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A Cyberknife-Specific Phantom for Dose Output and Energy Constancy Measurements


P Mone

P Mone*, T Holmes, Sinai Hospital, Baltimore, MD

SU-E-T-150 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To design and test a solid phantom for Cyberknife dose constancy measurement, beam energy constancy measurement, and laser-radiation center measurement that is small, lightweight and quick to setup by mounting it on the Cyberknife birdcage accessory.

Methods:
The phantom was designed and constructed to be small and light enough to mount on the birdcage but large enough to provide adequate backscatter.

We evaluated the phantom by comparing the dose output and ionization ratio obtained with it to values obtained using a conventional 30 cm by 30 cm Virtual Water phantom. The setup used was a 6cm diameter collimator, a source to chamber distance of 80cm and a chamber depth of 5cm. A second set of readings was obtained at a chamber depth of 10cm with all the other parameters constant. The time required for setup was also measured with a stop watch.

Results:
Ionization readings, corrected for temperature and pressure, obtained with a Farmer type chamber in the phantom was 49.93nC/300 MU and with the Virtual Water it was 49.41nC/300MU. The ratio of reading at 10cm depth to reading at 5cm depth was 0.8033 for the phantom and 0.7975 for the Solid Water phantom.

The time required to setup our phantom was 3 minutes 1 second while the time required to setup solid blocks was 6 minutes and 45 seconds.

Conclusion:
A phantom was designed and constructed to provide a fast and convenient means of measuring beam output constancy and beam energy constancy for the Cyberknife system. Readings obtained by using the phantom are within 1% of readings obtained using a conventional 30 cm by 30 cm Virtual Water phantom. The time required to setup our phantom was found to be half the time required to setup a conventional phantom.

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