Combined effects of radiotherapy and photodynamic therapy on an in vitro human prostate model

Human prostate cancer cells were evaluated for growth after photodynamic therapy, radiotherapy, and combined treatment. Indocyanine green was tested as a photosensitizer and radiosensitizer. Two human cell lines were used: PC-3 derived from prostate carcinoma, and EPN derived from normal prostate tissue. The light source used for the photoactivation experiments was a diode laser peaked at 805 nm. The light dose incident on cells was 108 J/cm(2). Ionizing radiation was produced by a linear accelerator, and the dose was 2, 4 and 6 Gy. Cytotoxicity was evaluated by measuring the colony forming ability of cells. Our results show that indocyanine green induces cell death by photoactivation, but it does not act as a radiosensitizer if used with ionizing radiation. The combined treatment of photodynamic therapy and radiotherapy produces an additive effect which does not depend on the sequence of the two treatments. Combined treatments could be more useful since they allow the reduction of the ionizing radiation dose to obtain the same effect as one obtainable by radiotherapy alone.