Extrapolation of Normal Tissue Complication Probability for Different Fractionations in Liver Irradiation

Purpose: The ability to predict normal tissue complication probability (NTCP) is essential for NTCP-based treatment planning. The purpose of this work is to estimate the Lyman NTCP model parameters for liver irradiation from published clinical data of different fractionation regimens. A new expression of normalized total dose (NTD) is proposed to convert NTCP data between different treatment schemes. Method and Materials: The NTCP data of radiation- induced liver disease (RILD) from external beam radiation therapy for primary liver cancer patients were selected for analysis. The data were collected from 4 institutions for tumor sizes in the range of of 8-10 cm. The dose per fraction ranged from 1.5 Gy to 6 Gy. A modified linear-quadratic model with two components corresponding to radiosensitive and radioresistant cells in the normal liver tissue was proposed to understand the new NTD formalism. Results: There are five parameters in the model: TD50, m, n, α/β and f. With two parameters n and α/β fixed to be 1.0 and 2.0 Gy, respectively, the extracted parameters from the fitting are TD50(1) = 40.3 ± 8.4Gy, m =0.36 ± 0.09, f = 0.156 ± 0.074 Gy and TD50(1) = 23.9 ± 5.3Gy, m = 0.41 ± 0.15, f = 0.0 ± 0.04 Gy for patients with liver cirrhosis scores of Child-Pugh A and Child-Pugh B, respectively. The fitting results showed that the liver cirrhosis score significantly affects fractional dose dependence of NTD. Conclusion: The Lyman parameters generated presently and the new form of NTD may be used to predict NTCP for treatment planning of innovative liver irradiation with different fractionations, such as hypofractioned stereotactic body radiation therapy.