Cancer Causes and Control 15: 1021–1025, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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The incidence of cancer in people with intellectual disabilities Sheena G. Sullivan1, Rafat Hussain2, Timothy Threlfall3 & Alan H. Bittles1,* 1 Centre for Human Genetics, Edith Cowan University, Perth, WA 6027 Australia; 2School of Health, University of New England, Armidale, NSW 2351 Australia; 3Western Australian Cancer Registry, Department of Health, Perth, WA 6004 Australia Received 5 February 2004; accepted in revised form 8 July 2004
Key words: intellectual disability, cancer, data linkage.
Abstract Objective: During the last 50 years there have been significant improvements in life expectancy among people with intellectual disability (ID), and so their incidence of age-associated diseases, such as cancer, is rising. The aim of this study was to compare the rate of cancer in people with ID with that found in the general population. Methods: Information on 9409 individuals registered with the Disability Services Commission of Western Australia was linked to the State Cancer Registry, with 200 cases of cancer detected over 156,729 person-years. Standardised incidence ratios (SIRs) and 95% confidence intervals were calculated for both sexes separately by 5-year age groups for the period 1982–2001. The same procedures were adopted in the estimation of SIRs for specific types of cancers. Results: The age-standardised incidence of all cancers in people with ID was not significantly different from the general population. However, males with ID were observed to have a significantly increased risk of leukaemia, brain and stomach cancers, and a reduced risk of prostate cancer, while leukaemia, corpus uteri and colorectal cancers were significantly higher in females. Conclusions: Health practitioners need to be aware that with improvements in life expectancy the incidence of cancer in people with ID is likely to rise. More proactive health promotion campaigns may be needed for people with ID, who are likely to be poor users of screening services and whose symptoms may not be reported until they are in more advanced, less treatable stages of disease.
Introduction Intellectual disability (ID) affects approximately 1–2% of the population of developed countries and is typified by deficits in intellectual functioning and adaptive behaviour, with onset prior to 18 years [1]. Current evidence suggests that the prevalence of ID is increasing globally, with the majority of those affected resident in less developed countries [2], where ID rates of 4% to over 8% have been reported [3, 4]. To date, limited information has been available on the incidence of common morbidities among this group, and in particular on diseases associated with advancing age * Address correspondence to: Prof A.H. Bittles, Centre for Human Genetics, Edith Cowan University, 100 Joondalup Drive, Perth WA 6027, Australia. Ph.: +61-8-6304-5623; Fax: +61-8-6304 5851; E-mail:
[email protected]
[5]. Historically, reduced survival among people with ID meant that their risk of cancer was lower than for the general population. However, in recent years their life expectancy has been rapidly increasing [6, 7], accompanied by a greater incidence of certain types of cancer [5, 8]. A specific investigation of the profile of cancers in people with ID is necessary because syndrome- and lifestyle-related factors may mean that their profile of disease differs from the general population. There are some specific conditions in which the risk of both cancer and ID are high. For example, persons with Down syndrome have an increased risk of leukaemia and testicular cancer [9–11]; WAGR syndrome (Wilm’s tumour, aniridia, genitourinary abnormalities and retardation) is typified by both cancer and ID [12]; tuberous sclerosis is associated with brain tumours and renal cell carcinomas [13]; neurofibromatosis is characterised by
1022 multiple cutaneous tumours and tumours of the peripheral and central nervous system which may metastasise [14]; and ataxia telangiectasia may cause ID and is associated with increased risk of cancer, particularly of the lymphoid system [15]. Furthermore, tumours of the brain (both malignant and benign) can cause ID. Particular lifestyle factors may also affect the incidence of cancer among people with ID. Thus, the high rate of Helicobacter pylori has been linked with an increased risk of gastrointestinal cancer among people with ID living in institutional settings [16, 17]. Conversely, cervical cancer in women with ID is rare as many are sexually inactive [18]. Changes in lifestyle associated with the movement of people with ID from large institutions into smaller community-based residences may, however, result in a greater tendency to engage in risky behaviour, including smoking, and result in increasing rates of respiratory and other smoking-related cancers [5]. Given the limited information on cancer in people with ID, the present investigation was conducted to document the incidence of cancer among individuals with ID living in Western Australia (WA).
