Impact of absence of consensual cutoff time distinguishing between synchronous and metachronous metastases: illustration with colorectal cancer

Staging is essential for scientific exchanges on colorectal cancer. Lack of a consensual definition for synchronous and metachronous metastases for colorectal cancer may introduce artifactual differences between epidemiological studies according to stage. We investigated how variations in the cutoff for the definition of synchronous metastases influenced the stage-specific distribution and incidence and the survival of stage IV patients. Between 2007 and 2013, a total of 4636 cases of colorectal adenocarcinoma were registered in the cancer registry of Burgundy. Age-standardized incidence by stage was estimated for each cutoff from 0 to 12 months, differentiating between synchronous and metachronous metastases. Net survival was calculated from the date of the diagnosis of metastasis. The incidence of stage IV colorectal cancer increased from 6.0/100 000 when considering metastases diagnosed within the first month to 7.1/100 000 when including metastases diagnosed until 12 months after the diagnosis of colorectal cancer. When the cutoff increased from 1 to 12 months, the relative variation in the proportion of cancers was +21% for stage IV, -12% for stage III, and -5% for stage II. Similarly, the 1-year net survival for metachronous group was over 10% higher than that for the synchronous group when the cutoff was over 5 months. An objective definition of the relevant cutoff to distinguish between synchronous and metachronous metastases is required for scientific epidemiologic exchanges. Survival in the metachronous group was significantly better than survival in the synchronous group when the cutoff between synchronous and metachronous was over 4 months after the primary diagnosis.