Estimate of Radiobiologic Parameters From Clinical Data for Biologically Based Treatment Planning for Liver Irradiation

Abstract

The Radiation Therapy Oncology Group (RTOG) is initiating a few new hypofractionation regimens (RTOG 0438) to treat liver cancer patients. To evaluate the radiobiologic equivalence between different regimens requires reliable radiobiologic parameters. The purpose of this work is to estimate a plausible set of such parameters for liver tumors and to design new optimized dose fractionation schemes to increase patient survival. A model was developed to fit clinical survival data from irradiation of a series of primary liver patients. The model consists of six parameters including radiosensitivity parameters alpha and alpha/beta, potential doubling time T(d). Using this model together with the Lyman model for calculations of the normal tissue complication probability, we designed a series of hypofractionated treatment strategies for liver irradiation. The radiobiologic parameters for liver tumors were estimated to be: alpha/beta = 15.0 +/- 2.0 Gy, alpha = 0.010 +/- 0.001 Gy (-1), T(d) = 128 +/- 12 day. By calculating the biologically effective dose using the obtained parameters, it is found that for liver patients with an effective liver volume of approximately 45% the dose fractionation regimens suggested in RTOG 0438 can be escalated to higher dose for improved patient survival ( approximately 80% at 1 year) while keeping the normal tissue complication probability to less than 10%. A plausible set of radiobiologic parameters has been obtained based on clinical data. These parameters may be used for radiation treatment planning of liver tumors, in particular, for the design of new treatment regimens aimed at dose escalation.