AbstractID: 7845 Title: Primary Beam Widths in Ceiling Shielding
For primary barriers, the NCRP recommends that the width of the primary barrier be
equal to the width of the primary beam at the barrier plus 1’ or 30 cm on each side. The purpose
of this is to (i) ‘guard against misalignment of the beam’ and (ii) take into account ‘small angle
scatter from the patient.’ For walls, against which the beam is directed horizontally and
orthogonally, for any point within the primary beam, the thickness of shielding required is only
marginally less than that at the central axis when inverse square and obliquity are taken into
account. This is due to the half angle of the primary beam being about 14°. This rule does not
apply to ceilings, because the geometry is entirely different. For normal room geometry, the
isocenter is closer to the ceiling than the walls and hence the angle subtended by the ceiling at
isocenter is >>14°. This leads to a large obliquity factor which can dominate the effect of the
primary beam. This implies that the width of the primary beam close to where the ceiling meets
the wall is considerably less than calculated.
We present a comparison of primary shielding thicknesses for a typical room
arrangement where the ceiling has to use laminated lead or steel due to height restrictions, with
and without the obliquity factor to demonstrate the change in the primary width. Such
considerations have a direct impact on the cost of the shielding.