The preceding DIN protocol has been revised by a German task group and its latest version was published in March as the national standard dosimetry protocol DIN March. Since then, in Germany the determination of absorbed dose to water for high-energy photon and electron beams has to be performed according to this new German dosimetry protocol.
The new German version has widely adapted the methodology and dosimetric data of TRS This paper investigates systematically the DIN protocol and compares it with the procedures and results obtained by using the international protocols. While only cylindrical chambers were used for photon beams, the measurements of electron beams were performed by using cylindrical and plane-parallel chambers.
It was found that the discrepancies in the determination of absorbed dose to water among the three protocols were 0. The determination of water absorbed dose was also checked by a national audit procedure using TLDs. The advantage of the new German protocol DIN lies in the renouncement on the cross calibration procedure as well as its clear presentation of formulas and parameters. In the past, the different protocols evoluted differently from time to time. Fortunately today, a good convergence has been obtained in concepts and methods.
In Germany, the dosimetry for linear accelerators is currently performed according to the new DIN March [ 1 ]. In this protocol, the absorbed dose in water is used as a measuring quantity for high-energy photon and electron beams. In order to better meet international consistency, the preceding DIN [ 2 ] was modified and adapted to the Technical Report Series No.
The description of the measurements and methods, the correction parameters for various chambers and the evaluation for uncertainties are the main parts of DIN March. The physical basics are illustrated in its appendix.
The new data of the correction factors and the interaction coefficients of the IAEA TRS are taken for the calculation of the absorbed dose in the new DIN, so that the international consistence of the dose calculation is assured. Although not all parameters were taken directly from TRS , the effects of the deviations from the TRS remain low [ 4 ].
The main difference between the old protocol and the new one lies in the electron dosimetry as well as in the evaluation of the uncertainties. In this work, the authors have applied the new DIN March to check the clinical dosimetry in reference to the TRS as the base for the future application of the DIN The measurements for the evaluation of the depth dose distributions were performed in a Wellhoefer water phantom Blue Phantom with the plane-parallel Roos chamber PTW.
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The cylindrical chamber CC Wellhoefer was used as a reference chamber. The determination of the absorbed dose in water was also done in the Blue Phantom with two cylindrical chambers PTW type as well as with two plane parallel chambers PTW type, Roos-Chamber.
For the absolute dosimetry, the Scanditronix-Wellhoefer-Assembly system was used only for the positioning of the measuring chamber. Positioning of the ionisation chamber was performed by adjusting the reference point of a specific chamber to the measuring depth as required in the respective protocol Table 1.
For the Roos-chamber, the reference point is located on the inner surface of the entrance window which is at the centre of the window PMMA thickness of the window: 1 mm.
Each chamber type is calibrated in a Co radiation beam with its reference point at measuring depth. The operating voltage for the cylindrical chamber is V, whereas the Roos-chamber is connected to a voltage of V.
The results were taken to additionally check the accuracy of this work. All ionisation chambers were calibrated with a Co source and thus provided with calibration factor N. For the determination of the absorbed dose in water D w for photon and electron beams, having the dosimeter reading M at the depth of measurement z ref , the following equation is generally applied to all protocols as:.
In the other protocols, this correction factor is embedded in the quality correction factor k Q as one of the constituent perturbation factors. The reference conditions for the determination of the absorbed dose to water are listed in Table 2. The different methods are presented in Table 3. The authors have applied the experimental method. The difference between the experimental method and the theoretical calculation is smaller than 0.
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The constants a 0 , a 1 and a 2 are listed in the corresponding protocol. The different methods for the determination of k P according to the different protocols are shown in Table 4.
However, care must be taken in case the calibration laboratory has not corrected for the polarity effect as documented in the certification. In this case, the subsequent treatment of the polarity effect depends on the facilities available to the user, and on what beam qualities must be measured:. If the user beam quality is the same as the calibration quality normally Co and the chamber is used at the same polarizing potential and polarity, then k pol will be the same in both cases and the user must not apply a polarity correction for that particular beam.
If the user beam quality is not the same as the calibration quality, but it is possible to reproduce the calibration quality, then the polarity correction [ k pol ] Q o that was not applied at the time of calibration must be estimated using one of the equations of Table 4 and using the same polarizing potential and polarity as was used at the calibration laboratory.
The polarity effect at the user beam quality, k pol , must be determined the same way using the polarizing potential and polarity adopted for routine use. A modified polarity correction is then evaluated as follows:.
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This is then used to correct the dosimeter readings for polarity for each beam quality Q. For the determination of [ k p ] Q o measurements are generally performed with Co beams.
This correction must always be included according to the DIN Protocol for cylindrical chambers for photon- and electron beams. For the calculation of k r , the following relation is given in DIN March :. The radiation quality must be determined before since calculated values of k Q are provided in tables as a function of the radiation quality.
TPR 20,10 can be measured directly according to its definition or by a depth dose measurement:.
