  Cloud computing allows for vast computational resources to be leveraged
quickly and easily in bursts as and when required. Using the Amazon Elastic
Compute Cloud and the Amazon Simple Storage Solution, we describe a technique
that allows for Monte Carlo radiotherapy dose calculations to be performed
using GEANT4 and executed in the cloud. Simulation cost and completion time was
evaluated as a function of instance count using compute instances acquired via
biding on the Elastic Compute Cloud spot market. Bidding for instances on the
instance spot market was found to be 35-60% of the cost of on-demand instances
of the same type. Using the technique, we demonstrate the potential usefulness
of cloud computing as a solution for rapid Monte Carlo simulation for
radiotherapy dose calculation.
