Commissioning of a dedicated commercial Co‐60 total body irradiation unit

Jay Burmeister, Adrian Nalichowski, Michael Snyder, Robert Halford, Geoff Baran, Brian Loughery, Ahmad Hammoud, Joe Rakowski, Todd Bossenberger

We describe the commissioning of the first dedicated commercial total body irradiation (TBI) unit in clinical operation. The Best Theratronics GammaBeam 500 is a Co-60 teletherapy unit with extended field size and imaging capabilities. Radiation safety, mechanical and imaging systems, and radiation output are characterized. Beam data collection, calibration, and external dosimetric validation are described. All radiation safety and mechanical tests satisfied relevant requirements and measured dose distributions meet recommendations of American Association of Physicists in Medicine (AAPM) Report #17. At a typical treatment distance, the dose rate in free space per unit source activity using the thick flattening filter is 1.53 × 10-3 cGy*min-1 *Ci-1 . With a 14,000 Ci source, the resulting dose rate at the midplane of a typical patient is approximately 17 and 30 cGy/min using the thick and thin flattening filters, respectively, using the maximum source to couch distance. The maximum useful field size, defined by the 90% isodose line, at this location is 225 × 78 cm with field flatness within 5% over the central 178 × 73 cm. Measured output agreed with external validation within 0.5%. End-to-end testing was performed in a modified Rando phantom. In-house MATLAB software was developed to calculate patient-specific dose distributions using DOSXYZnrc, and fabricate custom 3D-printed forms for creating patient-specific lung blocks. End-to-end OSLD and diode measurements both with and without lung blocks agreed with Monte Carlo calculated doses to within 5% and in-phantom film measurements validated dose distribution uniformity. Custom lung block transmission measurements agree well with design criteria and provide clinically favorable dose distributions within the lungs. Block placement is easily facilitated using the flat panel imaging system with an exposure time of 0.01 min. In conclusion, a novel dedicated TBI unit has been commissioned and clinically implemented. Its mechanical, dosimetric, and imaging capabilities are suitable to provide state-of-the-art TBI for patients as large as 220 cm in height and 78 cm in width.