Abstract
Background and purposeSmall radiation beams (<4cm width) are being increasingly used in the delivery of advanced techniques as Intensity Modulated Radiotherapy (IMRT) and Stereotactic Radiosurgery (SRS). Dose measurements in small beams present challenges not encountered for larger beams. A postal audit with Thermoluminiscent Dosimeters (TLD) was developed to check the doses in small photon beams. A validation test in real conditions was carried out in fourteen centres. Material and methodsThe TLD postal audit employs very small chips (1×1×1mm3) of TLD-100 inserted at 5 and 10cm of depth in a cylindrical PMMA phantom designed for this purpose. This experimental system is mailed to the audited centres to be irradiated with beams of 1 and 3cm of side or diameter. The prescribeddose is 1.5Gy at 10cm. The properties of this system were studied experimentally and by Monte Carlo (MC) simulation, before the external test. ResultsDeviations between the prescribed and measured absorbed doses are below 5% for 69% (1×1cm2 beam) and 64% (3×3cm2 beam) of the audited centres. When deviations are above 5%, their causes have been investigated and led to corrections. ConclusionThe developed postal audit is suitable to verify the absorbed doses in small photon beams with an accuracy of 2.9% (1s).
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