The California Cancer Registry (CCR) collects information about cancers diagnosed among California residents. This page explains how CCR responds when community members are concerned that cancer may be occurring more often than expected. CCR can evaluate patterns of cancer occurrence using population-based registry data. A registry review may identify an unusual pattern, but registry data alone usually cannot determine what caused individual cancers or whether a specific environmental exposure caused the pattern.
Whom should I contact with my questions or concerns about cancers in my community?
You may contact either your local health department or the regional registry covering the county where the community is located.
In most counties, the county health department should be contacted first. These departments will usually be familiar with local issues, and you may find that officials are already aware of the area you are concerned about. County health departments may respond to community cancer concerns themselves or refer callers to the regional registry depending upon the resources available in each county.
When contacting your regional registry, please include the community or geographic area of concern; the cancer type(s) or type(s); the approximate ages or age groups of the people affected; the years or time period of concern; and the reason you believe the pattern may be unusual. You may also describe any environmental or other concerns that prompted your inquiry.
Please do not email names, medical records, exact addresses, or other personally identifiable health information.
How does CCR evaluate a community cancer concern?
CCR reviews each inquiry to determine the cancer type (s), age groups, geographic area, time period, and any reported environmental concerns. Some questions can be addressed with existing information. Other concerns may require a more detailed registry review.
CCR’s approach is informed by and aligned with the Centers for Disease Control and Prevention (CDC) and the Agency for Toxic Substances and Disease Registry (ATSDR) Guidelines for Examining Unusual Patterns of Cancer and Environmental Concerns. These guidelines provide a systematic framework for documenting and responding to community concerns, determining whether additional assessment is warranted, and examining unusual patterns of cancer.
When a statistical assessment is warranted and feasible, CCR and the appropriate regional cancer registry may define the study population, geographic area, cancer type (s), and time period; verify registry-confirmed cases; estimate the expected number of cases using an appropriate comparison population; and compare the observed and expected numbers. The assessment may include a standardized incidence ratio, confidence intervals, and a descriptive review of time, geography, age, sex, race, and ethnicity, tumor characteristics, and other relevant factors.
Results are interpreted using scientific judgment. Statistical significance is one consideration, but it is not the only criterion. CCR also considers the size and precision of the estimate, the rarity and biological relationship of the cancers, the geographic and temporal pattern, latency, population mobility, potential exposure information, data quality, and community context.
Not every inquiry will lead to a formal statistical analysis. The scope and timeline depend on the concern, the availability and completeness of cancer and population data, the need to verify cases, the complexity of the analysis, and the required scientific review.
How can I find out more about a particular cancer?
To explore California cancer statistics, visit CAL*Explorer, an interactive website that uses California Cancer Registry data. CAL*Explorer allows users to view cancer incidence, mortality, and survival statistics; examine patterns by age, sex, race, and ethnicity, county or region, and other characteristics; and create or download customized graphs and tables.
For questions about California cancer statistics or the California Cancer Registry, please contact cdsrbhelp@cdph.ca.gov
Does the environment cause cancer?
Some environmental and occupational exposures can increase cancer risk. Cancer usually develops through a complex interaction of factors that may include age, inherited susceptibility, infections, behaviors, medical exposures, occupational exposures, social conditions, and contaminants in air, water, soil, food, or consumer products. The strength of the scientific evidence differs by cancer type and exposure.
An elevated cancer rate does not, by itself, show that an environmental exposure caused the cancers. CCR data generally identify where a person lived at the time of diagnosis, but they do not contain complete residential, school, occupational, behavioral, genetic, or environmental-exposure histories.
Evaluation of cancer incidence and evaluation of environmental conditions are related but distinct activities. If a concern involves a specific exposure, site, contaminant, or pathway, CCR may coordinate with or refer the matter to the appropriate local, state, or federal public health, environmental, or regulatory agency.
What if pets or other animals in the community have cancer or other illnesses?
California does not maintain a statewide, population-based animal cancer registry comparable to the CCR. Without consistent information about animal diagnoses and the number and characteristics of animals at risk, public-health officials generally cannot determine whether animal cancers in a neighborhood are occurring more often than expected.
Reports of cancer or illness in animals do not, by themselves, establish a human health risk or a shared environmental cause. Pet owners should consult a veterinarian about an individual animal. A community member may include specific animal-health and environmental information when reporting a community concern so that it can be considered or referred to the appropriate agency.
Why do population characteristics matter when cancer rates are compared?
Cancer occurrence varies by age, sex, race, and ethnicity, inherited susceptibility, screening and diagnostic practices, access to health care, occupation, environmental and medical exposures, behaviors, and social and economic conditions. The distribution of these factors differs across communities.
When possible, CCR accounts for available demographic differences, commonly age and sex, and sometimes race and ethnicity, when comparing observed and expected cancer incidence. Adjustment improves comparability, but it does not identify the cause of an observed difference and cannot account for factors that are not available in registry or population da
Why are cancer rates higher in some areas than others?
Cancer rates may differ across places or time periods for several reasons, and more than one reason may apply:
- Differences in the age and other characteristics of the population.
- Random variation, especially when the population or number of cases is small.
- Differences in screening, diagnosis, access to care, or completeness and timing of reporting.
- Differences in known or suspected risk factors, including behavioral, occupational, medical, social, genetic, infectious, and environmental factors.
- Population movement and differences between where an exposure may have occurred and where a person lived when diagnosed.
- Differences in the geographic boundaries, time periods, cancer definitions, or comparison populations used in the analysis.
A higher rate is a signal that requires interpretation; it does not, by itself, identify the reason for the difference.
Several people in my neighborhood have been diagnosed with cancer. Is this unusual?
