Light and Radiation Field Agreement

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Aim:

To determine the radiation and light field congruence at various gantry angles.


How to perform the test:

    A film is placed perpendicular to the beam central axis at the Isocenter. The edges of the light field and the crosshair are marked before the film is exposed. The difference between the edges of the light and radiation fields can be checked on this film, for example using a densitometer to establish the radiation field edge. The size of the radiation field is compared to the light field size and the radiation field center is compared to the crosshair position. This test is normally performed at a gantry angle of 0°, but possible movement of the light source as the gantry is rotated could be detected if the congruence of the radiation and light fields is checked at other angles. At least two field sizes (e.g. 10 x 10cm2 and 20 x 20cm2) and four different gantry angles (e.g. 0°, 90°, 180°, 270°)should be tested. These may be carried out on a rotational basis. Annually, this check should also be carried out at an additional SSD (other than reference SSD) at one gantry angle and one field size.

The above figure shows the tool to verify the light and radiation field congruence.


Tolerance:

1mm for field sizes < 10 x 10 cm

2 mm for field sizes < 20 x 20 cm2

1% for field sizes > 20 x 20 cm2

2 mm (deviation between the radiation field centre and the crosshair)

(at additional SSD) ---2 mm for field sizes < 20 x 20 cm2

1% for field sizes > 20 x 20 cm2

2 mm (deviation between the radiation field centre and the crosshair)


Questions;

1.What is the tolerance limit for the field size 5 x 5 cm2?

1mm

b) 2mm

c) .5

d) All


Answer:

a) 1mm



References:

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