THE DEVELOPMENT OF RADIO-OPAQUE, ISOTROPIC, FIBEROPTIC PROBES FOR LIGHT DOSIMETRY STUDIES IN PHOTODYNAMIC THERAPY

During photodynamic therapy of skin carcinomas, several small detector probes are inserted under the skin in order to monitor the light dose. The position of the probe must be precisely defined in order to yield accurate dosimetry measurements. The geometrical measurement of depth was found to yield optical parameters with a large sample variance, which led to the application of computerized tomographic scanning in determining the probe depth. This demands the inclusion of a radio-opaque contrast in the probe construction. Short, cylindrical diffusing-type probes were used initially, but their anisotropic response to light prompted the construction of a new type of probe, formed from a proprietary dental paste cured by blue (488 nm) light. Different mixtures of dental pastes, radio-opaque powders and scattering substances were tested for isotropy, radio-opacity and strength. A probe made up of 73% opaque dental paste and 27% barium sulphate powder (by mass) was found to satisfy the requirements for in vivo light dosimetry.