Results of a one year survey of output for linear accelerators using IMRT and non-IMRT techniques

This paper presents the results of a one year survey of treated fields for 3 treatment machines at our New Jersey regional center. One machine predominantly treated IMRT prostate patients using a sliding window technique. The others were not equipped to deliver IMRT. Information obtained for each treated field included patient number, modality, monitor units delivered, gantry angle, and time. Data was obtained directly from our record and verify system and analyzed using a spreadsheet. We studied workload (MU/week), patient load, and average MU per patient as a function of time, as well as angular distributions and number of treatment fractions per patient. We also calculated the fraction of time the beam was on during treatments. By the end of the survey year, the workload of the IMRT machine reached approximately 100,000 MU/week and that of the non-IMRT machines was approximately 40-45000 MU/week. This was due predominantly to the higher number of monitor units for IMRT plans. Patient loads were not significantly different for the 3 machines. Duty cycle was 14% and 16% for the non-IMRT machines and 27% for the IMRT machine. The difference in workload for IMRT treatments relative to non-IMRT treatments confirms an earlier study performed at our institution using a much smaller data sample. One needs to consider the increase in leakage associated with this higher workload when designing shielding for an IMRT room.