10602 Background: The AJCC/UICC staging system categorises tumour size in multifocal and multicentric breast cancer (MMBC) using the largest dimension of the largest focus of tumour. This under-estimates the total tumour burden and therefore may under-estimate the potential of these tumours to metastasise. This study tests the hypothesis that an aggregate measurement of tumour size in MMBC is more closely related to nodal status than the size of the largest focus. Methods: This prospective cohort study involved women with ipsilateral invasive breast cancer with known nodal status using data from the ACT & SENSW BCTG database from July 1997 to June 2004. Pathology reports were reviewed to obtain tumour size in the unifocal group and nodal status in all women. The histopathology of MMBC cases were reviewed measuring all tumour foci that were recorded as: (1) the diameter of the largest deposit (LD), (2) the aggregate diameter (AD) of all deposits, and (3) a calculation of the aggregate volume (AV) using the largest dimension of each focus. The MMBC dimensions were compared with unifocal cancers and against nodal status using a multivariate logistic regression analysis and compared against each other using a stepwise method to determine which method is most predictive of nodal involvement. Results: 795 women met the criteria for study entry: 139 (17.5%) had MMBC and 656 (82.5%) had unifocal disease. When adjusting for tumour size method of measurement in MMBC, there was strong agreement between the AD MMBC and unifocal groups in terms of nodal status (p=0.7). Conversely, there were statistically significant differences in nodal status using LD (p=0.008) and AV (p=0.02) measurements in MMBC when compared to the unifocal group. Within the MMBC group the aggregated diameter measure was most significantly related to nodal status (p=0.001). Conclusions: The results of this study confirm that the current tumour staging system in MMBC under-estimates the potential for nodal spread and that aggregate diameter is more clinically useful. Nodal status is used in this study as a surrogate for tumour behaviour. Further studies are underway to assess relationship of aggregate size of MMBC to relapse and survival. No significant financial relationships to disclose.