• Karen Lim, Philip Chan, Robert Dinniwell, Anthony Fyles, Masoom Haider, Young-Bin Cho, David Jaffray, Lee Manchul, Wilfred Levin, Richard P Hill, and Michael Milosevic.
• Radiation Medicine Program, Princess Margaret Hospital/Ontario Cancer Institute, University Health Network, Toronto, ON, Canada.
• Int. J. Radiat. Oncol. Biol. Phys. 2008 Jan 1; 70 (1): 126-33.

PurposeTo measure regression of cancer of the uterine cervix during external beam radiotherapy using magnetic resonance imaging, derive radiobiologic parameters from a mathematical model of tumor regression, and compare these parameters with the pretreatment measurements of tumor hypoxia.Methods And MaterialsA total of 27 eligible patients undergoing external beam radiotherapy for cervical cancer underwent weekly magnetic resonance imaging scans. The tumor volume was assessed on each of these scans and the rate of regression plotted. A radiobiologic model was formulated to simulate the effect on tumor regression of the surviving proportion of cells after 2 Gy (SP(2)), the cell clearance constant (clearance of irreparably damaged cells from the tumor [T(c)]), and accelerated repopulation. Nonlinear regression analysis was used to fit the radiobiologic model to the magnetic resonance imaging-derived tumor volumes and to derive the estimates of SP(2) and T(c) for each patient. These were compared to the pretreatment hypoxia measurements.ResultsThe initial tumor volume was 8-209 cm(3). The relative reduction in volume during treatment was 0.02-0.79. The simulations using representative values of the independent biologic variables derived from published data showed SP(2) and T(c) to strongly influence the shape of the volume-response curves. Nonlinear regression analysis yielded a median SP(2) of 0.71 and median T(c) of 10 days. Tumors with a high SP(2) >0.71 were significantly more hypoxic at diagnosis (p = 0.02).ConclusionThe results of our study have shown that cervical cancer regresses during external beam radiotherapy, although marked variability is present among patients and is influenced by underlying biologic processes, including cellular sensitivity to radiotherapy and proliferation. Better understanding of the biologic mechanisms might facilitate novel adaptive treatment strategies in future studies.

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