ANZ J. Surg. 2004; 74: 931–934
CONTINUING MEDICAL EDUCATION
CONTINUING MEDICAL EDUCATION
CANCER STATISTICS: EVERYTHING YOU WANTED TO KNOW ABOUT
THE CANCER REGISTRY DATA BUT WERE TOO AFRAID TO ASK
G
RAHAMG. G
ILESANDV
ICKYT
HURSFIELDVictorian Cancer Registry, The Cancer Council Victoria, Melbourne, Victoria, Australia
Australia and New Zealand (ANZ) have had complete population cancer incidence registration for decades and are beginning to develop clinical data systems to collect details of staging and treatment. Sadly, this bounty of statistical data is often under utilized and occasionally abused. Many are unaware of the data and statistical services available from their local cancer registry. To promote the use of ANZ population cancer data, the following paper describes the principal statistics that are available and identifies common problems with their use.
Key words: cancer, incidence, population registers, prevalence, survival.
Abbreviations: AACR, Australasian Association of Cancer Registries; AIHW, Australian Institute of Health and Welfare;
ANZ, Australia and New Zealand; IARC, International Agency for Research in Cancer; IACR, International Association of Cancer Registries; NCCI, National Cancer Control Initiative; NHMRC, National Health & Medical Research Council; PYLL, person years of life lost; YPLL, years of potential life lost.
INTRODUCTION
The object of this paper is to encourage the proper and increas-ing use of Antipodean cancer data, it beincreas-ing a cause of deep chauvinistic shame to observe Australians and New Zealanders making presentations at international conferences using Ameri-can or European Ameri-cancer information. Australia and New Zealand (ANZ) are unusually fortunate in having their entire populations covered by cancer registries. Full coverage of cancer incidence for ANZ commenced in 1982 with some jurisdictions going back further in time.1 In all States and Territories of Australia, cancer registration is mandated by law that usually makes it incumbent on pathology laboratories and hospitals to notify a central regis-try of cancer diagnoses. Cancer registries also receive data from other sources such as death certificate details from the Registrars of Births, Deaths and Marriages. The duplicate registration of information from more than one source is important to assure data completeness especially in larger populations with multiple notifying institutions
Cancer registration has been largely a passive process that has focused on gathering a few data items that are usually to be found in the primary source documents, particularly those items that usually are accurately recorded. This historical infrastructure is now evolving to resemble the SEER registry system2 in the USA, as national needs for increased clinical data on cancer patients have been identified and articulated principally by the National Cancer Control Initiative (NCCI).3 The NCCI has developed a minimum dataset for clinical data collection and this has been adopted by key national bodies including the National Health & Medical Research Council (NHMRC), the National Cancer
Strat-egies Group and the Australasian Association of Cancer Regis-tries (AACR).4 It is hoped that the next decade will see the collection of the NCCI minimum clinical data set widely imple-mented across Australia.
All ANZ cancer registries are members of the International Association of Cancer Registries (IACR) based at the Inter-national Agency for Research in Cancer (IARC) in Lyons, France and, as such, have satisfied essential criteria for data quality and their data are published in Cancer Incidence in Five Continents.5 The AACR meets regularly, with secretarial support from the Australian Institute of Health and Welfare (AIHW)6 to continu-ally improve the level of data commonality and standardization between the different State and Territory jurisdictions. All Aus-tralian cancer incidence data are pooled at the AIHW in order to facilitate the production of national cancer statistics.7
MEASURES OF INCIDENCE
The principal incident event eligible for registration is the diag-nosis of a primary invasive cancer. Some registries also collect
in situ tumours and tumours of uncertain behaviour but these are not commonly included in incidence reports. Non-melanocytic skin cancers also are rarely collected by registries on a routine basis because of the huge resources that would be required. Because a diagnosis date is rarely written in a medical record, this is usually determined by the date of key procedures that are recorded, e.g., date of biopsy or the date of the pathologist’s report. In a minority of cases, a diagnosis date is determined solely on clinical grounds. Great care is taken at cancer registries to avoid the registration of additional primary cancers in the same person that may be extensions or metastases of a previous cancer. There are internationally agreed conventions in regard to the def-inition of such multiple primaries;8 for example a second primary in the same organ has to have a different morphology according to Berg groupings.9 Although registries may record all diagnoses of primary cancers on their database, only the first primary cancer for a given organ site and morphology combination is used for reporting incidence.
G. G. Giles MSc, PhD; V. Thursfield BSc, Grad. Dip (Appl. stats.). Correspondence: Professor Graham Giles, Victorian Cancer Registry, The Cancer Council Victoria, Carlton, Vic. 3053, Australia.
Email: graham.giles@cancervic.org.au Accepted for publication 27 April 2004.
