Thermoluminescence dosimetry in the Prince of Wales Hospital, Sydney.

Thermoluminescence dosimeters (TLDs) were introduced in the Radiotherapy Department of the Prince of Wales Hospital in 1975. They are used in three different types of applications: 1. to confirm doses in complicated geometries (e.g. axilla, ear, nose), 2. to measure the dose delivered to critical organs (e.g. lens, scrotum) and 3. to monitor certain treatments like total body irradiations (TBI). Using four LiF chips (TLD 100; size: 3.1mm x 3.1mm x 0.9mm) per measuring point the accuracy of each measurement is +/- 5%. Approximately 200 TLD measurements are requested from the planning radiographers per year. Lens dose determination (32% of all cases) and total body irradiations are the main applications. Since 1988 the planning radiographers have been asked to state their expectation of the dose where it could be estimated. In 14% of these cases (7% of all requests) the expected and the measured dose differed by more than 20%. This was usually followed by revising the treatment set-up and a repeated TLD measurement. For about one third of all patients more than one dose measurement is required either to monitor the treatment or to investigate changes in the delivered dose after changes in the treatment set-up. Several changes to treatment modalities (e.g. boli in various treatments, a changed scrotal shield) were made due to TLD results. This is reflected in the still increasing number of TLD requests which demonstrates the benefit of TLDs in clinical practice.