Assessment of the Occurrence of Cancer
Freeport, Texas
2000-2015
May 2, 2018

Prepared by the
Texas Department of State Health Services

Investigation# 17006

 Table of Contents
Executive Summary .......................................................................... 1
Background ...................................................................................... 2
Methods ............................................................................................ 2
Data Sources .................................................................................... 3
Statistical Analysis ............................................................................. 3
Results ............................................................................................. 4
Table 1. ............................................................................................ 5
Discussion ........................................................................................ 5
Additional Information ..................................................................... 6
Figure 1. ........................................................................................... 7

Investigation# 17006

 Executive Summary
In 2010 and 2012, community concern prompted the Environmental
Surveillance and Toxicology Branch (ESTB) and Texas Cancer Registry (TCR)
of the Texas Department of State Health Services (DSHS) to examine the
occurrence of cancer in Freeport, Texas. Due to ongoing concerns, DSHS
conducted a follow-up analysis of cancers in the area, which is described in
this report.
DSHS followed the Centers for Disease Control and Prevention (CDC) and
Council of State and Territorial Epidemiologists (CSTE) 2013 guidelines and
agency protocol to investigate the occurrence of 11 types of all-age cancers
in a geographic area selected in collaboration with community members. In
accordance with these guidelines, the purpose of this assessment was to
determine whether the observed number of cancer cases is statistically
significantly greater than expected. It was not intended to determine the
cause of the observed cancers or identify possible associations with any risk
factors.
DSHS staff analyzed TCR data available for a 16-year period spanning from
2000 to 2015. United States Census data was used to estimate the
population in the selected geographic area, which consisted of six census
tracts. To evaluate the occurrence of cancer in the area investigated, the
number of observed cancer cases was compared to what would be expected
for the area based on cancer rates in Texas. Standardized incidence ratios
(SIRs) were calculated as the number of observed cases divided by the
number of expected cases in the area of concern for the 16-year period
(2000-2015). A 95 percent confidence interval (CI) was calculated for each
SIR to determine statistical significance.
The number of all-age leukemias was below the range of what is expected
based on cancer rates in Texas. The number of all-age liver and intrahepatic
bile duct, lung, nasopharynx/nose/nasal cavity and middle ear, and stomach
cancers was above the range of what is expected based on cancer rates in
Texas.

Investigation# 17006

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 Background
In 2010 and 2012, community concern prompted the Environmental
Surveillance and Toxicology Branch (ESTB) and Texas Cancer Registry (TCR)
of the Texas Department of State Health Services (DSHS) to examine the
occurrence of cancer in Freeport, Texas. Local residents were concerned
about emissions from a local catalyst recycling/reclaiming facility. Due to
ongoing community concerns, DSHS conducted a follow-up analysis of
cancers in the area, which is described in this report.
The Centers for Disease Control and Prevention (CDC) and Council of State
and Territorial Epidemiologists (CSTE) define a cancer cluster 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 time1. DSHS followed the CDC
and CSTE 2013 Guidelines for Investigating Suspected Cancer Clusters and
Responding to Community Concerns 1 and agency protocol 2 to investigate the
occurrence of cancer in this community.
The CDC and CSTE guidelines include four steps1. The first step is to collect
information about the community’s concerns. The second step, reported
here, is to determine whether the observed number of cancer cases is
statistically significantly greater than expected. It is important to note that
the data and statistical analysis conducted at this step cannot determine if
cancers observed in the community are associated with environmental,
lifestyle, or other risk factors.
The guidelines also provide additional steps that can be followed when
appropriate. The third step is to evaluate the feasibility of performing an
epidemiologic study to examine if exposure to a specific risk factor is
associated with the suspected cancer cluster, and the fourth step is to
conduct an epidemiologic study, if deemed feasible in step three. Many
factors are considered in making the determination to progress to steps
three or four. The CDC and CSTE guidelines state, “only a small fraction of
cancer cluster inquiries might meet the statistical and etiological criteria to
support a cluster investigation through all the steps outlined….”1
Methods
Consistent with the CDC and CSTE guidelines, DSHS collaborated with the
community to select the geographic area, time frame, and cancers to be
1

2

Centers for Disease Control and Prevention, Investigating Suspected Cancer Clusters and Responding to
Community Concerns. MMWR, 2013. 62: p. 22.
Texas Department of State Health Services, Protocol for Responding to Community Cancer Cluster Concerns.
Updated January 15, 2016. Available from: http://www.dshs.texas.gov/epitox/CancerClusters/Protocol-forResponding-to-Community-Cancer-Cluster-Concerns.pdf.

