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Optical field indication

6.1. Quick check

The quick check of optical field size indication for a 10 cm x 10 cm field must be carried out daily. In the case of a properly marked device this test can be carried out simultaneously with the daily dose check.

6.2. Variation with field size

The field size scale must be checked monthly over its full range. This can be easily carried out using a piece of squared paper placed at the isocentre. The collimator is set successively to the indicated 4, 10, 20 and 30 cm and the sizes are noted. If an experience is found that adjustment is required first the radiation field size should be set and the optical indication adjusted to suit to it using the optical field trimmers. In the case of small fields the tolerance is generally 2 mm in the field size and in the position of individual jaws. For modern precision radiotherapy a tolerance relative to individual jaw positions of 1 mm is usually achievable. The optical indication for the 30 cm field can be less accurate. Occasionally precision of the indicated field size should be checked at extended SSD (see Section Interpretation of alignment checks). For these tests a sheet of accurately ruled graph paper, 30 x 40 cm, can be used. Additional lines should be drawn on to this corresponding to field sizes of 5, 10, 20, 30 and 40 cm. It is convenient if the paper is coated with a transparent plastic to prevent damages. It also stiffens the graph paper so it can be placed vertically on the couch top using lead blocks as support.

6.3. Variation with collimator rotation

The positions of the field edges do not change when the collimator is rotated to the four cardinal angles (this should be checked). This can be carried out simultaneously with checking the crosswire. The check should also be performed at gantry angles other than 0o. A check of the alignment of the field indicated at gantry 0oand 180o can be useful. If the light bulb is loose, a discrepancy will be seen which is greater than normal.

6.4. Adjustment of optical indication

If the adjustment of the optical system is found necessary the first step is to check that the light source is accurately positioned on the axis of rotation of the collimator. It can be executed as described in Section 5 or using test (1) described in Section 4.3. This test should also be carried out at extended SSD (e.g. on the floor as it is possible for a wrong adjustment of the light source is made to compensate for a misalignment of the crosswire if the test is only carried out at one SSD). To allocate that the light source is at the same distance from the isocentre as the radiation source the field size indication at extended SSD should be checked.

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