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New integral quantities for high-energy x-rays small fields

The aim of this work package is to address the lack of metrological reference value to be used for very small radiation field in radiotherapy. Two proposals are made to overcome this problem: the definition of a suitable quality index for small irregular fields the introduction of a new quantity for radiotherapy similar to the Dose Area Product (DAP) used in diagnostic radiology.

**Task 2.1: Optimised integral quantities as beam quality parameter**

The aim of this task is to define an integral beam quality parameter such as the Dose Area Product Ratio (DAPR) at 10 20 cm, specifying the surface over which the absorbed dose at a point should be integrated the two depths at which measurements should be performed before taking the ratio of the resulting values. The definition should be optimal, taking into account the range of beam sizes for which the quality parameter may be used, the suitability of commercially available detectors with which to measure it , by an analysis of stopping power ratios S_{w,air}, its ability to discriminate between beam qualities for the purpose of ion chamber calibration. Since the most recent small field treatment machines (TomoTherapy, CyberKnife, TrueBeam) exclude flattening filters, it is important also to consider flattening filter free (FFF) beams.

**Task 2.2: Optimised integral quantities for reference dosimetry**

The aim of this task is to define at least one integral quantity reference geometry to replace absorbed dose at a point, suitable for use in reference dosimetry for small irregular fields, as the basis of traceable dosimetry for SRS, SRT IMRT. DAP_{w} is the integral of the absorbed dose to water over a large area (mean absorbed dose to water).

**Task 2.3: References in terms of DAP _{w}**

The aim of this task is to build dosimeters able to measure DAP_{w}, to establish references in terms of DAP_{w} to assure traceability in DAP_{w} clinical measurements.

**Task 2.4: TPS DAP _{w}**

The aims of this task are to derive DAP_{w} data from dose distributions calculated by Treatment Planning Systems (TPS) , by analysis of these data, to identify the scope for using measured integral quantities integral beam quality parameters in place of central axis depth dose data output factors, to determine beam model parameters when commissioning TPS for the planning of SRS, SRT IMRT.

Work package leader: Franck Delaunay (CEA)