Calibration of High Dose Rate Cobalt-60 Source Using a Farmer Type Chamber in A Locally Constructed Hollow Phantom

ABSTRACT

An important aspect of general quality assurance program for brachytherapy dosimetry is source calibration. Vendors assign large uncertainties of up to ±10 % for the stated source calibrated values. It is thus important to check the vendor stated calibration. There are three main methods of calibrating brachytherapy sources and they are the “Free- in – air” method, using a re-entrant(well ) type chamber and using a thimble (Farmer) type ionization chamber in a phantom. The main objective of this study was to compare values of source strength obtained by using the re-entrant chamber method and the thimble (farmer) type ionization chamber in a phantom method. A cylindrical hollow phantom was constructed from Poly methyl methacrylate (PMMA) having four holes located at a distance of 8cm from the centre and the angle between each of the holes being 90° from the centrally located source holder . The experimental set up followed International Atomic Energy Agency (IAEA) Technical Document 1274 (IAEA-TECDOC-1274) for the re-entrant chamber method and DIN 68909-2 in combination with DGMP report 13 for the thimble ionization chamber in a phantom method. Measurements carried out using the well type chamber deviated from the theoretical value of the source by -1.36% whereas those measured by the Farmer type ionization chamber in a locally constructed phantom deviated from the theoretical air kerma strength value by -2.1%. The difference in measured values from the two measuring methods was found to be0.8%.