Monte Carlo simulation of CT guided radiotherapy

A pilot study to correct for systematic errors in prostate position using daily CT scans is planned as part of a larger dose escalation study. For planning purposes, this project employed Monte Carlo techniques to simulate the pilot study in order to determine the optimum protocol (i.e. number of CT scans, when to check them, how many are necessary and the appropriate action level for correction) In the Monte Carlo simulation a number of test protocols were evaluated, using setup error and organ motion data previously measured by us, as well as estimations of measurement and correction error. For each protocol, the simulation was performed for 10000 patients, each receiving 45 fractions. The calculated residual systematic error distribution was evaluated. Based on Monte Carlo results we chose a protocol where the patient's systematic error is calculated on days 3 and 5 by averaging the last 3 CT scans and every 2 days thereafter until the patient stabilizes. This led to an average of 5.2 scans and 0.7 corrections per patient, with an average systematic error reduction of 28%. For validation, this protocol was applied to data gathered from 2 patients who had undergone multiple CT scans. Under this protocol, both patients would have required 7 scans, 1 and 2 corrections respectively, and would have had their systematic errors lowered by 50%, slightly higher than the averages predicted by the program.