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Small field dosimetry is particularly relevant to stereotactic radiation treatment. The accuracy of dose calculation for small static beams is critical to dose planning so would potentially affect the treatment outcomes in a heterogeneous medium. Our results have shown good agreement with plastic scintillation detector in both homogeneous and heterogeneous medium.

Purpose: Commissioning Monte Carlo beam data for clinical use within a treatment planning system (TPS) for Stereotactic Body Radiation Therapy (SBRT) should require validation both in homogeneous and in heterogeneous phantom materials. Compounding this task is the difficulty in accurately measuring small field sizes. This work outlines the use of a new lung phantom in conjunction with a new scintillation detector designed for small field measurement to verify the dose calculated in the TPS. Methods: The measurements were performed on a CyberKnife VSI. The Stereotactic Dose Verification Phantom (SDVP) from Standard Imaging (SI) was fitted with a newly designed 12cm lung material insert with a mass density of 0.28 g/cc. The scintillating detector was placed in a 1.5 cm diameter water‐equivalent insert, simulating the central dose measurement of a small tumor, which was then placed in the SDVP lung phantom and CT scanned. The new SI W1 pinpoint scintillator is nearly water equivalent but the 1mm diameter by 3mm long scintillating fiber was visible and was contoured in the MultiPlan TPS. After dose cross‐calibration with the SI A19 ion chamber in the basic SDVP water equivalent phantom, measurements were taken with the 5, 7.5, 10, 12.5, 15, 20, 30, and 60mm cones. For cross‐calibration to known larger field conditions, the SI A16 ion chamber was used to remeasure the 20, 30, and 60mm cones. Results: The measured dose matched the Monte Carlo dose within 2% for the three collimator sizes that were used with the A16. All measurements with the W1 matched Monte Carlo dose as well, with the exception of the 30mm cone, which was 3.18% deviant. Conclusion: The SDVP lung material inserts combined with the W1 scintillator Result in clinically acceptable correlation to the Monte Carlo dose calculated in the TPS for all CyberKnife fixed collimator sizes. Dr. Grimm received a consulting fee from Standard Imaging for guiding the research.

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