The outcome for metastatic colorectal cancer (mCRC) patients has improved substantially in recent decades. This has chiefly been observed in study populations, and predominantly in left-sided primary tumours, which is why we wanted to study if and how survival has improved in the background population. It has also been seen that certain molecular subtypes are more common in population-based materials, and, thus, we studied the prevalence and effects of different molecular alterations.

Paper I is a national population-based material of all 19 566 Swedish patients with a diagnosis of mCRC 2007-2016, 55% were male and 70% had synchronous metastases. Median overall survival (OS) for all patients was 14.0 months. An improvement could be seen over time, also in stratified analyses. OS was influenced by presentation of metastases, age, primary tumour location, and sex. All except sex remained statistically significant in a multivariable analysis. Differences of about one month in median OS were seen between healthcare regions, but these diminished over time.

Paper II included all 765 patients from the Uppsala Region with a mCRC diagnosis 2010-2020. Right colon primary tumours were seen in 38%, left colon in 27% and rectum in 34%. BRAF-V600E mutations (mt) and deficient mismatch repair (dMMR) had a poor OS and were more common in right colon primary tumours. Primary tumour location did not affect OS in subgroups according to mutations in RAS or BRAF, nor in a multivariable analysis. Molecular alterations seem to be more important than primary tumour location for prognosis.

Paper III studied KRAS-G12Cmt in three population-based and one real-world material. KRAS-G12C was seen in 2-4% of all tested and in 4-8% of all KRASmt. No differences in patient characteristics were observed between KRAS-G12C and other KRASmt. No differences in OS were seen between KRAS-G12C and other KRASmt, neither for all patients, nor in different treatment groups.

Paper IV studied atypical BRAFmt (aBRAFmt) in two population-based and one real-world cohort. aBRAFmt was seen in 1-4% of the adequately tested patients in the different cohorts. aBRAFmt patients were predominantly male, had dMMR less often, more rectal primary tumours, and less peritoneal metastases compared with BRAF-V600Emt. Serrated adenocarcinomas were seen in about half of the aBRAFmt. OS was significantly better for aBRAFmt than in BRAF-V600Emt, but worse than for RASmt and RAS&BRAFwt patients. Nine aBRAFmt received epidermal growth factor receptor inhibitors without responses.