Facility layout, shielding and interlocks

7.4.1. External beams 7.4.1.1. Layout

Initial considerations for an external beam treatment unit should include careful consideration of a number of points which will have an impact on the radiation aspects of the programme:

(a) The types of use and proximity of the work and public access spaces beyond the treatment room will play a major role in the amount of shielding required in each of the barriers. It is best to keep highly occupied areas as far away from the treatment room as possible, and conversely to surround that room with spaces that cannot be occupied or have very low and controlled occupancy (such as a roof, which can have access controlled by locks or signs to prevent entry).

(b) Ease of access to the treatment room by patients and for the installation and replacement of equipment is also an important factor. A maze is the most practical solution for 60Co external beam treatment rooms. If well designed, a maze makes a heavy door unnecessary. It should be noted that staff may enter the room a 100 times per day, and heavy doors may become impractical. Motor driven doors are expensive and slow, and are usually not necessary for 60Co irradiation rooms. For ventilation and electrical conduits, the maze also allows easy installation of ducts into the treatment room vault over the maze door, and only a minimal amount of scattered radiation will come through the ducts.

(c) Primary barrier widths should be about 0.67 times the distance from the source to the barrier, while their thickness is determined by the methods discussed in Appendix XV.

(d) It is necessary to supply an open access conduit for equipment cables and test cables near the unit control (as specified in the manufacturer’s accompanying documents). This can be done by ensuring that the line of sight of the conduit does not intercept any surface inside the room that can be struck by the primary beam.

(e) Care should be taken to ensure that there are no voids, including conduits, in any of the primary barriers.

(f) Any junction boxes in the secondary walls should be backed by 4 cm of steel with a 3 cm margin at the sides.

An example of a calculation spreadsheet for a 60Co therapy source is given in Appendix XV.

7.4.1.2. Interlocks and signs

In addition to layout and shielding considerations, there are safety interlocks and procedures that need to be incorporated into the radiotherapy programme:

(a) The door to the treatment room should have a fail-safe interlock to switch off the radiation beam (i.e. return the source to the shielded position) if the door is opened during a treatment. Restarting irradiation should require both closing of the door and activation of a switch at the control console. This is intended as a reminder to record the irradiation time given prior to opening the door.

(b) The door to the room should have a sign which indicates that the room contains radiation sources or radioactive materials.

(c) There should be a visible light at the door to the room that shows if the source is on. Typically, this will be red when the source is on and green when it is off.

(d) There should be a battery operated detector of scattered radiation inside the room that shows when the source is on.

(e) There should be emergency buttons located inside the room to shut off the radiation, and these should be reachable without passing through the radiation beam.

7.4.2. Brachytherapy 7.4.2.1. Layout and shielding

Low dose rate brachytherapy can be performed by manually loading the sources into the applicators, which have been placed into the patient, or by using a remote afterloading unit that stores the sources until they are needed and then drives them into position in the applicator.

The remote afterloader acts as its own storage safe and allows the sources to be retracted into the safe position whenever anyone, such as a nurse, needs to be near the patient. Therefore, staff exposures can be kept to a very low level.

With manually loaded sources there is a need for a shielded and locked container, which is usually kept in a locked room. Security for the sources is of the utmost importance. This room can also serve for loading the sources into the applicators.

For LDR type sources that are always stored in a locked shielded safe within the room, except while loading and unloading the applicators, the room itself does not need to be shielded. It will usually have a work area with an L block shield for the person loading the source to use while identifying and loading the sources into the applicator. Since the sources and their identifying marks are very small, it is useful to have a leaded glass viewing window on the L block along with a magnifying lens mounted to a light assembly.

The patient rooms used to house the LDR brachytherapy patients until they are ready to be discharged may not need to have shielding in their walls if mobile lead shields around the patient’s bed are made available.

A sink in this room can aid in the cleaning of the applicators. However, sinks have also led to loss of sources; for instance, when a patient has removed a source and disposed of it down the sink. This can be avoided by placing a filter to prevent any source from falling down the drain.

High dose rate remote afterloading units require some special considera- tions in their layout and shielding. Each of the walls, the ceiling and the floor of an HDR room is a primary barrier and shall be of adequate thickness to protect the staff and public, who must remain outside the room during the treatments. If the HDR source may be positioned anywhere inside the room, the resulting calculated barrier thickness can be very large since distance cannot be assumed to aid in the protection beyond any barrier. Thus, it is advisable to require the HDR unit to be located within a defined area of the room and to use a chain or electrical interlock to ensure that it cannot be turned on (i.e. the source driven outside its protective housing) unless the HDR unit is in that prescribed area. The room should be designed so that:

(a) There is an interlock on the door that will cause the source to be retracted into its shielded housing if the door is opened during the time the source is on.

(b) There is an indicator at the door to the room as well as at the treatment console of the source ‘on/off’ status.

(c) There is a battery operated detector of scattered radiation inside the room that shows when the source is on.

(d) There are emergency procedures for safely removing the source from the patient and quickly storing it in a safe location in the event that it does not retract all the way into its source housing when expected. This requires that a wire cutter sufficient to cut the source cable and a shielded storage container be located inside the treatment room.

(e) The door to the room should be marked to indicate the radioactive materials that are within, and there should be an indication of how to contact the person responsible for radiation safety in the event of an emergency.

Detailed information relating to HDR and LDR brachytherapy can additionally be found in Refs [8, 13].

7.4.2.2. Interlocks and signs

The doors to the source storage rooms need to be locked and have a sign indicating that there are radioactive materials stored within. There should also be an indication of the responsible person to contact in the event that entry is needed, for example, for fire safety purposes.