Mailed TLD dose intercomparison at a reference point of high energy photon beams among 11 institutions in Japan

To perform the multi-institutional cooperative studies of radiation therapy for malignant diseases, standardization of absorbed dose among the participating institutions must be established. To this end, site visits and a mailed TLD program are necessary during the first phase of quality assurance (QA) program. In this study, we examined a mailed TLD program for dosimetric intercomparisons among 11 participating institutions. We use a mailed TLD system that consists of TLD-100TM extruded disks (4.5 mm in diameter and 0.9 mm thick, Harshaw/Bicron) in an irradiation container made of water equivalent plastics named tough-waterTM (Kyoto Kagaku). These TLD disks are placed at 5 cm depth in a tough-water mini-phantom. The three irradiation containers and one mini-phantom were sent to 11 institutions. In each institution, an additional phantom of about 10 cm thick is put under the mini-phantom to provide sufficient back-scatter. Two Gy of institutional stated dose is irradiated to the TLDs. TLD output was measured for 42 beam through standardized field size of 10 cm x 10 cm. Thirty-six TLD values out of 42 beam data fall within ±3% difference between measured and institutional stated dose values. In the case of reused TLD, the measuring accuracy is seriously affected by the annealing procedures and its reproducibility. Thus, it is important to repeat annealing of TLD four or five times for initialization and also to select TLD by weight for better uniformity of sensitivity. It is clearly indicated that a mailed TLD program is easy to perform and necessary for QA program of multi-institutional cooperative study. (author)