Source of Support: None, Conflict of Interest: None
The NSD Concept was modified to TDF and TSD for normal tissues and tumour regressions respectively. However these concepts were empirical and TDF was not accurately applicable to late effects of normal tissues. Recent linear quadratic model predicts not only early and late reactions of normal tissues but also tumour control. Late stages of cancers of head and neck were treated with either 2.0, 2.17 or 2.2Gy per fraction five times a week for a total dose of 60, 65 and 55Gy respectively. The patients were inspected weekly by two radiotherapists independently to evaluate the normal tissue reactions and tumour regressions. ERD, for normal tissue reactions were evaluated for a/B = l0 and ERD, for tumour regressions with a/p=' 12Gy. The normal tissue reactions were plotted against ERD, and TDFs which show good fit to the data and confirm the value of sip = 10 and 12Gy and 100 TDF for the tolerance values. Tumour regressions Vs. ERD, and TSD data indicate the value of alp to be 12Gy.