TY - JOUR
T1 - Comments on a time-dependent version of the linear-quadratic model
AU - Tucker, Susan L.
AU - Travis, Elizabeth L.
N1 - Funding Information:
This work was supported in part by grants CA1 1430, CA29026, CA38106 and CA48672 from the National Cancer Institute, The National Institutes of Health, U.S.A. The authors thank Ms. Connie Seifert for her expert assistance in preparing this manuscript.
PY - 1990/6
Y1 - 1990/6
N2 - The accuracy and interpretation of the "LQ + time" model (E = D(α + βd) - γT) are discussed. Evidence is presented, based on data in the literature, that this model does not accurately describe the changes in isoeffect dose occurring with protraction of the overall treatment time during fractionated irradiation of the lung. This lack of fit of the model explains, in part, the surprisingly large values of γ/α that have been derived from experimental lung data. The large apparent time factors for lung suggested by the model are also partly explained by the fact that γT/α, despite having units of dose, actually measures the influence of treatment time on the effect scale, not the dose scale, and is shown to consistently overestimate the change in total dose. The unusually high values of α/β that have been derived for lung using the model ( ≈5 Gy) are shown to be influenced by the method by which the model was fitted to data. Reanalyses of the data using a more statistically valid regression procedure produce estimates of α/β more typical of those usually cited for lung ( ≈ 3 Gy). Most importantly, published isoeffect data from lung indicate that the true deviation from the linear-quadratic (LQ) model is nonlinear in time, instead of linear, and also depends on other factors such as the effect level and the size of dose per fraction. Thus, we do not advocate the use of the "LQ + time" expression as a general isoeffect model.
AB - The accuracy and interpretation of the "LQ + time" model (E = D(α + βd) - γT) are discussed. Evidence is presented, based on data in the literature, that this model does not accurately describe the changes in isoeffect dose occurring with protraction of the overall treatment time during fractionated irradiation of the lung. This lack of fit of the model explains, in part, the surprisingly large values of γ/α that have been derived from experimental lung data. The large apparent time factors for lung suggested by the model are also partly explained by the fact that γT/α, despite having units of dose, actually measures the influence of treatment time on the effect scale, not the dose scale, and is shown to consistently overestimate the change in total dose. The unusually high values of α/β that have been derived for lung using the model ( ≈5 Gy) are shown to be influenced by the method by which the model was fitted to data. Reanalyses of the data using a more statistically valid regression procedure produce estimates of α/β more typical of those usually cited for lung ( ≈ 3 Gy). Most importantly, published isoeffect data from lung indicate that the true deviation from the linear-quadratic (LQ) model is nonlinear in time, instead of linear, and also depends on other factors such as the effect level and the size of dose per fraction. Thus, we do not advocate the use of the "LQ + time" expression as a general isoeffect model.
KW - Effect of treatment time
KW - Fractionated irradiation
KW - Linear-quadratic model
KW - Lung response
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U2 - 10.1016/0167-8140(90)90141-I
DO - 10.1016/0167-8140(90)90141-I
M3 - Article
C2 - 2367690
AN - SCOPUS:0025360034
SN - 0167-8140
VL - 18
SP - 155
EP - 163
JO - Radiotherapy and Oncology
JF - Radiotherapy and Oncology
IS - 2
ER -