CLINICAL USE OF CARBON-LOADED THERMOLUMINESCENT DOSIMETERS FOR SKIN DOSE DETERMINATION

Purpose: Carbon-loaded thermoluminescent dosimeters (TLDs) are designe d for surface/skin dose measurements, Following 4 years in clinical us e at the Mater Hospital, the accuracy and clinical usefulness of the c arbon-loaded TLDs was assessed. Methods and Materials: Teffon-based ca rbon-loaded lithium fluoride (LlF) disks with a diameter of 13 mm were used in the present study. The TLDs were compared with ion chamber re adings and TLD extrapolation to determine the effective depth of the T LD measurement, In vivo measurements were made on patients receiving o pen-field treatments to the chest, abdomen, and groin, Skin entry dose or entry and exit dose were assessed in comparison with doses estimat ed from phantom measurements. Results: The effective depth of measurem ent in a 6 MV therapeutic x-ray beam was found to be about 0.10 mm usi ng TLD extrapolation as a comparison. Entrance surface dose measuremen ts made on a solid water phantom agreed well with ion chamber and TLD extrapolation measurements, and black TLDs provide a more accurate exi t dose than the other methods. Under clinical conditions, the black TL Ds have an accuracy of +/- 5% (+/- 2 SD). The dose predicted from blac k TLD readings correlate with observed skin reactions as assessed with reflectance spectroscopy. Conclusion: In vivo dosimetry with carbon-l oaded TLDs proved to be a useful tool in assessing the dose delivered to the basal cell layer in the skin of patients undergoing radiotherap y.