether stage II colon cancer patients should receive postoperative adjuvant chemotherapy or not.
Which Stage II Patients Should Receive Treatment?
The issues themselves are simple. Surgeons cure stage II colon cancer approximately 75% of the time, which of course is a fairly good number. However, this leaves 25% of the patients who will ultimately die of their disease. Postoperative adjuvant chemotherapy has been given to these patients and, in a series of clinical trials, has demonstrated a small but measurable improvement in outcome. The most important of these, and the only one that was adequately powered to answer this question, is the QUASAR (QUick And Simple And Reliable) clinical trial. In this trial, mostly stage II colon cancer patients were randomized to either fluorouracil (5-FU)-based therapy or observation. The results demonstrated an approximate 3% improvement in outcome when 5-FU was administered to this patient subset. Said another way, 97% of patients were exposed to chemotherapy without any benefit. So, it also raises the issue of whether or not all patients with stage II colon cancer should be exposed to chemotherapy, knowing that only 3% of them will benefit from the treatment (Figure 1). This is clearly an overtreatment of most of the patients given that 97% of them will not benefit from the therapy administered. However, given that the 3% are, in essence, cured of their disease, these are high-stakes gambles that most patients will accept.
In the world of breast cancer, there are several studies that point to even as low as 1% benefit rate that patients will accept as justification for receiving adjuvant chemotherapy. Similar studies have been performed in colon cancer but are not yet published. The world of decision-making in stage II colon cancer, in my opinion, is a lot like religion. You either believe that it is appropriate to treat these patients, treating them similarly to stage III patients and recognizing the benefit rate is less, or you believe that this is an inappropriate use of our resources and exposure of patients to toxicity and, therefore, unwarranted. You either believe or you do not.
Further confusing the arguments, we have long felt that stage II colon cancer was simply a step on the path from stage I on to stage III disease, that each of these stages represents a continuum of, in essence, the same cancer with new additional genetic abilities of metastasis being acquired. Newer evidence, however, contends that it is possible that stage II colon cancer is genetically different than stage III colon cancer, that they are in fact different cancers genetically. This raises a further complicating factor as to whether these patients should even be included into routine adjuvant clinical trials. Maybe that is why the results are different between the stages. There is a great deal of debate on an international level between cooperative groups as to whether stage II colon cancer should be included, with the NSABP (National Surgical Adjuvant Breast and Bowel Project) and European groups often including stage II colon cancer, whereas others, such as the Southwest Oncology Group (SWOG) and Cancer and Leukemia Group B (CALGB), not including stage II patients in their adjuvant trials. It is unclear who is right in this argument.
Let us begin by looking at our current ways of stratifying patients. Clearly what we need is a way to determine which patients are in the high-risk category and therefore should be given chemotherapy, and those who will not have recurrent disease and are therefore in the low-risk category. There has been a great deal of focus on this over the past several years, and as a result, I think we are left with more questions than answers.
Defining High-Risk Stage II Colon Cancer
Our current determination of high-risk and low-risk stage II colon cancer is purely based on histopathologic characteristics. Based on data that have come forward over the past 2 decades, we looked at patients with T4 lesions, those who have had obstruction or perforation, those with lymphovascular invasion, and those with poorly differentiated histology as being high-risk patients. These are all quite gross measures (if one considers our expanding understanding of the molecular biology of cancer) to determine who is in trouble and who is not. For example, one obvious flaw in this strategy of risk assessment is microsatellite unstable tumors. These tumors typically have poorly differentiated histologies that would put them in the high-risk category by that measure, but we know that MSI-H (microsatellite instability–high) tumors have a better prognosis and may actually not benefit from chemotherapy at all. Here our molecular biology insights are in stark conflict with our current risk assessments. So these gross measures of delineation of high and low risk based on purely histopathologic testing are not going to serve us well in the long term.
Lymph Node Count
A more recent focus has taken us to the lymph node count in a patient’s specimen, with a cutoff now being somewhat arbitrarily set at 12 lymph nodes. Data suggest that the more lymph nodes that are collected, the better a patient’s prognosis. This is most pronounced in stage II disease. It is unclear why some patients have more lymph nodes than others, with the debate being that it is possibly the surgeon’s fault, possibly the pathologist’s fault, or even possibly the patient’s fault. However, the data do suggest that those with higher numbers of negative lymph nodes have better prognosis. Is this nothing more than an appropriate assignment of the stage of the patient? It is certainly possible that as more lymph nodes are sampled, the likelihood of truly being a stage II vs a stage III increases.
Newer assays have been developed to try and look at this even further. Studies have been done looking at microscopic tumor deposits within lymph nodes; these studies have demonstrated that patients with microscopic disease do behave more like stage III patients. Therefore, these data suggest that we should analyze our tumors to this level if at all possible. This new assay is available but has not been widely incorporated into practice at present, but it is certainly something we need to continue to consider in making decisions for our patients.