The use of plane-parallel chambers in electron dosimetry without cross calibration

Modern IAEA, DIN and AAPM dosimetry protocols for external beam radiotherapy recommend the use of plane-parallel ionization chambers for the determination of the absorbed dose to water in (low-energy) electron beams. The recommended procedure relies on a cross-calibration of the plane-parallel ionization chamber, i.e. on the comparison with a calibrated cylindrical chamber in an electron beam of high energy, which has to be done by the responsible physicist in the hospital. The rationale for this is the assumed chamber-to-chamber variation of the perturbation factor (pwall)Co for plane-parallel chambers of the same type (variations of up to 4% have been reported in early studies) which forbids the specification of type-specific (pwall)Co values in dosimetry protocols. In routine measurements there is a certain risk that this cross-calibration procedure introduces erroneous results into the measurement of the absorbed dose in electron beams because all mistakes and uncertainties associated with one single dose measurement using a cylindrical chamber are carried on to the later dose measurement using the plane-parallel chamber. Due to this risk, it might be more advantageous to use plane-parallel chambers which have been calibrated directly at a PSDL or SSDL. This, however, requires the availability of type specific values of the perturbation factor (pwall)Co for plane-parallel chambers of frequently used types with adequate small uncertainty. In an extensive study we determined the overall perturbation factors pCo (which are usually assumed to be equal to (pwall)Co) for a total of 35 plane-parallel chambers of the Roos type (30 PTW 34001, 2 PTB FK6, 3 Scanditronix-Wellhoefer PPC40), 15 chambers of the Markus type (PTW 23343), and 12 chambers of the Advanced Markus type (PTW 34045). A total of 188 individual cross calibrations using these chambers were carried out at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, at the University Hospitals of the Universities of Freiburg and Tuebingen, and at the German Cancer Research Center (DKFZ) in Heidelberg using different types of linear accelerators, different electron energies, and different measurement equipment. The measurements were analysed according to the revised version of the German dosimetry protocol DIN 6800-2 which is very similar to IAEA TRS-398. The results obtained from these measurements are shown. Despite the larger variety of experimental conditions occurring in our study and the much larger number of chambers investigated we did not observe such large variations of the pCo values for different plane-parallel chambers of the same type as reported in earlier studies. The largest variation we observed was 1.0% for chambers of the Roos type; for chambers of the Markus and Advanced Markus types the variations were 0.9% and 0.6%, respectively. Furthermore we observed in our study that the variations of the pCo values which we obtained in repeated measurements using always the same plane-parallel chamber are comparable to the variation of the pCo values obtained for different chambers of the same type given above. This justifies the assumption that the mean pCo values obtained in our study from a large number of measurements are more reliable than a pCo value determined from a single cross-calibration measurement in the hospital. An analysis of uncertainty gives for the relative standard uncertainty of the pCo values determined in our study u(pCo) = 1.1%, which includes the uncertainties of all correction factors involved in the data analysis. This results in a relative standard uncertainty of the beam quality correction factor kQ (which is eventually needed for dose measurements using a plane-parallel chamber calibrated at 60Co) of u(kQ) = 1.3% - a value which is only 0.1% larger than the uncertainty given in TRS-398 for the beam quality correction factor for cylindrical chambers in electron beams. Hence the (supposed) large variation of pCo values for different chambers of the same type and the uncertainty of the generic pCo values determined in our study are no arguments against a calibration of plane-parallel chambers at 60Co and a measurement of the absorbed dose in electron beams without cross calibration. A replacement of the cross calibration procedure by the 60Co calibration of plane-parallel chambers facilitates electron dosimetry without increasing the uncertainty of measurements, it provides the possibility of conducting consistency checks between measurements with cylindrical and parallel plate chambers and thus represents an important contribution to QA in clinical dosimetry. The pCo values determined here will be recommended in the revised version of the German dosimetry protocol DIN 6800-2