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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.

1. What is a sentinel lymph node?

The sentinel lymph node is the first node draining a particular anatomical location. The location of a sentinel node is able to be reliably determined by a combination of the injection of a radiolabelled traced and blue dye. The status of the sentinel node is used as a marker of the status of the entire nodal basin.

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

Major morbidity from axillary dissection is uncommon. The problem that many women complain of is anaesthesia or paraesthesia in the axilla, lateral chest wall and medial arm which is related to division of intercostobrachial nerves. Disruption of the long thoracic nerve to serratus anterior or the nerve to latissimus dorsi results in a more significant function deficit. The medial pectoral nerve supplying pectoralis major is also at risk.
The rate of clinically significant chronic lymphoedema of the arm is as high as 10-15%. Seroma development in the wound is more common but usually resolves with repeat aspiration.
Shoulder stiffness usually responds to physiotherapy and it is part of the breast care nurse's and surgeon's role to discuss appropraiate exercises pre-operatively.

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

The false negative rate means that of all those axillas truely involved 7% will be falsely thought to be negative. It is the reverse of sensitivity (93%). This will lead to incorrect downstaging of the patient resulting in potential undertreatment with adjuvant therapies.
One other issue needs to be considered in order to make sense of the false negative rate. That is the incidence of involvement of the axilla in early breast cancer. If only 20% of patients with early cancers have axillary disease and 93% of these will be correctly detected then only 1.4% (7% of 20%) of all patients will have an incorrectly staged axilla.

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

You will need to draw up a table with 4 potential result types. True positives, false positives, false negatives and true negatives

Sensitivity equals true positives divided by true positives + false negatives =TP/(TP+FN)

Specificity equals true negatives divided by true negatives + false positives =TN/(TN+FP)