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 included in this analysis. The following all-age cancer types were included in
the analysis: bladder, brain and other nervous system, Hodgkin lymphoma,
kidney and renal pelvis, leukemia, liver and intrahepatic bile duct, lung,
nasopharynx/nose/nasal cavity and middle ear, non-Hodgkin lymphoma,
prostate, and stomach cancer. Community members also requested that
DSHS analyze childhood leukemias alone. However, because there were less
than six cases of childhood leukemia observed in the area of concern during
this time frame, these cancers could not be included in the analysis per
agency protocol. Complete TCR cancer data are available for 1995 to 2015.
DSHS evaluated 16 years of available cancer data in accordance with
community concerns. The geographic area investigated was selected to
encompass the entire area of concern. The six census tracts comprising the
area investigated are shown in Figure 1.
This document outlines the results from step two of the CDC and CSTE
guidelines, and only addresses the question, “Is there a statistically
significant excess of cancer in the area of investigation?”
Data Sources
For each cancer type, the number of cases observed from 2000 to 2015 in
the area included in the investigation was obtained from the TCR (Incidence
– Texas, 1995-2015, SEER*Prep 2.5.3). The TCR is responsible for the
collection, maintenance, and dissemination of high-quality Texas populationbased cancer data, and meets national CDC timeliness and data quality
standards, as well as North American Association of Central Cancer Registry
certification standards. All-age cancers were defined according to Site
Recode ICD-O-3/WHO 2008 Definitions 3. Statewide cancer rates for the
same time period were also obtained from the TCR.
Population estimates for 2000 to 2015 were calculated using linear
interpolation based on population counts obtained from the United States
Decennial Census 4 for the years 2000 to 2010. This method, outlined by the
United States Census Bureau 5, assumed population growth occurred in a
linear manner.
Statistical Analysis
To determine if a statistically significant excess of cancer existed in the area
investigated, the number of observed cancer cases was compared to what
3 National Cancer Institute, Surveillance, Epidemiology and End Results Program. Site Recode ICD-O-3/WHO 2008
Definition. Available online: http://seer.cancer.gov/siterecode/icdo3_dwhoheme/index.html
4

United States Census Bureau. American FactFinder. 2012; Available from:
http://factfinder2.census.gov/faces/nav/jsf/pages/index.xhtml

5

US Census Bureau. Methodology for the Intercensal Population and Housing Unit Estimates: 2000 to 2010. 2012;
Available from: https://www2.census.gov/programs-surveys/popest/technicaldocumentation/methodology/intercensal/2000-2010-intercensal-estimates-methodology.pdf

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 would be expected for the area based on cancer rates in Texas.
Characteristics such as race, sex, and age are closely related to cancer. To
ensure that differences between the numbers of observed and expected
cancer cases are not simply due to differences in these demographic
characteristics, the expected numbers of cancer cases were calculated by
multiplying the age-, sex-, and race-specific cancer incidence rates of Texas
residents (reference population) by the number of people in the
corresponding demographic groups in the area of investigation.
Standardized incidence ratios (SIRs) were calculated to determine if an
excess of cancer exists in the area. The SIR is the number of observed cases
compared to (divided by) the number of expected cases for each cancer
type. A SIR greater than 1.00 indicates that the observed number of cases
of a specific cancer type is higher than expected and a SIR less than 1.00
indicates that the observed number of cases of a specific cancer type is
lower than expected.
Few, if any, communities will have exactly the same rate as the average
state rate for a similar population; most will be higher or lower. Therefore,
95 percent confidence intervals (CI) were calculated for the SIRs to
determine if the observed number of cases was statistically significantly
different than expected. If a 95 percent CI (range) includes 1.00, no
statistically significant excess (or reduction) of cancer is indicated. If a 95
percent CI does not contain 1.00, the SIR is outside the expected range and
is statistically significant. When using a 95 percent CI, 5 percent of SIR
values calculated is expected to be statistically significantly higher or lower
than the state average due to random chance alone.
In all cases, when results are described as significant or not significant,
DSHS is referring only to statistical significance, with the understanding that
all cases of cancer are significant to the individual, the family, and friends of
the individuals who are affected.
Results
Table 1 presents the number of observed cases, the number of expected
cases, the SIRs, and the corresponding 95 percent CIs for each cancer type
evaluated in the area of investigation. The number of all-age bladder, brain
and other nervous system, Hodgkin lymphoma, kidney and renal pelvis,
non-Hodgkin lymphoma, and prostate cancers was within the range of what
is expected based on cancer rates in Texas. The number of all-age leukemias
was below the range of what is expected based on cancer rates in Texas.
The number of all-age liver and intrahepatic bile duct, lung,
nasopharynx/nose/nasal cavity and middle ear, and stomach cancers was
above the range of what is expected based on cancer rates in Texas.
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 Table 1. Standardized Incidence Ratios (SIRs) and 95 percent Confidence
Intervals (CIs) for Selected All-Age Cancers in Freeport, TX, 2000-2015.
Cancer Type

