Of course, the alignment of the radiation field is fundamental in radiotherapy. In most cases (except when fitting fields) the radiographers set the field size using the numerical field size scale and assume that this field is centred on the crosswires.
The basic check: place a film at the isocentre, set a mid range field size, mark the position of the light field and the crosswires and expose the film. The marks can be made either by applying pressure to the film (before exposure) with a biro or hard pencil, or by pricking the corners of the field with a pin, or using a jig with radioopaque inserts.
The dose given should be such as to cause an optical density of 1.5. A low sensitivity film should be used as, with short exposures, undue weight can be given to any transient misalignment of the radiation beam before the beam steering servos begin to act. The positions of the field edges are establidhed with a densitometer. Discrepancy of the radiation and light fields can be caused either by a misalignment of the optical system or by misalignment of the radiation beam. For monthly checks it is sufficient to carry out this test for one field size (e.g. 10 cmx10 cm or 15 cmx15 cm). The alignment of small and large fields should also be checked every half a year or when adjustments are made. In analysing these checks the relationship of the radiation field to the field centre (as indicated by the crosswires) is more fundamental than the coincidence with the optical field. Treatment verification films are conveniently envelope wrapped but have a small linear dose-density range. Fine grain graphic line films have better linearity and background density but need reusable black plastic envelopes. They are thinner than X-ray film and can get stuck in some automatic X-ray film processors. It can be avoided by attaching a leading X-ray film using special metallic adhesive tape. A slab of PMMA of size 25 cmx25 cmx5 mm can be useful for checking the alignement of the radiation and light beams where the centre and fields of size 5 cmx5 cm, 10 cmx10 cm, 15 cmx15 cm and 20 cmx20 cm are marked. Lead markers (tin-lead solder) can be inserted into holes drilled on the central cross and along the edges of each field. The radiation and light field should match within 2 mm. The centre of the radiation field should also be within 2 mm of the central axis of the machine. The latter case should be easily achievable, therefore in the precision radiotherapy a tolerance of 1 mm must be achievable.
Several devices with multiple detectors - designed for checking beam flatness - are available (see Section 11.2). These usually have diodes or ionisation chambers at the centre of the field and at the edges of a 16 cm square. For flatness and symmetry checks a 20x20 cm field should be set, but if a 16x16 cm field is set the dose measured at the edges should be 50 per cent that of on the central axis. Since the variation of dose with distance from the centre between the 20 per cent and 80 per cent points on the beam profile is roughly linear and the beam penumbra is typically 6 mm, a 10 per cent difference from 50 per cent indicate an error of 1mm in the position of the field edge. Because devices are not normally designed for this purpose they should be individually calibrated with film to determine the field edge position.
Establishment of the radiation isocentre is incontestably a complex procedure that does not lend itself to a quick check. The simplest method is to expose a film which is usually marked with the gantry vertical and then irradiated with the gantry at 180o. By comparing the alignment of this film with one described in the previous paragraph it is easy to state the relative movement of the radiation field. This is usually experienced along the direction of the gun target axis due to sag of the gantry. It is evident that the film should be placed in the plane of the isocentre because otherwise an error in the gantry angle is incorrectly detected as an error in the radiation isocentre.
A useful method for checking the radiation isocentre is the ‘star film’. A film is placed vertically in a PMMA holder in the plane of rotation of the gantry with its centre at the isocentre. As narrow a field as possible is set and the film is irradiated with the gantry at 0o, 120o and 240o (these directions can be others). There can be holes drilled at the centre and at 120o increments around the centre of the perspex holder. The holes can be used to pierce the film to assist analysis. The image on the film will be a star shape and it is relatively easy to draw lines at the centre of each beam. All of these should meet at the film centre. It will be noted that this method gives no information about errors in the direction orthogonal to the plane of rotation. A very precise estimation of the fundamental isocentric accuracy of the gantry becomes possible with the development of stereotactic radiotherapy. With a circular applicator attached to the head of the machine and a ball bearing suspended at the isocentre by rotating the gantry films exposed at appropriate angles. The displacement of the image of the ball bearing relative to the circular field is easily observed. Of course, this test can be carried out with a small field produced by the original collimator.
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