Effects of plastic protective caps on the calibration of therapy beams in water.

Plastic 60Co buildup caps have been widely used to protect ionization chambers when calibrating high-energy x-ray and electron beams in water, and have been used consistently by the Radiological Physics Center. Recent calibration protocols base their calculations on a theory that assumes that no protective cap is used during calibration in phantom. The change in ionization within the chamber due to the presence of a protective cap has been investigated for acrylic and polystyrene caps of various wall thicknesses, using 60Co and x-ray beams from 6-25 MV and electron beams from 7-18 MeV. The change has been shown to be small, no more than 0.5% for x rays and 0.7% for electrons using acrylic 60Co caps. The change for polystyrene is seen to be as much as twice that for acrylic. Empirical correction factors to compensate for this effect have been determined. A basis in theory for photons is suggested by an extension of the theory in recent protocols. The effect for electrons is explained only qualitatively.