The Project is structured into four technical work packages (WP):
- WP1: Primary and secondary (transfer) standards for the absorbed dose rate to water due to eBT devices at 1 cm depth of water;
- WP2: Traceability for superficial (skin) external treatment;
- WP3: Characterisation and calibration of detectors for 3D dose distribution measurements;
- WP4: 3D dose distribution measurements and comparison with vendor-supplied dose maps.
WP1: Primary and secondary transfer standards for the absorbed dose rate to water due to eBT devices at 1 cm depth of water
The general acceptable levels of accuracy in radiotherapy, which in a simplified manner can be stated as ‘the dose delivered to the dose specification point’, may vary between 3.5% and 5% according to IAEA. Instead, a significantly higher uncertainty ± 10-15% (k = 1) is currently reported for eBT procedures.
The aim of this work package is to carry out pre-normative research needed to harmonise and simplify dosimetric procedures for clinical use of eBT in internal radiotherapy, to reduce uncertainties in dose determination to the level recommended by the IAEA. This will be fulfilled by establishing primary standards dedicated to the assessment of the absorbed dose rate to water at 1 cm depth due to eBT sources in Task 1.1, by establishing simple and robust secondary transfer instruments and calibration procedures in Task 1.2 for dissemination of the absorbed dose rate to water, and by validation of these new standards as traceability chains in Task 1.3. The results of this WP will provide input to all other WPs.
WP2: Traceability for superficial (skin) external treatment
The aim of this work package is to establish and validate dosimetric traceability for eBT devices used for superficial (skin) treatment in terms of absorbed dose to water at the surface of a water phantom. The basis of this traceability route will be formed by existing formalisms for low energy x-rays (e.g. IAEA TRS-398, DIN 6809-4 and NCS-10) and implemented for commercial eBT equipped with surface applicators.
In Task 2.1, ion chambers will be calibrated in terms of air kerma in matched beams of conventional x-ray tubes and compared to direct calibration in x-ray beams of the actual eBT devices. The absorbed dose to water at the surface of a water phantom will be established by applying average mass-energy absorption coefficients and backscatter factors based on available and measured eBT x-ray spectra and Monte Carlo calculations. X-ray spectra from eBT devices will be taken from available data, complemented with measurements and Monte Carlo calculations.
In Task 2.2, Monte Carlo models will be used to convert measurements with ion chambers calibrated in terms of air kerma at the applicator surface to absorbed dose to water at the phantom surface for specific eBT devices. Calculated conversion factors and developed methods will be validated by measurements. The dose at the phantom surface will be transferred to the dose at 1 cm depth by Monte Carlo calculations and measurements with radiochromic film. The target uncertainty for the dose to water at the surface is 3.5 % (k = 1). The target uncertainty for the dose to water at 1 cm depth is 5 % (k = 1).
Finally, in Task 2.3, the dose at 1 cm will be compared with the dose measured with the new PTB primary standard, developed in WP1, with the aim of establishing traceability via the new primary Dw-route.
WP3: Characterisation and calibration of detectors for 3D dose distribution measurements
The aim of this work package is to characterise and calibrate different radiation detectors for the measurement of 3D dose distributions in close vicinity to various commercially available low energy eBT X-ray devices with or without fitted applicators. The previous EMRP project HLT09 MetrExtRT focussed on point source eBT devices, i.e. INTRABEAM® (Carl Zeiss, Germany) and Axxent (iCAD Inc, USA). This project will be dealing with additional commercially available eBT devices such as the Papillon50 and Papillon+ (Ariane Medical Systems Ltd, UK), Esteya® (Elekta Brachytherapy, The Netherlands) and WOmed systems (Eckert & Ziegler, Bebig, Germany).
All these low energy eBT X-ray sources generate different photon spectra because of different source configurations and operating voltages. In order to measure 3D dose distributions close to eBT sources (0.2 cm to 5 cm) for the verification of eBT treatment plans in WP4, the energy dependence of different types of detectors needs to be investigated and quality factors for various beam qualities (as expected between 0.2 cm and 5 cm distance from the source) need to be determined in WP3. Additionally, volume averaging correction factors need to be calculated for the finite sized detectors.
Once fully characterised and calibrated in the relevant eBT beams or ‘eBT-equivalent’ X-ray beams, which will be established in WP1, the detectors will be used in WP4 for the actual measurement of 3D dose distributions in water or water-equivalent phantoms.
The three tasks in this work package will focus on the characterisation and calibration of small volume online detectors (Task 3.1), Fricke gel dosimeters (Task 3.2) and alanine dosimeters (Task 3.3) for eBT sources.
The aim is to achieve uncertainties (k = 1) for the calibration coefficients of not more than: 1 % – 2 % (at NMIs) and 2 % – 3 % (at clinic) for the scintillation detectors and for the small volume ionisation chambers, 3.5% for the gel dosimeter and 2.5% for the alanine pellets. This will enable more accurate 3D dose distribution measurements close to eBT sources compared to currently used methods.
WP4: 3D dose distribution measurements and comparison with manufacturer-supplied dose maps
The aim of this work package is to compare the manufacturer-supplied dose map for all of the eBT devices considered in WP3. This WP will have input from the work performed in previous WPs. To achieve the above-specified aim, the considered manufacturer-supplied dose values will be compared with ones obtained though this project using both the primary and secondary standards established and validated in WP1 and the detectors characterised and calibrated in WP3. To this end:
– the characterised and calibrated detectors will be used to measure in terms of absorbed dose to water the dose map close to the considered eBT source in a given layout (Task 4.1),
– the different absorbed dose to water maps, obtained using the various detectors considered for a given eBT system and an associated layout, will be compared both with each other and with the one supplied by the manufacturer (Task 4.2).