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In the German protocol, this second method must be used. Consequently, for the determination of the dose depth curve above 10 MV, a lead plate, about 1 mm thick, must be positioned between the focus and the measuring chamber. When using the same positioning as for the reference dosimetry, the dose depth curve is shifted 0. The corresponding k Q -value can be interpolated for the cylindrical chamber PTW The values of Table 6 in DIN March are approximated by a polynomial of 4 th degree, in order to calculate the correction factor k Q by putting any value of the quality index into the equation:.
Formalism for the determination of the quality correction factors for photon beams k Q z ref and Electron beams k E z ref. For all protocols, a summary is given in table 6 for the determination of the radiation quality correction factor.
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Effective point of measurement and gradient correction for the cylindrical chamber PTW In all protocols the R 50 of the energy depth curve represents the quality index for the radiation quality. The measurement of the dose depth curves is done with a cylindrical chamber by a displacement correction of 0. This correction is not to be applied with plane parallel chambers.
The correction factor for radiation quality k E consists of a quality-specific factor k E' and a chamber-specific factor k E" , which are determined separately in the DIN-Norms, whereas in the TRS there is no split up. The determination of the absorbed dose to water can be done in a quality index R 50 dependent reference depth z ref.
The R 50 can be calculated from the ion depth dose curves as follows:. From the half value depth R 50 , the energy dependent reference depth can be calculated as:. DIN defines the following equation for the determination of energy quality specific factor k E ' in reference depth z ref :. Chamber quality specific factor k E " is given in the DIN , Table 8, for different cylindrical chambers with different reference conditions in dependence of half value depth R These values are approximated for the cylindrical chamber PTW by a polynomial of 3 rd degree which deviate only of 0.
Chamber specific factor k E " for plane parallel chambers equals the inverse value of the chamber specific perturbation factor p Co for Cobeams, which is, according to DIN March , Table Therefore, the time-consuming cross calibration measurement for the determination of k E " for all known chambers is not necessary in DIN March.
It is only necessary for the introduction of new types of chambers. The IAEA proposes, on the other hand, to perform a cross calibration for plane parallel chambers by using a cylindrical chamber instead of using the inaccurate values of Table The authors have performed the cross calibration with the highest recommended energy of each accelerator 14 and 21 MeV-Electrons.
Thereby, the calibration factor of the Ross-chamber will be newly determined. The exact procedures are presented in literature [ 3 , 6 ]. According to AAPM TG, the quality correction factor k E for electrons as one of the constituent correction factors in equation 1 is substituted by:. P g r Q is an additional component of k E and is the gradient correction factor in an electron beam that is dependent on the ionization gradient at the point of measurement.
For cylindrical chambers, P g r Q is a function of the radius of the cavity; it is equal to 1 for plane-parallel chambers. A cross calibration for plane parallel chambers is also recommended by the AAPM. A summary of the formalism for the determination of the radiation quality correction concerning electrons is given for all three protocols in Table 6. These values are deduced from multiple measurements followed by a calculated average value with every chamber type for each protocol.
In Figure 1 , the deviations of the dose values for photon beams measured with a cylindrical chamber are presented with respect to DIN March. The measurements are done for both photon and electron beams with cylindrical chambers whereas the plane parallel chambers are only used for the electron beams. Figure 1 shows the deviations for photon beams for both linear accelerators. Figures 2 and 3 show the deviations by the electron beams, each for one accelerator.
DIN EN (Entwurf) Teil 1: Teil 3: Teil 4:
The deviations for all chambers are given in relation to the values of the Roos-chamber according to protocol DIN March. The measuring values of all chambers according to all protocols are generally smaller than the values of the Roos-chamber according to DIN March. The largest deviation is about As observed, the deviation of the dose values between the cylindrical and the plane parallel chambers varies between 0.
For the electron beams, it shows the tendency that the measuring values of the Roos chamber deviate from each other for only a maximum of about 0. For photon beams, a deviation of max.
In contrast to that work the current deviations are decreased by more than a half for photon beams whereas the values for the electrons progressed a little. The uncertainties in the measured values consist of a series of independent, single uncertainties. The total uncertainty is calculated from the geometrical sum of the individual uncertainties.
In the protocol AAPM TG 51 no uncertainties are given for the determination of the correction factors for the radiation quality. For the determination of dose for photon and electron beams with cylindrical chambers, the total standard uncertainties is estimated according to TRS a maximum of 1.
The influence quantities which contribute to an important part to the total uncertainty are presented in Table 7 [ 8 ]. While in the IAEA and the AAPM , a cross calibration measurement for plane parallel chambers with a cylindrical chamber is recommended, an experimental calibration factor for the different plane parallel chambers is given in DIN March so that the relative inaccurate individual cross calibration measurement is spared for the user.
In DIN March , an analysis of the measurement uncertainties is discussed for the first time. In connection with the TRS , measurements of electron beams are now also done in z ref according to DIN