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The angle of obliquity is used in radiation shielding calculations to account for the longer path length x rays will see when obliquely incident on the protective barrier. According to the National Council on Radiation Protection and Measurements (NCRP), use of the angle of obliquity is explicitly assumed for primary radiation, so that an angle of obliquity for secondary radiation is never addressed. However, in the example section of the latest report, it specifically recommends against using an angle of obliquity for scattered radiation. To check this assumption, the existence or not of an angle of obliquity for scattered radiation has been investigated for bremsstrahlung x-ray beams of 4, 6, 10, 15, and 18 MV and for barriers consisting of concrete, lead, and steel using a Monte Carlo approach. The MCNP Monte Carlo code, v4.2C, has been used to generate scattered radiation at 30 degrees from a water phantom and incident on a secondary barrier at the same angle relative to the normal to the barrier. The barrier thickness was increased from zero to a thickness sufficient to reduce the fluence (f4 tally) to <10(-3). A transmission curve was created for each energy-barrier material combination by normalizing to zero thickness. The results for the first tenth-value layer (TVL) in concrete (5 energies) show an average angle of obliquity of 21.7 degrees +/- 5.6 degrees , and for the first two TVLs averaged 29.7 degrees +/- 3.9 degrees . The results for the first TVL in lead (3 energies) show an average angle of obliquity of 27.7 degrees +/- 4.0 degrees , and for the first two TVLs averaged 20.5 degrees +/- 5.8 degrees . There are no data in the NCRP reports for 30 degrees scattered radiation attenuated by steel with which to make a comparison.

The calculation of shielding barrier thicknesses for radiation therapy facilities according to the NCRP formalism is based on the use of broad beams (that is, the maximum possible field sizes). However, in practice, treatment fields used in radiation therapy are, on average, less than half the maximum size. Indeed, many contemporary treatment techniques call for reduced field sizes to reduce co-morbidity and the risk of second cancers. Therefore, published tenth value layers (TVLs) for shielding materials do not apply to these very small fields. There is, hence, a need to determine the TVLs for various beam modalities as a function of field size. The attenuation of (60)Co gamma rays and photons of 4, 6, 10, 15, and 18 MV bremsstrahlung x ray beams by concrete has been studied using the Monte Carlo technique (MCNP version 4C2) for beams of half-opening angles of 0 degrees , 3 degrees , 6 degrees , 9 degrees , 12 degrees , and 14 degrees . The distance between the x-ray source and the distal surface of the shielding wall was fixed at 600 cm, a distance that is typical for modern radiation therapy rooms. The maximum concrete thickness varied between 76.5 cm and 151.5 cm for (60)Co and 18 MV x rays, respectively. Detectors were placed at 630 cm, 700 cm, and 800 cm from the source. TVLs have been determined down to the third TVL. Energy spectra for 4, 6, 10, 15, and 18 MV x rays for 10 x 10 cm(2) and 40 x 40 cm(2) field sizes were used to generate depth dose curves in water that were compared with experimentally measured values.

The x-ray leakage from the housing of a therapy x-ray source is regulated to be <0.1% of the useful beam exposure at 1 m from the source. It is to be expected that the machine leakage in the backward direction would be less because the gantry and stand contain significant amounts of additional metal to attenuate the x rays. A reduction in head leakage in this direction will have a direct effect on the thickness of the shielding wall behind the linear accelerator. However, no reports have been published to date on measurements in this area. The x-ray leakage in the backward direction has been measured from linacs having energies of 4, 6, 10, 15, and 18 MV using a 100 cm ionization chamber and Al2O3 dosimeters. The leakage was measured at nine different positions over the rear wall using a 3 x 3 matrix with a 1-m separation between adjacent horizontal and vertical points with either the leftmost or rightmost column aligned with the target and isocenter. In general, the leakage is less than the canonical value, but the exact value depends on energy, gantry angle, and measurement position. There is significantly greater attenuation directly behind the gantry stand for all energies. Leakage at 10 MV for some positions exceeded 0.1%. Additionally, neutron leakage measurements were made for 10, 15, and 18 MV x-ray beams using track-etch detectors. The average neutron leakage was less than 0.1% except for 18 MV, where neutron leakage was more than 0.1% of the useful beam at some positions.

Measurements have been made of the transmission factors through lead for scattered radiation produced by 4, 6, and 10 MV bremsstrahlung x-ray beams incident on a polystyrene phantom; these measurements cover the range of scattering angles between 30 degrees and 135 degrees. The results show that the tenth value layer (TVL) for scattered radiation decreases sharply with increasing scattering angle and increases slightly with increasing beam energy, although at large scattering angles there is little energy dependence. TVLs range from 3.5 cm to 0.3 cm for 4 MV at scattering angles between 30 degrees and 135 degrees, from 3.8 cm to 0.6 cm for 6 MV, and from 4.2 cm to 0.7 cm for 10 MV, respectively, at scattering angles between 30 degrees and 120 degrees. Monte Carlo calculations, performed at 4 MV to simulate the transmission of scattered radiation, are in good agreement with the experimental measurements.

