LINEAR-QUADRATIC MODEL AND ITS APPLICATIO NS TO RADIOTHERAPY

The linear-quadratic model, based on a biological concept, has gained popularity these recent years, essentially due to its concordance with clinical data. It is recommended to use it for dose equivalencies in clinical radiotherapy when fractionation is altered. In this review, w e discuss the various modalities of its applications in the daily prac tice of radiotherapy. Although its primary application has concerned e quivalencies for late effects, it has been subsequently enriched with the adaptation of other factors such as the time, the interval between fractions, and the dose rate. These improvements have made it useful for equivalencies concerning early effects and for tumour control in e xternal radiotherapy when there is modification of the standard regime n, and in brachytherapy as well. It is emphasized that, in general, th ere is no unique equivalency for all effects when fractionation is alt ered. The use of this model should be preferably confined to clinical trials, or under certain conditions leading to deviation from the esta blished protocol. Despite the superiority of this model on previous on es, it contains limits in utilization, especially concerning its valid ity for a restricted range of doses per fraction. As for any model, it relies on constant values, necessitating a proper validation. The int er-patient and intra-tumour heterogeneities represent other shortcomin gs which call for the implementation of reliable individual tests.