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Incidence can be reported in various ways; as a count, a crude rate, a standardized (age-adjusted) rate or as a cumulative risk. These metrics are usually presented in an annual format and are commonly broken down by sex and 5-year age group. Occasion-ally they are also made available by geographical region, country of birth and socioeconomic status. Crude rates are simply the annual count (numerator) divided by the population for that year (denominator) and multiplied by a constant, commonly 100 000 is used for cancers and 1000 000 for childhood cancers. In order to compare incidence rates from different populations, standard-ized rates are produced by multiplying the age-specific rates by a set of age-specific weights in order to adjust for possible differ-ences due to the different age structures of the populations to be compared. For international comparisons, the most useful set of weights is the World Standard Population10 and these weights are used, for example, in the volumes of Cancer Incidence in Five Continents.5 On the other hand, comparisons within populations particularly in developed countries that have an aged population structure compared with the youthful structure of the World Standard Population, a set of weights that more closely resembles the actual population structure is more useful in that it does not give undue weight to younger cases while discounting older cases. In Australia, the AIHW has commonly used the 1991 census population as well as the World Standard Population to standardize rates for publication. When inspecting published
rates it is therefore important to ascertain which standards have been used – the US SEER system2 uses the 1971 US census pop-ulation. If in doubt, consult Cancer Incidence in Five Continents5 or make friends with your local cancer registry, as it will usually have easy access to comparative data from other countries.
An often neglected aspect of rate estimates is their precision, as rates based on small numbers will be more variable and less reliable than those based on large numbers. When inspecting published rates, one should look for information about their asso-ciated standard errors and 95% confidence intervals (estimated by taking 1.96 standard errors either side of the rate estimate). Should the confidence intervals of two different rate estimates overlap, any difference between the rates is unlikely to be statisti-cally significant.
The cumulative rate of a cancer (at a given age) is a measure of the proportion of all persons who have, by that age, been affected by this cancer. It is a directly standardized incidence rate that approximates the actuarial or cumulative risk.10 This is the sum over each year of age of the age-specific incidence rate taken from birth to age 74. This rate is expressed as a percentage, e.g., 33%, or as its reciprocal, the ‘lifetime risk’, e.g., 1 in 3. Other end points than 74 years may sometimes be used, e.g., 0–14 for child-hood cancers, or increased to 80 years to reflect the increasing life expectancy in developed countries – this should be checked when making comparisons.
CANCER STATISTICS 933
MEASURES OF PREVALENCE
The primary importance of prevalence estimates is to gain an understanding, at any given point in time, of the proportion of people in a given population who remain alive after having had a diagnosis of cancer at any time in the past. It follows that preva-lence can only be estimated accurately by population cancer registries that have had a long period of operation – therefore esti-mates have not been common until recently.11 Prevalence data are useful to those charged with planning for the provision of health services for example continuing therapy including treatment of disabilities, regular medical consultations, screening for recur-rences and second primary cancers, and for long-term counselling and support. Total prevalence is often inadequate to meet these needs, and estimates of patients with active disease ‘active preva-lence’ are now being separately estimated.12
MEASURES OF MORTALITY
Cancer mortality rates are available from national sources such as the Australian Bureau of Statistics13 and the AIHW.6 Rate estima-tion from mortality data is carried out in the same way as for incidence, and uses the same standard population weights for age adjustment. Note that some cancer registries code cause of death data separately from the national collections. Although these reg-istry mortality procedures might be more accurate in attributing some deaths to cancer or more specific in regard to subtypes of cancer, they will vary between different registries and it is impor-tant to check published data before making comparisons. The Cancer Epidemiology Centre at The Cancer Council Victoria updates the long-term Australian cancer mortality trends every 5 years.14
Person years of life lost (PYLL), sometimes referred to as years of potential life lost (YPLL), is an estimate of the impact of cancer mortality on the community in terms of loss of life. It is estimated by subtracting for each cancer death the person’s age at death from their expected age at death obtained from appropriate population life tables. This indicator is useful to compare the impact of cancers that occur more often in older people with those that have a more youthful distribution. As with cumulative rates, different values of life expectancy might be used in estimat-ing PYLL and this should be checked before makestimat-ing compari-sons. In Australia in 2000,7 prostate cancer and breast cancer were similar in numbers of incident cases and cumulative rate, but the PYLL for prostate cancer was only 5,783 years compared with 28,305 years for breast cancer.
