Linear quadratic model--estimation of alpha/beta ratio for mucosal reaction.

In radiotherapy to ensure that appropriate tumor lethal dose is delivered without exceeding normal tissue tolerance, concepts like NSD and its derivatives were used in the past. Due to their short coming these concepts are now replaced by a linear quadratic model. As this model is based on the recent radiobiological data, it is claimed to be able to predict the biological responses of tumors as well as normal tissues more accurately. However, the accuracy of prediction depends on the precise values of various parameters such as alpha/beta, repair constant mu and repopulation factor that are involved in the linear quadratic model. For most of the tumors and normal tissues, values of the parameters are not yet accurately known. We have estimated alpha/beta value for mucosal reactions, i.e. normal tissue early reactions in head neck cases treated with twice daily fractionation schedules and observed it to be 7.90 Gy for slight mucositis whereas it is 7.68 and 8.11 Gy for patchy and confluent mucositis, respectively.