A simple method for use in QC of CT for radiation therapy treatment planning.

A simple procedure for monitoring constancy of spatial measurement and CT number determination from CT images used in radiation therapy treatment planning is described. The procedure uses low-Z material rods glued to the underside of the CT table insert and does not require a special phantom. Measurements are made on the same patient images used for treatment planning. Deviations from predetermined baseline values outside quality control limits of +/- 2 mm in spatial resolution and +/- 20 CT numbers in density can be detected with a confidence level of 97% or better.