The effect of multiple primary rules on cancer incidence rates and trends.

PURPOSE: An examination of multiple primary cancers can provide insight into the etiologic role of genes, the environment, and prior cancer treatment on a cancer patient's risk of developing a subsequent cancer. Different rules for registering multiple primary cancers (MP) are used by cancer registries throughout the world making data comparisons difficult. METHODS: We evaluated the effect of SEER and IARC/IACR rules on cancer incidence rates and trends using data from the SEER Program. We estimated age-standardized incidence rate (ASIR) and trends (1975-2011) for the top 26 cancer categories using joinpoint regression analysis. RESULTS: ASIRs were higher using SEER compared to IARC/IACR rules for all cancers combined (3 %) and, in rank order, melanoma (9 %), female breast (7 %), urinary bladder (6 %), colon (4 %), kidney and renal pelvis (4 %), oral cavity and pharynx (3 %), lung and bronchus (2 %), and non-Hodgkin lymphoma (2 %). ASIR differences were largest for patients aged 65+ years. Trends were similar using both MP rules with the exception of cancers of the urinary bladder, and kidney and renal pelvis. CONCLUSIONS: The choice of multiple primary coding rules effects incidence rates and trends. Compared to SEER MP coding rules, IARC/IACR rules are less complex, have not changed over time, and report fewer multiple primary cancers, particularly cancers that occur in paired organs, at the same anatomic site and with the same or related histologic type. Cancer registries collecting incidence data using SEER rules may want to consider including incidence rates and trends using IARC/IACR rules to facilitate international data comparisons.