Parameters of the Lyman Model for Calculation of Normal-Tissue Complication Probability: A Systematic Literature Review

Abstract

Purpose

The Lyman model is one of the most used radiobiological models for calculation of Normal Tissue Complication Probability (NTCP). Since its introduction in 1985 many authors have published parameter values for the model based on clinical data of different radiotherapeutic situations. We attempted to collect the entirety of radiobiological parameter sets published until now and to provide an overview of the data basis for different variations of the model. Furthermore, we sought to compare the parameter values and calculated NTCPs for selected endpoints with sufficient data available.

Methods and Materials

A systematic literature analysis was performed searching for publications that provided parameters for the different variations of the Lyman model in the Medline database using PubMed. Parameter sets were grouped into 13 toxicity-related endpoint groups. For three selected endpoint groups "reduction of saliva ≤ 25% twelve months after irradiation of the parotid", "symptomatic pneumonitis after irradiation of the lung" and "bleeding ≥ grade 2 after irradiation of the rectum", we compared parameter values and analyzed differences in calculated NTCP values.

Results

A total of 509 parameter sets from 130 publications were identified. We detected considerable heterogeneities regarding the number of parameters available for different radiooncological situations. Furthermore, for the three selected endpoints we found large differences in published parameter values. These translate into great variations of calculated NTCPs, with maximum ranges of 35.2%-93.4% for the saliva endpoint, of 39.4%-90.4% for the pneumonitis endpoint and of 5.4%-99.3% for the rectal bleeding endpoint.

Conclusions

The detected heterogeneity of the data basis as well as the large variations of published radiobiological parameters underline the necessity for careful interpretation when using such parameters for NTCP calculations. Appropriate selection of parameters as well as validation of values is essential when using the Lyman model.