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The quantitative analysis of the cell dose-survival curves, the randomness of cell killing by radiation, the probabilistic basis of the response to irradiation of tumors and normal tissues, the understanding of the biological mechanisms underlying to this response, the rationale for dose-time and fractionation in radiotherapy, the introduction of the concepts of tumor control probability (TCP) and biologically effective dose (BED), the relationships TCP-dose, BED-alpha/beta BED-fraction size and BED-treatment time, the problems associated with the accelerated regeneration of surviving tumor clonogens during the course of fractionated radiotherapy, the new demands of knowledge on oncology and radiation biology derived from heterogeneous dose distributions in conformal radiation therapy programs and the definition of the biological basis of normal tissues tolerance to reirradiation are, probably, the most important contributions of radiobiology to clinical radiotherapy in the last twenty five years. Radiotherapy is today a scientific discipline based on the interplay of mathematics, physics, biology and oncology. The knowledge of the basic concepts of radiobiology is essential for daily radiotherapy practices and for all oncologists. The most efficient use of multimodality treatments in cancer therapy cannot be done without a clear understanding of these principles.

Major changes in cancer radiotherapy have followed a greater understanding of the biological effects of radiation on tumours and normal tissues. Clinical radiotherapy is today a solid body of knowledge with well defined scientific foundations. Key concepts in current radiobiology include lethal and sublethal injuries, dose-effect coefficients, alpha/beta ratios, acute and late response, biologically equivalent dose, fraction dose, irradiation time and tumour regeneration between others. Effects of irradiation time and dose per fraction on tumours versus normal tissues are of special importance. Dose per fraction must be considered for analysis of effects in normal late-responding tissues. In contrast, both dose per fraction and irradiation time influence the response to radiation of malignant tumours and acute-responding tissues. Finally, the ability to quantify relationships between radiation dose and biological effect has been of particular value in the development of radiotherapy. This is illustrated by the growing use of high doses per fraction for the treatment of some cancers.

Although complementary, the functional method (FDG-PET) is significantly superior to the structural method (CT) for detection of mediastinal tumour disease.