Method of developing an interlock system for linac-based stereotactic radiosurgery

Abstract

An interlock system was developed to enhance safety in stereotactic radiosurgery procedures on a linear accelerator. This system assists in 1) avoiding any collision between the gantry and a floor-mounted radiosurgery stand, 2) leaving the field size to that which was used for a regular treatment prior to a radiosurgery procedure, and 3) preventing accidental motion of the couch vertically, laterally, or longitudinally after the radiosurgery head frame is attached to a floor-mounted stand. Linacs (Varian Clinac 6/100 and Clinac 2100C, Varian, Palo Alto, CA) have gantry, couch angle, and collimator encoding potentiometers which supply input signals to the digital readout systems on the console, gantry, and record management system (RMS). Our approach was to tap into the analog signals derived from those potentiometers. Our interface circuit compares these signals to desired adjustable motion limit values and generates stop signals to the gantry rotation motor or the couch rotation motor control circuits when the limits are reached. In our departments, a 5 × 5 cm field size is used for the secondary collimator (jaws). When it is larger or smaller than this, our system creates a “customer interlock” which ties into the accelerator interlock logic and prevents initiation of radiation until corrected. The interface circuit consists of several LM339N quad comparators, ILQ-74 quad optosiolators, N4400 transitors, and logic elements. To bypass this gantry, couch angle, and collimator limits during regular operation, a switch was installed in parallel with interlock motion control relays, so that operation of these relays will not affect gantry rotation and the normal limits will apply. The front panel of the interface control box also includes switches for independent disabling of all couch motions. We found the system to be very useful in minimizing possible human errors and is easy to activate. The system allows easy limit adjustments to any gantry angle within the normal preset range, and it can be connected to the linear accelerator with minimal modification to the existing hardware. Both centers have treated over 100 patients with this system in use. The use of this device is limited to those performing radiosurgery with linear accelerators. Radiat. Oncol. Invest. 4:287–291, 1996. © 1997 Wiley-Liss, Inc.