AbstractID: 7121 Title: Evaluation of Quality Assurance by Utilizing Positional
Variation in IMRT
The stated objective of delivering an accurate maximum radiation dose to a tumor site
with the maximum sparing effects on the normal tissues requires tremendous efforts on
the part of planning a treatment in Intensity Modulated Radiation Therapy(IMRT). This
challenge requires extreme accuracy when the target volume consists of smaller irregular
nodules wrapped around critical structures coupled with positional uncertainties. We
have observed that moving the couch in a symmetrical order does not produce
symmetrical results in dose. Such a misleading scenario may result in unwanted outcome.
A study was carried out to evaluate the effects of symmetrical changes in couch
movement on a 15 MV Linac with a multi-leaf collimator at SSD =100. The detector was
placed at 10 cm below the surface in a Lucite phantom. Measurements were made with
gantry both at 900 and 2700 and a difference of only 0.2% was found in dose for the
symmetrical gantry rotation. With the gantry position fixed at 2700 the couch was raised
1 millimeter (mm) above its central axis and measurements were made. It was observed
that there was only 2.30% change in dose. Then the couch was lowered 1 mm below the
central axis and measurements were made again. A change of 7.65% in dose difference
was observed. This shows that a symmetrical change in target position with fixed SSD
does not necessarily result in a symmetrical change in dose. Technique developed along
these lines may be incorporated into regular QA program.