Quantifying association between liver tumor incidence and early-stage liver weight increase - An NTP data analysis

Two-year toxicology and carcinogenesis rodent studies conducted at the National Toxicology Program (NTP) are used to identify potential adverse health effects in humans due to chemical exposure, including cancer. Liver tumor, the most frequently diagnosed tumor type of chemically induced neoplastic effects documented in NTP's carcinogenicity studies, is usually difficult to be detected at early stage due to the inconspicuous symptoms. However, the abnormal growth of liver cells can lead to liver weight increase, so it is hypothesized that liver tumor incidence is associated with early stage liver weight increase. In this study, the association between liver weight increase and liver tumor incidence are quantified by (1) calculating the correlation coefficient of and (2) building quantitative linear relationship between benchmark dose estimates derived from these two types of data collected from NTP studies. Together with 151 chemical/species/sex combinations of liver tumor data showing positive evidence collected from 76 NTP long-term studies, short-term liver weight data reported in the same NTP report were extracted to be paired with the liver tumor data for the analyses. Results show that the estimated correlation coefficients (as high as 0.78) along with the adequately fitted linear models suggest that the association between relative liver weight increase and aggregated liver tumor incidence are relatively strong. Additional analyses focused on some more specific situations (e.g., specific tumor type or specific strain/sex combination) further confirmed the strong association. Given the design of this study, the interpretation of the findings is not that liver weight increase can be used to predict liver tumor incidence, instead, evident increase in liver weight might be used as a reason to prioritize the test article for a two-year toxicology and carcinogenesis study.