Abstract P1-09-08: Does adverse tumour biology contribute to inferior outcomes for Indigenous Australians diagnosed with breast cancer?
Authors: Redfern A , Lee E , Bulsara M , Threlfall T , Harvey J , Cordell D 
Published: May 2016

Background: Analyses across multiple Australian states have consistently demonstrated significantly inferior breast cancer survival for Indigenous patients (IPs). Studies compensating for increased remoteness, socioeconomic disadvantage and later presentation demonstrate a residual unexplained detriment. This survival disadvantage is confined to the first five years, akin to the inferior outcomes demonstrated by higher risk biological breast cancer subtypes. We postulated that a preponderance of such higher risk subtypes could explain the disparate mortalities.

Methods: The distribution of breast cancer subtypes in Western Australian IPs diagnosed between 2001 and 2010 was assessed to explore the contribution of adverse prognostic subtypes to poorer outcome. This was a retrospective cohort study of Indigenous women (n=114) and 3:1 age and remoteness matched non-Indigenous women (n=310) diagnosed with invasive, non-metastatic, unilateral breast cancer, who underwent definitive local treatment. Subtypes were assigned as luminal A, B, HER2 enriched and triple negative by ER, PR, HER2 and tumour grade comparisons. Differences in basic tumour demographics and biological sub-types were analysed and racial survival discrepancies explored within biological subtype cohorts.

Results: Hazards for overall and breast cancer-specific mortality in IPs were 4.07 (95% CI 2.55-6.49) and 4.19 (95% CI 2.42-7.25). IPs were significantly more likely to have grade 3 tumours (41 v 25%, p&lt;0.001), LN positive disease (39 v 27%, p&lt;0.001) and larger tumours (median 20 v 10 mm, p&lt;0.001). No significant differences in proportions of classical histological sub-types (ductal v lobular) or in tumours showing ER, PR or HER2 positivity were observed. There were no significant differences in biological sub-type proportions although IPs were diagnosed with numerically more non-Luminal A subtypes (56 v 44%, p=0.08), accounted for by increased Luminal B (21 v 15%) and HER2 enriched (10 v 5%) sub-types. The significant relative five-year survival deficit for IPs noted overall (94 v73%, p&lt;0.0001) was observed for each sub-type with the exception of HER2 positive patients. This extended from the relatively low risk luminal A sub-type where oral anti-estrogens are the mainstay of treatment (98 v 82%, p=0.0002) to the high risk triple negative sub-type where intravenous chemotherapy is the standard adjuvant therapy (94 v 50%, p=0.0014).

Conclusions: The contribution of adverse tumour biological subtype to poorer outcomes for Indigenous women is modest. Indigenous women with almost all biological subtypes fare significantly worse than their non-Indigenous contemporaries. Ongoing work includes more detailed biological comparisons of RNA expression and DNA mutation between groups as well as an exploration of potentially disparate treatment patterns.

Citation Format: Redfern A, Lee E, Bulsara M, Threlfall T, Harvey J, Cordell D. Does adverse tumour biology contribute to inferior outcomes for Indigenous Australians diagnosed with breast cancer?. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-09-08.