Reportable Cancer Cases
The Oregon State Legislature established the Oregon State Cancer Registry (OSCaR) in 1995. OSCaR is a population-based reporting system that collects and analyzes information about cancer cases occurring in Oregon. The first cancer cases were collected for diagnosis year 1996.
Reportable cases include all cancers except specific forms of common, curable skin cancer and in situ cervical cancers. An in situ stage cancer is a tumor that is malignant but does not invade or penetrate surrounding tissue. These types of skin cancers and cervical cancer are considered 100% curable if the patient receives treatment. In addition, in situ cervical cancers are poorly distinguished from a number of benign conditions.
- Incidence rate demonstrates the risk of developing cancer.
- Mortality rate demonstrates the risk of dying from cancer.
Invasive Versus Total Counts
Cancer rates are based on invasive cases which exclude all in situ cases with the exception of bladder cancer. This means that the count of invasive cases reported in cancer rate tables may differ from the count of total cases reported elsewhere.
Crude Versus Age-Adjusted
The crude rate is the number of cases divided by the population. It is called the crude rate because it is not comparable with any other population, nor is it comparable with the same population over time. It does not convey the true risk of cancer to a population. Because the risk of developing cancer increases with age, rates are age-adjusted to a national population standard. These adjusted rates convey the risk of cancer for populations with different age distributions. Cancer rates that are age-adjusted allow comparison of rates among different populations (i.e., counties or states). Crude rates should not be used for comparison purposes.
County level rates are published annually. However, interpretation of inter-county differences in the rates of cancers must be made with caution. There are many reasons why it is difficult to know if the differences in county rates reflect true differences in county risk.
- Areas with different risks or exposures are difficult to compare because the cause of many cancers are unknown.
- County rates could be higher due to personal behaviors, i.e. increased lung cancers in counties with greater tobacco consumption, rather than environmental hazards.
- During the generally long interval between exposure and diagnosis of cancer, a person may have changed jobs or residences. This means they may have been exposed in one county and diagnosed in another.
These and other components can make it difficult to evaluate potential geographic clusters. In addition, the more populated area (county) could mask any potential variation in a small geographic area (city). OSCaR is currently developing a Geographical Information System (GIS) to allow analysis of data for small areas. This is a new direction for the cancer registry, and only limited data are currently available.
Rates based on small numbers of cases or on a sparsely populated geographic area must be evaluated carefully. Chance variation is a common problem when the numbers used to calculate rates are extremely small. Large fluctuations often occur in rates based on small numbers that do not reflect real changes in the cancer burden of the underlying population.
Therefore, if the number of cancer events (new cases or deaths) is ten or fewer, the calculated rate is considered unstable. A carat (^) in a table denotes an unstable rate. When it is considered unstable, the rate is not shown. The number of cases, however, is still shown. Therefore, county rates for cancers with low incidence, such as cervical cancer, are calculated using a 5-year aggregate number.