WE-G-BRB-03: On the Detectability of Jaw Decalibrations with the DAVID System

To evaluate if the recently described problems with the calibration of jaws at Varian linear accelerators can be detected with the DAVID system.

Recently Varian Clinac field size issues resulted in a widespread medical device correction notice, stating that incorrect jaw positioning due to excessive connector resistances may result in clinically relevant deviations from the intended field size. Since field size checks via the light field, separately for each administered field segment, are not practicable, it might be helpful to know that an independent, permanently active field size surveillance system is routinely used in our department since four years. The DAVID system (PTW-Freiburg, Germany) is a translucent multiwire transmission ionisation chamber, placed in the linac accessory holder. Each detection wire of the chamber is positioned in theprojection of the midline of the associated MLC leaf pair, resulting in a signal proportional to the leaf pair aperture. High lateral resolution has been achieved by a deconvolution software (Looe et al, PMB 55, 2010, 3981- 3992). MLC decalibrations are detected by comparison between measured and reference signal profiles.

The side wings of the DAVID system signal profiles are sensibly influenced by the position of the backup jaw blocks limiting the field size in the direction at right angles to the MLC travel direction. Systematic variation of the backup jaw positions for several field sizes and IMRT plans has revealed high sensitivity of the signal profile side wings with respect to jaw decalibrations.

The DAVID system is able to detect, without any delay, suddenly appearing jaw decalibrations down to positioning errors less than 1mm. Use of the DAVID system provides the independent and permanent surveillance of correct field sizes including the position of the backup jaws, making checks of light-field congruence for each field segment unnecessary.