Introduction: A dosimetric audit is a quality assurance process (QA) that allows the accuracy of dosimetric and geometric precision to accurately control the delivery of doses. They can be used to evaluate the accuracy of radiation delivery at different radiotherapy centers. Differences in prescriptive doses, depending on the severity of the differences, have potential consequences not so important, severe and even fatal. Measuring the absolute output of linear accelerators (LINACs) under reference conditions is a standard method for verifying the accuracy of dose delivery. In this study, Radiotherapy centers in Iran were audited by the secondary standard dosimetry laboratory (SSDL). The absolute photon output of the accelerators and some of the relative factors such as wedge factor, shield tray factor, total scatter factor was measured using a Farmer chamber and the results were compared with the reported values by the radiotherapy physicist of centers. The differences between SSDL and the inspected center were reported.
Materials and Methods: Measurements were performed at 178 on-site visits from 61 radiotherapy centers (including 98 LINACs) between March 2007 and October 2017. Some participating centers were visited more than once. Photon absolute dose measurements were carried out according to TRS-398 protocol using Farmer ionization chamber (TW30013). Action levels were determined including ±3% for absolute dose and ±2% for relative factors.
Results: Absolute dose differences between SSDL and local centers were in the range of -3.5% up to 4%. The differences in the values of the wedge factor between the amounts measured by SSDL and the local centers were -3.5% up to 3.36%. The minimum and maximum differences in the total scatter factor were -3.4% up to 3.7%. The differences in the obtained tray factor values were -1.4% up to 2.5%. The difference between SSDL and radiotherapy centers at the reported absolute dose values in 11 beam measurements was greater than the permissible limits. Moreover, 299 beam measurements were within the tolerance.
Conclusion:
Differences in measurements between SSDL and radiotherapy centers could be due to the following: I. Lack of qualified medical physicists in some radiotherapy centers, II. Error in calculating beam quality factor (kQ) for some chambers, III. Mistake in the report of the reference depth and reference source to surface distance (SSD).