Subjects and methods The subjects were identified through the Disability Services Commission (DSC) of WA. The Commission has maintained a large database comprising demographic and diagnostic information on all clients referred for services since 1953 and is estimated to service 70% of persons eligible for ID services in WA [19]. Level of disability is determined by clinical assessment and, where possible, by adaptive behaviour and IQ testing, with a score of 70 or less on both tests accepted as indicative of ID. Diagnosis is made according to the Heber classifications [20]. By the end of 2001, the DSC database contained information on 14,311 individuals. Cases of cancer were identified by linkage of the DSC database with the WA Cancer Registry. Mandatory notification of cancer has been in place in WA since August 1981, with details of all neoplasms forwarded to the office of the Executive Director, Public Health and added to the state Cancer Registry (http:// www.health.wa.gov.au/wacr/). For the purposes of the present study, cases were classified according to the International Classification of Diseases, ICD-10AM [21]. On the basis of the information available, a total of 9409 individuals (5490 males and 3919 females) from the DSC database were considered to be eligible for linkage with the Cancer Registry. From the original 14,311 individuals, 592 were excluded because they had died
S.G. Sullivan et al. prior to 1982, 3422 were considered ineligible for ID services after assessment, and for a further 889 the referrals process had not been completed so there was no evidence that they would be eligible for services. Inclusion in the study began on 1 January 1982 for those born before 1982, or on the date of birth for those born after 1982 (complete registry records were available from 1982 onwards). The exit date for the study was set to 31 December 2001, except for individuals who had died or left the state and for whom person-years were censored to the date of death or emigration. Standardised incidence ratios (SIRs) and 95% confidence intervals were computed to compare the observed and expected rates of cancer among people with ID, based on information available for the general population from the WA Cancer Registry for the period 1982–2001. The expected number of cases was estimated for each of the 5-year age groups by applying the WACR age-, sex- and period-specific rates for cancer incidence. The same procedures were adopted in the estimation of SIRs for specific types of cancers. The analysis was undertaken using PAMCOMP [22] and SPSS (Version 7) software. Ethical approval for the study was obtained from the Confidentiality of Health Information Committee at the Department of Health, WA, and from the Human Research Ethics Committees of the Disability Services Commission and Edith Cowan University.
Results A total of 156,729 person-years were generated for the 9409 individuals with ID whose records were linked to the Cancer Registry. The mean age of people in the sample was 30.8 years (SD ¼ 16.3) and the median age was 30.7 years. This compares with the median age for the general population in WA of 34.2 years [23]. Overall, there were fewer cases of cancer observed among people with ID and a similar pattern was observed when sex-specific incidence ratios (IR) were estimated (Table 1). For males, 103 cancers were detected over 90,660.6 person years, compared to an expected number of 349.2 cases (unadjusted IR ¼ 0.29, 95% CI ¼ 0.24–0.36). For females, the observed number of cancer cases was also significantly lower than the expected number, with 97 cancers detected over 66,068.3 person years compared with 215.8 expected (unadjusted IR ¼ 0.45, 95% CI ¼ 0.36–0.55). The markedly reduced rates of cancer reported in the total ID population result in part from small sample numbers, especially in the older age groups who are at greater risk of cancer. When the ratios were
