Beam Quality Specification of High-Energy Photon Beams

Project Description

The experimental part of the project was started in November 2000. The 4 chambers to be calibrated at each laboratory were so far measured at LNHB, METAS and NPL at different accelerator energies in the range from 4 MV to 21 MV and at Co-60 radiation.In winter 2002/2003 the chambers were measured at 4 additional qualities at METAS. These were non-standard qualities with special filtrations.In summer 2003 LNHB recalibrated 3 of the chambers in the 60Co beam.The analysis of the resulting experimental calibration coefficients and correction factors from LNHB (4 energies, including Co), METAS (12 energies), NPL (8 energies) is not yet completed.Parallel to the measurements some MC calculation work in order to simulate the METAS accelerator beams was initiated.In spring 2004 a second measurement campaign was initiated. The laboratories involved in the 2004 experimental work are ARPANSA, ENEA, METAS and NRC. Due to a mechanical problem one of the 4 chambers can no longer be used and was replaced by another chamber of the same type.The schedule for the remaining measurements is as follows:

• May/June 2004: NRC
• July/August 2004: ARPANSA
• September/October 2004: ENEA

The experimental work is planned to be completed by end of 2004. The specification of the quality of a high-energy photon beam has been subject of several studies due to its relevance in radiation dosimetry. However, no beam quality specifier has been found that satisfies the requirements of being unique for the entire range of photon energies used in radiotherapy and for all types of accelerators used in hospitals and standard laboratories, as well as being easy to measure.

It is therefor proposed to investigate the characteristics of the existing -and of some new beam qualitiy specifiers in more detail in order to determine the optimal beam quality specifyer.

A set of at least 4 ionisation chambers will be sent to the participating laboratories where they will be calibrated against the local primary standards in terms of absorbed dose to water in a Co-60 beam and in several predefined accelerator beams. The high-energy photon beam should be characterized using different specifiers like TPR, Ý, LAI, DAI(Pb,H20) etc.

The result of this chamber circulation is a set of calibration factors for each chamber, each beam quality, each - specifier and each laboratory. An analysis of the data should determine the beam quality specifier leading to the most consistent set of calibration factors/kq.

In addition to the experimental work a modelling of the different accelerator beams using Monte Carlo techniques should be firmly incorporated into the project.

Final Report 2009-10-13

The specification of the quality of a high-energy photon beam has been subject of several studies due to its relevance in radiation dosimetry. However, no beam quality specifier has been found that satisfies all the requirements of being unique for the entire range of photon energies used in radiotherapy and for all types of accelerators used in hospitals and standard laboratories, as well as being easy to measure. The scope of this EUROMET project was to compare the two most widely used beam quality specifiers TPR20,10 and %dd(10)x and determine which one best describes a high-energy photon beam and can therefore be used to provide consistent and unique calibration coefficients.
A set of  four cylindrical ionisation chambers belonging to METAS (2 of type NE2571A, 2 of type NE2561/2611A) were calibrated in terms of absorbed dose to water in the Co-60 beams (calibration coefficient ND,W) and in several high-energy X-ray beams (calibration coefficients ND,W,Q) at the participating laboratories (ENEA, LNHB, METAS, NPL, NRC and PTB). Beam quality measurements were performed in the accelerator beams in order to compare the efficacy of TPR20,10  and %dd(10)x as beam quality specifiers. In total 28 accelerator beams were measured providing 123 calibration coefficients ND,W,Q.

Results:

• The high-energy photon beams of the two research accelerators (METAS, NPL) are clinic-like
• For each laboratory a set of measured beam quality correction factors kq was deterimend. The rms deviations of these measured kq factors to a sigmoidal fit through all exprimental data using either TPR20,10 or %dd(10)x as the independent variable were in both cases <0.3% and did not significantly differ. Consequently neither of the two investigated beam quality specifiers (TPR20,10 and %dd(10)x) can be preferred as they turned out to be equivalent. They therefore describe the beam qualtiy equally well
• The two beam quality specifiers and easily be converted from one to the other
• For any two given participant laboratories the absorbed dose to water calibration coefficients agree within the uncertainty of the comparison. The primary standards used in this project are therefore consistent.
   

The project was successfully completed and a paper will be submitted to PMB.