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Case 11: Statistically speaking

A 52 year old woman has a 25mm mass in her left breast. It feels malignant and this is confirmed by core biopsy. She has no palpable lymph nodes in the axilla and basic staging investigations are normal. She raises the question of sentinel lymph node biopsy (SNB) to avoid an axillary dissection. Her lymphoscintigram is shown below.

Breast lymphoscintigraphy

1. What is a sentinel lymph node?

2. What are the common or important risks of axillary dissection you would discuss?

3. She asks you about the 7% false negative rate. What does the figure mean?

4. How would you calculate the sensitivity and specificity for a test?