July 6th, 2017

This report is in response to several inquiries to the Oklahoma Central Cancer Registry, Oklahoma
State Department of Health in 2016-2017 requesting an investigation into the occurrence of
cancer in Bokoshe, Oklahoma. Please do not hesitate to contact us with any further questions or
concerns at 405-271-5601 or communications@health.ok.gov
Respectively submitted,
Oklahoma Central Cancer Registry
Oklahoma State Department of Health
1000 NE 10th street
Oklahoma city, OK 73117

 Summary Report of Cancer Incidence within Bokoshe
Background
The Oklahoma Central Cancer Registry (OCCR) collects information on all reported cases of cancer
diagnosed or treated among Oklahoma residents. Currently, there are eighteen years, diagnosis
years of 1997 through 2014, of complete data available for analysis. This data can be used to
determine if cancer is occurring at an unusually high rate in certain geographical areas throughout
the state. This data cannot determine a cause or assess exposure to any substance. The OCCR
conducts investigations of suspected cancer clusters upon request.
This report is in response to several inquiries concerning cancer incidence rates in and around
Bokoshe due to the disposal of fly ash at a site located just outside of Bokoshe. Bokoshe is located
in Le Flore County in eastern Oklahoma and is in the zip code 74930. The concerns were of both
the release of fly ash into the air during its transport, as well as the results of leachate into the
groundwater and potential subsequent contamination of drinking water sources for the residents.
A cancer cluster is defined as a greater than expected number of cancer cases that occurs within
a group of people in a geographic area over a defined period of time. Comparison with the
occurrence within the state population or similar community population defines if the
occurrence of cancer was greater than expected. In a cancer cluster, cases are the same type of
cancer or cancers with the same causes. Often, the cancer types are rare or occur in an
uncharacteristic demographic group. Consideration is given to the presence of a specific
environmental concern associated with the type of cancer.
Methods
In this analysis, cancer cases diagnosed between 2005 and 2014 were included. There is
typically a lag period of years to decades between exposure and cancer development.
Therefore, more current years of data may reflect exposures that have occurred over the past
decades. The data was split into five year time frames; 2005- 2009 and 2010-2014, to show
whether there were time trends among the cancer types of concern.
Standardized Incidence Ratio (SIR) analysis was used to determine if the occurrence of cancer in
Le Flore County and the 74930 were statistically higher than the occurrence of cancer in the state
of Oklahoma and whether it had changed over time. SIR analysis can determine if the number of
observed cancer cases in a particular geographic area is higher or lower than expected, given the
population and age distribution for the area of interest. The SIR was obtained by dividing the
observed number of cases of cancer by the “expected” number of cases. The expected number was
the number of cases that would occur in Le Flore County or 74930 if cancer occurred at the same
rate as in the state of Oklahoma (comparison population). The SIR takes into account whether a
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 community’s population is older or younger than the comparison population since cancer rates
increase strongly with age.
The Fisher exact test was used to calculate the 95% Confidence Intervals (CI) in OpenEpi.¹CI is the
range of rates that contains the actual rate 95 times out of 100. The fewer cases the rate is based
on, the greater the uncertainty, and therefore the wider the CI. The CI calculated around the SIR
are used to determine how likely it is that the number of observed number of cases is high or low
by chance. Specifically, if the CI includes 1.0, then the difference between the observed and
expected number of cases is likely to have occurred by chance. If the CI does not include 1.0, then
the difference between the observed and expected number of cases is not very likely to have
occurred by chance.
Results
There are two substances within fly ash that have been identified by multiple agencies as
carcinogenic to humans; arsenic and chromium VI. In occupational settings, chromium VI has
been shown to result in an increased risk of lung cancer if inhaled. For those exposed to
chromium VI in drinking water, there is some evidence of a possible increased risk for mortality
from stomach cancer.2 Studies have shown evidence that inhalation of arsenic in occupational
and some environmental settings may result in an increased risk of developing lung cancer.
When individuals are exposed to high concentrations of inorganic arsenic through ingestion,
there is evidence that suggests this may increase risk the development of skin cancer, and
possibly internal tumors, specifically bladder, kidney, liver, lung and prostate.3
Based on the published evidence, the OCCR reviewed the data for the following cancer types:
stomach, lung and bronchus, urinary tract, urinary bladder, kidney and renal pelvis, melanoma,
liver, and prostate.
The table below (Table 1.) displays SIR and 95% CIs calculated for the different cancer types at zip
code and county level. If the CI includes 1.0, then the difference between the observed and
expected number of cases is likely to have occurred by chance. If the CI does not include 1.0, then
the difference between the observed and expected number of cases is not very likely to have
occurred by chance. The fewer cases the rate is based on, the greater the uncertainty, and
therefore the wider the CI.

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 Table 1. Standardized incidence ratios (SIR) by cancer type, Le Flore County and Bokoshe
Zip Code: 74930, 2005-2014
Cancer type
Stomach
Lung & Bronchus
Urinary Tract
Urinary Bladder
Kidney & Renal Pelvis
Melanoma
Liver
Prostate

Le Flore County
2005-2009
SIR (95% CI)
0.91 (0.17-1.50)
1.07 (0.80-1.11)
0.82 (0.49-0.90)
0.71 (0.31-0.87)
1.00 (0.48-1.19)
0.93 (0.50-1.06)
0.61 (0.08-1.10)
0.31 (0.21-0.44)

Le Flore County
2010-2014
SIR (95% CI)
0.75 (0.10-1.35)
0.89 (0.63-0.94)
0.52 (0.27-0.61)
0.54 (0.63-0.94)
0.51 (0.18-0.69)
0.52 (0.23-0.63)
0.65 (0.12-1.08)
0.31 (0.20-0.47)

74930
2005-2009
SIR (95% CI)
0.94 (0.12-17.4)
0.95 (0.12-2.86)
0.63 (0.02-2.90)
0.08 (0.10-1.46)
0.92 (0.04-5.66)
1.57 (0.24-5.46)
0.80 (0.10-14.73)
0.24 (0.01-1.36)

74930
2010-2014
SIR (95% CI)
N/A
1.18 (0.14-4.26)
0.44 (0.02-4.09)
0.45 (0.06-8.28)
0.46 (0.06-8.56)
1.84 (0.37-8.55)
N/A
0.22 (.01-1.36)

Conclusion
In calculating the SIR and the 95% CIs for both Le Flore and Bokoshe zip code (74930) among
cancers associated with both arsenic and chromium VI, it was concluded that the rates are not
statistically higher than the state of Oklahoma, and it has not changed over time. The Oklahoma
State Department of Health takes inquiries into cancer rates elevation seriously and the OCCR is
committed to reassess the data in future years to determine if the incidence of cancer increases
over time.
References
1. Dean AG, Sullivan KM, Soe MM. OpenEpi: Open Source Epidemiologic Statistics for Public Health,
Version. www.OpenEpi.com, updated 2013/04/06, accessed 2017/05/19.
2. Agency for Toxic Substances and Disease Registry. (2012). Toxicological Profile for Chromium.
Retrieved from https://www.atsdr.cdc.gov/ToxProfiles/tp7.pdf
3. Agency for Toxic Substances and Disease Registry. (2012). Toxicological Profile for Arsenic. Retrieved
from https://www.atsdr.cdc.gov/toxprofiles/tp2.pdf

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