Sunday, July 13, 2008

Neoplasms: 6

This is the sixth blog in a series of blogs on neoplasia.

In the past few blogs, I've been trying to explain the disconnect between cancer survival data and cancer death rate data. The cancer survival data seems to indicate that we're making enormous improvements in cancer treatment. The cancer death rate indicates that Americans are dying from cancer at about the same rate as they had been a half-century ago.

Several days ago, I listed over a dozen biases in cancer survival data that contribute to an overly optimistic sense of medical progress.

In this and the next few blogs, I thought I'd review some of these biases. The purpose of this exercise is to explain that the interpretation of survival data is enormously complex and that survival data is probably not the best way to gauge progress in the field of cancer research.

Today, let's look at lead time bias.

Suppose there were a cancer, cancer Y, that is uniformly deadly. Once it is diagnosed, the average survival, after the best available treatment, is three years. Nobody who has this cancer lives beyond five years.

Dr. Detecto is a pathologist who has invented a very sensitive method for detecting cancer Y at a very early stage. Dr. Detecto can detect cancer Y a full four years earlier than any previous method of detection. Unfortunately, there is no effective treatment for cancer Y, even when it is detected early. All patients with cancer Y will die. Because cancer Y patients are now detected four years earlier, the natural course of disease results in an expected death 7 years (3 years plus the 4 years lead time) later. When we study 5-year survival after diagnosis, we find that the five year survival is now 90%.

The newspaper headline reads, "New, improved detection technique for cancer Y improves 5-year survival from 0% to 90%."

Of course, detecting the cancer four years earlier only increased the time between diagnosis and death. It did not extend, by even a single minute, the age at death of patients with cancer Y.

Has Dr. Detecto made a useless discovery, and is the survival data fraudulent? No. Tumors are best treated when they are detected early. In the case of cancer Y, there was no immediate benefit for early detection. Nonetheless, the set of early cancers provided cancer researchers with a group of tumors that might have an improved response to alternate cancer therapies. This would require clinical trials. The survival data from the early-diagnosed group can be very misleading. It is the responsibility of trialists and journalists to interpret survival data cautiously.

We'll continue discussing the biases in survival data in the next blog.

-Copyright (C) 2008 Jules J. Berman

key words: cancer, tumor, tumour, carcinogen, neoplasia, neoplastic development, classification, biomedical informatics, tumor development, precancer, benign tumor, ontology, classification, developmental lineage classification and taxonomy of neoplasms

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