AbstractID: 7107 Title: A direct method to measure scatter and primary component in exit megavoltage
radiotherapy beams
Scatter from the patient in the megavoltage treatment beam exiting the
patient is difficult to eliminate. It introduces artifacts in megavoltage CT
reconstruction. Also, knowledge of the scatter component in the exit beam is
required in order to perform accurate dose reconstruction, which is a process
whereby the dose into a patient CT representation at the time of treatment is
inferred from the exit dose detected.
We developed two complementary methods to measure the scatter and
primary photon dose at the detector. One or several MLC leaves are used to
block a portion of a fan (slit) beam emitted from a linac. A constant
thickness solid water phantom placed at the isocenter is used to scatter
photons towards a detector. Thus, the photons scattered into the detector
behind the closed leaf can be directly measured. By changing the position of
the closed leaves, we can measure the profile of scatter at the detector. In the
other method, only one or several leaves are opened. The primary photon
dose is measured directly and the scatter is obtained by subtracting the
primary from an exposure with all leaves open. The work will be used to
reduce artifacts in megavoltage CT reconstruction and to verify the process
of dose reconstruction.
These methods were tested on two different radiotherapy systems: a Varian
linac and MLC system with film used to measure scatter; and a 4-MV Orion
linac with a NOMOS-MLC and xenon detector. On each system, both
methods for estimating scatter exhibited similar distributions.