Time for radiobiology in the nuclear medicine community

Radiobiology is the science of the interaction of ionizingradiations with living tissues. To begin with we have beenunaware of the potential hazard resulting from the exposureto ionizing radiations and for decades no health control orphysical measurement of absorbed dose has been put intopractice for exposed medical staff. Following the observa-tion that ionizing radiations may have harmful effects,radiobiology has lived a period of great interest, culminat-ing with the study of atomic bomb survivors and, morerecently, with the Chernobyl accident. Modern radiobiologyfocuses on two major subjects: development of radiobio-logical models for the optimization of therapies using highdoses of ionizing radiations and assessment of risk in thefield of low doses.Radiobiology of high doses of ionizing radiationsIn recent years radionuclide therapy (RNT) has beenincreasingly used and this has prompted us to elaboratemodels to predict toxicity and efficacy of treatment. Atreatment planning approach to RNT more and more willeventually require incorporation of radiobiological consid-erations of therapeutic radiopharmaceuticals [1, 2]. Thedevelopment of radiobiological models for treatmentplanning, however, is still at an early stage of developmentas compared to external beam radiotherapy (EBRT). Todate, most radiobiological models of RNT have been basedon the extrapolation of data obtained following homoge-neous exposures to acute single or fractionated doses ofEBRT and have assumed that EBRT and RNT administereddoses have biological equivalence.Several differences exist, however, between RNT andEBRT: in RNT the radiation dose is delivered to the tumourcells by continuous, but declining, exposure with a lowdose rate (LDR) that is a function of the initial uptake,cellular distribution, residence time, radionuclide half-lifeand energy. The average dose rate for RNT is typically ofthe order of 2–8 Gy/day, and the maximum absorbed dosemay be up to 50 Gy delivered over a period of many days.This is in marked contrast to the situation with EBRT,where the dose is delivered at a constant high dose rate(HDR), typically 1–5 Gy/min [3].Emerging evidence suggests that the mechanisms bywhich cells respond to LDR radiation exposures arefundamentally different from those occurring at HDR withEBRT [1, 4]. The combination of prolonged response,limited toxicity and the ability to treat on multipleoccasions suggests that the mechanism of action of LDR