Observed

Expected

SIR

95% CI

Bladder
Brain and other nervous system
Hodgkin lymphoma
Kidney and renal pelvis
Leukemia
Liver and intrahepatic bile duct
Lung
Nasopharynx/nose/nasal cavity
and middle ear
Non-Hodgkin lymphoma
Prostate
Stomach

57
20
11
59
31
46
291
9

46.9
22.7
8.7
58.3
45.0
32.2
179.3
3.8

1.22
0.88
1.26
1.01
0.69†
1.43*
1.62*
2.36*

(0.92,
(0.53,
(0.63,
(0.77,
(0.46,
(1.04,
(1.44,
(1.07,

69
193
34

58.2
180.4
21.8

1.19
1.07
1.56*

(0.92, 1.50)
(0.92, 1.23)
(1.08, 2.18)

1.57)
1.36)
2.26)
1.3)
0.98)
1.90)
1.82)
4.47)

*Indicates observed number of cancer cases is statistically significantly higher than expected
†Indicates observed number of cancer cases is statistically significantly lower than expected

Discussion
Consistent with the second step of the CDC and CSTE guidelines for
investigating suspected cancer clusters, the primary purpose of this step
(assessment) is to determine whether the observed number of cases is
statistically significantly greater than expected1. It is not intended to
determine the cause of the observed cancers or identify possible associations
with any risk factors.
The assessment step in a cancer cluster investigation has several inherent
limitations, and results should be interpreted with these limitations in mind.
Cancer is not a single disease, but rather many different diseases. Different
types of cancers vary in etiologies (causes or origins) and may not share the
same predisposing factors. Cancers may be associated with a variety of
factors such as genetics, lifestyle, and socioeconomic status. Because cancer
is common, cases might appear to occur with alarming frequencies within a
community even when the number of cases is within the expected rate for
the population.
Additionally, cancer incidence data are based on residence at the time of
diagnosis. As people move, it becomes more difficult to determine whether
living in the area of investigation is associated with an excess of cancers,
because residential history is not tracked. Latency (the time period elapsed
between exposure and illness onset) adds to the complexity of this step in
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 the investigation. For most adult cancers, a period of 10 to 40 years can
elapse between the beginning of an exposure to a cancer-causing agent and
the development of a clinically diagnosable case of cancer. It is possible that
former residents who developed cancer no longer lived in the area at the
time of diagnosis, and these cases would not be included in this assessment.
It is also possible that new people have moved into the area and then were
diagnosed with cancer; these cases are included in this assessment.
Conclusion
The observed number of liver and intrahepatic bile duct, lung,
nasopharynx/nose/nasal cavity and middle ear, and stomach cancers were
statistically significantly greater than expected. The observed number of allage leukemias was statistically significantly less than expected. The
observed numbers of the remaining six cancer types were within the range
of what was expected, based on cancer rates in Texas. DSHS will update this
report upon request when new data become available.
Additional Information
For additional information about cancer clusters, visit the Centers for
Disease Control and Prevention, “About Cancer Clusters,” web page at
http://www.cdc.gov/nceh/clusters/about.htm.
For additional information on cancer risk factors, visit the American Cancer
Society, “What Causes Cancer?” web page at
http://www.cancer.org/cancer/cancercauses/index.
Questions or comments regarding this investigation may be directed to the
DSHS Environmental Epidemiology and Toxicology Unit, 1-800-588-1248,
epitox@dshs.texas.gov.

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 Figure 1. Selected Census Tracts (2010) for Freeport, Texas.

 

 

 

 

 

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