Source of Support: None, Conflict of Interest: None
The application of linear quadratic (LQ) model to biological effects of radiation is known for at least three decades. In case of radiotherapy in order to ensure that appropriate tumour lethal dose is delivered without exceeding normal tissue tolerance, concepts like NSD and its derivatives were used in the past. Due to their shortcomings these concepts are now replaced by a linear quadratic model. As this model is based on the recent radiological data, it is claimed to be able to predict the biological responses of tumours as well as normal tissues more accurately. However, the accuracy of prediction depends on the precise values of various parameters that are involved in the linear quadratic equation. For most of the tumours and the associated normal tissues, values of the parameters are not yet accurately known. The parameters involved are alp the repair constant and repopulation factor. From radiobiological data on animals, in-vitro experiments on tumour cells and clinical data values of these parameters have been compiled and presented in this paper. It is found that the parameters are very much different from tissue to tissue and site to site. Large number of clinical data will have to be analysed to obtain precise values of these parameters so that LQ model can really be as accurate as it is promised to be.