Monte Carlo methods are used in many areas including random number generations, nuclear reactor designs, traffic flows, oil-well explorations, economics etc.
PEREGRINE is a software/hardware system that implements the Monte Carlo method for the specific purpose of simulating medical radiation delivery. The major goals in its development have been to design a system specifically tailored to radiation therapy applications and make Monte Carlo transport fast enough to be practical for day-to-day treatment planning.
It simulates the transport of neutrons, photons, electrons and protons through a patient using a geometry derived from a computed tomography (CT) scan.
This will help doctors to choose a treatment that maximizes the radiation dose to the tumor and minimizes the dose to normal tissues. Monte Carlo method is used here because it is the most accurate way to simulate radiation transport on a computer.
Further advantages are accurate dose calculations for all materials, all geometries, simultaneous determination of dose deposition and image formation and detailed information of how dose is deposited in matter.

www.nuc.berkeley.edu/courses/classes/NE39/Vujic-cancer.ppt

www.mgnet.org/~douglas/Classes/cs521-s01/monte-carlo/application.ppt