SU‐E‐T‐300: Linac Bunker Access Control System for An Exactrac/Novalis Imaging Systems

Abstract

Purpose: To implement and assess the radiological safety of a modified Last‐Person‐Out (LPO) interlock circuit in the bunker of a robotic Novalis/Exactrac stereotactic radiosurgery linac allowing the Exactrac imaging system to operate with the bunker door open. Methods: The Exactrac floor mounted X‐ray imaging system is designed to be used with the bunker door open, however access control is a concern in bunkers not designed for open door operation. The door interlock contact in our Exactrac generator was identified and a LPO circuit for the Exactrac imaging system interlocked with the operator console was installed. This allowed for operation of the Exactrac imaging system with the door open while providing access control using a light shower (Banner‐Engineering) installed at the maze entrance. To assess the radiological safety for operators, a large volume ionization chamber (PTW32003) used in conjunction with an electrometer (Keithley6517A) was placed at the location of the bunker door at the entrance to the maze. 100 radiographs (50 from each X‐ray tube) were taken with the Exactrac X‐ray imaging using a cranial standard (100 kVp, 80 mA and 100 ms) and a 20 × 20 × 20 cm3 solid water phantom placed at isocentre to simulate the patient. This corresponds to 2 to 3 times our typical daily load. Results: The Exactrac LPO system runs in parallel to the Novalis Last‐Person‐Out system that remains unaffected by the modification. Our measurements revealed a yearly equivalent dose at the open door of 4 micro‐Sv per year, and a committed dose of 0.5 micro‐Sv per year (assuming one‐eighth occupancy factor). Conclusions: An access control system was implemented on our Novalis/Exactrac system allowing the imaging system to be safely used with the treatment door open. Prior to use, the federal regulator (CNSC) was notified and staff training was provided.