Dosimetric verification of source strength for HDR afterloading units with Ir-192 and Co-60 photon sources: Comparison of three different international protocols.

Before clinical use of a brachytherapy source, regulations or recommendations by medical physics societies require an independent measurement of its air kerma strength by a qualified medical physicist. Currently, in addition to Ir-192, also HDR-Co-60 sources are increasingly coming into operation. However, the existing dosimetry protocols do not provide any guidelines for Co-60 sources. The purpose of this work was therefore to compare air kerma rate measurements as recommended by different dosimetry protocols for Ir-192 HDR sources and to test their applicability to Co-60 sources. Dosimetric verification of HDR afterloading source specification was performed according to three protocols, DIN 6809-2 (1993) in combination with DGMP-Report 13 (2006), IAEA-TECDOC-1274 (2002) and AAPM Report 41 (1993) for the nuclides Ir-192 and Co-60. Measurements of the sources reference air kerma rate were performed with 3 different methods (with a cylindrical chamber both in a solid phantom and in free air, and with a well chamber) and evaluated using all three protocols for each type of source and method of measurement. The measurements with all protocols and methods show deviations from the certified specification smaller than about 1.2% for Ir-192 and 2.5% for Co-60-Sources. The measurements with the well chamber showed the lowest deviations from the certificate value. Air kerma rate measurements for Co-60 HDR sources using the existing protocols are possible with accuracy sufficient to verify source calibration as provided by the source certificate. However, extension of the protocols by correction factors for measurement with Co-60 sources would be helpful.

[Dosimetry of HDR afterloading machines with Ir-192- und Co-60-sources: comparison of different international protocols]. / Dosimetrie an HDR-Afterloading-Geräten mit Ir-192- und Co-60-Strahler: Vergleich verschiedener internationaler Dosimetrieprotokolle.

The air kerma rate in air at a reference distance of 1 meter from the source is the recommended quantity for the specification of gamma ray source in brachytherapy. The absorbed dose for the patients is directly proportional to the air kerma rate. Therefore the air kerma rate should be determined before the first use of the source on patients by a medical physicist who is independent from the source manufacturer. The air kerma rate will then be applied in the calculation of the dose delivered to patients. In practice, high dose rate (HDR) Ir-192 afterloading machines are mostly used in brachytherapy treatment. Currently HDR-Co-60 increasingly come into operation, too. The essential advantage of the use of Co-60 sources is its longer half-life compared to Ir-192. In addition, the purchasing and disposal costs are lower. The use of HDR-Co-60- afterloading machines is also quite interesting for developing countries. This work describes the dosimetry at HDR afterloading machines according to the protocols DIN 6809-2 (1993) in relation to the DGMP-Report 13 (2006), IAEA-TECDOC-1274 (2002) and AAPM Report 41 (1993) with the nuclides Ir-192 and Co-60. We have used 3 different measurement methods (with a cylindrical chamber in solid phantom and in free air and with a well chamber) in dependence of each of the protocols. We have shown that the standard deviations of the measured air kerma rate for the Co-60 source are generally larger than those of the Ir-192 source. The measurements with the well chamber had the lowest deviation from the certificate value. In all protocols and methods the deviations stood for both nuclides by a maximum of about 1.2% for Ir-192 and 2.5% for Co-60-sources respectively.

[Determination of absorbed dose to water for high energy photon and electron beams--comparison of different dosimetry protocols]. / Bestimmung der Energiedosis in Wasser für hochenergetische Photonen- und Elektronenstrahlung--Vergleich verschiedener Dosimetrieprotokolle.

The determination of absorbed dose to water for high-energy photon and electron beams is performed in Germany according to the dosimetry protocol DIN 6800-2 (1997). At an international level, the main protocols used are the AAPM dosimetry protocol TG-51 (1999) and the IAEA Code of Practice TRS-398 (2000). The present paper systematically compares these three dosimetry protocols, and identifies similarities and differences. The investigations were performed using 4 and 10 MV photon beams, as well as 6, 8, 9, 10, 12 and 14 MeV electron beams. Two cylindrical and two plane-parallel type chambers were used for measurements. In general, the discrepancies among the three protocols were 1.0% for photon beams and 1.6% for electron beams. Comparative measurements in the context of measurement technical control (MTK) with TLD showed a deviation of less than 1.3% between the measurements obtained according to protocols DIN 6800-2 and MTK (exceptions: 4 MV photons with 2.9% and 6 MeV electrons with 2.4%). While only cylindrical chambers were used for photon beams, measurements of electron beams were performed using both cylindrical and plane-parallel chambers (the latter used after a cross-calibration to a cylindrical chamber, as required by the respective dosimetry protocols). Notably, unlike recommended in the corresponding protocols, we found out that cylindrical chambers can be used also for energies from 6 to 10 MeV.

[Dose distribution of asymmetric fields: comparison of the Helax-TMS with our developed 2D-program ASYMM]. / Dosisverteilungen asymmetrischer Felder: Vergleich des Bestrahlungsplanungssystems Helax-TMS mit dem selbstentwickelten 2D-Berechnungsprogramm ASYMM.

The purpose of this investigation was to compare the commercial 3D-treatment planning system Helax TMS to a simple 2D program ASYMM, concerning the calculation of dose distributions for asymmetric fields. The dose calculation algorithm in Helax-TMS is based on the polyenergetic pencil beam model of Ahnesjö. Our own developed 2D treatment planning program ASYMM, based on the Thomas and Thomas method for asymmetric open fields, has been extended to calculate the dose distributions for open and wedged fields. Using both methods, dose distributions for various asymmetric open and wedged fields of a 4-MV Linear accelerator were calculated and compared with measured data in water. The agreement of the Helax-TMS and the ASYMM with the experiment was good, whereas ASYMM showed a better accuracy for larger asymmetric angles. The explanation for this result is based on the consideration of beam hardening within the flattening filter and edges for different asymmetric settings in ASYMM algorithm. The TMS, however, owns the diverse possibilities that the 3D calculation and corresponding representation provide and holds better application opportunities in clinical routine.