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OCR for page 34
Characterizing Exposure ... Final Report
It became evident during the conduct of the study that the project would not be possible,
because of the difficulty of identifying specific people for study. As delineated in the discussion
of Projects ~ and 3, initial attempts to use NPRC uncovered substantial barriers to data access
and poor-quality data. The Columbia University researchers concluded that those problems
could not be resolved soon enough to pursue this project, and with the assent of the
committee it was abandoned.
The researchers concluded, more generally, that it was unlikely that such biomarkers as
TCDD will play a major role as quantitative exposure surrogates in future studies of the health of
Vietnam veterans. They stated that the most important reason is that the time that has elapsed
since spraying is many multiples of the half-life of dioxin in human serum or adipose tissue; any
future assays will thus almost certainly be dominated by false negatives. Additional constraints
include high cost, lack of biomarkers for non-TCDD-contaminated herbicides, and the potential
for selection bias. A more detailed discussion of the issue of using biomarkers in validating
exposure-assessment models is in the committee's interim report (IOM, 2003~.
PROJECT 5:
ANALYSIS OF IARC TISSUE SAMPLES OF SELECTED VIETNAMESE FOR DIOXIN
AND DIBENZOFURAN LEVELS IN ARCHIVED ADIPOSE TISSUE
This project was intended to assess the correlation between the exposure opportunity index
(the E4 ROT) developed in Project ~ and a direct measurement of dioxin in the adipose tissue of
Vietnamese. It was suspected that this population might have higher body burdens of dioxin than
US Vietnam veterans by virtue of having lived for long periods in areas that were sprayed.
The project was carried out in collaboration with the World Health Organization's
International Agency for Research on Cancer in Lyon, France. Since 1993, lARC has been
carrying out a case-control study of soft-tissue sarcoma and non-Hodgkin lymphoma in Ho Chi
34
OCR for page 35
Characterizing Exposure ... Final! Report
Minh City (formerly Saigon). The study is hospital-based at the Oncology Centre of Ho Chi
Minh City, which treats 5,000 inpatients per year, one-third of whom are residents of Ho Chi
Minh City and the remainder from elsewhere in Southern Vietnam. Steven Stellman was
collaborating with the {ARC researchers on this study before the start of the exposure-assessment
research effort.
{ARC researchers have published two papers addressing the research (Verger, et al., 1994;
Kramarova, et al., 19981. Exposure-opportunity scores used in those papers were calculated by
using an earlier formulation of the EO] developed by the Columbia University researchers
(Stellman and Stellman, ~ 9864.
In March 1999, the lARC study manager visited Vietnam to attempt to resolve some of the
data-quality issues that had arisen in the work. Her trip report raised questions for the Columbia
University researchers regarding the reliability of some subjects' residence data. The database
was reviewed, and the researchers chose to analyze data on 266 subjects whose residential data
appeared most reliable. Those data were geocoded,~9 EOIs were estimated, and the subjects
were rank-ordered from highest to lowest putative exposure.
IARC shipped two batches of 25 adipose tissue samples each—chosen randomly from
among the highest and lowest EOI estimates—to the Midwest Research Institute, a US analytic
testing laboratory. However, the laboratory received only one batch; the other was lost in transit
and could not be relocated by the shipper. On arrival, sample identifications were verified against
the chain of custody and stored. Standard laboratory quality control and sample blinding were
applied. Dioxin congeners in all samples were found to be below the limit of detection or at
background concentrations.
The residential coding problems and the preliminary experience with dioxin testing
suggested to the researchers that further testing of samples from the lARC archive was unlikely
to generate informative results. In consultation with the committee, they chose to end the
project.
i9 That is, the location in geographic space was converted into computer-readable form.
I ~
Representative terms from entire chapter:
university researchers
