Linear Accelerator

Getting the Necessary Shielding Right

The use of heavy concrete or ordinary concrete in conjunction with steel to construct the primary barrier of the room is the basic design requirement leading to the construction of the safe facility meant for the use of the medical linear accelerator.

Concrete and Concrete Materials

Today heavy concrete are widely used method for the protection against radiations in Radiation Therapy. Traditionally, it has a limiting effect of heavy aggregates to the baryte gravels and sands. A considerable improvement in the characteristics has occurred, producing concrete with a much higher specific gravity, increasing the density from 3 to 4 if not more to 5.4 which can be realised. These new performance are necessary for widening the common use in radiation therapy.  

Concrete & Metal as a combination

If not carried out correctly, the metal layer could potentially result in a source of photoneutron production where it presents a problem of radiation exposure beyond its shield.  The problem resulting from photoneutron production in the shielding occurs only for the primary beam barriers and not for the secondary barriers. It becomes more pronounce for larger field. To do calculations of photoneutron production, it is necessary to fold the incident photon spectrum into the cross-section curve for photoneutron production as a function of photon energy. 

 

Assuming that bare iron or steel plate or slab is considered, and the neutron production from a lead plate of  1.8MeV for a 15MV x-ray beam to 2.2MeV for a 25MV x-ray beam, the neutron from an iron plate would have a lower average energy because of the high threshold energy. A conservative treatment is to consider 2.2MeV in all cases. If the metal is very thick, the neutron penetrating to the other side will be significantly degraded in energy. A significantly good example is to consider that all the neutrons are created in the first x-ray Tenth-Value-Layer (TVL) and then decrease the average energy in the remaining thickness accordingly to National Council on Radiation Protection recommendation.

The neutrons will be produced in circular areas, typically of the order of 4.5 meters and 3 meters away from the isocentre for the walls and ceilings, respectively. This yields radii of 137cm and 100cm respectively. They are nearly uniform sources, too large to treat as point sources when you are close to them.  At one meter, a lead plate would give a fluence-to-dose of 14 rem/week. For iron plate, it would give 1.7 rem/week. 

Clearly, the above present a true problem that must be alleviate in some manner. This can be accomplished by the use of some neutron shielding materials after or before the metal plate. Preferably one does both the options. Since the photoneutron production is isotropic, if the metal plate is on the inside surface of the room, the neutron will add a small amount of whole body neutron dose to the patient and a certain amount of room activation. X-ray shielding on the inside of the metal plate will virtually always be concrete. The neutron production will be decreased simply by the x-ray attenuation of the intervening concrete. The usual x-ray TVL's are adequate since pair production is not important in concrete.  Note the neutron dose inside the room is attenuated very fast with the inside concrete layer, since the neutron TVL's are less than half of the x-ray TVL's. One x-ray TVL of concrete inside the metal plate will decrease the neutron production by a factor of 10 and will attenuate the resultant neutrons by a factor of 100, in the point source approximation. 

Designing the Room 

Fig (b) Good Design

Fig (a) Bad Design

  

 

                              

Fig (c), Typical Room Plan 



In planning the room size, we try to accommodate as many makes and models as possible during the planning stage.

In addition, maximum couch and gantry movements (both vertical & horizontal) must be considered.  These will decide how big is the room size and its minimum ceiling height.

Fig (d), Cross-Sectional View of AA

Base plate for the Linac has to ne planned in advance where the provision of the base pit is necessary.

Coolant pipes for the supply and return of cooling media to the heat exchanger are to be considered as well at the point of planning the pit and its size. Not forgetting the trunking for the electrical services to be provided as well.



 

 

Ensure that upon sizing up the room and its height, there is a need to look at how the gantry and its associate equipment can be brought into the room. We can always use a template scale to size to manoeuvre the gantry and its equipment such that we do not neglect the part of accessing the room for the purpose of installing the system.   

Others to be considered include the room lighting, laser positing light, observation cameras, intercom system, etc.

Information on this page is provided for interest only on a "best efforts" basis and does not 
constitute personal advice. Always discuss medical conditions and related matters with your doctor.
 

Call, McCall Associates, Woodslide, CA; Jeffrey H. Kleck, Varian Associates, Palo Alto, CA.