Background: The study of sentinel lymph nodes (SLN) assessed by One Step Nucleic Acid Amplification (OSNA, Sysmex, Kobe, Japan) creates a new variable, Total Tumor Load (TTL). This variable is defined as the total number of CK19 mRNA copies in all positive SLN (copies/microL). The latest edition of the Spanish Oncological Gynecology Society (SEGO) Guideline (2017) proposes a complete axillary lymph node dissection (ALND) when TTL is 15,000 copies or more in early breast cancer. In our center we are using OSNA to ascertain if there is axillary node involvement but the decision to proceed to ALND is based on Z0011 criteria. We want to determine if there is a correlation between clinical outcomes and TTL values, between TTL and pathological variables and if TTL is a useful tool to decide when to complete an ALND. Methods: Clinicopathological and follow up data were obtained from all patients with invasive breast cancer and SLN assessed by OSNA between 2011 and mid-year 2014 at our center. Results: A total of 167 patients underwent SNB assessed by OSNA with an average follow-up of 64,9 months. 166 were female and 1 male. Age range 27-87 years (mean 59.23). Tumor size range 27- 85 mm (mean 17). 86,8 % were ductal, 9,5 % lobular and 3,5 % other. 54.49% were luminal A, 25.14% luminal B, 11.9%, triple negative, 4.9%, Her2 positive and 4.19%. luminal B-Her2 positive.TTL was equal to 0 in 92 cases and greater than zero in 75 cases.40 cases showed a TTL higher than 15,000 copies. Only 12 cases met Z0011 criteria and had ALND.As of now, 3 patients have had locoregional relapse and 5 metastatic disease. 7 have died, only two from metastatic breast cancer. Conclusions:

Using Z0011 criteria, we have adequate clinical outcomes with a low rate of ALND

If we had based the axillary management on TTL values we would have multiplied the number of ALND by a factor of 2,4 (from 12 to 29).

We have observed a tendency to higher TTL in luminal phenotypes and to lower TTL in HER2 positive and triple negative subtypes.

Work is in progress to increase our sample size