The relative risk of second primary cancers in Queensland, Australia: a retrospective cohort study

The number of people being diagnosed with cancer worldwide is rising sharply, mainly as a result of population growth, ageing and increases in the prevalence of several lifestyle factors related to cancer risk [1, 2]. Combined with improvements in survival, due in part to both earlier detection and better treatment, more people are living with a diagnosis of cancer than ever before, and this increasing trend is likely to continue [3].

One of the consequences of surviving cancer is the increased likelihood of being diagnosed with a second primary cancer. Among other things, these second cancers may be the result of lifestyle choices, genetics, environmental exposures (all of which may also be related to the first cancer) and late effects of treatment [3, 4].

For the patient and their treating clinician, quantifying and characterising the risks for second malignancies has important implications for screening (particularly when effective screening methods such as mammography are available), prevention strategies and counselling [3, 5, 6]. Identifying cancers which have an elevated likelihood of occurring together is also a useful starting point for investigating possible shared aetiologies and mechanisms of carcinogenesis [7].

This study reports for the first time the relative risks of a second cancer for people diagnosed with a first primary cancer in Queensland, Australia.

A retrospective cohort design was used for this analysis. De-identified case records were obtained from the Queensland Cancer Registry (QCR), a population-based registry covering the entire state. All public and private hospitals, nursing homes and pathology services throughout Queensland are required by law to notify the QCR about any patients diagnosed with cancer, except for non-melanoma skin cancers [8].

The study cohort included all Queensland residents diagnosed with a first primary invasive cancer between 1982 and 2001 who survived for a minimum of 2 months. Since classifications for childhood cancers are different to adult cancers [9], we decided to restrict the cohort to people who were 15 years or older at the time of first diagnosis. A small number of records were excluded because information was missing for age (n = 20), or because the person was known to have had a first primary cancer diagnosed prior to 1982 (n = 76).

The cohort was followed up until 31^st December 2006 (allowing a potential minimum of five years and a maximum of 25 years after the initial diagnosis) to ascertain the occurrence of second primary invasive cancers. Histologically similar cases of cancer at the same body site were included, unless the medical record indicated that the tumour was recurrent or metastatic. Synchronous primary cancers (those diagnosed within 2 months of the first primary cancer) [10] were excluded because they were more likely to have been diagnosed as a result of detection bias [6]. Third or subsequent primary cancers were not considered in this analysis.

Cancers of various body sites were analysed in a separate group if they averaged more than 100 eligible first primary cancers per year. These cancers are shown in Table 1. Cancers of the head and neck (ICD-O-3 topography codes C00-C14 and C30-C32) were grouped together prior to analysis, as were cancers of the colon and rectum (C18-C20 and C218). All remaining types of cancer were analysed collectively in the category labelled "Other", including cases where site was ill-defined or unknown.

Person years at risk (PYAR) among people diagnosed with a first primary cancer was calculated as the time from 2 months after diagnosis until 31 December 2006, date of death or date of diagnosis of a second primary cancer, whichever came first. Data were stratified by type of first primary cancer, type of second primary cancer, sex, age at first diagnosis, period of first diagnosis and follow-up interval. Analysis by period of first diagnosis was restricted to five years of follow-up to allow more consistent comparisons across time. The expected number of second primary cancers in each stratum was calculated by multiplying the sum of PYAR by the cancer-specific incidence rate experienced by the general Queensland population, matched by sex, age group and time period. Standardised incidence ratios (SIRs) were then obtained by dividing the observed number of cases of second primary cancer by the expected number. The SIR is thus used to estimate the risk of a cancer patient developing a second primary malignancy relative to the incidence of cancer among the general population. Confidence intervals (CIs) for the SIRs were derived from the Poisson distribution [11] and calculated at the 95% level of certainty.

All analyses were conducted using SAS v9.2 for Windows. Data required for this study was non-identifiable so no ethics committee approval was necessary.

The basic characteristics of the study cohort are summarised in Table 1. Among the 204,962 eligible cancer patients, a total of 23,580 second invasive primary cancers were observed during 1,370,247 years of follow-up (median follow-up = 5.5 years per person, interquartile range = 1.3 to 10.2 years per person). In terms of absolute numbers, second primary cancers were more common among males and increased with older age, which is consistent with the distribution of first primary cancers. About one in ten (10.6%) of the second primary cancers were diagnosed within a year of the first diagnosis, while more than one in five (20.6%) were diagnosed at least 10 years afterwards. The highest proportions of second primary cancers occurred following an initial diagnosis of melanoma (21.6%), colorectal cancer (12.9%), prostate cancer (12.7%), or female breast cancer (12.6%).

Cancer patients in our study cohort were at significantly higher risk of a second diagnosis compared to the underlying incidence rates experienced in the entire population of Queensland. Although there was some variation in the size of the estimated relative risks, a consistent pattern of increased risk following all first primary cancers combined was seen for both males and females, and across all age groups at first diagnosis, time periods of first diagnosis and follow up intervals.

