Volume 22, No. 1 - 2001 

[Table of Contents]

Emigration Patterns of Cancer Cases in Alberta, Canada

Juanita Hatcher and Marilou Hervas

Abstract

Cancer registries are a unique source of data for population-based analysis of survival of cancer cases, but information on current vital status is essential. This paper describes a method to determine the last known vital status of cases and the emigration pattern of cancer cases diagnosed in Alberta. Data from the Alberta Cancer Registry (ACR) for the years 1985-1993 (83,446 cases) were linked to the Alberta Health Care Insurance Plan (AHCIP) registration file to identify cases that had left the province and the date they emigrated. Ninety-nine percent of the ACR cases linked correctly to the AHCIP registration file. Three percent of cases had left Alberta by March 1998. For the first five years of follow-up between 0.6% and 0.8% of cases alive at the beginning of each year of follow-up left the province in the succeeding year. Seven percent of those diagnosed under 45 years of age left the province compared to less than 2% of those aged 65 and over. There was no difference in emigration patterns between the sexes. The cancer sites with good prognosis tended to have the highest proportion of emigrants.

Key words: cancer; emigration

Introduction

The primary function of cancer registries is to identify and register all incident cancer cases occurring within their jurisdiction and to record information related to the death of each cancer case.¹ Cancer registries are thus a unique source of data for analyzing the survival of the population of cancer cases.² As many cancer registries do not actively follow up the incident cases, however, the residency and current vital status of those not known to be dead are often unknown, as is the impact of emigration on survival.

In Alberta, Canada, the problems caused by lack of active follow-up were overcome by linking the Alberta Cancer Registry (ACR) to the Alberta Health Care Insurance Plan (AHCIP) registration file maintained by Alberta Health and Wellness (AHW). This enabled the ACR to identify those cancer cases that had left the province and the date that they left. This date would be used as a censoring date in survival analysis, permitting the determination of the extent of emigration of cancer cases from Alberta and the potential impact on survival analysis.

This paper describes the pattern of emigration of cancer cases diagnosed in Alberta.

Data

AHW maintains a historical database of all residents of the province of Alberta who are/were registered in AHCIP, which insures all required medical care available in the provinces. The contract holder pays the premiums, and benefits are provided for all members of the immediate family. All residents of Alberta, except serving members of the Royal Canadian Mounted Police and the Canadian Military, inmates of federal penitentiaries and Status Indians, are eligible to register in AHCIP. The federal government pays the AHCIP premiums for Status Indians and other exceptions who are included in the AHCIP registration file. Only 200-300 of the approximately 3 million who are eligible to register choose not to. The AHCIP registration file includes data on more than 99% of the Alberta population.

The AHCIP registration data include a unique personal identifier, the Personal Health Number (PHN) and/or the AHCIP number, surname, initials, date of birth, gender and postal code, as well as the date and reason for any changes in coverage by AHCIP. Prior to 1994, the individual identifier (AHCIP number) consisted of an eight-digit base number to identify the contract, and a three-digit individual identifier. If a person changed the contract under which he or she was covered, by, for example, leaving the family home to marry, his or her AHCIP number would change. The PHN, introduced in 1994, is unique to an individual and remains the same for life. AHW has assigned a PHN to people who were initially registered before 1994, and a conversion file exists to link pre-1994 AHCIP numbers to the new PHNs. AHW is usually informed of people who leave the province by the medical plan of the province to which the person has emigrated. The AHCIP file is linked to the vital statistics file on a weekly basis to identify those who have died in the province. The estate of a dead person may also notify AHW when it receives the bill for the premiums.

The ACR is mandated under the Alberta Cancer Program Act to capture information on every incident of invasive cancer diagnosed in Alberta.³ Pathology reports and death information from the Alberta Registries vital statistics file are the two main sources for identifying incident cases of cancer in Alberta. The ACR includes identifying information, (PHN, AHCIP number, current surname, previous surnames, first names, date of birth, gender and postal code); information on the incident tumour; and for those cases known to have died, date and cause of death. The Alberta Registries vital statistics file also provides information on cases that have died in Alberta. The ACR would not generally be informed of the deaths in other provinces. There has been no follow-up information on cases that are not known to have died.

