Work Package 6 (lead by VSL)

Methods for verification of treatment planning systems (TPS) in anthropomorphic phantoms

The aim of this WP is to develop, validate, compare and characterise measurement methods and uncertainty budgets for the 2D/3D verification of the treatment planning system. These methods will be based on one dosimeter or on a combination of several dosimeters which have been characterised in WP5. The verification of TPS will be carried out in an anthropomorphic phantom to be as close as possible to the clinical conditions. Critical irradiation conditions will be chosen for which the TPS calculations and the measurements have known issues such as high dose gradients encountered in dynamic rotational treatment modalities and dose distributions in the presence of heterogeneities. Once the methods have been validated a series of recommendations or guidelines will be written and disseminated to the European radiotherapy community.

 

Task 6.1: Development of measurement methods for 2D/3D TPS verification in anthropomorphic phantoms

The aim of this task is to develop measurement methods for TPS verification of (complex) dose distributions in anthropomorphic phantoms. The methods will consist of either a combination of a point-like detector with a 2D detector or a 3D gel dosimeter applied in an anthropomorphic phantom. Since both the phantom and the properties of the radiation fields are different in the dosimetry of complex fields in anthropomorphic phantoms than in calibration conditions, corrections need to be investigated. These corrections will include the influence of phantom materials (different densities and materials), steep dose gradients and multiple beam directions.

Specifically for the verification of external electron beam radiotherapy dose distributions in the first centimetres from the surface of the phantom are important. Since commercial water filled anthropomorphic phantoms with free 3D positioning, the orientation of 2D and point detectors and with thin (2 mm – 4 mm) phantom surface are not available, for this treatment modality, a new water filled semi-anthropomorphic phantom will be developed. The anthropomorphic phantom for the breast area will contain the critical organs such as lung and heart inserts. In addition, the phantom will allow the accurate positioning of detectors at the interface area of combined photon and electron radiation fields.

Description of activities:

  • Design and construction of two phantoms for breast, and head and neck regions for verification of external electron beam dosimetry.

  • Determination of correction factors for single photon and electron beams in water and non-water material anthropomorphic phantoms using point-like detectors and 2D-detectors (storage foils), to correct for dose gradients. Simple phantoms made of water and non‑water material will be used for the detector testing.

  • Determination of correction factors for multiple photon and electron beams from different directions in water and non-water material anthropomorphic phantoms for point-like detectors and 2D-detectors, to correct for the influence of the change in phantom shape with beam direction and material with respect to the calibration conditions. The same phantoms described above will be used.

  • Software for reading and unfolding the raw 3D MRI gel dosimetry data (relaxation time). And validation of the measurement protocol by comparison with ion chamber measurements in simple cases under standard conditions.

Task 6.2: Validation of measurement methods in anthropomorphic phantoms for TPS verification

The aim of this task is to validate the 2D measurement methods developed in task 6.1, which use a combination of a point-like detector and a 2D detector, and the 3D gel dosimetry methods. The final validation will take place in typical clinical complex dose distributions using photon and electron beams. Validation will be performed by comparison of the dose distributions measured with the different 2D and 3D techniques. Comparison of dose distributions will be performed with the TPS available at the JRP-Partner institutes.

Description of activities:

  • Characterisation and classification of  typical dose distributions used in clinics for photon and electron beams with respect to shape, size and dose gradients, to identify plan-class specific reference fields as proposed by Alfonso et al. (Med. Phys 35, p5179) in close cooperation with several clinical collaborators.

  • Comparison of different combinations of point-like detectors such as diamond detector and 2D detectors (such as radio chromic film and storage foils) with a set of different TPS calculated complex dose distributions for photon beams in anthropomorphic phantoms to validate the characteristics of the detectors in WP5 and the correction factors of task 6.1.

  • Comparison of complex dose distributions (from the classification in the first bullet) in typical clinical electron beam treatments in anthropomorphic phantoms of the breast and head & neck regions measured with radio chromic film and point-like detectors such as diamond detector, with clinical TPS calculations, and with Monte Carlo calculations. The calculations will be performed in close cooperation with clinical colaborators.

  • Validation of a 3D gel dosimetry system in clinical complex dose distributions in anthropomorphic (for example a head) phantoms – comparison with TPS calculations.

  • Validation and comparison of the 2D and the 3D dosimetry systems to validate both methods. VSL will use radio chromic film, PTB will use storage foils and CEA and REG(UdA) will use a 3D gel dosimetry system.

  • Evaluation of the uncertainties associated with the used combinations of point detector and 2D detector, and the used 3D gel dosimeters. for verification of complex dose distributions in anthropomorphic phantoms for photon and electron beams,

  • Onsite pilot dosimetry audit of clinical rotational IMRT dose distributions using anthropomorphic phantoms and a combination of radio chromic film and point-like detectors.

  • Comparison of measured and EPID reconstructed dose distributions in phantoms. VSL will use radio chromic film to measure the dose distribution. NPL will perform the EPID based reconstruction of the dose distribution.

  • Validation of the measurements of the dose to the tumour volume using alanine/EPR in combination with dose-per-pulse measurements with different scintillators for identification of deviations between predicted time dependent dose rates and actual delivered dose rates in clinical conditions for rotational IMRT irradiations.

Task 6.3:  Recommendations on the traceable verification of TPS calculated complex dose distribution

The aim of this task is to write and disseminate measurements protocols and guidelines for good practice use of the methods developed and validated in task 6.2 and 6.1. These recommendations will be made available for the European medical physics community at conferences dedicated to medical physics and by publishing the report on the JRP web site. The recommendations will include the required correction factors, the recommended phantoms and materials, and the uncertainties assessed for the different methods, and will cover the following measurements:

  • TPS verification of electron beam dose distributions using a combination of radio chromic film and point-like detectors.

  • TPS verification of complex photon beam dose distributions using a combination of a point-like detector and a 2D detector.

  • TPS verification using 3D chemical gel dosimeters.

  • Validation of EPID dose reconstruction methods by direct in phantom measurement of absorbed dose.

 

Work package leader: Jacco de Pooter (VSL)