Background: Long-term daily use of aspirin has been associated with reduced cancer mortality. To explore this association, we compared tumour TNM characteristics among aspirin users with those among non-users.
Methods: From the Swedish Cancer Register, we identified patients diagnosed with colorectal, lung, prostate and breast cancers between 2006 and 2009 and matched them to the Swedish Prescribed Drug Register to obtain information on low-dose aspirin use prior to diagnosis. Contingency table and logistic regression analyses were used to test for association and obtain odds ratios (ORs).
Results: We identified 17,041 colorectal, 9766 lung, 29,770 prostate and 20,299 breast cancer patients. The proportion of low-dose aspirin users was ~26% among colorectal, lung and prostate cancer patients and ~14% among breast cancer patients. Adjusted for age, gender, education level and place of residence, low-dose aspirin use was associated with lower tumour extent (T) for colorectal and lung cancers (P<0.0001) but not for prostate and breast cancers. The adjusted OR of aspirin use for the T4 vs T1 categories was ~0.7 (95% confidence interval (CI) 0.6-0.8). For all cancers, we found no evidence of association of aspirin use with nodal involvement (N). Except for a borderline result in prostate cancer (OR ~0.9; 95% CI 0.8-1.0), aspirin use was associated with a lower rate of metastatic disease (ORs ~0.6-0.8). Among the histological subgroups of lung cancer, significant differences in tumour extent were observed most clearly within the adenocarcinoma subgroup (OR ~0.6, 95% CI 0.5-0.8), although numbers of other subtypes were more limited; and there was a significant reduction of ~20-30% in the odds of metastasis among the aspirin users across the subgroups.
Conclusion: Use of low-dose aspirin in the year prior to diagnosis was found to be associated with lower tumour extent and fewer metastatic disease for colorectal and lung cancers. For these cancers, the benefit of aspirin use appears to be during both early and late cancer progression.