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  • All participants gave written informed consent for the use of their clinical and environmental data. The collection of data was approved by the Human Research Ethics Committee of the Sydney South West Area Health Service. Participants were selected from eligible individuals who submitted self-completed questionnaires to the Australian Motor Neuron Disease DNA Bank, centred at the University of Sydney. The DNA Bank collected clinical, demographic and environmental data from patients with sporadic MND (SMND) and controls (individuals without SMND), recruited via MND Associations in each state of Australia. The duration of data collection was between 2000 and 2011. Inclusion criteria for this study included being a white Australian resident over the age of 26 years. Cases were those individuals diagnosed with SMND for whom clinical notes and special investigations were available from treating neurologists. Patients with SMND were classified as having SALS if they fulfilled the probable or definite revised El Escorial criteria for ALS (with both upper and lower motor neuron signs) [32]. Patients with a progressive lower motor neuron disorder with no upper motor neuron signs were classified as having sporadic progressive muscular atrophy (SPMA); those with a progressive upper motor neuron disorder without lower motor neuron signs were classified as having sporadic primary lateral sclerosis (SPLS); and those who had a progressive upper and lower motor neuron disorder involving only the bulbar muscles were classified as having sporadic progressive bulbar palsy (SPBP). Controls consisted of individuals without SMND who were partners or friends of SMND patients, or community volunteers. Identification of cancer was based on individuals' responses to the questions asking “List any significant past medical illness or health problems” and “List any previous injuries and/or surgeries”. It was assumed a diagnosis of cancer was a significant enough event in the life of participants for almost full reporting. Keywords used to identify malignancies were “cancer”, “cancerous tumour”, “malignant tumour”, “malignancy” or specific individual malignancies such as “melanoma”. In addition, reporting a surgery or procedure that would be undertaken exclusively for a particular cancer was included as a cancer item (e.g., patients having undergone a radical prostatectomy were assumed to have had prostate cancer). No benign tumours were classified as cancers. For all cancers identified, the following details were noted: (1) year of diagnosis; (2) tumour type (e.g., carcinoma or sarcoma); and (3) the organ affected. Diligence in answering the questionnaire: The presumably non-relevant questions “Have you ever had a pet?” and “If yes, list the types of pets you have had” were included to compare the diligence of the SMND and control groups in answering the questionnaire. Data were analysed using SPSS v20. Chi-square and Fisher's exact tests from contingency tables were used to determine odd ratios (ORs) with 95% confidence intervals (CIs) for developing SMND, in those with and without cancer. Unconditional logistic regression analyses were used to correct for confounding of covariates. Unconditional logistic regression gives inaccurate estimates if a covariate has a strong association with both the dependent and independent variable [33]. Multiple regression analyses showed significant colinearity between age and both SMND and cancer incidence, so age was not corrected as a covariate in the analysis. Due a statistically significant difference in average age between SMND and control females initially, a nested cohort was created in which female controls were randomly matched to a female SMND patient within a 5-year age range at a 1∶1 ratio. This resulted in almost identical mean ages between the two cohorts for both males and females (Table 1), and allowed age to be excluded from the logistic regression analysis. Table data removed from full text. Table identifier and caption: 10.1371/journal.pone.0103572.t001 Gender, age and smoking status in SMND and control individuals. N: number, SD: standard deviation. The potential covariates contributing to the risk of cancer were considered to be gender and smoking history. Subgroup analyses were undertaken in: (1) 10-year age groups from 41 to 90 years (this excluded 68 participants aged below 40 years and two aged over 90 years); (2) smokers and non-smokers; (3) different types of tumours; and (4) the four SMND clinical subgroups. Within the smoker and non-smoker subgroups, smoking-related cancers (of the oral cavity, throat, oesophagus, stomach, pancreas, lung, cervix, bladder, kidney and colon) and non-smoking-related cancers [34] were considered separately. Where numbers were insufficient for logistic regression (<5 individuals per cell), contingency tables were used to calculate ORs, 95% CIs and Fisher's exact p-values [35].
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