Our data offers further evidence of significant associations between particular types of first and second primary cancers that have been previously documented elsewhere in Australia and around the world [6, 12‐16]. Some examples include the mutual increased risks between head and neck, lung and oesophageal cancers and the relationship between melanoma and both prostate and female breast cancers.

Much of the elevated risk of being diagnosed with a second malignancy can be attributed to risk behaviours (such as smoking, harmful levels of alcohol consumption and poor diet), inherited susceptibilities and/or the medical treatment that cancer survivors have received [3, 4, 6]. On occasions, treatment can also have the opposite effect of reducing the risk of subsequent diagnosis. As was noted in another recent study of second primary cancer [13], the overall lower SIRs observed among prostate cancer survivors are mainly due to extremely low rates of reoccurrence as the organ is often completely removed as part of treatment. Given that prostate cancer is the most common cancer diagnosed for males in Queensland, this also helps to explain why the total relative risk is lower for male survivors compared to female survivors.

Another potential explanation for the difference in risk between cancer survivors and the general population is that some demographic factors may not be comparable between the two groups. Although the relative risks were based on calculations matched by sex, age group and time period, it is possible that other qualities, such as socioeconomic status, may vary between people who have been diagnosed with cancer and those who have not. This is likely to be the reason for at least part of the reduced risk of lung cancer (which has higher incidence among lower socioeconomic groups) for males following melanoma (which is more common in segments of the community with higher socioeconomic status) [6, 17]. A similar explanation could be used for the deficit of lung cancer after a diagnosis of prostate cancer. In contrast, the high reciprocal relative risks of melanoma with both female breast cancer and prostate cancer may correlate with the higher incidence for each of these malignancies among more affluent populations [18‐20].

One of the interesting relationships that emerged from the analysis was the variation in relative risk of second primary cancer by age at first diagnosis following cancer of the brain and central nervous system. Younger survivors (15-49 years old) had an increased risk of being diagnosed with a second primary cancer while survivors aged 65 years or over had a decreased risk. This is most likely due to the various histopathological subtypes of brain tumours which are more common in the different age groups. For example, glioblastoma tends to be diagnosed at an older age and is associated with poor survival, allowing a limited time for treatment-related second primary cancers to appear [6].

The reasons behind an increased relative risk of second primary cancer following certain types of cancer among survivors who were diagnosed more recently are unknown. Tsukuma et al. [16] described a similar pattern in Japan, and suggested that apparent increases in risk may be due to improved follow-up and surveillance of cancer patients. Another possible cause is the change in treatment modalities over time [4, 21].

Second primary cancers that arise due to the effects of treatment for the initial cancer might be expected to occur many years after the first diagnosis. However, the increase in relative risk remained fairly consistent irrespective of time since diagnosis, in accordance with results published elsewhere in Australia [13] and the United States [6].

The main strengths of this study include the extensive population-based coverage achieved by the QCR for the reporting of cancers among Queensland residents, combined with a high level of histological verification (88% in 2006) [8], which is important for distinguishing between new primary cancers and metastases of an existing cancer. Since all data used in this study have been collected prospectively for administrative purposes, and coded independently of the hypotheses, the opportunity for recall or information bias has been removed.

Increased medical surveillance of newly-diagnosed cancer patients may introduce a detection bias for second primary cancers. The likelihood of this happening has been reduced by only considering metachronous primary cancers, with a two-month window between first and second diagnosis. It is also possible that some second primary cancers were incorrectly classified as first primary cancers, especially for cancers diagnosed soon after the establishment of the QCR in the early 1980s; we are unable to quantify the impact of this on the observed results.

While acknowledging that the study cohort and comparison population were not independent, with the population containing people already diagnosed with cancer, this proportion was less than 0.5% in any given year. Finally, as a result of the large number of comparisons made, the possibility that some of the SIR estimates have been spuriously identified as statistically significant needs to be considered, particularly those based on small numbers of primary or secondary cancers, and these results should therefore be interpreted with due caution.

Our results demonstrate that cancer survivors in Queensland, Australia, like those in other countries, are confronted by a very real, ongoing risk of developing a second primary cancer that is significantly higher than the incidence of cancer experienced by the general population. Some first and second primary cancers share common aetiologies, making it imperative that cancer patients adopt a healthier lifestyle in order to lessen their chances of subsequent diagnoses [4, 22]. Recent studies have reported little difference in the health behaviours of cancer survivors compared to the wider community [23‐25], suggesting that health promotion efforts may need to be specifically targeted at people diagnosed with cancer due to their subsequent increased risk. Further work is needed to determine exactly how beneficial changes to lifestyle may be in regard to the risk of developing a second primary cancer [26]. Even so, some second primary cancers will be unavoidable, and as the number of cancer survivors continues to grow, the importance of ongoing medical supervision and screening to detect second primary cancers at an earlier stage and thereby improve the effectiveness of treatment will remain critical.