Cancer Slope Factor: Definition & Calculation

Instructor: Stephanie Katz

Stephanie has been a nurse for thirteen years and holds a Masters of Science in Nursing with a concentration in Nursing Education.

In this lesson, we'll tackle the cancer slope factor. You'll learn about importance of this value, how to calculate it using a dose-response curve, and how to interpret the result.

What Is the Cancer Slope Factor?

Pop quiz: Is the cancer slope factor the scariest ski slope on Earth, or is it a term used in the field of toxicology to describe how cancer-causing a substance may be?

Answer: The cancer slope factor is a single number that is calculated by looking at toxicology data on how cancer risk rises with increased exposure to a certain compound. Toxicology is the study of the effects of harmful substances on plants, animals, and the environment. This may seem dry, but determining the cancer slope factor is a critical component when evaluating the safety of a substance.

Example

You are a brilliant scientist who just discovered a potential cure for a fatal illness in children. Your new drug, called Brillient, works flawlessly in the lab against this awful disease, and early data from your clinical trials shows that every child who takes Brillient is cured of their illness.

However, as a part of your research you are required to perform toxicology tests on your new drug to see what the possible harmful effects may be as larger and larger doses are given. You find that when mice are given small doses of Brillient, there are no major harmful effects. But as you give larger and larger doses to the mice, you find that they start developing cancer. The higher the dose, the more mice get cancer. You plot this information on a graph showing the amount of Brillient given at each assessment and the risk of cancer at each dose. This is a dose-response curve for cancer risk.


The dose-response curve plots the amount of Brillient given on the x axis and the risk of cancer as a percentage on the y axis.
The dose-response curve in a toxicology assessment


This matters, because if Brillient can cause cancer in mice at large doses and you can't show that this risk is minimal for humans at low doses, then you won't get many customers lining up for your drug, even if it is a miracle cure.

Performing the Calculation

There are several ways to calculate cancer slope factor, and each way yields different results. This lesson demonstrates the Linearized Multistage Model, which is used by the Environmental Protection Agency (EPA). The Food and Drug Administration (FDA), which would oversee your drug development, uses multiple very complex models that are beyond the scope of this lesson.

Step 1: Find the lowest dose that caused any cancer in mice and plot that on your dose-response curve.

Step 2: Assess the confidence interval of that data point. You can do this with a variety of online calculators.


A 95% confidence interval shows the upper and lower bound of what a resample would yield. If you tested this dose again, you would be 95% certain that the number would end up within the confidence interval range.
Confidence Interval


Step 3: Plot a line from the point (0,0)—meaning zero cancer risk at zero dose—to the high point of the confidence interval from step 2.

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