GENERALIZED LINEAR-QUADRATIC MODEL AND DOSE RATE TERM IN RADIOTHERAPY PREDICTION

The Linear-Quadratic model (LQ) is widely used to study cell response to radiation dose and to predict clinal results in radiotherapy. The LQ model uses alpha and beta parameters to assess radiossensitivity of tissues, being a simplified model in its classical formulation. The dose-rate effect in cell response is decribed in the Generalized Linear-Quadratic Model (gLQ). This model assumes that dose-rate have only influence in sublethal damages (beta parameter). Here, the doserate parameter (G) of gLQ model was evaluated using numerical simulation in a range of low dose-rate up to ultra High dose-rate (uHDR) treatments. Matlab was used to analyse the dose-rate term as a function of dose-rate (0.001 – 3000 Gy/min) and of the probability of sublethal lesions converted to lethal lesions (β2). The influence of G on breast tumours response was evaluate in the survival fraction (SF) and in the tumour control probability (TCP). The classic LQ model was used comparatively for standart fractionation and hypofractionation. G present 42% maximum variation with β2 ranging between 0-1, presenting a plateau from 0.1 Gy/min for all β2. The dose range for a 50% TCP was 47 to 62 Gy and 39 to 62 Gy, for standart and hypofractionation protocols, respectively. The broadening of the control window seen between protocols, however, is due to the delivered dose between treatments. The dose-rate term in gLQ model aims to correct the cells response for dose-rate effects. However, its modeling does not have sensitivity for dose-rates above to 0.1 Gy/min. Hence, a correction is require in gLQ model for higher dose-rates.