Radiation Skyshine From A 6 MeV Medical Accelerator

I. Introduction

Skyshine radiation emanating from medical accelerator facilities is a phenomenon not well understood. A comprehensive analysis of skyshine has not been reported in the applicable literature for 6 MV X-rays by the NCRP in Reports 49, 51, 79 or 147 or the Institute of Physics and Engineering in Medicine in Report 75.^(1,2,3,4,5) Where literature is available for skyshine, it exists for higher energy X-ray beams only.^(6,7,8) It is defined by the National Council on Radiation Protection and Measurement as radiation scattered back to Earth by the atmosphere above a radiation-producing facility.⁽⁶⁾ The purpose here is to determine the appropriate technique for separating skyshine measurements from leakage and scatter radiation transmitted through the wall adjacent to the area where the measurements were made. Skyshine measurements are obtained in this research from a clinically operating facility.

II. Materials and Methods

Data was obtained at a facility housing a Varian Medical Systems, Inc. (Palo Alto, CA) Model 6EX particle accelerator. Radiation measurements were taken in the adjacent parking lot, immediately lateral to the position of the isocenter. This vault wall constitutes a primary barrier for the facility. The total distance from the isocenter of the machine to the barrier is 2.44 m. The barrier includes only 0.51 m of concrete; an amount needed for an operational workload at about a third of typical capacity. Thus, the total distance from the isocenter to the outside of the barrier is 2.95 m. The ceiling of the vault is also shielded with 0.51 m of concrete. This original shielding design provided for a distance of 2.3 m from the machine isocenter to the exterior surface of the roof. This arrangement is similar to that published in NCRP Report 151 or originally published by McGinley.^(6,7,8)

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.

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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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