Room: Exhibit Hall
Purpose: To implement an alternative method for determining the electron density of the ArcCheck phantom (Sun Nuclear Corporation, Melbourne, FL) in the treatment planning system via evaluating the correlation between measured and TPS predicted dose difference in ArcCheck vs. liquid water.
Methods: ArcCheck density was first determined via the vendor suggested method. The alternative method began with ion chamber measurements taken at different central axis positions of the ArcCheck multiplug, and at the same spatial point within liquid water with the corresponding equivalent depth, calculated from the physical depth in the ArcCheck multiplied by the electron density of the ArcCheck phantom modeled in Eclipse. Both sets of measurements were compared against dose predicted by Eclipse AAA algorithm (Varian Medical System, Palo Alto, CA). The electron density of the ArcCheck was then iteratively adjusted in Eclipse to minimize the percent difference between the measured over predicted dose for ArcCheck vs. water at the central ArcCheck multiplug position.
Results: Technique for determining the electron density from the vendor yielded a value of 1.148 for the Eclipse AAA algorithm, which is applied for all energies. The alternative method produced energy-specific electron density values of 1.125, 1.138, and 1.127 for 6X, 15X and 6X-FFF, respectively. This reduced the average percent difference between measured over predicted dose in ArcCheck vs. water from 0.91%, 0.30%, and 0.89% for 6X, 15X and 6X-FFF, respectively, to 0.41%, 0.06%, and 0.43%. Preliminary study on the repeatability of the experiment demonstrated the average percent difference to be within 0.25% of the values stated above, which can affect the modeled electron density up to Â±0.006.
Conclusion: For Eclipse AAA algorithm, utilizing the corresponding dose measurements in liquid water as the reference to determine dose in PMMA for ArcCheck can result in more accurate and realistic modeling of the ArcCheck phantom.