Survival trends of right- and left-sided colon cancer across four decades: a Norwegian population-based study.

Background: Patients with right-sided (RCC) and left-sided (LCC) colon cancer differ clinically and molecularly. The main objective was to investigate stage-stratified survival and recurrence of RCC and LCC across four 10-year periods. Methods: Patients diagnosed 1977-2016 with colon adenocarcinoma were included from the Cancer Registry of Norway. Primary tumour location (PTL) was defined as RCC if proximal and LCC if distal to the splenic flexure. Multivariable regressions were used to estimate hazard ratios (HR) for overall survival (OS), recurrence-free survival (RFS), survival after recurrence (SAR), and excess hazard ratios (eHR) for relative survival (RS). Results: 72224 patients were eligible for analyses (55.1% [n=39769/72224] had RCC). In 1977-1986, there was no difference between LCC and RCC in OS (HR=1.01; 95%-CI, 0.97-1.06; P=0.581) or RS (eHR=0.96; 95%-CI, 0.90-1.02; P=0.179). In 2007-2016, LCC had significantly better OS (HR=0.84; 95%-CI, 0.80-0.87; P<0.001) and RS (eHR=0.76; 95%-CI, 0.72-0.81; P<0.001) compared to RCC. The gradually diverging and significantly favourable prognosis for LCC was evident for distant disease across all time periods and for regional disease from 2007 onwards. There was no difference in RFS between LCC and RCC in patients <75 years during 2007-2016 (HR=0.99; 95%-CI, 0.91-1.08; P=0.819), however SAR was significantly better for LCC (HR=0.61; 95%-CI, 0.53-0.71; P<0.001). Conclusions: A gradually diverging and increasingly favourable prognosis was observed for LCC patients with advanced disease over the past four decades. Impact: Current PTL survival disparities stresses the need of further exploring targetable molecular subgroups across and within different PTLs to further improve patient outcomes.