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Characterisation of the radiation fields of commercial radiation therapy instruments

15.01.2010

For radiation therapy in hospitals, compact electron accelerators are used. Similar to an X-ray tube, the high-energy electron beam hits a metal sheet of high mass density (source). Thereby, bremsstrahlung of high energy is generated. The energy spectrum of the high-energy photon radiation cannot be measured directly. For the radiation characterisation, the depth-dependence of the energy transfer in water (absorbed dose to water) is thus determined in hospitals and calibration laboratories.

Figure 1 : Left: Photon energy flux density of the Elekta PreciseTM accelerator of PTB with multi-leaf collimator at a source distance of one meter (radiation quality 10 MVX, field size 10 cm x 10 cm). The colour scale represents thelogarithm of the energy flow density. Small picture: Component of unscattered bremsstrahlung with the signature of the multi-leaf collimator. Right: Photonspectrum of the effective radiation beam at a source distance of one meter.

For the two clinical accelerators [1] which are available at PTB, computer models were elaborated taking into account confidential data of manufacturers. By using approved software [2] of the national Canadian Metrology Institute NRC, these models can be used to calculate - in two steps - the characteristics of the radiation emitting from the device (Figure 1) as well as the radiation propagating in water and the energy dose absorbed there. The depth dose curves calculated or measured in this way can be adapted to a newly developed model function with a deviation of only 0.02% (Figure 2).

Figure 2 : Left: Simulation of a central depth dose curve for the radiation quality of 10 MVX and a field size of 10 cm x 10 cm. The deviation from the model functionis plotted below. The quotient of the dose values at a water depth of 10 cm and 20 cm is indicated. Right: Central depth dose curve measured with an air-filled ionisation chamber (Roos chamber). The statistical deviations are much smaller than in the case of the simulation so that a systematic deviation of 0.1% results in the dose build-up region.

With these calculations it is now possible to precisely determine the dose quotient for two different water depths, and thus to calculate the index Q used in different standards [3, 4] for the characterisation of the radiation quality. This index hence determines something like an average photon energy but is, at the same time, however, dependent on the field size. The precision achieved during the determination of the dose quotient is sufficient to detect a change of the accelerator’s electron energy of only 80 keV at an energy of 20 MeV.

By comparing the depth dose curves and the horizontal profiles obtained by simulation and measurement, the computer model can be verified. For photon qualities (effective accelerating voltage of the electron beam used) of 6 MV, 10 MV, this comparison has already been carried out successfully by means of own measurements and literature data [5]. The photon qualities 25 MV, 15 MV, 8 MV and 4 MV are being prepared. Thus, those characteristics of the radiation fields which are not directly measurable are exactly known and can be taken into account for the calibration of dosimeters. Control measurements with calibrated dosimeters for the determination of the absorbed dose to water are a serviceable tool to rule out - already at an early stage - any possible malfunctions of accelerators for radiation therapy which might have negative consequences for patients.

Literature

  1. "Richard-Glocker-Bau eingeweiht", www.ptb.de/de/org/6/nachrichten6/2008/60608_de.htm
  2. BEAMnrc: "A Monte Carlo Simulation System for Modelling Radiotherapy Sources" http://www.irs.inms.nrc.ca/inms/irs/BEAM/beamhome.html
  3. DIN 6800-2 "Dosismessverfahren nach der Sondenmethode für Photonen- und Elektronenstrahlung Teil 2: Ionisationsdosimetrie", Deutsches Institut für Normung e.V., Beuth Verlag, 2008
  4. IAEA TRS-398 "Absorbed dose determination in external beam radiotherapy: An international code of practice for dosimetry based on standards of absorbed dose to water"
  5. Technical Report Series 398, International Atomic Energy Agency, Vienna. 2004 http://www.pub.iaea.org/MTCD/publications/PDF/TRS398_scr.pdf  British Journal of Radiology, Supplement 25 (1996)