A programme for dose quality audits for high-energy radiotherapy beams in non-reference conditions in Argentina

Under the framework of the IAEA Coordinated Research Project on 'Dose quality audits for high energy radiotherapy beams in non-reference conditions', the External Audit Group (EAG) of Argentina is running a programme in order to offer a service of TLD audits. This programme involves several steps, some of which are currently being performed: acquisition and characterization of a new PTW PCL3 reader; processing of TLD powder and determination of calibration, fading, linearity, energy and holder correction factors; dose audit in non-reference conditions, on axis and off axis, for high energy photon beams; dose audit in reference and non-reference on axis for high energy electron beams. A new reader PTW PCL3 was acquired, including a manual dispenser for TLD powder, an oven FIMEL ETT for TLD annealing, and a number of cupels to dispense the powder. The powder in use is TLD100 LiF(Mg,Ti), Rexon TLD Systems, 80-200 mesh. The characterization included the intensive operator training in loading powder in polyethylene capsules and dispensing aliquots of powder in the cupels. The cupels were weighed and a selection was made under the criteria of 2 SD in order to reduce the final uncertainty. The mass of aliquots is dependent on total mass of powder loaded in the dispenser, i. e. the mass of powder loaded in each capsule. In order to reduce the standard deviation of readings in each capsules the environmental conditions of room, the stabilization time and other requirements were established prior to starting the reading sessions. Following careful procedures, the readings showed a standard deviation of 0.6% for four readings per capsule. The annealing procedure is as recommended in the literature: 1 h at 400 deg. C and 24 h at 80 deg. C. The powder annealed is sieved using 80 and 200 mesh sieve. Fading shows a variation of 4.5% in the first 20 days with stable behaviour after this time. The linearity correction factor varies from 1.016 at 1.5Gy to 0.982 at 2.5Gy. The energy correction factors for photons and electrons, respectively, are shown. To obtain reliable values, more determinations of this factors are being performed. The complete programme for audits in Co-60 beams and high energy X ray beams consists of the following steps: - TLD audits for photon beams in reference condition photon beams in reference and non reference conditions on-axis (two depths, three different open field sizes and two wedged field size); - Photon beams in reference and non-reference conditions off-axis (open and wedged fields with TLDs located on main transversal axis at reference depth). The audits for electron beams included similar steps: irradiation at a reference depth, zref, on-axis (6-9 MeV, 10 MeV; 12-18 MeV) at two field sizes; electron beams at two depths, (zref and R50 on-axis) at two field sizes. A postal audit for Co-60 beams in reference condition was performed on 23 machines as the first step of a complete audit. For this step the participating centre is requested to irradiate each of the three capsules separately to a dose of 2 Gy to water, in a water phantom, at the central axis of a vertical irradiation beam, at 5 cm depth. The field size used is 10 cmx10 cm at either source-to-surface distance (SSD) or source-to-capsule distance. All participants have to complete a data sheet giving the method used for the absorbed dose determination. Once the TL-dosimeters return, the measurements corresponding to a batch are made on the same day. From each capsule 4 TL-reading are obtained. The mean value is determined for each capsule. The reading is referred to reference readings in order to compensate reader fluctuations and using the linearity factor and the holder factor (constant equal to 1.01) the dose is obtained. The overall uncertainty of the dose determination is 2.4% (3% including the dosimetry protocol). Six of the 23 participating machines obtained the dose deviation outside the interval ±5%. Corresponding follow-up is being made. The others (17 machines) will participate in the next step (in reference and non-refer ence conditions on-axis). This paper will include the results of audits in non-references conditions, on-axis and off-axis, for Co-60 machines and at least 10 Linear Accelerators (X ray and electron beams) operating in the country