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Mechanical alignment checks


4.1. Mechanical isocentre – definitive checks

The isocentre is the common centre of rotation of the gantry, the collimator and treatment couch. Because of mechanical limitations, the isocentre is not a single point, so it is defined as the centre of the smallest sphere which contains the axes of rotation of these three components. The diameter of this sphere may not be bigger than 2 mm, but it depends on the design and age of the accelerator. The isocentric accuracy in older machines might limit the types of feasible treatments set-up and in such circumstances clinical practice must be reviewed to make sure that set-up methods achieve the best results possible.
Isocentric techniques are based on that the machine can rotate about the isocentre which is placed at the centre of the tumour without having to move the patient between fields. The initial setting of the isocentre is usually guided either by a distance indicator from the anterior field or by the laser isocentre markers. The point of rotation of the gantry can move due to machanical inaccuracies, abrasion, cluck, etc.
To achieve the best geometric accuracy of treatments in such case it might be suitable to set the indicated isocentre position partly below the true isocentre position as indicated. It may not be set to the centre of rotation from the AP direction (Woo et al 1992). There is also often a misalignment of the central axis in the longitudinal direction between the gantry 0° and gantry 180° positions. The most satisfactory way to determine the isocentre is to attach a sharp rigid pointer to the front of the accelerator. It needs to be adjustable both in the distance from the source and in the other two orthogonal directions. First step is to set up this so that the centre of the pointer does not move when the collimator system is rotated. The collimator is rotated through its complete interval and the centre of rotation is noted. (A convenient way to perform this is to use a spring loaded pointer and a piece of carbon paper. As the collimator is rotated the pointer marks out its movement on the carbon paper.) To set the accurate rotation of the collimator is relatively easy, therefore it can be adjusted so that the pointer movement is less than 0.5 mm.
Then a second fixed pointer is set up usually on the top of the couch. The distance of the tip of the gantry-mounted pointer from the source is adjusted so that there is the minimum movement of its tip relative to the fixed pointer as the gantry is rotated. The fixed pointer is moved to the optimum position and the importance of the horizontal sector is kept in mind as discussed above. Now the fixed pointer defines the mechanical isocentre, and the optical and mechanical indicators can be adjusted accordingly. It is convenient that a reference pointer can belong to each machine whereby the isocentre indication may be checked and adjusted easily and accurately.
After the fixed pointer has been located the size of the sphere containing the isocentre (isosphere) can be determined. Observation of the relative movement of the two pointers can be very useful for diagnosis of problems if the movement is found to be significant. Simple check of the alignment of the optical system could also be carried out by watching the relative movement of the shadows of the pointer and the cross wires when the gantry is rotated.


4.2. Couch rotation axis

The axis of rotation of the couch should also be within the isosphere. This is often forgotten when determining the isocentre position. If the pointer is rigidly fixed to the couch, the couch can be rotated and the displacement of this pointer relative to the gantry can be measured. However, the indication of the isocentre should not be changed on the basis of this, rather this should be adjusted, if it is necessary to match the gantry and collimator determined isocentre. The accuracy of the couch rotation is specially important when such treatments are carried out where treatment planes are being set using couch rotations or with multiple arc stereotactic radiotherapy. In such techniques errors of couch rotation should be corrected by moving the patient, although this makes the treatment more time consuming.

4.3. Mechanical isocentre – quick checks

Several simpler methods can be chosen for quick checking the isocentre indication. These
applies the optical indication of the isocentre. It should be noted that errorss relative to the optically indicated isocentre will often be caused by a misalignment of the light bulb rather than the misalignment of the gantry. Some of the checks are complemented each other and the combination of these should be used for routine checking:

1. The collimator is rotated at 0° position of the gantry and the crosswire indication is marked on a piece of paper at different collimator angles. The points must be within a 1 mm diameter circle. This can also be carried out with the reference mechanical front pointer (see in Section ”definitive checks”). Adjustment of the crosswire is described in chapters „optical field indication” and „interpretation of alignment checks”
2. The gantry is rotated with a small angle and the couch height is changed until the shadow of the crosswires does not move across the couch. Then the rotation axis of the gantry is on the surface of the couch. However, this check can be misleading because it bases the position of the rotation axis for gantry angles close to 0°. Because of the reasons explained above (mechanical isocenter quick checks), this is less important than with the gantry close to the zero.
3. A white plate marked with a black cross which can be rotated about a horizontal line placed on the couch (see simulators QC). The rotation axis of the plate must coincide with the indicated position of the isocentre and the central cross should be aligned with the crosswires. With the rotation of the gantry the alignment of the crosswires to the cross can be observed. This test is especially recommended. If the field size is set so that the field is smaller than the plate, movements of the collimator jaws with gantry angle can also be monitored. By rotating the collimator at different gantry angles it can be confirmed that the crosswire gives an accurate indication of the axis of rotation of the collimator at all gantry positions.
4, A square block is used with crosses marked on its faces. It is placed so that the upper surface of this should be half the thickness of the block above the isocentre and centred on the crosswires. When the gantry is rotated to the horizontal position the centre of the field should be aligned with the centre of the block. The block can be placed in a track on a levelling plate and the alignment of the lateral lasers can be checked as well as the indication of 90° and 270° on the gantry rotation scale by moving the block horizontally.

4.4. Distance indicator

If the position of the isocentre has been determined the mechanical and optical distance indicators can be adjusted. For quick checking methods (3) and (4) in the previous section can be used. The mechanical distance indicator is probably more stable than the optical distance indicator and should only be altered if a substantial reason exists by determining the isocentre position as described in Section definitive checks (see above) The linearity of the distance indication set for the isocentre distance should be checked by moving the couch a known distance.

4.5. Front pointer and back pointer

The front and back pointers can also be checked after the position of the isocentre has been established.

4.6. Calibration of gantry and collimator rotation scales

The gantry rotation scale can be checked using a spirit level placed on a surface which is perpendicular to the axis of the beam. The resolution of the scale is only 1°, but this should not be more than 0.5°. The collimator rotation scale should be tested in a similar way.

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