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Cancer and intellectual disability Table 1. Age and sex-specific SIRs with 95% CI for all cancers in people with intellectual disability Males p-y
Females
Obs
Exp
SIR
95% CI
p-y
Obs
Exp
SIR
95% CI
All cancers 90,660.6
103
349.2
0.29
0.24–0.36
66,068.3
97
215.8
0.45
0.36–0.55
By age 0–4 5–9 10–14 15–19 20–24 25–29 30–34 35–39 40–44 45–49 50–54 55–59 60–64 65–69 70–74 75–79 80–84 85+ Total
15 3 1 2 6 2 9 10 8 13 7 8 4 11 3 1 0 0 103
2.08 1.05 1.14 2.22 3.57 6.14 7.27 8.26 8.65 9.21 9.33 8.39 7.28 5.56 4.38 2.41 1.85 1.63 90.42
7.23 2.87 0.88 0.90 1.68 0.33 1.24 1.21 0.93 1.41 0.75 0.95 0.55 1.98 0.68 0.41 0.00 0.00 1.14
4.04–11.92 0.59–8.38 0.02–4.89 0.11–3.25 0.61–3.65 0.04–1.18 0.57–2.35 0.58–2.23 0.40–1.82 0.75–2.41 0.30–1.55 0.41–1.88 0.15–1.41 0.99–3.54 0.14–2.00 0.01–2.31 – – 0.93–1.38
6091.9 6682.7 6855.3 6708.2 6800.7 7019.8 6633.8 5863.0 4605.3 3264.5 2160.6 1349.7 881.4 557.3 333.7 146.2 72.1 42.2
7 0 1 4 1 5 8 10 16 9 12 8 3 5 3 1 3 1 97
1.07 0.64 0.80 1.41 2.45 4.56 7.17 9.66 11.92 12.69 11.69 9.32 7.83 6.21 4.32 2.30 1.23 0.80 96.09
6.57 0.00 1.25 2.84 0.41 1.10 1.12 1.03 1.34 0.71 1.03 0.86 0.38 0.81 0.69 0.43 2.44 1.25 1.01
2.64–13.53 – 0.03–6.95 0.77–7.27 0.01–2.28 0.36–2.56 0.48–2.20 0.50–1.90 0.77–2.18 0.32–1.35 0.53–1.79 0.37–1.69 0.08–1.12 0.26–1.88 0.14–2.03 0.01–2.42 0.50–7.14 0.03–6.94 0.82–1.23
9521.3 10,064.6 9731.8 9489.4 10,154.2 10,455.5 9509.1 8047.6 5618.0 3636.9 2131.3 1085.0 577.0 297.5 171.6 76.4 51.9 41.8
standardised by 5-year age groups the effect of age was reduced, and the cancer risk in the ID group did not differ significantly from the general population for males (SIR ¼ 1.14, 95% CI ¼ 0.93–1.38) or females (SIR ¼ 1.01, 95% CI ¼ 0.82–1.23). For certain age groups the cancer risk did, however, appear to differ significantly. For example, in the 0–4 year age group the SIRs were markedly higher for both sexes: males (SIR ¼ 7.23, 95%
CI ¼ 4.04–11.92); females (SIR ¼ 6.57, 95% CI ¼ 2.64– 13.53). The age-standardised SIRs for some of the more common cancers among people with ID are shown separately by sex in Table 2. While the age-standardised SIRs for most cancers appeared to be lower than the reference population for the period 1982–2001, the results were not statistically significant. Among the
Table 2. Age standardised SIRs for selected cancers (all ages) in people with ID 1982–2001 Males (90,660.6p-y)
Females (66,068.3p-y)
Cancer type
obs
exp
SIR
95% CI
Obs
exp
SIR
95% CI
Lung Skin (melanoma only) Stomach Colorectal All lymphomas All leukaemias Myeloma & other plasma cell tumours Breast Cervix uteri Corpus uteri Ovary Prostate gland Testis Thyroid gland Brain Unknown primary site
5 10 7 13 4 14 1 0 – – – 2 7 2 9 2
7.90 17.33 2.19 9.38 6.02 4.23 0.73 0.14 – – – 8.86 4.89 1.19 3.26 3.11
0.63 0.58 3.19 1.39 0.66 3.31 1.36 0.00 – – – 0.23 1.43 1.69 2.76 0.64
0.21–1.48 0.28–1.06 1.29–6.59 0.74–2.37 0.18–1.70 1.81–5.56 0.03–7.60 0.00 – – – 0.03–0.82 0.58–2.95 0.20–6.09 1.26–5.24 0.08–2.32
2 13 1 9 1 12 2 21 2 8 3 – – 3 4 4
4.56 14.74 1.24 2.91 3.65 2.58 0.67 30.25 5.68 2.68 2.87 – – 3.65 2.63 2.12
0.44 0.88 0.80 3.10 0.27 4.64 2.98 0.69 0.35 2.98 1.04 – – 0.82 2.28 1.41
0.05–1.58 0.47–1.51 0.02–4.49 1.42–5.88 0.01–1.52 2.40–8.11 0.36–10.77 0.43–1.06 0.04–1.27 1.29–5.87 0.22–3.05 – – 0.17–2.40 0.84–4.97 0.29–4.13
1024 statistically significant results for males, there was an increased incidence of leukaemia (SIR ¼ 3.31, 95% CI ¼ 1.81–5.56), cancer of the stomach (SIR ¼ 3.19, 95% CI ¼ 1.29–6.59) and brain cancer (SIR ¼ 2.76, 95% CI ¼ 1.26–5.24), while the incidence of prostate cancer was lower (SIR ¼ 0.23, 95% CI ¼ 0.03–0.82). For females, besides significantly higher age-standardised SIRs for leukaemia (SIR ¼ 4.64, 95% CI ¼ 2.40–8.11), an increase in the incidence of colorectal cancers (SIR ¼ 3.10, 95% CI ¼ 1.42–5.88) and cancer of the corpus uteri (SIR ¼ 2.98, 95% CI ¼ 1.29–5.87) was observed (Table 2).
