Quality Assurance
QA FOR LINEAR ACCELERATOR Radiation Survey
Radiation protection survey involves the measurement of head leakage, area survey and test of interlocks, warning lights and emergency lights.
The survey is evaluated on the basis of clinical use, by taking into account the workload, use factor and occupancy factors. The detail procedure of survey is explained in chapter five under area survey.
Jaw Symmetry
To study jaw symmetry, a machinist’s dial indicator is used. First the gantry is set at horizontal and jaws open to a large field size. The feeler of the dial indicator is made to touch the face of the one of the jaws and the indicator reading is noted. Now the collimator is rotated to 180 degrees and the feeler is touching the opposite jaw and the dial reading is again noted. The difference between the two readings is noted. The symmetry error is ½ of the difference in readings. The procedure is repeated for the second jaw.
Spirit level is used to check the collimator angle. The tolerance for symmetry error is 1 mm.
155 Coincidence
Collimator Axis, Light Beam Axis and Cross-hairs Coincidence Gantry is set at vertical and the SSD is set at 100 cm. A graph paper is fixed on the couch and the field size is kept as 10 × 10 cm. Switch on the light field and mark the edges of the light field, intersection of diagonals and the position of the cross hairs images. Rotate the collimator through 180 degrees and mark the above parameters in the graph paper. Check the coincidence of light field edges, intersection of diagonals and position of cross hair images. If there is a misalignment it should be adjusted to bring down to coincidence.
Optical and Radiation Beam Congruence
A therapy verification film back is placed on the couch with SSD of 100 cm.
A field size of 10 cm x 10 cm is set and collimator angle is set to 0 degree.
The light beam is made on and the light field edges and the centre is marked with lead wires or radio opaque markers. A plastic (2-5 mm) sheet is placed over the film pack to give electronic buildup and eliminate electron contamination. The film is exposed so that a optical density of around 1 is achieved. The procedure is repeated for a collimator angle 90, 180 and 270 degrees. The coincidence of the optical and radiation beam is checked visually or by cross beam optical density profiles (Fig. 6.6). The tolerance is
± 3 mm.
Mechanical Isocenter Collimator Rotation
A graph sheet is fixed on the couch and front pointer is put on the accessory mount with a gantry angle of 0 degree. With the pointer extended, the SAD is set to 100 cm. Now the pointer tip position is marked on the graph sheet. The collimator is rotated to 90,180 and 270 degree and each time the pointer tip position is noted. A sharp edge or wiggler may be attached to the end of the pointer rod to have effective observation. The tolerance of the isocenter is ± 2 mm diameter.
Gantry Rotation
In the above procedure, another horizontal rod with fine pointer is positioned by means of a stand. The stand should be kept away to avoid gantry collision. The horizontal rod tip and the front pointer tip are made to coincidence at 100 cm SAD with gantry position of 0 degree. The gantry is rotated to 90,180 and 270 degrees and the displacement between front pointer tip and the horizontal rod tip is observed. The tolerance of the isocenter is ± 1mm.
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Fig. 6.6: Optical and radiation field field congruence:
9 MeV electron and 6 MV photon beams (For color version see plate 1)
Radiation Isocenter Collimator
The gantry is set to vertical, 0 degree with a SAD of 100 cm. A ready pack film is kept flat on the couch. The upper jaws are fully opened and the lower jaws are closed to have a narrow slit of beam. A build up sheet is placed over the film and it is exposed to create a density of about 1. The collimator is rotated to different angles (4-8 angles) and each time the film is exposed. The procedure is repeated for upper jaws of narrow slit, while the lower jaws are wide open. The developed film will show the star pattern with dark centre region (Fig. 6.7). Using a film marker lines are drawn through the middle of the slit images, which will show clearly the intersection point. The lines should intersect with in a ± 2 mm diameter circle.
Gantry
A ready pack film is sandwiched between two plastic sheets and it is kept on the couch vertically. This means that the plane of the film should be perpendicular to the plane of the couch top. A slit beam is created by moving the jaws optimally, parallel to the gantry axis. The film is exposed for different gantry angles (12 to 30 degree) and the final star pattern is obtained (Fig. 6.8). The lines should intersect with in a ± 2 mm diameter circle.
157 Table
The above procedure is repeated. The gantry and the collimator is in fixed position. The table is rotated (4-8 times) to different angles and each time the film is exposed. A final star pattern is obtained and it is examined (Fig. 6.9). The lines should intersect with in a ± 2 mm diameter circle.
Fig. 6.7: Mechanical isocenter verification: Collimator rotation
Multiple Beam Alignment Check
When more than one beam is used misalignment may occur. This may be due to (i) focal spot displacement, (ii) asymmetry of collimator jaws, and (iii) displacement in the collimator rotation axis or the gantry rotation axis (Lutz et al,1981). To check the beam misalignment a split field test is recommended. A ready pack film is sandwiched between buildup sheet and is exposed twice. First the one half (region 1) of the field is exposed, by blocking the other half (region 2). The gantry is rotated through 180 degree
Fig. 6.8: Mechanical isocenter verification: Gantry rotation
Fig. 6.9: Mechanical isocenter verification: Couch rotation
Isocenter shift by gantry rotation
The shift is found to be with in 2 mm
The shift is found to be with in 2 mm Isocenter shift by table rotation
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and exposed again by blocking region 1. The relative shift of the two images is the indicator of the misalignment.
Photon Beam Data Energy
Photon beam energy is specified by the depth dose distribution. A central axis depth dose curve measured with a suitable ion chamber in a water phantom can be compared with published data (BJR 25). The ion chamber should have a small internal diameter (< 3 mm), to minimize displacement correction. It is advisable to compare depth dose ratios for depths beyond dose maximum, instead of absolute values of depth dose. The recommended depth for depth ratios are 10 and 20 cm. The acceptance criteria is specified in terms of depth dose variance for 10 × 10 cm field size,100 SSD, at 10 cm.
The acceptable difference is ± 2 % from the published data. The measured depth dose data is to be used for clinical dose calculations.
Field Flatness
Field flatness for photon beams is defined as the variation of dose relative to the central axis over the central 80 % of the field size at a depth of 10 cm in a plane perpendicular to the central axis. The AAPM -TG 45 specified flatness in terms of maximum percentage variation from the average dose across the central 80 % of the width at half maximum (FWHM) of the profile in a plane transverse to the beam axis. It is given by the relation:
F M m
=M m− + ×100
where M and m are the maximum and minimum dose values in the central 80% of the profile. The tolerance limit is ± 3 %. The flatness should be checked for 10 cm and Dmax depths, for maximum field sizes. Beam profiles are generated for inplane, cross plane and diagonal directions and checked for flatness for each given field size.
Field Symmetry
The profile generated with the above procedure can be used for checking the field symmetry. Usually the profile is folded at the centre and hence the two peripheral halves should be compared at the reference depths. This should not differ more than 2 % at any pair of points located symmetrically with respect to the central ray.
Electron Beam Data Energy
The electron energy is specified by practical electron range (RP) and the most probable energy (EP)O as per AAPM-TG 25. The RP is the depth of the
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