Objectives: The aim of this study is to describe the technique, develop a formula to define its limits of accuracy, and report on its clinical application.

Methods: Using the positioning laser grid lines on the computed tomography scanner, a small metallic marker is placed on the scalp at the intended biopsy site. The distance between the tumor and the inner table of the bone is measured. In the operating theater, the patient is positioned such that the tip and the bridge of the nose are aligned in a horizontal position. The patient's two eyebrows or the orbital canthi are aligned in a vertical line. Simultaneously, the posterior borders of the two pinnae are aligned vertically. Gravity is used to guide the biopsy needle through the marked burr hole into the target. Seven patients had biopsies. One was for targeting the craniopharyngioma cyst to place an Ommaya catheter. The fraction of error or error fraction (EF), was developed for verification of its limits of accuracy.

Results: All the biopsies were diagnostic and the Ommaya catheter was correctly sited. The EFs at α = 5° were all predictive of the limits of accuracy of this technique.