Optical Linacroom Access Control.

By Niels de Graaff.

Optical Linacroom Access Control. (Also called the Last Man Out system) 04 10 2001 12:18PM

The Access Control Setup is as follows: We have a Controlroom and a Linacroom. The connecting labyrinth between both is about 6-7 meter long. On both sides of this labyrinth a pushbutton has been mounted: P1 on the Linac side and P2 on the Controlroom side.

In the labyrinth on about 40-50 cm distance from each pushbutton we have set of two Infrared optical barrier switches. For security reasons each switch consists of a transmitter and a receiver. The barrier switches appear in the drawing as contacts F1....F4

These barrier switches are mounted one at about 60 cm and another at about 120 cm above floor level. This way we expect that almost everybody who passes the labyrinth will be 'seen' by at least two optical switches. This includes children, and ambulance people wearing reflective clothes.

The AccessControl system has three phases:

The 'FREE' PHASE where everybody can enter the Linac room as radiation is impossible.

The 'CLOSING ACCESS' PHASE which allows everybody (except the patient) to leave the Linacroom before radiation can start. This under the assumption that the 'closing access' procedure has been followed correctly. This procedure is simple but very strict. It ends with a confirmation by pressing pushbutton P2. (Thereby effectively forming a Last Man Out action....)

The 'RADIATION' PHASE where radiation is possible when the responsible therapist wants to start the treatment. If someone presses pushbutton P1 or walks through one of the optical barriers in the RADIATION PHASE the radiation will immediately be terminated and it is impossible to simply re-start: the system switches back to the safe (FREE) phase. The only way to re-start the radiation is to re-follow the procedure again.

Description of the procedure: The last therapist leaving the Linacroom before an intended treatment can be started will have to press pushbutton P1.

This indicates a start of the 'CLOSING ACCESS PROCEDURE'.

She/he then walks through the labyrinth to the controlroom thereby passing the optical barrier switches which should react but don't do anything more in this phase.

She/he then presses pushbutton P2 within about 6 seconds. This is the final confirmation that radiation may start. Anyone walking through one of the barrier switches or who presses the pushbutton P1 will immediately terminate possible radiation, and a re-run of the procedure will be forced. It is clear that the one who presses the pushbutton P2 has the responsibility to be sure that no one (except the patient) is in the Linacroom at that time.

Technical aspects. The system is supposed to be built with electrical hardware like relays, electromechanical switches. It is of course possible to develop a system based on a microcontroller, but the philosophy intended here asks for an easy to understand, maintainable and simple system. In our perception we don't like 'black boxes' in this situation. And, in the end, we will have to present a real free-floating electro-mechanical contact to the Linac anyway.......

The circuitry normally will be installed in the 'Interface Cabinet' where all the electrical power comes in where the connection will be made to the Linac system itself. There you can mount the relays etc. This is also the place to mount the yellow indicator lamp: there you can check whether the optical system functions. Normally there are LEDs on those Optical units which show the function. But remember: Those LEDs are activated locally, this does NOT mean that the relay contacts are OK: they could (!) get sticky giving you a false idea of security....

Speaking of barriers: As nothing can compare to a simple metal door with simple electromechanical switches as it is a PHYSICAL barrier, any optical barrier is less. Consequently, as we will not use a mechanical door for our new treatmentrooms, outmost care has been taken to improve the safety, security and the reliability of the system: a number of (seemingly) redundant tricks have been used as extra relay contacts.

Simply, what we want is: Only radiation when all components and procedures are OK. If anything fails, NO radiation!

However we have to fight against a physical law: the more secure the more complex a system becomes. But the more complex a system (circuitry and components) is the more vulnerable it is in the end. We have to find a balance.....

And, reflecting the recent discussion about LMO systems on the Linac-Eng list: There is NO system that absolutely can prevent that some person remains in the treatmentroom when the Last (Wo)man going Out has not checked the room.

OK, we are supposed to be professionals and should work with maximal responsibility. But we remain human beings........... And routine and workpressures sometimes can make you blind...lets face this!

On the other side we feel obliged to make the risk of unwanted radiation on our personel as low as possible, following the ALARA principle. I think this is a very reasonable and sound principle we should follow.

We did our best.

I anybody has a better idea please contact us!!!! degraaff@radth.ruu.nl