Measurement of the perturbation effect correction factors p_Co for flat chambers

For a large number of flat chambers of types Roos, Markus and Advanced Markus, the perturbation correction factors p_Co were determined. As the result of these investigations, the new version of standard DIN 6800-2 states type-specific p_Co-values for these flat chamber types.

Modern dosimetry protocols as, for example, DIN 6800-2 or IAEA TRS-398, recommend the use of flat chambers to measure the water absorbed dose in electron radiation fields. Compared to cylindrical ionization chambers, these chamber types have the advantages of an improved depth resolution and - in particular in low-energy electron radiation fields (nominal energy smaller than 10 MeV) - a clearly reduced in-scattering effect (fluence perturbation effect).

The recommended procedure for dose measurement with flat chambers in electron radiation fields deviates, however, from the procedure usually applied in clinical dosimetry: flat chambers are not calibrated in the ⁶⁰Co radiation field for indication of the water absorbed dose; but the user must first perform a cross calibration with a calibrated compact chamber in a high-energy electron radiation field before dose measurement is carried out.

In literature, this deviating procedure is explained by the strongly specimen-specific scattering of the perturbation correction factor p_Co of flat chambers (deviations of 3%...4% are reported), as a result of which no type-specific p_Co-values with sufficiently small measurement uncertainty can be indicated in dosimetry records. This statement is based on investigations on NACP chambers of older generations; in newer papers, reference is made to a clearly lower variation of the perturbation correction factors for chambers of modern construction types.

To investigate this in more detail, the perturbation correction factors p_Co were determined for three modern types of flat chambers frequently used in clinical dosimetry by "cross calibration" at PTB in cooperation with the university hospitals of Freiburg and Tübingen and the German Cancer Research Centre in Heidelberg. Within the scope of this study, a total of 35 Roos chambers (30 PTW 34001, 3 Scanditronix-Wellhöfer PPC40, 2 PTB FK6), 15 Markus chambers (PTW 23343) and 12 Advanced-Markus chambers (PTW 34045) were investigated. For many of these chambers, the perturbation correction factor p_Co was determined several times in all participating institutes so that a total of 111 measurements were performed with Roos chambers, 40 measurements with Markus chambers and 37 measurements with Advanced-Markus chambers.

The scattering of the perturbation correction factors p_Co observed in these measurements for different chambers of the same type (see table) is clearly smaller than the value indicated in literature, although in our study a by far greater number of chambers was investigated under different experimental conditions (e.g. on electron accelerators of different types).

Table: Values obtained by experiment and relative standard deviations of the perturbation correction factors p_Co for the three chamber types investigated.

The results of this study show that indication of type-specific perturbation correction factors p_Co for the types of flat chamber types investigated is possible and reasonable. The measurement uncertainty of the mean values of p_Co stated in the table is only a little larger than the measurement uncertainty of the respective correction factors for cylindrical ionization chambers.

The values obtained in this study for the perturbation correction factors (see Table) are included in the new version of standard DIN 6800-2 in which the procedure of dose measurements with cylinder chambers and flat chambers will for the first time be unified.