SU‐F‐T‐293: Experimental Comparisons of Ionization Chambers with Different Volumes for CyberKnife Delivery Quality Assurance

Abstract

Purpose:To evaluate the practicality use of ionization chambers with different volumes for delivery quality assurance of CyberKnife plans,Methods:Dosimetric measurements with a spherical solid water phantom and three ionization chambers with volumes of 0.13, 0.04, and 0.01 cm3 (IBA CC13, CC04, and CC01, respectively) were performed for various CyberKnife clinical treatment plans including both isocentric and nonisocentric delivery. For each chamber, the ion recombination correction factors <I>Ks</I> were calculated using the Jaffe plot method and twovoltage method at a 10‐cm depth for a 60‐mm collimator field in a water phantom. The polarity correction factors <I>Kpol</I> were determined for 5–60‐mm collimator fields in same experimental setup. The measured doses were compared to the doses for the detectors calculated using a treatment planning system.Results:The differences in the <I>Ks</I> between the Jaffe plot method and two‐voltage method were −0.12, −0.02, and 0.89% for CC13, CC04, and CC01, respectively. The changes in <I>Kpol</I> for the different field sizes were 0.2, 0.3, and 0.8% for CC13, CC04, and CC01, respectively. The measured doses for CC04 and CC01 were within 3% of the calculated doses for the clinical treatment plans with isocentric delivery with collimator fields greater than 12.5 mm. Those for CC13 had differences of over 3% for the plans with isocentric delivery with collimator fields less than 15 mm. The differences for the isocentric plans were similar to those for the single beam plans. The measured doses for each chamber were within 3% of the calculated doses for the non‐isocentric plans except for that with a PTV volume less than 1.0 cm3.Conclusion:Although there are some limitations, the ionization chamber with a smaller volume is a better detector for verification of the CyberKnife plans owing to the high spatial resolution.