Work Package 5 (lead by PTB)

Characterisation of detectors for verification of TPS calculated complex dose distributions

The aims of this WP are to study the use of point like detectors to overcome the lack of primary st ards for very small radiation fields, to characterise 2D 3D detectors to be used for verification of complex dose distributions in photon electron beams to optimise the use of diamond EPID detector for in vivo dosimetry.

 

Task 5.1: Investigation of point-like detectors for dose measurements in complex dose distributions

Up to now there is no absolute (primary) measurement method available for very small fields. As mentioned in WP2 one possibility is to introduce a new quantity to overcome that. Another possibility is to use existing secondary st ards with well suited correction factors in order to derive Dw in very small fields, this is the final goal of this task.

Point-like detectors such as ionization chambers, semiconductor detectors (diodes), or diamond detectors are traditionally used for absorbed dose measurements in radiation therapy. Due to their widespread use in clinics these types of dosimeters will also be investigated here.

In large radiation fields with homogeneous dose distributions (with dimensions larger than the size of the detector), the detector properties of interest are mainly related to the long-term stability of the response, the variation of the response with the radiation energy or the magnitude of the signal. However, in complex dose distributions generated by modern photon electron irradiations large dose gradients (changes of dose up to 90 % over a distance of a few millimetres) are present additional detector properties come into play. These are for example the spatial resolution of the detectors, the angular or directional response, the linearity (or non-linearity) of response with respect to the dose, dose rate or dose per pulse, or the perturbation of the radiation field by the detector in order to make this detector able to measure absorbed dose in very small fields. The experience knowledge of the manufacturers of dosimeters for radiation therapy (e.g. PTW Freiburg IBA Dosimetry) will be taken into account in the work in this task.

In order to be able to recommend suitable types of detectors for dosimetry in complex dose distributions to obtain the input data needed in WP6 the following investigations will be undertaken:

  • Measurement of the spatial response function of ionization chambers (i.e. by scanning the detector with a slit beam or by comparing the results of measurement of steep dose gradients), semiconductor diodes, in high-energy photon (6 MV 10 MV).

  • Measurement of the spatial response function of ionization chambers (i.e. by scanning the detector with a slit beam or by comparing the results of measurement of steep dose gradients), semiconductor diodes, single crystal CVD diamond detector electron beams (6 MeV 15 MeV).

  • Measurement of the directional response function of dosimeters based on ionization chambers, semiconductor diodes, in photon (6 MV 10 MV).

  • Measurement of the directional response function of dosimeters based on ionization chambers, semiconductor diodes, single crystal CVD diamond detector in electron beams (6 MeV 15 MeV).

  • Determination of beam quality correction factors (kQ) for single crystal CVD diamond detector in electron beams of different energies field sizes:

  • The kQ factors will be calculated by Monte-Carlo methods in the energy range from 6 MeV to 18 MeV for field sizes from 3 cm × 3 cm to the reference size 20 cm × 20 cm.

  • The values calculated above for the reference conditions (field size 20 cm x 20 cm) will be compared with measured kQ factors for two beam energies at PTB’s clinical accelerators.

  • Measurement of perturbation factors for ionization chambers, diodes alanine detectors in different phantom materials in high-energy photon beams.

  • Evaluation of the uncertainty for absorbed dose measurements using the diamond detector in reference non-reference conditions for electron photon beams comparison with the requirements given in ICRU report 24.

The investigations in this task will be carried out with commercially available ionisation chambers of types PTW 31010 PTW 31014 a diode of type PTW 60012. The diamond detector to be used is built at Tor Vergata Rome University (collaborator) was commissioned as an absorbed dose dosimeter in reference conditions in high-energy photon beams within the iMERA-Plus JRP T2.J07 ‘EBCT’.

 

Task 5.2: Investigation of 2- 3-dimensional dose detectors

The aim of this task is to characterise the properties of 2-dimensional (radio chromic film of type gafChromic EBT2 EBT3, storage foil system “KodakCR 2000RT”) 3-dimensional (chemical gel) dose detectors in order to evaluate their potential for dose measurements with high spatial resolution (either alone or in combination with point-like detectors – see WP6).

Description of activities:

  • Determination of dose response curves of flexible storage phosphor plates over the range 0.1 Gy to 8 Gy radio chromic film in electron beams of energies 6 MeV, 9 MeV 16 MeV.

  • Measurement of the homogeneity the directional response function of storage foils in high-energy photon electron beams.

  • Measurement of the response of radio chromic film for photons for different angle of incidence.

  • Evaluation of accuracy spatial resolution of point plane film readers. The uncertainty of dose measurements in electron beams using radio chromic films will be evaluated with respect to the uncertainty requirements (2.5 %) given in ICRU reports 24 48.

  • Investigation of the dispersion effect for a chemical gel after irradiation:

    • The chemical composition of the gel will be optimised by grafting on the gelatine chains specific lig s adapted to bind iron(III) /or used thickening agents to decrease water mobility concentration in the gel.

    • Determination of the dose energy response of the optimised chemical gel irradiated with photons of different energies (X-rays, Ir-192, Cs-137, Co-60, …)

    • Evaluation of the uncertainty of the absorbed dose measurements using the optimised chemical gel, the target st ard uncertainty is 2 %.

 

Task 5.3: Development of detectors for in-vivo dosimetry

The aim of this task is to study the traceability to primary st ards of in-vivo measurements, based on diamond detector Electronic Portal Imaging Detector (EPID) detectors. A new point like real-time dosimeter will be developed based on the diamond detector technology available at the collaborator Rome University “Tor Vergata”. The basic properties of EPID will be investigated in this task.

Description of activities:

  • Construction of a prototype in-vivo dosimeter for high-energy photon electron beams based on the single crystal diamond detector. This work will be done in close cooperation with the Rome University “Tor Vergata” (collaborator).

  • Characterisation in terms of energy, angular temperature dependence of the dosimeter response in photon electron beams determination of correction factors to be applied to the new in-vivo diamond dosimeter.

  • Evaluation of uncertainty of dose measurements using the in-vivo diamond dosimeter check of its compliance with the uncertainty requirements in ICRU report 24.

  • Characterisation of the Elekta iView EPID in terms of energy angular response for both primary scattered photon radiations, by means of Monte-Carlo modelling measurements.

 

Work package leader: Ralf-Peter Kapsch (PTB)