AbstractID: 7547 Title: Pre-treatment Quality Assurance for IMRT treatments of
Pancreatic Cancers.
We have recently initiated an IMRT service by implementing the Inverse planning module within our CMS-FOCUS treatment
planning system and delivering the treatments using a Varian 600CD Linac. A Step and Shoot method with a Millennium 120 MLC is
utilised.
There are three aspects to our QA protocol: 1) checking the monitor units, 2) checking the ‘dose maps’ of each beam used in the plan
and 3) measuring the dose distribution in the plane of the machines CAX. The measured MU comparisons agree within 1.5% (using a
standard Farmer chamber) when ‘dosimetry unfriendly’ segments are removed. Film dosimetry is performed, for parts 2 and 3, using
KODAK EDR2. Calibrations are performed which enable comparison of measured and planned absolute doses ,in general to within
2mm. This process has been possible by utilising the QA tools provided within our treatment planning system, which include the
ability to easily write ASCII files of 1-D dose profiles from selected planes and/ or the 2-D dose planes.
We have implemented an independent check calculation which computes a ‘modulation factor’ based on the Leaf sequence files or
MLC instruction files that are ultimately transferred to the Linac for the treatment delivery. Currently, this simple calculation verifies
the corresponding factor given by the planning system, within 3%. We plan to develop this algorithm to produce a fully independent
MU checker to reduce the amount of required measurements.