A pressurized ionization chamber (Fluke Biomedical, Cleveland, OH, Model 451P) was chosen for all measurements. The device was identified as being suitable due to its fast response time to radiation with capabilities that include auto-ranging, auto-zeroing, high X-ray sensitivity and resolution of exposure rate down to 10.32 nC kg-1 hr-1 (40 μR h-1).
The linear accelerator gantry was positioned so the X-ray beam was directly toward the ceiling of the vault. For International Electrotechnical Commission (IEC) 61217 geometry scaling, the gantry angle was 180°, with collimator angle and couch angle each set at 0°.(9) Annual machine calibration was completed prior to this work, resulting in 1.00 cGy MU-1 delivery 100 cm from the source, at the depth of maximum dose clinically in a 10 x 10 cm2 beam. At the machine isocenter, the accelerator was programmed at a fixed dose rate of 6.67cGy s-1 (400 MU min-1). The Varian Model Millennium 120-leaf multileaf collimator (MLC) was fully retracted in standby mode during each survey. The square field size was set to 40 x 40 cm2 initially.
Measurements were made in a vacant parking lot adjacent to the therapy facility. The detector remained in the transverse central-axis plane (gantry plane) of the linear accelerator at all times. Each measurement was taken at a fixed height of 1.8 m above the ground. The detector was pointed towards the roof at a point observed to be 2 m above the edge of the building, so as to detect air scattering radiation emanating from the roof of the building. For all measurements in this study, no phantom was required to be placed at isocenter.
In order to consider potential relationships which might exist between our readings and the distance from the lateral barrier, measurements were taken in 1.52 m (5 ft) increments. Between the initial distance of 1.52 m and the final distance of 15.2 m, a total of 10 measurements were taken. As found in IPEM Report 75, the mean energy of skyshine radiation has been reported to be between 120-250 keV.(5) Based on this energy range, the ratio of absorbed dose to exposure is 3.76 μSv nC-1 kg (0.971 rem R-1).(10) Since the ratio is almost one, it was assumed that the exposure rate was identical to the dose rate. Likewise, in order to consider potential relationships which might exist between our readings and field size in use, this process was repeated for a variety of field sizes at each distance. The open field beam sizes considered were 5 x 5cm2, 10 x 10 cm2, 20 x 20 cm2, 30 x 30 cm2 and 40 x 40 cm2.
Michael Gossman, MS, DABR, RSO, is a Board Certified Qualified Expert Medical Physicist - Currently the Chief Medical Physicist & RSO of Radiation Oncology in Ashland, KY - a Medical Consultant to the U.S. Nuclear Regulatory Commission (U.S. NRC) - and an Accreditation Site Reviewer for the American College of Radiation Oncology (ACRO). He is the highest ranking scientist in the medical community. His expertise involves the safe, effective and precise delivery of radiation to achieve the therapeutic result prescribed in patient care by radiation oncologists.
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