Transverse profile (radiation therapy)
As part of the physical measurements on radiation therapy devices, a transverse profile shows the dose distribution perpendicular to the central beam at a certain water depth. A water phantom, which is centered on the axis of rotation of the jet diaphragms, is used to measure transverse profiles. The measuring probe of the water phantom is moved horizontally during profile measurements. (Basically on the depth dose curve ) Usually, measurements are taken along the main planes of the device ( sagittal and transversal , also called inplane and crossplane) and diagonally to these.
The field size, its symmetry and homogeneity result from the measured cross profile
and the width of the penumbra. These are important measures for the quality of the irradiation system and are therefore defined as equipment quality features according to the standard DIN EN 60976 and specified by the respective equipment manufacturers. Other important criteria are the stability of the transverse profile when the device is moved and the maximum absorbed dose ratio at a certain measuring depth.
Cross profiles are measured to generate data in the clinical use of treatment planning systems. During the prescribed regular constancy tests, the calculated transverse profiles are compared with the measured ones. Regardless of the device, the shape of the cross-section depends on the focus-surface distance, the measurement depth, the field size, and the type and energy of the radiation. The standardized measuring depth for transverse profiles of the X-ray beam is 10 cm.
The beam generated at the target of the linear accelerator (electrons become brake X-rays ) is more intense inside than outside. If a homogeneous field is needed, it must be weakened internally; for example by a compensating body . A radially symmetrical and completely homogeneous beam whose energy dose would be the same at every point of the irradiation field would be ideal.
Quality features
- The field size is defined as the distance between the points with 50% of the absorbed dose measured in the center of the field. The further down you measure, the wider the beam becomes, as it spreads divergently in space. A cross-section at a specific focus distance shows whether the actual field size corresponds to the selected setting.
- The homogeneity describes in the balanced area (e.g. 8 cm within a 10 cm field) the ratio of the highest and lowest dose value in percent. (D max - D min ) / (D max + D min ); ideally 0%. Another evaluation method is to relate the value to the dose in the center: (D max - D min ) / D 0
- Symmetry is specified like homogeneity, but curve points that are symmetrical to the center of the field are considered. Another evaluation method is to put the area of the profile halves in relation.
- The width of the penumbra (also called penumbra) on both sides of the transverse profile is another important quality criterion. It is caused by beam scattering in the irradiated medium, but device properties such as focusing the beam on the target and the geometry of the apertures also influence the penumbra. According to the norm, the penumbra is defined as the distance between the points with 80% and 20% dose.
Slightly different specifications apply to electron fields.
See also
literature
- Hanno Krieger: Radiation sources for technology and medicine. 2nd edition, Springer Spectrum, Wiesbaden, 2013. ISBN 978-3-658-00589-4 .
- Rolf Sauer: Radiation Therapy and Oncology. 4th edition. Urban and Fischer, Munich 2010, ISBN 978-3-437-47501-6 .
- Schäfer B., Hödl P. (1999) Quality assurance in radiation therapy. In: Medical-technical assistance in modern radiation therapy. Springer, Berlin, Heidelberg, ISBN 978-3-540-63834-6
Individual evidence
- ↑ Standard DIN EN 60976: 2007 Medical electron accelerators - Equipment quality features (IEC 60976: 2007); German version, Chapter 9