Estimation of linear quadratic (LQ) model parameter alpha/beta (α/β) and biologically effective dose (BED) for acute normal tissue reactions in head and neck malignancies

Purpose: Linear-Quadratic (LQ) model has been widely used for describing radiobiological effectiveness of various fractionation schedules on tumour as well as normal tissues. This study estimates α/β for acute normal tissue reactions using Fe-plot method. Methods: 50 cases of locally advanced head and neck squamous cell carcinoma (stage III and IV) treated with external beam radiotherapy were included in this study. Patients were randomly distributed into Hyper-fractionation (HF) arm (1.2 Gy/fraction, twice daily, 6 hours apart) and conventional fractionation (CF) arm (2 Gy/fraction, once daily) with 25 cases in each arm. α/β and BED were calculated for acute normal tissue reactions using Fe-plot method. Results: In our study, the estimated values of α/β for RTOG (Radiation Therapy Oncology Group) grade 1, 2 and 3 skin reactions were 11.2 Gy, 10.1 Gy and 9 Gy respectively. Estimated values of α/β for RTOG grade 1, 2 and 3 mucosal reactions were 9.7 Gy, 8.0 Gy and 9.1 Gy respectively. For Hyper-fractionation arm, calculated BED values for grade 1, 2 and 3 skin reactions were 54.45 Gy 11.239 , 66.90 Gy 10.114 and 73.43Gy 9.001 respectively and for grade 1, 2 and 3 mucosal reactions were 33.5 Gy 9.797 , 57.8 Gy 8.011 and 70.8 Gy 9.106 respectively. For conventional fractionation arm, calculated BED values for grade 1, 2 and 3 skin reactions were 54.09 Gy 11.239 , 66.88 Gy 10.114 and 73.33 Gy 9.001 respectively and for grade 1, 2 and 3 mucosal reactions were 33.52 Gy 9.797 , 57.68 Gy 8.011 and 70.73 Gy 9.106 respectively. Conclusion: LQ model and the concept of BED provide an excellent tool to compare different fractionation schedules in radiotherapy. The estimated values of α/β for acute reacting normal tissues are in good agreement with the available literature.