Discussion The study demonstrates the value of data linkage, without which it would not have been possible to collect detailed data on the incidence of cancer in the ID population. By linkage with Cancer Registry data the age-standardised risk of cancer among people with ID was found to be comparable to that in the general population of WA. However, the risk varied by site and type of cancer. The increased risk of leukaemia was partly due to the higher incidence of these cancers in people with ID due to Down syndrome; and leukemias accounted for 65% of the cancers in the Down syndrome group, which is comparable to the findings of two Scandinavian studies [10, 11]. Our results also indicate quite variable risks of cancer between the sexes, particularly with reference to stomach cancers, which have been among the more common gastro-intestinal cancers reported in people with ID [5]. This is partly due to the increased risk of H. pylori infection among institutionalised samples [16, 17, 24]. In the present study the age-standardised incidence of stomach cancer was significantly higher among males with ID compared to the incidence in the general male population of WA, but no equivalent relationship was seen in females with ID. The majority of the sample have never lived in large institutions, and many of those who did live in hospitals in the past were moved into group homes and hostels in the 1980s. Unfortunately, the absence of adequate longitudinal information on residential patterns precluded an analysis of the risk of stomach cancer by type of accommodation. Similarly, it was not possible to determine whether the higher incidence among males with ID was related to their duration of stay in institutionalised settings. In contrast, colorectal cancers were significantly more common among women with ID, while men showed no significant increase in risk. Turner and Moss [25] attributed the higher incidence of cancers of the colon
S.G. Sullivan et al. and rectum in men with ID to obesity and it is plausible that the underlying risk association is similar for women. An increased risk of colorectal cancer in women has been linked to diet with a high glycemic load and/or presence of hyperinsulinemia or insulin resistance [26], but the lack of detailed medical histories of our study sample makes it difficult to explain the reasons for the observed results. The point estimate for brain cancers indicated an almost 3-fold increase in risk among people with ID, although the results were only significant for men. As brain tumours can be a cause of ID and may be associated with the clinical course of the disease, it might be expected that they would be seen at higher frequency in the ID group [14, 15]. In this sample intracranial neoplasm was the diagnosed cause of ID for two people, and a further three clients had conditions which may increase the risk of brain tumours, i.e., neurofibromatosis (n ¼ 1) and tuberous sclerosis (n ¼ 2). With regard to sex-specific cancers, an increased risk of ovarian and corpus uteri cancers has been linked to the practice of chemical or surgical sterilisation to control menstrual flow among women with ID [18]. While it was not possible to verify this association in our study, there was a significantly increased incidence of cancers of the corpus uteri. Breast cancers were observed at a reduced rate, despite the presence of significant risk factors such as nulliparity, but this may be related to reduced uptake of breast screening and hence lower detection in the ID group [27]. Likewise, the lower incidence of prostate cancers in men may be linked to low detection by screening. Although testicular cancer has been reported to