AbstractID: 7346 Title: QA and the Gap Measurements for Dynamic IMRT QA and the Gap Measurement for Dynamic IMRT Intensity modulated radiation therapy (IMRT) presents a challenge for patient dose verification as well as the continual quality assurance process (QA). This is particularly true for dynamic IMRT treatments generated by moving multi-leaf collimators. In addition, a machine’s MLC specific parameter, the “end gap” or “effective gap” is a user determined input parameter used in commissioning treatment planning systems for IMRT. We propose a new simple method of establishing the “effective gap” which can also be used for establishing dose reference conditions that can be used as a part of a periodic QA procedures for IMRT treatments. The method relies on obtaining phantom based ion chamber measurements for an open 10x10 field and a series of dynamically generated 10x10 fields. The later fields are generated by moving slits of different widths subject to the same incident fluence as the open 10x10field. The ratios of dynamically generated field dose to the open 10x10 field dose are a linear function of the slit width. The extrapolated zero value of the ratio represents the “effective gap”, which is then used as a parameter in treatment planning optimization. The data gathered can then be used as a reference condition of an ongoing Quality Assurance program of IMRT.