Tuesday, May 20, 2014

Analysis of CTC Concentration in Blood

As my partner had illustrated in his own blogpost, there is believed to be a negative correlation between CTC count and the severity of cancer. The study of CTCs is relatively new and there is a lack of general consensus on what dangerous levels of CTCs actually are. Therefore, most studies determine a concentration of CTCs in the blood at which the patients are believed to be at risk. This is accomplished through a series of tests in which the experimenters take a series of CTC concentrations that they believe to be relevant and note at which concentration the hazard ratio appears to significantly increase. A hazard ratio is a description of the relative risk of a complication occurring with or without a specific event. In CTC studies the event would refer to CTC concentration while the complication could be death or cancer progression. However, the identity of these variables is the choice of the researchers. Once a CTC concentration is determined to be related to a relevant change in hazard ratio, it is labeled as the cutoff value. This cutoff value is then used to mark the change between high and low CTC counts.

This article from the New England Journal of Medicine details the difference in survivability for patients with high and low CTC counts based on a cutoff level determined by a hazard ratio. This particular study also included the training set and validation set. The training set displayed here is a patient set used to test the cutoff level which utilizes all the patients used to select the cutoff level. The results of this set display the change in patient survivability at high and low CTC concentrations based on cutoff level. The validation set is used to verify that the results of the training set are replicable through the use of different patients under the same conditions. Therefore, the results of the validation set are expected to be similar to that of the training set. In this test once the cutoff value was validated, patients from both sets were pooled together and placed into a full set. This set is simply a group of all patients which was utilized over a longer period of time than the training and validation sets. Due to the larger patient pool and the longer time in the study, it should represent a more patient survivability more accurately than the validation and training sets.

The study was designed with about 177 breast cancer patients. The CellSearch and CellSpotter Systems were used to identify, isolate, and enumerate CTCs from patient blood draws. The cutoff level was determined using a training set consisting of 102 patients over a period of 40 weeks while the remaining 75 patients were used in the validation test for 40 weeks. Patients from both sets were then pooled into a full set in which blood was drawn over a period of 80 weeks. The results of the study are displayed in the graphs below.

Figure 1Kaplan–Meier Estimates of Probabilities of Progression-free Survival and Overall Survival in Patients with Metastatic Breast Cancer for Those with <5 Circulating Tumor Cells per 7.5 ml of Whole Blood and Those in the Group with ≥5 Circulating Tumor Cells per 7.5 ml of Whole Blood before Initiation of a New Line of Therapy.
Figure 1. Kaplan-Meier Estimates of Patient Survival since Baseline Blood Collection in Patients with Metastatic Breast Cancer. Plots A, B, and C represent the data for progression free survival. Plots D, E, and F display the data for overall survival. The hazard ratio for panel A is 1.97, B is 1.81, C is 1.95, D is 3.98, E is 5.22, and F is 4.39. The cutoff value between high and low CTC counts was determined to be at 5 CTCs per 7.5 mL of blood.
As illustrated by the figure, patients with a CTC count of ≥ 5 CTCs per 7.5 mL of blood had a lower probability of both progression free and overall survival than those with < 5 CTCs per 7.5 mL of blood for all sets of data. Furthermore, each validation set appears to confirm the cutoff value because of the similarities in both the lines’ curvature and the median survival values. Thus the cutoff value apparently displays a significant change in survivability based on CTC concentration.
Because of its use in determining the cutoff range and its relation to the risk of complication, one could expect that the magnitude of the hazard ratio could make an accurate measurement of patient survivability by itself. Interestingly, this value seemed to have no effect on the patient survival across different sets. This was exemplified by what appeared to be a significant difference in hazard values between the training and validation sets for overall survival. The training set had a ratio of 3.98 while the validation set had one of 5.22. Nonetheless, the appearance of both data curves was similar. For the < 5 CTCs sets, the median overall survival was above 40 weeks and the curvatures of the lines were almost identical. Both of the ≥ 5 CTCs sets had a median overall survival of over 40 weeks and had reached approximately a 55% survival by week 40. However, the curve of the training set was higher than that of the validation set between 20-25 weeks. Nonetheless, the plots matched each other in all other areas. The experimenters likewise did not believe that the difference meant anything significant. Thus, it would seem that hazard value can only be used to estimate the relative degree of risk within a specific set of people and that its use outside of these sets is fairly limited.

Cristofanilli, Massimo, G. Thomas Budd, Matthew J. Ellis, Alison Stopeck, Jeri Matera, M. Craig Miller, James M. Reuben, Gerald V. Doyle, W. Jeffrey Allard, Leon W.M.M. Terstappen, and F. Hayes. "Circulating Tumor Cells, Disease Progression, and Survival in Metastatic Breast Cancer." New England Journal of Medicine 351.8 (2004): 781-791. Web. 18 May 2014.