WHY DID TREATMENT RATES FOR COLORECTAL-CANCER IN SOUTH EAST ENGLAND FALL BETWEEN 1982 AND 1988 - THE EFFECT OF CASE ASCERTAINMENT AND REGISTRATION BIAS

Background We had two aims in undertaking this study, as follows: (1) to describe regional and district trends in incidence and treatment fo r colorectal cancer in South East England from 1982 to 1988; (2) to ex amine the effect of registration practice and case ascertainment on di strict variations in incidence and treatment using data on death certi ficate only (DCO) registrations, mortality and stage. Methods We inclu ded all cases registered by the Thames cancer registry diagnosed with colon or rectal cancer between 1982 and 1988 and resident in 28 distri cts in the two South Thames regions. Indirect standardized incidence r atios were calculated for the districts and a chi(2) test for trend wa s carried out. Results In the SE England regional analysis, between 19 82 and 1988 there was a significant increase in the incidence of cases of colon and rectal cancer in the over-75s, but treatment rates remai ned unchanged. Treatment rates fell significantly in the under-65s alt hough incidence rates remained unchanged. Age is a strong predictor of nontreatment. Between 1982 and 1988 the relative risk of not receivin g treatment increased for all ages over 65 years. DCO registrations ac counted for 22 per cent and 15 per cent of all colon and rectal cancer cases, respectively, between 1982 and 1988. The proportions rose (bet ween 1982 and 1988) from 10 and 8 per cent to 25 and 19 per cent in co lon and rectal cancer, respectively. DCO registration rates increased over time and in all age groups in South East England for both colon a nd rectal cancer between 1982 and 1988, but the largest increase was i n the over-75s. Thirty-two per cent of colon and 25 per cent of rectal cases were unstaged. Although the proportion of unstaged cases remain ed constant over time, they were increasingly the result of DCO regist rations. Errors in the registry staging data rendered those cases whic h were staged unusable. In the district analysis, there were significa nt variations in age-standardized incidence, treatment and DCO registr ation ratios across the 28 districts for men and women with colon and rectal cancer between 1982 and 1988. DCO registrations show a negative correlation with treatment for both colon and rectal cancer (p < 0.05 ) and with incidence for only rectal cancer. Conclusions We report sig nificant differences in age-standardized incidence and treatment ratio s across 28 districts in South East England, some of which can be acco unted for by differences in registration practice. There is a complex relationship between DCO registrations and incidence and treatment for both colon and rectal cancer. DCO registrations are a good proxy for under-ascertainment of incidence in rectal cancer but not colon cancer , and are a good proxy for under-ascertainment of treatment in both co lon and rectal cancers. Information from the cancer registry can be us ed to examine registration and treatment rates across districts. Howev er, if variations are to be adequately explained, meticulous data coll ection on stage and quality control are essential.