Radiobiological studies with low energy ionizing radiation - dosimetric aspects

Abstract: Background: During the last 20 years both in vitro and in vivo models have been developed for studies of the cell killing characteristics of low energy electrons, e.g. Auger electrons. This thesis deals with radiobiological effects of radiation from indium isotopes in V79 cells in vitro (Paper I-III) and the rat testis in vivo (Paper IVV). The focus has mainly been on studies of the accuracy and limitations of the radiation dosimetry in both models. Material and Methods: Thermo luminescent dosimetry (TLD) was used for measurement of the absorbed dose to cells exposed to photon induced Auger electrons or X-ray photons at ultrahigh dose rates. A rat testis model was developed and intratesticularly exposed to 110In, 111In and X-ray photons. The biokinetics and dosimetry were carefully evaluated by use of a scintillation camera, HPGe semiconductor detector and TL Dosimetry. Results: In none of the in vitro experiments could an enhanced biological effect attributed to Auger electrons be proved. It was concluded early in the work that the dosimetry was a matter of decisive importance for assessment of radiobiological results and that every single irradiation occasion needs to be scrutinized. Although the rat testis model was demonstrated to be an appropriate model for radiobiological studies in vivo, there was no difference in the sperm count survival when exposed to the Auger emitting radionuclide 111In compared to 110In and X-ray photons. Discussion and Conclusion: Radiobiology must rely on a radiation quantity such as absorbed dose to compare the effects between different experiments and laboratories. This investigation has shown the complexity of determining the absorbed dose in radiobiological experiments. The accuracy depends on several factors, especially the experimental setup and the accurate calibrations of the TLDs. For the testis, the kinetics, mass, volume and radionuclide localization influence the accuracy of the absorbed dose and thus the RBE. The overall question raised by this thesis is the concept of absorbed dose and its use in diagnostic and therapeutic nuclear medicine. Many of the results point in the direction of the inadequacy of using the mean absorbed dose concept for radionuclides.