NCRP Report No. 49, published in 1976, describes how to calculate the shielding for the medical use of x rays and gamma rays for energies up to 10 MV, including primary, scattered, and leakage radiation. However, in that report, data for scattered radiation for linear accelerators exist only for 6 MV, and leakage radiation is assumed, incorrectly, to be equivalent to primary radiation. Since the publication of that report, linear accelerators with energies up to 25 MV have been widely used in the radiation therapy community. Thus, there is a need to measure additional data for all energies in the range 4-25 MV. In this study, measurements were made of the "a" factor for 4, 6, 10, and 23 MV x rays at scattering angles between 30 degrees and 135 degrees. The results show that the 6 and 10 MV "a" factor data are consistent with published data, and the 23 MV data are also consistent with recently published data at 18 and 25 MV. The data show that, in general, the "a" factor decreases with energy; the exception is that 23 MV data show a sharp increase at low scattering angles, much greater than at other energies.

An addition has been made to an existing software program for calculating shielding thicknesses of barriers for megavoltage radiation therapy beams. This addition calculates the shielding thicknesses required for the roof of a single-story building when adjacent, multi-level buildings are occupied.

A dedicated electron-only intra-operative suite has just been completed at the Massachusetts General Hospital. This suite is located on the 3rd floor with the shielding consisting entirely of lead and borated polyethylene, except for concrete in the floor and ceiling to support the lead and on the finished floor. The radiation protection barriers for this facility were calculated on the basis of a maximum permissible dose of 10 microSv wk(-1) for photons and 10 microSv wk(-1) for neutrons, based on a quality factor of 10 (this factor is specified in the regulations of the Commonwealth of Massachusetts). This even split was predicated on the basis that, except for the primary beam, the neutron leakage from the machine was generally about the same as for x rays. The initial survey showed that the neutron dose equivalent outside all barriers, except below the floor, was within the calculated values, < 10 microSv wk(-1). However, the neutron dose equivalent measured directly below the floor was found to be 35 microSv wk(-1) for the highest electron energy, a difference of > 25 microSv wk(-1). The neutron dose equivalent, for neutrons produced by bremsstrahlung photon interactions in the lead and based on a 3% photon background in the electron beam, was calculated to be 25 microSv wk(-1), in good agreement with this difference.

The attenuation of 60 Co gamma rays and photons of 4, 10, and 18 MV x-ray beams by concrete, steel, and lead has been studied using the Monte Carlo technique for angles of incidence 0, 30, 45, 60, and 70 degrees. Transmission factors have been determined down to <2x10(-5) in all cases. The results show that deviation from the obliquity factor increases with angle but is not significant for angles <45 degrees. At a 70 degree angle of incidence and a transmission factor of 10(-5), the obliquity factor varies between 1.2 and 1.9 for concrete, between 1.4 and 1.7 for steel, and between 1.4 and 1.5 for lead for the range of energies investigated. This amounts to an additional 86 and 50 cm of concrete, 25 and 23 cm of steel, and 8 and 14 cm of lead for 60 Co and 18 MV x rays respectively. The results for 60 Co in concrete and lead are in good agreement with previously published experimental work. Fits to the data using mathematical models allow reconstruction of all data curves to better than 1% on average and 7% in the worst single case.

Shielding calculations for door thicknesses for megavoltage radiotherapy facilities with mazes are generally straightforward. To simplify the calculations, the standard formalism adopts several approximations relating to the average beam path, scattering coefficients, and the mean energy of the spectrum of scattered radiation. To test the accuracy of these calculations, the Monte Carlo program, ITS, was applied to this problem by determining the dose and energy spectrum of the radiation at the door for 4- and 10-MV bremsstrahlung beams incident on a phantom at isocenter. This was performed for mazes, one termed "standard" and the other a shorter maze where the primary beam is incident on the wall adjacent to the door. The peak of the photon-energy spectrum at the door was found to be the same for both types of maze, independent of primary beam energy, and also, in the case of the conventional maze, of the primary beam orientation. The spectrum was harder for the short maze and for 10 MV vs. 4 MV. The thickness of the lead door for a short maze configuration was 1.5 cm for 10 MV and 1.2 cm for 4 MV vs. approximately less than 1 mm for a conventional maze. For the conventional maze, the Monte Carlo calculation predicts the dose at the door to be lower than given by NCRP 49 and NCRP 51 by about a factor of 2 at 4 MV but to be the same at 10 MV. For the short maze, the Monte Carlo predicts the dose to be a factor of 3 lower for 4 MV and about a factor of 1.5 lower for 10 MV. Experimental results support the Monte Carlo findings for the short maze.