The results from external TLD audit in reference and non-reference conditions in Bulgarian radiotherapy centres

Full text: The necessity of national and international dose intercomparisons is brought about by the high degree of precision and accuracy required in radiotherapy. The aim of the intercomparisons is to check and improve the dosimetry uniformity and accuracy in order to achieve consistency and comparability of radiation dose delivery to the patients. According to the International Commission on Radiation Units and Measurements (ICRU) the dose delivered to the target volume should be known at least to within ±5%. Some investigations indicate that even better accuracy is preferable. Since 1975 the Laboratory of Clinical Dosimetry and Ionizing Radiation Metrology (SSDL-Sofia) has been conducting a postal dose intercomparison programme for all Co60 teletherapy units in the country. The method used is similar to the IAEA method. The first run was carried out in 1975 and 11 runs have been accomplished so far. Of 285 results, 251 (88%) deviation between dose stated by the participants and the dose measured by our laboratory were within ±5% and three (3) deviations were outside ±10%. Since 2004 SSDL-Sofia has been developing an operating procedure for the photon beam audits on-axis in non-reference conditions including three radiotherapy centres. Instruction sheets, data sheets and result reporting forms are prepared under the IAEA methodology. Deviation between dose stated by the participants and the dose measured by our laboratory were within ±5%, excluding one result regarding wedge transmission factor, where the deviation was greater then 5%. When the absolute value of the deviation is grater then 5%, the participant is asked to take urgently the necessary steps in order to find the source of the error. If the errors fail, we make a site inspection and the unit is thoroughly checked and if necessary recalibrated or explained through the audit methodology. This year SSDL-Sofia will organize a national run of external TLD audits for all radiotherapy units in non-reference conditions on-axis and a pilot run for selected groups of radiotherapy centres in non-reference conditions off-axis using a new IAEA TLD holder with horizontal arm for dose measurements under IAEA methodology. The results will be reported in November 2006. The required high accuracy in therapy level dose measurements and determinations can be achieved only each step in the sequence of dosimetric procedures - from the PSDL to the irradiation of the patient is performed very carefully with the highest possible precision. (author)