Loss of field flatness in AECL teletherapy units form misadjustment of the collimator

Atomic Energy of Canada Limited 60Co teletherapy units have adjustable collimator hinges to adapt to source diameters from 0.75 to 2.50 cm. Adjustment of the collimator hinge to the manfacturer's settings can cause a loss of radiation field flatness as high as 20% for large fields. The recommended settings appear to clip the top corners of tall source capsules. Opening the hinge adjustment restores field flatness, without changing penumbra or the inverse square law behavior of the teletherapy unit.