Advanced Physics for Brachytherapy 2018

The MC method …

Do I know the probability distribution of possible tracks? A photon “moves” from phase space element to phase space element. A photon track is composed of the sequential phase space elements or photon states, S j ( r j ,E j, Ω j ) just before each interaction j.

The probability of occurrence of each photon state j only depends on the probability of occurrence of state j-1.

Or in other words: the probability that the photon interacts at r j-1 , the probability that a specific kind of interaction occurs and the probability that during this interaction the photon is scattered in direction Ω j with energy E j given Ω j-1 and E j-1 . These probability distributions are known in Physics….!

r

r

j-2

j

Ω

Ω

j-1

j

r

j-1

Hence, the only component missing is a method to RANDOMLY sample from the above, known, probability distributions.