Journal of Medical Physics
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Year : 1999  |  Volume : 24  |  Issue : 4  |  Page : 205-209

Application Of Lq + Time Bioeffect Dose Model To Spinal Cord Myelitis In Radiotherapy

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S Manimaran

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There is increasing evidence from animal and clinical studies suggesting that prolongation of overall treatment time (OTT) during radiotherapy is associated with a reduced probability of normal tissue complications, attributable to repopulation during the course of treatment. Radiation spinal cord myelitis limits the dose to tumours in the head and neck, thoracic and upper abdominal regions, resulting in reduction of tumour control probability. The bioeffect of a physical dose depends on the nature of the tissue, fractionation scheme, dose rate and treatment time. The absorbed dose needs to be translated into a bioeffect dose, which takes into account treatment varibles and the radiobiological characteristics of the relevant tissue. The purpose of the present study was to correlate Bioeffect Dose (BED) with probability of spinal cord myelitis, taking into account overall treatment time for human compiled retrospective clinical data. Clinical data of time dose fractionation schedules and incidence of myelitis for 3233 patients compiled from the data of Cohen (1981), Wara (1975), Abbatucci (1978) and Jeremic (1991) was used for the analysis. Radiotherapy schedules were 19 to 76 Gy in 2 to 46 fractions in 7 to 71 days with the dose per fraction of 0.96 to 10.0 Gy. BED values were evaluated with Orton's equation, with alp of 2.5 Gy, K of 0.45 Gylday and To of 0, 10 and 20 days. BED values indicated statistically significant correlation with probability of spinal cord myelitis for the four clinical situations considered i.e. no influence of OTT, influence of OTT with three different onset times of proliferation/repair of 0, 10 and 20 days. In order to limit the probability of spinal cord myelitis to acceptable level of 5.0%, tolerance BED values of 84, 81, 86 and 90 Gy were obtained for the four clinical situations. The BED concept of Linear Quadratic model is useful as a prognostic indicator for normal tissue late complication of spinal cord myelitis. However, its clinical validity needs to be tested with prospective clinical studies.

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