1Medizinische Klinik und Poliklinik I, Universitätsklinikum Carl Gustav Carus der TU-Dresden, Dresden, Germany; 2The Cancer Research Company Section of Medicine and the GI Unit, The Royal Marsden Hospital, Sutton, UK; 3Department of Internal Medicine and Oncology, Azienda Ospedaliera S. Maria, Terni, Italy; 4University Hospital, Department of Oncology, Uppsala, Sweden; 5Academisch Ziekenhuis Utrecht, Department of Internal Medicine, Utrecht, The Netherlands; 6Hospital Universitario Reina Sofia, Cordoba, Spain; 7University Hospital, Department of Internal Medicine, Vienna, Austria; 8Hôpital Ambroise Paré, Service de Hepato-Gastroent. Concologie Digestive, Boulogne Cedex, France; 9ASTRA-ZENECA Pharmaceuticals, Macclesfield, UK; 10Laurentius Hospital, Roermond, The Netherlands; 11EORTC Data Center, Brussels, Belgium; 12Medical School Hannover, Department of Internal Medicine, Hannover; 13Robert-Rössle-Clinic Charité, Campus Berlin-Buch, Berlin, Germany
Received 6 July 2001; accepted 27 August 2001.
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Abstract |
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Patients with metastatic colorectal cancer are usually offered systemic chemotherapy as palliative treatment. A multivariate analysis was performed in order to identify predictors and their constellation that allow a valid prediction of the outcome in patients treated with 5-fluorouracil (5-FU)-based therapy.
Patients and methods
A total of 3825 patients treated with 5-FU within 19 prospective randomised and three phase II trials were separated into learning (n = 2549) and validation (n = 1276) samples. Data were analysed by tree analysis using the recursive partition and amalgamation method (RECPAM). A predictor could only enter the RECPAM analysis if the number of patients with missing values was <33.3% within a node, and the minimal node size was set to 50 patients. Twenty-three potential predictors were grouped into subsets of laboratory variables (11 parameters), tumour-related variables (seven parameters) and clinical variables (five parameters). In the first step, tree analysis was performed separately for each predictor subset. The selected prognostic parameters of the resulting partial models (the winners) were entered into the general model. The classification rule from the data of the learning set was applied to the independent validation set.
Results
Winners of the subgroup analysis for laboratory variables were: platelets 400 x 109/l, alkaline phosphatase
300 U/l, white blood cell (WBC) count
10 x 109/l and haemoglobin <11 x 109/l, and all predicted a worse outcome. Negative predictors within the subgroup of tumour parameters were: number of tumour sites more than one or more than two, presence of liver metastases or peritoneal carcinomatosis, which predicted a worse outcome. Furthermore, presence of lung metastases, a primary rectal cancer and presence of lymph node metastases all predicted a better outcome in the multivariate setting. Among the clinical parameters only performance status of ECOG 0 or 1 predicted better outcome. In the final regression tree, three risk groups could be identified: low risk group (n = 1111) with a median survival of 15 months for patients with ECOG 0/1 and only one tumour site; intermediate risk group (n = 904) with a median survival of 10.7 months for patients with ECOG 0/1 and more than one tumour site and alkaline phosphatase <300 U/l or patients with ECOG >1, WBC count <10 x 109/l and only one tumour site; high risk group (n = 534) with a median survival of 6.1 months for patients with ECOG 0/1 and more than one tumour site and alkaline phosphatase of
300 U/l or patients with ECOG >1 and more than one tumour site or WBC count >10 x 109/l. The median survival times for the good, intermediate and high risk groups in the validation sample were 14.7, 10.5 and 6.4 months, respectively.
Conclusions
Patients can be divided into at least three risk groups depending on the four baseline clinical parameters: performance status, WBC count, alkaline phosphatase and number of metastatic sites. Any molecular or biological marker should be validated against these clinical parameters and decisions for more or less intensive treatments may be studied separately in these three risk groups. Also, clinical trials should be stratified according to the three risk groups.
Key words: colorectal neoplasm, metastatic disease, multivariate analysis, prognosis
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Introduction |
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Clinical trials, although using similar patient selection criteria, often display a surprising heterogeneity in survival rates [3], which is usually explained by differences in patient characteristics or prognostic factors. Patients entering randomised trials are usually stratified according to their performance status, but in addition other prognostic factors or their constellation may have the potential to determine the survival of patients to a greater extent than any promising antineoplastic agent or drug combination.
