Build-up Caps to be Used in In-vivo Thermoluminescence Dosimetry

Thermoluminescence dosimeters (TLDs) are widely used for quality assurance in radiation therapy. To avoid steep dose gradients, such measurements are usually made at either the depth of maximum dose Dmax or at the exit skin surface, to assess in vivo the entrance and exit doses respectively. The build -up caps to be used have, among others, to induce minimal perturbation to the treatment field and the registered dose to be as close as possible to that to be assessed in order to reduce the size of the possible errors of the measurement. The aim of the present study was to assess the influence on the TL signal of the geometrical characteristics of cylindrical caps made of various materials at various photon fields. Cylindrical caps of various sizes made of plexiglas, aluminum, copper and stainless steel were constructed to accommodate four 3.1x3.1x0.9 mm LiF: Mg, Ti TLDs per cap. The relationships between the doses registered by the TLDs and the entrance and exit doses measured in phantoms with ionization chambers in 6 and 15 MV fields, were determined. The deviations from the dose registered by the TLDs and those measured by ionization chambers, increased with increasing material density. The ratio of the measured dose to the expected dose was equal to ± 2.5%. The signal increased with build-up cap atomic number. On the other hand, build-up caps made up of Cu were found to be optimum for high-energy photon fields and caps made up of Plexiglas for low energy fields.