Apart from its obvious use in mapping, geocoding of cancer cases allows linkage to area-based data, such as population density and measures of socio-economic status (e.g. percentage unemployed). This is described in more detail in sections 2.2.4.2 and 2.2.4.3. This type of information is not, in general, accessible at the level of the individual cancer case in Ireland, and has to be inferred from area-based measures.
Northern Ireland
NICR routinely collects address information for registered cancers, allowing small geographic areas to be assigned to individual cancer registrations. This is accomplished through an electronic process which uses the postcode that accompanies the majority of NI addresses along with a postcode-to-electoral ward lookup file known as the Central Postcode Directory (CPD). This is maintained by the Northern Ireland Statistics and Research Agency (NISRA) and updated annually (Northern Ireland Statistics and Research Agency, 2010a). Addresses with an unknown, incomplete or invalid postcode cannot be assigned an electoral ward.
Republic of Ireland
The National Cancer Registry attempts to code all addresses of cancer cases to the level of electoral division (ED), the smallest area for which census data can be obtained. Unlike NI, addresses in RoI do not have postcodes. Each address is therefore assigned to an ED by means of matching Registry address information to other data sources. This process of geocoding is carried out for the most part by matching Registry patient address data to the GeoDirectory database which provides a list of official postal addresses and location details for every residential and commercial property in the country (www.geodirectory.ie). Data matching is carried out using software developed by the Registry, with some manual coding for any remaining unmatched records. Additional resources used include address tables from census surveys, supplied by the Central Statistics Office (CSO), as well as manually locating addresses on maps provided by Ordnance Survey Ireland (OSI). Using a combination of these resources, almost all complete patient addresses can be assigned to a particular ED.
For some cases it is impossible to assign an address confidently to a single ED, usually because the address is incomplete or ambiguous (4.4% of all RoI cancers included in this report; Table 2.2). The number of EDs to which the address could potentially belong is usually small (2 or 3) and, for analysis, these cases were assigned at random to one of the possible EDs, with the possibility of assignment weighted by the population of the alternative EDs.
NI and RoI records with unknown ED or ward
At the end of the geocoding process, a number of registrations in both RoI and NI remained which could not be assigned to any ED/ward (3.3% of all cancers included in this report; 3.6% in RoI and 2.7% in NI) (Table 2.2). For these registrations, a fraction of the cases of each cancer type was allocated in proportion to each ED (RoI) or ward (NI) weighted by population. In NI almost all of these cases were non-melanoma skin cancers, but in RoI all cancer sites had a similar percentage of cases with unknown ED.
Table 2.2 Number and percentage of cases not assigned to an ED or ward, and of cases assigned to multiple EDs
cancer site | cases assigned to more than one ED (RoI only) | cancer cases not assigned to an ED or ward |
|---|
| | | | | Ireland | RoI | NI |
|---|
| | number | % RoI cases | % all cases | number | % | number | % | number | % |
|---|
non-melanoma skin cancer | 3443 | 4.6% | 3.3% | 4896 | 4.7% | 2803 | 3.8% | 2093 | 6.9% |
breast | 1045 | 4.0% | 2.7% | 948 | 2.4% | 839 | 3.2% | 109 | 0.9% |
colorectal | 1136 | 4.5% | 3.0% | 985 | 2.6% | 885 | 3.5% | 100 | 0.8% |
lung | 783 | 3.5% | 2.3% | 808 | 2.4% | 749 | 3.4% | 59 | 0.5% |
prostate | 1282 | 5.2% | 3.9% | 1128 | 3.4% | 983 | 4.0% | 145 | 1.7% |
non-Hodgkin's lymphoma | 270 | 4.2% | 2.8% | 305 | 3.1% | 254 | 4.0% | 51 | 1.5% |
stomach | 256 | 4.1% | 2.7% | 270 | 2.9% | 250 | 4.1% | 20 | 0.6% |
melanoma of the skin | 280 | 4.3% | 3.1% | 365 | 4.0% | 281 | 4.3% | 84 | 3.1% |
bladder | 248 | 4.1% | 2.8% | 222 | 2.5% | 208 | 3.4% | 14 | 0.5% |
head and neck | 199 | 3.8% | 2.5% | 190 | 2.4% | 168 | 3.2% | 22 | 0.8% |
leukaemia | 258 | 4.6% | 3.4% | 247 | 3.2% | 230 | 4.1% | 17 | 0.8% |
pancreas | 243 | 4.9% | 3.5% | 173 | 2.5% | 164 | 3.3% | 9 | 0.4% |
kidney | 205 | 4.7% | 3.2% | 129 | 2.0% | 120 | 2.7% | 9 | 0.4% |
oesophagus | 203 | 4.8% | 3.2% | 169 | 2.7% | 151 | 3.6% | 18 | 0.9% |
ovary | 183 | 4.4% | 2.9% | 165 | 2.7% | 154 | 3.7% | 11 | 0.5% |
brain and other central nervous system | 168 | 4.4% | 3.2% | 155 | 2.9% | 150 | 4.0% | 5 | 0.3% |
corpus uteri | 129 | 3.8% | 2.5% | 133 | 2.5% | 113 | 3.4% | 20 | 1.1% |
cervix uteri | 75 | 2.8% | 2.0% | 90 | 2.4% | 81 | 3.0% | 9 | 0.8% |
all cancers in this report | 10406 | 4.4% | 3.0% | 11378 | 3.3% | 8583 | 3.6% | 2795 | 2.7% |
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