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.