be more common among men with ID [5] we were unable to demonstrate any significant excess rate which again could be due to the paucity of the sample. As with previous studies, wide confidence intervals resulting from small incident numbers has limited interpretation of the results for many of the cancers [8, 18, 24]. For example, although the incidence of lung and other smoking-related cancers appeared to be lower in the ID cohort than in the general population, the difference was not statistically significant. Nonetheless, it has been documented that the risk of these cancers is greater among people with a mild or moderate level of ID, who are more likely to smoke than severely affected clients [28]. Six of the seven lung cancers in our study sample were indeed detected in people with mild ID, with cases of lip, tongue and laryngeal cancer only detected in mildly and moderately affected clients. Skin cancers were similarly more rare in people with ID, though not significantly. Once again, it is probable that their exposure to risk factors (in this case, sunlight) has been reduced because of their closely monitored lifestyle.
Cancer and intellectual disability Although the incidence of cancer in our study population of ID did not differ significantly from that in the general population, health providers need to be aware that the incidence of cancer in people with ID is likely to rise with continuing increases in their life expectancy. Health promotion campaigns targeted at the general population may have little impact on this group, since individuals with ID often have poor literacy skills or are unable to understand the content of health education, and may be incapable of self-advocacy [29]. Thus, it is improbable that they will use the screening services available for detecting common cancers, such as prostate and breast cancer, and will also be less likely to report symptoms to their doctor at early, more treatable stages of disease [30]. Given the current emphasis on independent and community living for people with ID, health promotion and cancer detection may become harder to facilitate as more individuals move away from institutional contact with specialist medical services. Hence a more proactive approach on the part of health practitioners may be needed to detect cancer in people with ID than would be the case with the general population. Acknowledgement We wish to thank the Disability Services Commission of Western Australia for their continuing support during the course of this project. References 1. Wen X (1997) The Definition and Prevalence of Intellectual Disability. Canberra: Australian Institute of Health and Welfare. Report No.: DIS-2. 2. WHO (2000) Healthy Ageing: Adults with Intellectual Disabilities. Summative Report. Geneva: World Health Organization. Report No.: WHO/MSD/HPS/MDP/00.3 3. Roeleveld N, Zielhuis GA, Gabreels F (1997) The prevalence of mental retardation: a critical review of recent literature. Dev Med Child Neurol 39: 125–32. 4. Durkin MS, Hasan ZM, Hasan KZ (1998) Prevalence and correlates of mental retardation among children in Karachi, Pakistan. Am J Epidemiol 147: 281–288. 5. Hogg J, Northfield J, Turnbull J (2001) Cancer and People with Learning Disabilities: The Evidence from Published Studies and Experiences from Cancer Services. Kidderminster: Bild Publications. 6. Bittles AH, Petterson BA, Sullivan SG, et al. (2002) The influence of intellectual disability on life expectancy. J Gerontol A Biol Sci Med Sci 57: M470–M472. 7. Patja K, Iivanainen M, Vesala H, Oksanen H, Ruoppila I (2000) Life expectancy of people with intellectual disability: a 35-year follow-up study. J Intellect Disabil Res 44: 591–599. 8. Jancar J (1990) Cancer and mental handicap. A further study (1976–85). Br J Psychiatry 156: 531–533.
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