Intensity Modulated Radiation Therapy (IMRT) offers the opportunity to achieve a better compromise between target volume coverage and sparing organs at risk. Used generally for prostate and head-and-neck cancer treatments, its routine clinical implementation is partially held back by the time required to perform pre-treatment quality control. The purpose of this study is to evaluate and then optimize the pre-treatment quality control in IMRT, using Statistical Process Control (SPC). For prostate and head-and-neck cancers, we retrospectively evaluated the treatment process quality using SPC tools. We found that using control charts during quality controls would have allowed us to detect deviations, such as multileaf collimator calibration, more quickly. The control charts also confirmed that our processes are only subject to random causes of variability, which means they are statistically controlled. The capability analysis showed that the treatment process is not capable for head-and-neck treatments, and is barely capable for prostate treatments. So, after identification all the factors affecting our processes, we will test their influence using the design of experiments and then try to act on those factors, to make both processes more capable. The final step is to consider reducing quality controls according to predefined statistical risks, instead of choosing them only by experience or intuition, in order to optimize the benefit/risk ratio. Key words: quality control, Statistical Process Control, radiotherapy