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Absolute Dose Calibration of the First Stereotactic Breast Radiotherapy Device: GammaPod

Y Mutaf

Y Mutaf1*, P Hoban2 , Y Niu2 , B Yi1 , C Yu1,2 , K Prado1 , (1) University of Maryland School of Medicine, Baltimore, MD, (2) Xcision Medical Systems, Columbia, MD


TH-CD-304-4 (Thursday, July 16, 2015) 10:00 AM - 12:00 PM Room: 304

Purpose: A dedicated stereotactic breast radiotherapy device, GammaPod, was developed to treat early stage breast cancer. The first clinical GammaPod unit is currently being commissioned to treat its first patient in summer 2015. We provide an absorbed dose calibration formalism following AAPM protocols with modifications for the calibration medium and unique geometry of GammaPod. We also provide cross-comparison of the dosimetric calibration using different types of dosimeters.

Methods: GammaPod is comprised of a rotating hemi-spherical source carrier containing 36 Co-60 sources and a concentric rotating tungsten collimator providing beam diameters of 1.5 and 2.5 cm. A solid breast phantom made of ultra-high-molecular-weight polyethylene (density 0.935 g/cm3) with an effective depth of 7cm is used for absolute dose calibration. Due to the non-water equivalent medium, we utilized the former AAPM TG-21 protocol, but cross-compared the calibration with TG-51 calibrated OSLD and radiochromic film dosimeters. Polyethylene medium and chamber-specific calibration parameters are derived and provided for Exradin A-18 and Capintec PR-05P ionization chambers.

Results: The first clinical unit was installed with a total Co-60 activity of 2672 Ci in November 2014. Maximum dose rate was determined to be 2.76 Gy/min and 2.63 Gy/min for 2.5 and 1.5 cm collimators respectively at 7 cm depth. The determination of Ngas and appropriate polyethylene dosimetric quantities (stopping power and absorption coefficient ratios) is performed using NIST data tables and supplied as reference for calibration of subsequent units. Calibration crosscheck was accomplished with OSLD and radiochromic films, both with secondary TG-51 based calibrations, yielding relative dose differences less than 1%.

Conclusion: The first GammaPod treatment unit for breast stereotactic radiotherapy was successfully installed and calibrated for commencement of patient treatments in 2015. An absolute dose calibration formalism was established for the unique geometry of GammaPod and the method is validated with different detectors.

Funding Support, Disclosures, and Conflict of Interest: P Hoban, Y Niu and C Yu are employees of Xcision Medical Systems.

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