Dose profile

A cross profile shows as part of the radiation therapy, the dose distribution perpendicular to the central beam along one of the axes box in a defined water depth. Symmetry and homogeneity of the cross section is a measure for the quality of the compensator and thus Feldhomogenisierungstechniken through the irradiation unit.

For prescribed in the clinical use of treatment planning systems regular constancy tests, among other calculated and measured cross sections are compared. The nature of the depth dose curve is on the focus - skin distance, field size, and the energy of the radiation. The further one goes into depth, the wider the beam, as it is divergently propagates in space. The attenuation in matter can it be lower, and the penumbra are flatter. Flat, so large penumbra are unsuitable for radiotherapy, since in this region takes place underdosing or the healthy tissue gets an unnecessary radiation dose applied. The more energy of the linear accelerator or a nuclide, the more photons are directed forward, and the smaller the spread laterally. As used in linear accelerators, a target ( electrons to bremsstrahlung ), the beam must be balanced. This is homogenized with a compensation body or " Flatnesfilter ". An undesirable side effect are caused dents in the cross section. This means that the dose is missing, as the compensation body is thicker in the middle. In 10 cm depth a completely homogeneous beam is expected.

For the measurement of cross sections to use a water phantom.

Quality characteristics

• The homogeneity describes in the 50% range ( according to ICRU Report 50) the value of Dmax, Dmin and D0. Dmax is the maximum dose value, D min is the lowest and D0 is always located on the central beam. To obtain homogeneity, one must subtract from Dmin Dmax and divide by D0. Thus we obtain a result which is always greater than / equal to 1. In 1, the transverse profile is completely homogeneous, which is not possible in practice.

• The symmetry describes the relationship between right and left of the central beam. One side of the curve is simply mirrored and should be with the other blankets. If it does not, it is not symmetrical. This also points Dmax, Dmin and D0 measured and the following formula can be established: (Dmax - Dmin) / D0 * 100