A variety of clinical parameters such as performance status [4], elevated lactate dehydrogenase, white blood cell (WBC) count [5], serum albumin [4], elevated liver transaminases [6], level of haemoglobin [7] or platelets, pathological grading [8] or localisation of the primary [9], or tumour markers like carcinoembryonic antigen (CEA) [8] have been identified as prognostic markers in some studies that seldom included more than 400 patients. There is no consensus and general acceptance about the importance of various prognostic factors. We collected a large database of patients with metastatic colorectal cancer who were eligible for clinical trials and treated with a 5-fluorouracil (5-FU)-based regimen, in order to identify predictors and their constellation that would classify patients according to their predictor values into a few clearly separated groups with different risks.
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Materials and methods |
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We preferred RECPAM over the classical Coxs regression [31] approach for the following reasons:
(i) RECPAM is directly focussed on the derivation and characterisation of clearly separable risk classes rather than on fitting regression equations to the data.
(ii) Interactions between the predictors, if present in the data, are entered automatically into the model.
(iii) Missing values of active predictors are replaced by surrogate variables in a special way, newly adapted for each node. Thus all patients may contribute to the derivation of the model.
(iv) Internal validation is incorporated into the model by applying pruning techniques and the Akaike Information Criterion (AIC) as an approximate cross-validation procedure. It was applied to select the tree with the smallest AIC as the best honest tree, but also to select the simplest acceptable tree by applying the elbow rule to the curve of AIC versus number of nodes within the pruning procedure [30]. Thus, prediction bias due to over-fitting is generally avoided.
External validation. In order to add an external validation to the analyses, we split the population randomly into a learning (n = 2541) and a validation (n = 1276) set. The RECPAM prediction model was derived using only the learning set and it was then applied to the independent validation set. Thus the predicted survival curves within the validation clusters could be compared with the observed curves.
Details of the RECPAM approach. The prediction model was derived in two steps: first a RECPAM subtree was derived for each of the above-mentioned subset of predictors (clinical, tumour-related and laboratory parameters). In the second step, only those parameters that were selected for the subtrees (the winners) were chosen as candidates for the general model. By this restriction the possibility of over-fitting was further diminished. In addition, the results of the subtree models are of interest in their own right.
The RECPAM method itself was applied with the following restrictions: the minimal node size was set to 50 patients throughout the tree, and at each node only those variables with <33% missing values were accepted for the next split.
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Results |
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For the tumour-related parameters the number of tumour sites of more than one and also of more than two, and presence of liver metastases or peritoneal carcinomatosis predicted a worse patient outcome. In contrast, in well-defined subpopulations, presence of lung or lymph node metastases and rectal cancer as the primary tumour were all associated with a better outcome. Among the laboratory parameters, platelets >400 x 109/l, alkaline phosphatase of >300 U/l, WBC count >10 x 109/l and haemoglobin <11 g/dl all predicted an inferior survival probability. The winners of each subgroup analysis were entered into the general predictive model.
General predictive model
The general model without amalgamation is displayed in Figure 1 and Table 4. With the knowledge of eight different parameters the individual prognosis of a patient can be estimated, ranging from a median of 4 months up to >16 months. The strongest predictor for survival is the performance status. The number of involved tumour sites is separating the cohort learning sample at seven of 15 possible nodes, thus indicating the importance of tumour load for the prognosis of patients with metastatic colorectal cancer. The WBC count (cut point at 10 x 109/l), location of the primary tumour, level of alkaline phosphatase (cut point of 300 U/l), platelets (cut point of 400 x 109/l), haemoglobin (cut point of 11 x 109/l), and presence of peritoneal involvement all entered the general model and influence the prognosis at various nodes in various constellations with other parameters.
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Figure 3 shows the survival curves of the three different risk groups for the learning and validation samples. In both samples the three risk groups are clearly separated with a high level of concordance. Also, the survival curves for the good, intermediate and high risk groups of the validation set lie within the 95% confidence limits of the learning set for the first 24 months at least. The P value for the three risk goups in the validation set is <0.0001.
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Discussion |
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Earlier attempts to identify prognostic factors identified variables consistent to our findings. Most investigators re-ported the performance status [5, 8, 3234] as a major prognos- tic factor, but the WBC count (10 x 109/l) [5, 35, 36], haemoglobin [6], alkaline phosphatase [5] and site of the primary tumour (i.e. colon or rectum) [9] were also identified by some.
Our general model, however, has some limitations. In approximately two-thirds of cases we had no information on weight loss, level of lactate dehydrogenase (LDH), serum protein, albumin or pathological tumour grading. In addition, in 50% of patients information on bilirubin, transaminases, CEA levels and tumour-related symptoms were missing. Serum levels of LDH, CEA, bilirubin transaminases and albumin and weight loss were all identified as prognostic factors in smaller cohorts of patients [5, 8, 32, 3739]. The RECPAM analysis uses surrogate markers in case of missing values in certain patients. This is a useful tool in order not to exclude too many patients with missing values or alternatively not too exclude too many variables from the analysis. We determined that no variables should have >33.3% of patients with missing values within a node that separates the population by this variable. Hence, all variables with a moderate (50%) or high (75%) rate of missing value still had a chance to enter the model; however, the likelihood was lower compared with the chances of other variables, unless they had a marked impact on prognosis.
