Evaluation of incorporated iododeoxyuridine cellular radiosensitization by photon activation therapy

Halogenated pyrimidines such as iododeoxyuridine (IdUrd), when incorporated into cellular DNA, sensitize cells to radiation. A technique known as photon activation therapy (PAT) has recently been proposed to further enhance IdUrd radiosensitization. This approach relies on the emission of high linear energy transfer Auger electrons as a result of photoelectric absorption of photons with energies slightly above the K shell binding energy for iodine. Thus, additional IdUrd radiosensitization has been predicted for this technique using photons either generated externally from orthovoltage units or with implants of radioisotopes with emissions of the appropriate energy range. We have tested this prediction by comparing the radiosensitization of Chinese V79 cells, which have incorporated IdUrd (16% replacement of thymidine with IdUrd), to megavoltage (15 MV) versus orthovoltage (100 KVp) radiation. Enhancement ratios at 1% survival levels for 15 MV vs. 100 KVp radiation were 1.8 and 1.95, respectively. This modest 10-15% additional enhancement was much less than that predicted by PAT. The discrepancy between experimental data and the theoretical predictions of PAT are discussed.