Methods

All invasive cancer cases diagnosed in Alberta between January 1, 1985, and December 31, 1993, inclusive were identified from the ACR. A file containing PHN, AHCIP number, surname, initials, gender and date of birth for each individual diagnosed with cancer was submitted to AHW. This file included records for residents and non-residents of Alberta. A hierarchical deterministic linkage strategy was used to link the ACR to the AHCIP registration file, linking on the AHCIP number, the AHCIP base number and surname, gender and month and year of birth (identical). All links were checked using the other identifying information common to the two files.

Discrepancies between the ACR and the AHCIP records were investigated further by reviewing the patient's chart. Errors in the ACR were corrected, and AHW was notified of suspected errors in its data set. The agreement in the vital statistics of the linked cancer cases in the two files was also checked. Where the ACR had recorded that a case had died, but the AHCIP record indicated the case was alive, the death information held by the ACR was confirmed. In cases where AHCIP indicated the case had died but the ACR had no record of death, the case was deemed to be dead for subsequent analysis and the death date taken as that recorded by AHCIP. The ACR also had death information on some cases that AHCIP deemed had left the province.

Cases for which the first invasive primary (excluding non-melanoma skin cancer) occurred between 1985 and 1993 and which were resident in Alberta at the time of diagnosis were identified from the linked cohort. The percentage of cases that had left the province as determined from AHCIP was examined for each cancer site. Crude, age specific and age-standardized percentages were estimated. The standard population used was the total population of cancer cases used in the analysis. The percentage emigrating each year up to five years past diagnosis was estimated using the revised ACR vital status definitions. Person-years at risk were calculated as the time between diagnosis and date of last follow-up, death, emigration or for those still alive and resident in Alberta, as of March 31, 1998. All follow-up periods were terminated at five years to standardize the potential length of follow-up. The reduction in person-years at risk produced by censoring emigrants at the time of emigration rather than as of March 31, 1998, was investigated.

Results

Of the 83,446 cases that were resident in Alberta for at least one diagnosis between 1985 and 1993, 82,466 cases (98.8%) linked correctly to the AHCIP registration file. There was no difference in linkage rate between those who were registered as dead (98.8%) and those who were not registered as dead (98.8%).

The following results are presented for the 82,466 cases who were residents in Alberta at the time of diagnosis between 1985 to 1993, and who linked successfully with the AHCIP registration file (Table 1). Of the 45,925 cases that were registered as dead on the ACR, 97.3% were also registered as dead on the AHCIP registration file, 1.8% were registered as still alive and 0.9% were registered as having left the province. Of the 36,541 cases that were not registered as dead on the ACR, 1.3% were registered as dead on the AHCIP registration file and 5.6% had left the province. Of the 44,689 cases who were confirmed as dead on both the ACR and the AHCIP registration file, 574 (1.3%) of the dates of death do not agree between the two files. In 95% of these cases the AHCIP registration death date is later than that of the ACR. The discrepancies can be quite large: only 31% have differences of less than one year.

The following results are presented for the 76,164 cases of a first primary invasive cancer (excluding non-melanoma skin cancer) diagnosed between 1985 and 1993 while resident in Alberta. On average, 3.0% of the cases had left Alberta by 1998. (Table 2) This is somewhat lower than the age-standardized five-year emigration rate from Alberta between 1991 and 1996 of 3.5%.⁴ For the cohort of cases diagnosed between 1985 and 1993, and followed until March 1998, the cancer sites with the highest proportion of emigrants were testicular cancer (9.0%), cervical cancer (6.5%) and melanoma skin cancer (6.3%). The cancer sites with the lowest proportion of emigrants were prostate cancer (1.9%), lung cancer (1.6%) and pancreatic cancer (0.9%) (Table 2). Age standardization removes these differences except for those cancers with short survival rates, lung and pancreatic cancer, and for prostate cancer. In general, younger cancer cases were more likely to emigrate than older ones, but there is little difference in emigration rates among cancer sites. (Table 3).

For the first five years of follow-up, between 0.6% and 0.8% of all cases alive at the beginning of each year of follow-up left the province during the succeeding year (Table 4). This pattern continues after five years of follow-up. There are no marked differences in the patterns of emigration among the cancer sites. There are no differences in emigration patterns between the sexes.

The person-years at risk for the first five years of follow-up is reduced on overall by 1.7% of the total person-years at risk if all persons not known to be dead are assumed to be alive. The largest effect (3.5% reduction) is seen for cancer of the testis, and the smallest (0.8% reduction) for cancer of the prostate.