A formalism and methodology for measurement and control of LINAC isocenter

Despite the acknowledged need for a stable reference point for LINAC isocenter quality assurance (QA), no standard for such a reference point has been established. This paper introduces a practical and robust technique for measuring and tuning LINAC isocenter within a stable reference frame based on the collimator axes of rotation.

We develop a framework based on physical isocenter, a refinement of the approach by Skworcow et al. The physical isocenter provides a relatively stable, first principles spatial point from which other LINAC parameters can be referenced. An optical tracking system was used to measure the collimator axes with high precision and an isocenter cost function was implemented to ensure a unique isocenter location. The same optical tracking system was used to (a) align the couch axis to the physical isocenter, (b) align the radiation beam to the collimator axes, and (c) position a marker precisely at the physical isocenter to demonstrate the effectiveness of the approach.

The framework was successfully demonstrated on an Elekta LINAC. The physical isocenter was shown to be repeatable with a standard deviation of 0.03 mm for the position and 0.03 mm for the radius. The couch axis was aligned to physical isocenter within 0.07 mm. The average collimator to beam axis distance before beam alignment was 0.19 and 0.10 mm after. All these steps were performed within 3 h, showing that the method is efficient when applied to isocenter optimization. The time required to measure physical isocenter and guide a marker to it for day-to-day isocenter QA was under 10 min.

We have presented a modular, practical framework for isocenter characterization and optimization based on physical isocenter, which is a stable and fixed reference point.