Detector alignment with beam central axis

For small field dosimetry, it is essential to ensure accurate alignment of the detector with respect to the beam’s central axis given the sharp maximum and steep gradients in lateral beam profiles. Figure 16 [90] illustrates that alignment based on laser beams or the machine’s light field with typical tolerances of 1 mm is not accurate enough for the measurement of field output factors for small fields. After initial alignment based on lasers or light field, further refinement of the alignment is thus required. This requires measurement of profiles in two dimensions at the measurement depth.

6.2.3.1. Alignment of real time detectors

With real time detectors, the alignment of the detector with the central axis of the beam can be achieved using the scanning system. These scans are

FIG. 16. Demonstration of the influence of clinical set-up accuracy: the beam laser (solid vertical line) is calibrated with a misalignment tolerance of less than 1 mm from the beam’s central axis in a field of 5 mm width, but this does not ensure a negligible underestimation of the profile maximum (reproduced from Ref. [90] with the permission of the American Association of Physicists in Medicine).

performed at slow speed, with an appropriate step size for the field (of the order of 0.1 mm for the smallest fields) and with attention to potential effects of hysteresis of the scanning system. The alignment can be performed based either on the centre of the two 50% profile levels or on the profile maximum assuming that the beam profile is symmetric. At the same time this provides a measurement of the FWHM field size specification. Given that tiny changes in the collimator position can result in substantial changes of the absorbed dose to water at the centre of the field, this alignment procedure and FWHM determination is performed every time the field has been set or re-set by moving the collimator for MLC based radiotherapy machines. The alignment has to be performed in two orthogonal directions, and this may require an iterative procedure to determine the centre of the field, accounting for the possibility of tiny phantom misalignments.

Note that for the measurement of depth dose profiles along the beam axis, the centre of the field has to be determined at different depths and, based on that information, the phantom and scanning system needs to be accurately aligned with the beam central axis (CAX correction). For the procedures in this COP, which are restricted to field output factor and lateral beam profile measurements, this is not critical; however for the measurement of lateral beam profiles, it is advised that they be measured at the same depth at which the output factors are determined.

6.2.3.2. Lateral alignment of off-line detectors

The main problem with setting up an off-line detector is that the radiation-induced signal cannot be observed immediately, and any radiation exposure during alignment of the detector contributes to the signal. Thus, the detector itself cannot be used to detect the centre of the field. Various methods have been described to deal with this alignment problem, of which three are discussed below. For MLC based radiotherapy machines, this alignment procedure and FWHM determination is performed every time the field has been set or re-set by moving the collimator.

(a) Off-line detector set-up using attachment system

A specially constructed attachment system on the scanning arm in a scanning phantom can be used to allow a real time detector to be replaced with a passive detector. This requires very accurate machining of the real time and off-line detector holders to ensure accurate positioning of the reference point of both detectors at the same location. Often, ancillary parts (e.g. so-called stop thimbles for ionization chambers) can facilitate the insertion of detectors in their holders with the required positioning

accuracy. The user is referred to the product catalogue of the ionization chamber manufacturer.

(b) Off-line detector set-up using film

This method is particularly suited for measurements in a solid water equivalent phantom [177]. After preliminary lateral positioning of the detector insert in a phantom slab at the correct SDD and aligned with the beam axis based on lasers and/or light field, a radiochromic film is inserted between the slab with the detector insert and the slab further away from beam source to quantify any necessary additional lateral displacement of the phantom. To enhance contrast, a dummy detector made of a high Z material, or with a high Z material bead at the location of the reference point, could be inserted. If necessary the procedure is repeated iteratively.

Once alignment within the required tolerance has been achieved, the detector is inserted and the amount of phantom material necessary to position the detector’s reference point at the measurement depth is added on top or in front of the slab containing the detector insert. It is essential with each handling for taking away or adding slabs that the lateral alignment of the slab be adequately maintained.

In addition to this alignment procedure, a radiochromic film could be inserted behind the slab containing the detector for each detector irradiation, provided the detector contour can be clearly resolved on the exposed film.

This enables a volume averaging correction to be made for each individual detector retrospectively, based on the measured lateral beam profiles and the measured position of the detector with respect to the field. Ideally, a slab with a special insert would be designed for this purpose, such that the slab containing the detector does not have to be removed for every irradiation.

(c) Off-line detector set-up using an electronic portal imaging device

This method is suited for measurements in both a water phantom and a solid water equivalent phantom [100]. After initial positioning of the detector insert in the water phantom or a phantom slab at the correct SDD and aligned with the beam axis based on lasers and/or light field, an image of the irradiation set-up is taken using an electronic portal imaging device (EPID) below or behind the phantom to quantify any necessary additional lateral displacement of the detector holder in the scanning system or of the solid phantom. An example of such an EPID image is shown in Fig. 17.

To enhance contrast, a dummy detector made of a high Z material, or with a high Z material bead at the location of the reference point, is inserted.

If necessary the procedure is repeated iteratively. Once alignment within the required tolerance has been achieved, the detector is inserted in the detector holder on the scanning system and moved to the previously determined measurement point in the water phantom or, in the case of a solid phantom, the correct amount of phantom material to position the detector’s reference point at the measurement depth is added on top or in front of the slab containing the detector insert. It is essential with each handling for taking away or adding slabs that the lateral alignment of the slab be adequately maintained. The EPID image usually does not have sufficient resolution to work out retrospectively the volume averaging correction for each individual detector based on the measured lateral beam profiles and the measured position of the detector with respect to the field.