AbstractFor the stereotactic irradiation of intracranial lesions with a linear accelerator, the use of a tertiary collimating system can be expected to confer several benefits in comparison to the use of the standard linac collimating jaw system. Moving the collimator closer to the patient should lead to reduced penumbra and reduced susceptibility to positioning error. In addition, the use of a circular rather than a square aperture should yield a more advantageous dose distribution for irradiation techniques which follow spherical geometry. An investigation using film dosimetry with a tissue‐equivalent head phantom demonstrated that the tertiary system was superior to the standard jaw system as expected. © 1993 Wiley‐Liss,