325P - Impact of breast tumour location on axillary nodal involvement, chemotherapy use, and survival

Background

Axillary level I-II node (ALN) status is used to stratify breast cancer risk and inform systemic therapy decisions. We sought to investigate the association of breast tumour location with ALN involvement, use of chemotherapy, and breast cancer-specific survival (BCSS).

Methods

Patients diagnosed with stage T1-2 M0 breast cancer from 2004 to 2019 were identified from the SEER 17 registries database. Tumour locations were classified as outer quadrant, inner quadrant, or central. Covariates were age, race, median household income, year of diagnosis, tumour size, histology, grade, and ER/PR/HER2 status. Propensity score weighting with Bayesian additive regression trees was used to balance covariates between tumour locations. Doubly robust estimation was then performed with multivariable logistic and Cox regressions to assess the association of tumour location with binary outcomes and BCSS, respectively.

Results

A total of 453,387 patients with a median follow-up of 70 months were eligible for analysis, of whom 293,070 (65%) had outer quadrant tumours, 127,237 (28%) had inner quadrant tumours, and 33,560 (7%) had central tumours. After propensity score weighting, the rates of ALN metastases for outer quadrant, inner quadrant, and central tumours were 28.2%, 20.2%, and 32.6%, respectively. Chemotherapy was given to 38.0%, 36.5%, and 38.8% of patients. Ten-year BCSS were 91.6%, 91.0%, and 90.5%. Compared to outer quadrant tumours, inner quadrant tumours had lower odds of ALN metastases (OR 0.62; 95% CI 0.61-0.63; P<0.001) and central tumours had higher odds (OR 1.30; 95% CI 1.26-1.33; P<0.001). Similarly, inner quadrant tumours had lower odds of receiving chemotherapy (OR 0.90; 95% CI 0.88-0.91; P<0.001) while central tumours had higher odds (OR 1.07; 95% CI 1.04-1.10; P<0.001). BCSS was worse in both inner quadrant (HR 1.07; 95% CI 1.04-1.10; P<0.001) and central (HR 1.10; 95% CI 1.04-1.15; P<0.001) tumours.

Conclusions

The risk of ALN metastases from breast cancer varies with tumour location. Inner quadrant tumours have worse BCSS than outer quadrant tumours despite a lower risk of ALN metastases. Reliance on ALN staging without consideration of tumour location may lead to treatment decisions that are inconsistent with breast cancer risk.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.