Work Package 1 (lead by ENEA)

Dosimetry in terms of absorbed dose to water for therapy with medium energy x-rays: traceability st ardisation 

The aim of this work package is to reduce the uncertainty for clinical reference dosimetry of medium-energy x-rays below 2% (1SD) from the current value of about 3%. New beam specifiers st ardised calibration procedures will be developed a set of Dw primary st ards operative at the same beam qualities measurement conditions will be established. A comparison of all Dw st ards, two new ones provided within this JRP three already existing will be used to demonstrate that developments in this WP have resulted in a more robust system with a lower uncertainty (below 2%) for reference dosimetry for medium-energy x-rays.

Finally, recommendations for a new unified dosimetry protocol will be developed.

 

Task 1.1: Stan​dardisation of calibration procedures for dosimetry for therapy with medium-energy x-rays

The aim of this task is to choose radiation qualities, measurement conditions new beam quality specifiers suitable for dosimeter calibration Dw determination for medium-energy x-rays.

Description of activities:

  • A survey on the range of medium-energy x-ray beam qualities beam specifiers used in therapy will be undertaken with the aim of selecting the most relevant beam qualities to be used for dosimeter calibrations.

  • Beam quality specifiers will be investigated. Measured photon fluence spectra will be used to determine the dosimetric beam parameters (i.e. mean energy, mean mass energy absorption coefficients). These parameters will be analysed as a function of HVL or percentage depth dose ratios in order to identify the best beam specifier.

 

Task 1.2: Development of two new primary st ards for the realisation of absorbed dose to water in medium-energy x-radiation qualities

The aim of this task is to design build two new primary st ards for absorbed dose to water (a water-graphite calorimeter an extrapolation chamber) measure the absorbed dose to water under the reference conditions selected in the task 1.1.

Description of activities:

  • Design construction of a new water-graphite calorimeter as a new primary st ard for absorbed dose to water for medium-energy x-rays.

  • Determination of correction factors for the water-graphite calorimeter such as the correction factors accounting for perturbations introduced by non-water materials, gap effect, conversion from absorbed dose to graphite to absorbed dose to water, heat transfer from or into the calorimeter core.

  • Absolute Dw measurements with detailed uncertainty budget by means of the water-graphite calorimeter in the new defined x-radiation qualities (see task 1.1).

  • Design construction of a graphite extrapolation chamber as a new primary st ard for absorbed dose to water for medium-energy x-rays.

  • Determination of corrections factors conversion coefficients for the newly developed graphite extrapolation chamber for the realisation of Dw.

  • Absolute Dw measurements with detailed uncertainty budget using the extrapolation ionisation chamber in the newly defined x-radiation qualities (see task 1.1).

 

Task 1.3: Comparison of dosimetry based on Nw calibration factors with dosimetry based on Nk calibration factors

The aim of this task is to organise undertake an international comparison of dosimetry based on Nw calibration factors with dosimetry based on Nk calibration factors, using transfer st ards calibrated by the JRP-Partners in this comparison. The calibration coefficients will be established in terms of absorbed dose to water derived from existing air kerma st ards using the methods described in the IAEA TRS 398 dosimetry protocol. 

Description of activities:

  • Preparation of a technical protocol for performing the comparison using radiation qualities according to task 1.1 two secondary st ards based on ionisation chambers commercially available for medium energy x-rays.

  • Calibration of the two selected secondary st ards in terms of Dw Ka according to the technical protocol.

  • Absorbed-dose-to-water calibration coefficients, based on air kerma free-in-air absorbed-dose-to-water primary st ards, will be compared the improvement in accuracy in medium-energy x-ray dosimetry due to the availability of Dw primary st ards for dosimeter calibrations will be estimated.

 

Task 1.4: Guidelines for a dosimetry protocol for therapy with medium x-radiation

The development of new reference radiation qualities (task 1.1), the availability of additional Dw primary st ards (task 1.2) results of the comparison (task 1.3) will lead to a proposal for an update of the existing recommendations for medium-energy x-ray dosimetry in radiotherapy. The aim of this task is to prepare draft guidelines that can be used to promote the first harmonised European dosimetry protocol for dosimetry for therapy with medium-energy x-rays. The guidelines developed in WP1 will be presented at national international meetings/congresses of the medical physics community.

Description of activities:

  • Drafting of guidelines for a dosimetry protocol for therapy with medium-energy x-radiation beams based on Dw st ards.

 

Work package leader: Massimo Pinto (ENEA)