Work Package 4 (lead by PTB)

Brachytherapy

The aims of this WP are to characterise two different types of miniature x-ray sources, to establish a primary st ard in terms of absorbed dose to water, for these sources to investigate different transfer st ards in order to provide recommendations on a calibration chain. A robotic based high precision positioning system within a water phantom measuring techniques applying alanine, scintillators x-ray storage foils will be developed to determine the 2D or 3D dose distribution in a water phantom in the presence of clinical applicators.

 

Task 4.1: Source characterisation

The aim of this task is to characterise the radiation fields emitted by both miniature x-rays tubes in terms of energy spectrum or absorbed dose to water. Four complementary measuring systems will therefore be characterised experimentally in this task:

  • Spectrometer: necessary for accurate determination of the miniature X-ray tubes emission spectra

  • Scintillator: used for accurate measurements of the absorbed dose to water at an arbitrary point up to 10 cm from the source in water

  • X-ray storage film: used for accurate relative 2D measurements of dose distribution at an arbitrary distance from the source

  • Radiochromic gels: used for determination of relative 3D dose distribution close to the source (< 3 cm).

Description of the activities

  • Characterisation of the HP-Ge spectrometer including the development of the unfolding pile-up reconstruction algorithm needed because of the high dose rate close to the sources.

  • Spectrometry measurements around the X-rays sources using the HP-Ge spectrometer at discrete radial azimuthal angles.

  • Characterisation of the storage films in terms of absorbed dose to water for photons with energies between 8keV – 50 keV.

  • Characterisation of the dose distribution of the radiation field in a water phantom with scintillator with X-ray storage films.

  • Characterisation of the energy dependence for low energy photons spatial resolution of radiochromic gel evaluated by optical computed tomography.

  • Direct 3D measurement of the radiation field with the radiochromic gel, the source being immersed in the gel.

 

Task 4.2: Monte Carlo simulations of the miniature X-ray tubes for the characterisation of the measuring devices

The aim of this task is to model the radiation sources the measuring devices using Monte Carlo codes in combination with the source characterisation undertaken in task 4.1 to optimise the model taking into account that small variations of the model differences in the data may have a significant effect on the results. This may also hold for the Monte Carlo calculation algorithms, therefore both JRP-Partners the collaborator University Mannheim, all of which have experience in different Monte Carlo Codes (EGSnrc (PTB), MCNP (CMI)), Geant4 (University Mannhiem), will cooperate in several Monte Carlo problems to study these issues.

Description of the activities:

  • Creation of detailed Monte Carlo models of both micro X-ray tubes (MXT). The models will be created in the MCNPX Monte Carlo code used in the following steps. 

  • Monte Carlo calculations for the characterisation of the HPGe-detector applied in task 4.1 with EGSnrc (PTB) MCNPX (CMI) also to study the impact on the response function of the detector of the choice of atomic databases the Monte Carlo algorithm. PTB will lead the deliverable CMI will deliver complementary results for comparison.

  • Monte Carlo calculations of the change of the energy spectra with depth in order to apply the energy dependence correction to the data measured by the scintillator, the x-ray storage foil the radiochromic gels used for the characterisation of the MXT in the task 4.1. CMI will lead the deliverable PTB University Mannheim will deliver complementary results for comparison.

  • Monte Carlo studies to accompany the investigations of the phantom materials (task 4.3): Quality of the characterisation of phantom materials by Monte Carlo codes databases. PTB will lead the deliverable CMI will deliver complementary results for comparison.

  • Monte Carlo simulations of absorbed dose to water values in defined geometries delivered by both micro X-ray tubes in geometries with without the applicators in order to support the aim of the task 4.4: Recommendations of a Calibration chain. CMI will lead the deliverable PTB University Mannheim will deliver complementary results for comparison.

 

Task 4.3: Development of primary st ards for miniature x-ray tubes in terms of Dw

The aim of this task is to reconstruct the extrapolation chamber previously used as a primary st ard for I‑125 in the iMERA-Plus JRP T2.J06. A new water equivalent phantom material for PTB’s primary st ard (Grovex II), RW1, must be selected for the lower energies of the prominent fluorescence lines of the MXT. The selection will be supported by experiments, namely the determination of the mass attenuation coefficients of water the phantom materials in question. Finally the extrapolation chamber will be established as a primary st ard.

Description of the activities:

  • Experimental determination of the mass attenuation coefficients of the phantom material, selection of phantom material database used by Monte Carlo.

  • Reconstruction of the extrapolation chamber with the selected phantom material (re)-establishing the extrapolation chamber as a primary st ard for the miniature x-ray tube (I-125), including comparisons with other st ards, developing an uncertainty budget.

 

Task 4.4: Recommendations on a calibration chain

A calibration chain must be established to transfer calibrations for absorbed dose to water to the clinics. In addition to the outcome of tasks 4.1–4.3, the investigations will enable firstly a statement as to whether both miniature x-ray tubes can be treated equally with respect to basic dosimetry or if a separate dosimetric procedure must be established for each of the devices secondly recommendations to be made on the design of transfer-chambers the structure of such a calibration chain.

 

Task 4.5: Development of measuring techniques for the verification of treatment planning systems in HDR-Brachytherapy

The development of measuring techniques for the verification of treatment planning systems (TPS) of HDR-Brachytherapy is the topic of task T4.5. In Ir-192 HDR-Brachytherapy, applicator related shielding effects are the greatest limitations of the current AAPM TG-43 dosimetry protocol, which plays the role of an “informal” world-wide st ard.

In most publications the results of the TPS are only compared to Monte Carlo calculations. It is therefore the aim of this task to develop measuring techniques (alanine, scintillators x-ray storage foils) to determine the 2D or 3D dose distribution in a water phantom in the presence of clinical applicators. From a dosimetric point of view this means moving a step away from st ard fields to a complex field that is closer to clinical conditions.

Description of the activities:

  • Establishing of a robotic 3D measuring system in a water phantom for these 3 devices.

  • Characterisation of the measuring devices to achieve an uncertainty below 2 % (k=1) calibration of the devices traceable to the PTB primary st ard. (scintillators, alanine x‑ray storage foils).

 

For two typical clinical treatment modalities in the water phantom, measurements will be made with the three devices the results compared with each other with those of treatment plans of Monte Carlo calculations. The criteria for agreement between the three measuring systems, the Monte Carlo calculations the TP-systems will be 3 % (k=1).

 

Work package leader: Ulrike Ankerhold (PTB)