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Linear quadratic modeling of increased late normal-tissue effects in special clinical situations
Bleddyn Jones1, Roger G Dale, Andrew M Gaya
1Birmingham Cancer Centre, Queen Elizabeth University Hospital, Birmingham, UK. b.jones.1@bham.ac.uk
Purpose:
To extend linear quadratic theory to allow changes in normal-tissue radiation tolerance after exposure to cytotoxic chemotherapy, after surgery, and in elderly patients.
Methods:
Examples of these situations are analyzed by use of the biologic effective dose (BED) concept. Changes in tolerance can be allowed for by: estimation of either the contribution of the additional factor as an equivalent BED or the equivalent dose in 2-Gy fractions or by the degree of radiosensitization by a mean dose-modifying factor (x).
Results:
The estimated x value is 1.063 (95% confidence limits for the mean, 1.056 to 1.070) for subcutaneous fibrosis after cyclophosphamide, methotrexate, and fluorouracil (CMF) chemotherapy and radiotherapy in breast cancer. The point estimate of x is 1.18 for the additional risk of gastrointestinal late-radiation effects after abdominal surgery in lymphoma patients (or 10.62 Gy at 2 Gy per fraction). For shoulder fibrosis in patients older than 60 years after breast and nodal irradiation, x is estimated to be 1.033 (95% confidence limits for the mean, 1.028 to 1.0385). The equivalent BED values were CMF chemotherapy (6.48 Gy3), surgery (17.73 Gy3), and age (3.61 Gy3).
Conclusions:
The LQ model can, in principle, be extended to quantify reduced normal-tissue tolerance in special clinical situations.
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