It is not possible to determine whether a pattern is unusual from the number of people reported alone. A meaningful assessment requires a clearly defined population, geographic area, time period, cancer type or group of related cancers, and an estimate of how many cases would normally be expected.
Cancer is common, and different cancer types can have different risk factors and causes. Several people living near one another may have unrelated cancers, and cancer counts can vary from place to place and year to year by chance. At the same time, reports of rare, similar, or otherwise concerning cancers may warrant further review. Community members are encouraged to report a concern so that CCR can determine what information and assessment are appropriate.
What does it mean when cancer incidence is higher than expected?
CCR may compare the number of cancers observed in a defined population with the number expected if that population experienced the cancer rates of an appropriate comparison population. One commonly used measure is the standardized incidence ratio (SIR), calculated as the observed number of cases divided by the expected number of cases.
An SIR greater than 1.0 means that more cases were observed than expected; an SIR below 1.0 means that fewer cases were observed than expected. A confidence interval describes the statistical uncertainty and precision of the estimate. Rare cancers and small populations often produce wide confidence intervals.
Statistical significance can indicate whether a pattern is unlikely under a specified statistical model, but it does not establish causation, rule out chance, or, on its own, determine whether additional assessment is warranted.
What is a cancer cluster or an unusual pattern of cancer?
CDC and the Agency for Toxic Substances and Disease Registry define a cancer cluster as a greater-than-expected number of the same or etiologically related cancers occurring within a group of people or geographic area over a defined period of time.
The term “unusual pattern of cancer” is broader. A pattern may warrant further assessment even when it does not meet every element of the formal cluster definition, for example, when the geographic boundaries of a plausible exposure do not match standard census or political boundaries.
Neither term explains why the cancers occurred. A statistical or geographic pattern must be interpreted together with information about the cancers, population, time period, possible exposures, data limitations, and other relevant evidence.
What are some common misconceptions about community cancer concerns?
Misconception 1: A statistically elevated cancer rate proves that environmental contamination caused the cancers.
Correction: Statistical analysis can indicate whether the incidence is higher than expected, but it cannot identify the cause on its own. Exposure, toxicologic, clinical, and other evidence would be needed.
Misconception 2: If cancer incidence is not statistically elevated, there cannot be an environmental health concern.
Correction: A population-based analysis may have limited statistical power, especially for rare cancers or small communities, and it cannot determine whether an individual was exposed or harmed. Environmental concerns may require evaluation by the appropriate environmental or regulatory agency, regardless of the cancer incidence result.
Misconception 3: A CCR review begins by testing the air, water, or soil.
Correction: CCR evaluates cancer occurrence using registry and population data. Environmental sampling, exposure assessment, source investigation, and enforcement are separate functions performed by the agencies with the relevant authority and expertise.
Misconception 4: Scientists can identify the exact cause of a person’s cancer.
Correction: For most individual cancers, the exact cause or combination of causes cannot be determined. Known risk factors can inform population-level interpretation, but usually cannot establish individual causation.
Why is it difficult to determine whether an environmental exposure caused a community cancer pattern?
Community cancer assessments face several scientific and data limitations:
- Latency: Many cancers develop years or decades after a relevant exposure.
- Population mobility: People may move between the time of exposure and diagnosis. CCR generally has the residence reported at diagnosis, not a complete residential history.
- Small numbers: Rare cancers and small populations can produce unstable rates, wide confidence intervals, and limited statistical power.
- Exposure information: Registry data do not contain individual measurements of most environmental, occupational, school, or household exposures, and historical environmental data may be incomplete.
- Multiple causes and susceptibility: Cancer usually reflects a combination of factors, and people can respond differently to the same exposure.
- Population-level inference: A geographic association between cancer and an environmental measure does not necessarily show that the people with cancer experienced the exposure.
- Privacy: Detailed small-area data may need to be suppressed, combined, or described generally to protect patient confidentiality.
These limitations do not mean that community concerns are unimportant. They explain why public health agencies must use a systematic approach, clearly communicate uncertainty, and coordinate cancer incidence and environmental evaluations when appropriate.
What can and cannot be learned from CCR data?
CCR data can be used to count registry-confirmed cancer diagnoses, calculate incidence rates, compare observed and expected numbers, and examine patterns by cancer type, age, sex, race and ethnicity, time, and geography when the data support those analyses.
Why might CCR be unable to release case counts or detailed maps?
Cancer registry information is confidential. CCR may release statistical summaries only when they do not identify individual patients or sources of information. For rare cancers, small populations, small geographic areas, or combinations of detailed characteristics, even a small count or precise map may create a risk that an individual could be identified.
To protect privacy, CCR may suppress a count, combine categories or geographic areas, use a broader range or description, or withhold detailed results. Privacy review may therefore affect the level of detail that can be included in a public response or report.
What happens if a review identifies a pattern that warrants additional follow-up?
CCR will communicate the findings, methods, limitations, and planned next steps. If the cancer pattern warrants additional follow-up, CDPH may coordinate with county or local health departments and the appropriate state or federal public health, environmental, occupational, or regulatory agencies.
The agencies involved will depend on the specific concern, the suspected exposure or pathway, the geographic area, and the legal authority and technical expertise required. CCR’s role remains focused on population-based cancer surveillance and analysis.
How long does a community cancer review take?
CCR will acknowledge an inquiry within 7–10 business days. Some inquiries can be addressed with existing information. A formal assessment may take several weeks or months, depending on the complexity of the concern, the availability of finalized cancer and population data, the need to verify cases, the analytic scope, and the required scientific, privacy, legal, and communications review.
When a review is extensive, CCR will identify a point of contact and provide updates when feasible.