Highlights
- New rules for counting multiple primary cancers has increased the number of cancer cases registered in the Ontario Cancer Registry from 2010 onwards.
- The change in number of cancer cases does not mean that more people in Ontario are being diagnosed with cancer.
- The impact of the new rules varies by cancer site.
In October 2014, the Ontario Cancer Registry (OCR) implemented a new set of rules for identifying multiple primary cancers, which has led to an increase in the number of reported cases for certain cancer types. The OCR is the provincial database containing information about all newly diagnosed cancer cases in Ontario, except for basal cell and squamous cell carcinomas of the skin, and its new rules are applied to cases diagnosed from 2010 onwards.
Multiple primary cancers are 2 or more distinct cancers that occur in 1 person and are different from metastases, which develop when cells from a primary cancer spread to other parts of the body. The new rules, which follow standards for counting multiple primary cancers defined by the National Cancer Institute’s Surveillance, Epidemiology and End Results (NCI SEER) Program, replaced rules that were a modified version of the International Association of Cancer Registries (IACR) standards. With the new rules, the number of newly diagnosed cancer cases registered by the OCR in 2010–2011 is 5.8% higher than the number of cases that would have been reported using the old rules.
The increased number of cancer cases due to the new rules varies by cancer site (i.e., part of the body where the cancer occurs). For example, while the number of prostate cancer cases diagnosed in 2010–2011 is roughly the same (an increase that is 0.8% higher under the new rules vs. the old rules), the change is much greater for melanoma (a 15.2% increase), female breast cancer (a 13.4% increase) and colorectal cancer (a 7.5% increase).
Part of the reason there is a relatively large increase for melanoma, female breast cancer and colorectal cancer is that the new rules provide a greater opportunity for new cancers to be captured in these sites compared with prostate. The old IACR-based rules are much more conservative than the new NCI SEER rules, allowing only 1 cancer per organ to be registered in the OCR. However, according to the new rules, the reporting of separate cancers in different paired organs (e.g., right and left breasts), as well as different colon subsites (e.g., ascending and descending colon) and skin melanoma subsites (e.g., skin on the arm and leg) is allowed. The new rules also allow the registration of subsequent primary cancers in an organ after a specified period of time (e.g., 5 years for breast cancer) and the registration of different types of cancer in the same organ as separate primary cancers.
Adoption of the new NCI SEER rules brings the OCR in line with current Canadian and U.S. standards for tracking cancer incidence by more fully capturing the diagnosis and treatment patterns experienced by people with cancer in Ontario. The increase in the number of cancer cases counted under the new rules compared with the old rules does not mean that more people in Ontario are being diagnosed with cancer.