Any multivariate analysis faces the problem that only those variables that have been put into the model come out as important predictors. It is possible that other potential parameters or constellation of variables may better describe and separate the prognosis of patients with metastatic colorectal cancer. Nevertheless, our learning sample clearly identifies three separated risk groups validated in the validation set. Whether other variables including those with a high rate of missing values might add any information to our model remains speculative and at present less important, as we were able to clearly separate patient risk groups with the available data. The difference in median survival in the three groups is also of definite clinical relevance.
Our model was based on patients who received several forms of 5-FU-based treatment, i.e. bolus 5-FU, 5-FU with modulation or infusional 5-FU regimens. Whether this model also applies to patients receiving 5-FU in combination with CPT-11 or oxaliplatin as their first-line treatment is unknown. In other words, it may by speculated that a more effective treatment may alter the predictive value of prognostic factors. Interestingly, the number of involved organ sites, performance status and WBC count have all been identified as important prognostic factors in a recent randomised trial with the use of CPT-11 and 5-FU as first-line treatment [39]. Also, two studies with the use of 5-FU in combination with oxaliplatin found the number of involved organ sites, performance status and alkaline phosphatase as independent prognostic factors consistent with our findings [40]. In addition, similar results have been reported in two studies for patients receiving CPT-11 as second-line treatment after 5-FU failure [41, 42]. In a multivariate Coxs regression analysis performance status, haemoglobin, WBC count and alkaline phosphatase were identified in both studies and number of involved tumour sites, aspartate aminotransferase and right-sided primary tumours of the colon in at least one of these two trials. In summary, the prognostic factors identified within our model appear to be a consistent finding, irrespective of the type of cytostatic treatment and whether patients receive first- or second-line therapy.
What are the potential implications of our findings?
First, our three risk groups could serve as a useful stratification tool for clinical trials. Four parameters separate patient survival by a median of 5 months. Indeed, very often multivariate analysis performed in controlled randomised trials demonstrates a larger influence on survival for certain prognostic factors than for the treatment that was under investigation [19, 39, 43]. This is of special importance as most clinical trials demonstrate only a small difference, if any at all, in median survival, which seldom exceeds 23 months.
Secondly, to use the four variables for a more precise description of patients entering non-randomised trials also appears useful. Patients entering phase II trials may often represent a more favourable subgroup. Thus, a median survival of 12 months in a patient cohort that belongs to the high risk category may be very promising, but more disappointing for a low risk group.
Thirdly, our model allows for an estimate of expected individual patient survival, and every fifth patient in our population had a a median survival of only 6 months, despite receiving 5-FU-based treatment. Prospective trials should answer the question of whether this high risk group of patients might benefit from a more intensive first-line treatment. Preliminary retrospective data, presented by Knight et al. [44], indicate the opposite, namely that patients with a favourable prognostic profile, rather than those with a poor one identified by levels of LDH, did benefit more from intensified treatment. Nevertheless, only a prospective randomised trial would help to identify the groups of patients that benefit most from intensified combination treatment with CPT-11 or oxaliplatin added to modulated 5-FU.
Fourthly, the level of haemoglobin has also been identified by the Nordic Group as a risk factor. Recently, trials using erythropoietin to increase the haemoglobin level suggest the possibility of an improved outcome for patients with haematological malignancies and solid tumours receiving erythropoietin in addition to chemotherapy [45, 46]. Our investigation implies that further study of the effects of erythropoietin in patients with metastatic colorectal cancer and low haemoglobin levels is necessary.
Finally, markers identified as predictors of response to chemotherapy such as thymidilate synthetase or thymidine phosphorylase [47, 48], or molecular prognostic parameters for survival, such as proteins involved in the cell cycle regulation and apoptosis [49], should be validated against our simple model, as all of them need sophisticated and expensive laboratory investigations and thus are less easily obtained compared with our simple clinical parameters.
In summary, patients with metastatic colorectal cancer are very heterogeneous in respect to survival after a standard cytostatic drug treatment. Patients entering randomised trials for metastatic colorectal cancer may be stratified by at least four simple parameters. Also, the EORTC GI group will initiate studies for high risk, intermediate and low risk patients separately. Our model may serve as a point of reference for any other models using different parameters.
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Footnotes |
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