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Volume 11, Number 2, 2006

Isoeffect calculations based on linear quadratic equations for head and neck cancers

S. Manimaran 1, V. Ramasubramanian 1, K. Thayalan 2


Background The linear quadratic model has led to various methods for the calculation of isoeffect relationships in radiotherapy. In this model, the tissue sensitive parameters a and b usually appear as a ratio, a/b. These parameters are used to describe the response of normal tissues to radiation insult. Different radiation induced biological end points in specific tissues and organs are associated with the characteristics of the a/b ratio. The linear quadratic model has been used clinically to address questions relating to changes in fractions in treatment schedules.
Aim The process of treating cancer with ionizing radiation is complex and subject to dosimetric errors which may potentially result in early or late complications. Our objective was to correct such errors through the application of the incomplete repair linear quadratic model.
Materials/Methods Repair mechanisms are affected if, owing to dosimetric error, excess dose is delivered in single or multiple fractions. Corrections for such errors were simulated, for different clinical situations, in order to avoid late fibrosis in head and neck cancers.
Results NSD, CRE, and TDF approach could not predict, onset of proliferation, overall treatment time, late and early complications, but linear quadratic model calculations predicts isoeffective schedules successfully with above parameters.
Conclusions In head and neck cancers, a number of parameters influence the results of treatment. Isoeffect calculations show the risk factors responsible for fi brosis and spinal cord damage and therefore may be used to calculate dose reductions for all remaining fractions, rather than applying shielding.

Signature: Rep Pract Oncol Radiother, 2006; 11(2) : 91-95


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