The radiation beam is aimed at the tumour with an accuracy of a few millimetres, and the field edges are sharply defined. At the edges of the radiation field, the radiation intensity decreases from 80% to 20% over a distance of around 5 mm. To ensure the quality and safety of the radiation treatment it is essential that the patient remains accurately in the planned treatment position during the irradiation. For this purpose the part of the body to be irradiated can be fixed in the predetermined position on the treatment table in a customised plastic mask.
For the production of plastic masks, in 1969 a mould room is set up in which plaster casts of the patients are made. First, the body part to be fixated is covered in plaster bandages. After hardening this first plaster cast is taken away and used as a mould to make a solid plaster cast of the body part. The solid cast is then used to form a plastic mask.
Mould room in the Sarphatistraat
Instrument maker, Ruud Fontein, makes a plastic mask. In the background solid plaster casts of heads of patients can be seen. On these casts, the plastic mask is shaped out of a thermoplastic sheet, using vacuumforming at high temperature.
The construction of this fixation mask is quite similar to that of the original mask in 1970.
On the transparent material, the radiation fields are marked up while the patient is on the treatment simulator, with reference lines on which the accelerator has to be aligned. Within these lines the plastic is cut away as far as possible in order to maintain the skin sparing effect of the high energy radiation.
Fixation mask in 2004
Over 40 years, the fixation method has not changed much but has become more effective. This mask now covers a larger part of the body in order to improve fixation for the same type of head and neck irradiation as shown above. Only a few reference lines are marked on the mask as the execution of the treatment is done automatically. After an accurate first alignment, the treatment machine turns around the patient, automatically adjusts the different radiation fields and irradiates with the programmed dose.
The making of the masks has become simpler and the irradiation possibilities have improved. Masks are now made from plastic which can be shaped directly on the patient at luke warm temperatures, leaving no remains on the skin, such as was the case with the old plaster method. The elaborate plaster cast and vacuum shaping have been abandoned. The material is thin and perforated. It is therefore no longer necessary to cut out parts in order to spare the skin and it is easier for the patient to wear as the skin can breathe. The patient lies on a carbon fibre reinforced support through which radiation can pass in all directions.
On the table a mask with solid plaster cast can be seen according to the system which was used in 1969.
In the front lies a customized shielding in cast metal which has been made for an individual patient; in this case for a large irradiation of the upper body to treat morbus Hodgkin. The shielding is made in various parts, which shield lungs and other organs.
The radiation oncologist draws the shape of the required shielding onto X-ray film. From this a mould is cut out in a block of polystyrene foam. The shielding blocks are then cast in this mould from a lead-bismuth alloy which melts at 93°C.
The different parts of the shielding are fixed on a perspex plate which slides into a holder under the treatment head of the accelerator. The distance from the radiation source to the patient is in this case about 178 cm as at this distance the radiation field is large enough to irradiate the upper body.
The photo shows a casting mould being cut from polystyrene foam using a heated wire which is attached to a pivot at 178 cm from the X-ray film. In this way, the mould and the resulting shielding blocks will have diverging edges at the same angle as the field edges of the diverging radiation beam. This ensures sharp edges of the radiation field which is produced by the shielding block.