MEASURES OF SURVIVAL
Crude survival is the simple proportion of a given series of cancer patients that survive a particular time from diagnosis, usually 5 years. The relative survival ratio is a measure of net survival that is usually interpreted as the proportion of patients who would have survived to a certain time (usually 5 years for cancer) if the cancer they had was the only cause of death in the patient popula-tion. It is defined as the ratio of observed survivors in a cohort of cancer patients to the proportion of expected survivors in a com-parable group of cancer-free individuals. Population life tables stratified by age, sex and calendar year are generally used to cal-culate expected survival. Unlike cause-specific survival, which depends on accurate coding of cause of death, relative survival measures the excess mortality experienced by cancer patients
irrespective of whether their deaths are ‘attributed’ to their cancers or not. It does require that the excess mortality is caused by the cancer, and not to some other factor (e.g., smoking) that might be related to the onset of cancer and to excess mortality from other causes. Cancer survival information is available for a number of cancer registries and there has been a recent national publication on this topic.15
ISSUES OF ACCESS AND DATA QUALITY Cancer registries all produce formal annual statistics usually pub-lished at least a year after a calendar year of notifications is deemed to be complete. Such cancer registry reports usually present tables of incidence data by broad topographical rubrics of the International Classification of Diseases for Oncology16 by sex and by 5-year age group, along with summary crude and age-standardized rates. The registry database contains more details, including more recent notifications, not only on cancer topogra-phy sub sites, but also on tumour morphology. These data are usually made available on request and free of charge. The registry databases also include demographics such as geocodes that permit analysis by local government area, hospital catchments and health department regions.
Clinical and epidemiological researchers often need to match series of patients or unaffected persons to the cancer registry in order to follow up new diagnoses and deaths. Because accurate matching requires the use of full names and dates of birth, having the person’s consent is a usual requirement. However, as consent is difficult to obtain in certain situations, record matching can still be performed, providing all relevant ethics committees give their approval.
Data quality is another important issue. Population registers are passive recipients of data abstracted from source documents by third parties and, as such, are prone to error. Having registry data used extensively by external researchers and analysts is a valuable way of increasing data quality. To judge whether regis-tries meet international accreditation standards, the IACR inspect the proportion of cases identified solely from death certificates, the proportion of cases for which there is evidence of histological examination, and the mortality to incidence ratio.
Death certificate only is the term used for the proportion of cases registered for which no information was available other than a statement on the death certificate that the deceased died from or with cancer. A high percentage suggests incomplete inci-dence notification, and such diagnoses might be less accurate. Registries take great pains to seek additional information for cancers first notified by death certificate in order to identify pos-sible missed registrations. If no further information is available, the cancer is registered as ‘death certificate only’ based on infor-mation provided on the death certificate, the date of diagnosis being taken as the date of death.
The proportion of cases registered for which there is evidence that the diagnosis was based on a histological examination of the primary tumour is another quality indicator. A low percentage suggests incomplete registration of pathology reports and conse-quently poorer verification of diagnoses and incomplete registra-tion of cancers for which this is often the only source of notification, such as melanoma. The higher the proportion histo-logically verified for cancers of less accessible sites, like brain and pancreas, the more confident one can be that the neoplasm existed and that it was primary rather than metastatic.
The mortality to incidence ratio is the ratio of the number of deaths attributed to a specific cancer with the number of new
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cases of the same cancer diagnosed during the same period in the same population. If registration is complete and the incidence of the cancer is not changing rapidly, the mortality to incidence ratio should reflect long-term survival. If survival proportions are comparable in two populations, a more complete case ascertain-ment is suggested by a lower mortality to incidence ratio.
INCREASING CLINICAL RELEVANCE Population cancer registries in ANZ were developed to measure incidence and have therefore attempted only to collect a minimal dataset for every case of cancer diagnosed in the com-munity. The minimum dataset required for incidence has not included in its scope any details of prognostic indicators or treat-ment. Recognizing the absence of local information on cancer management in our region, attempts have been made in recent decades to address this issue. These activities range from the type of professional audit taken up, for example, by the breast surgeons17 to the notable attempt in South Australia to capture clinical data on cancer via a network of hospital units18 and the ‘patterns of care’ model based on surveying clinicians involved in the care of a random sample of cases drawn from the popula-tion cancer registry.19,20
As mentioned above, the NCCI has developed a core clinical data set after a national consultation process. This dataset has been adopted in principle by cancer registries and professional societies, and plans are being drawn to implement its collection in several jurisdictions. Given that the resources can be found to fund the data collection, it will probably take a decade for Australia to approach population coverage. Meanwhile, profes-sional audits and patterns of care surveys will probably remain the major source of information on cancer management and outcomes.
SUMMARY
In comparison with many other parts of the world, the ANZ oncology community is well served by population cancer regis-tries. Although historically focused on obtaining compete cover-age of incident cases, recent proposals to increase registries’ value by augmenting their collections with more clinically rele-vant data, are being seriously considered. Nothing is more pleas-ing to those charged with runnpleas-ing registries than seepleas-ing their hard-won data presented at scientific meetings. The reader is reminded that most registries have more data available than are commonly reported in their statistical publications. They are also the best source of local cancer data and have access to data from other registries around the globe with which to make compari-sons. The AIHW website in reference 6 contains links to all State and Territory cancer registries – click on one today!
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