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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Gamma knife radiosurgery

Problem

The best available medical treatment for some diseases involves destroying or removing abnormal tissue. A gamma knife destroys those dangerous tissues without first cutting through sensitive healthy structures to reach the target. The gamma knife is used to treat patients who have intracranial tumors (or “brain cancers”) and other dangerous conditions inside their skulls.
A gamma knife is a helmet-like device with 200 samples of radioactive cobalt-60. As the Co-60 (half-life of 5 years) decays to stable nickel-60, it gives off beta- and gamma-radiation that travel away from the atom in random directions.
Figure 1. Co-60 decay chain
The Co-60 sources are embedded in collimators, devices that shape the radiation into a thin beam traveling in one direction.
Figure 2. Simplified schematic of a gamma knife showing only three of its 200 Co-60 sources and collimating devices with beam paths crossing at a brain tumor (green)
The gamma knife works because gamma ray photons do not deposit their energy in tissue quickly. Where beta ray electrons will deposit almost all of their energy in the first few centimeters of tissue, gamma ray photons deposit energy very slowly as they travel through the body. Depositing energy by both beta- and gamma rays ionizes atoms in the cells. If the rate of ionization is low, cells can usually repair the damage done by reactive ions. Large doses of ionization can overwhelm a cell’s ability to compensate and it will not survive. The cells along the paths of the gamma knife’s beams are generally able to repair themselves except at the points where many beam paths cross, yielding a much higher rate of ionization that kills the cells and destroys the tissue.
Gamma knife sources are replaced every ten years. What amount of radiation per unit time does a new gamma knife deliver relative to one that is about to have its Co-60 replaced?
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