In patients with breast cancer, the presence of nodal metastases limits the therapeutic options and also indicates worse prognosis. When a potentially "early" curable cancer has been detected, the next most critical step is therefore to determine whether the nodal basins are involved as part of the staging process. The TNM classification system has been revised to better reflect the prognostic implications of the discovery of lymph node metastasis in the various nodal basins draining the cancer-containing breast.1
Ultrasound (US) is more sensitive than physical examination in the detection of axillary nodal metastases and can visualize high axillary, infraclavicular, and internal mammary lymphadenopathy that cannot be assessed with palpation and mammography.
A few points must be kept in mind when using imaging modalities in general and US in particular to detect lymph node metastases from breast cancer:
- With all recent imaging modalities, the criteria for the diagnosis of lymph node metastasis remain to be defined (and evaluated).
- There are multiple nodes in the axilla, and a one-to-one correlation between the nodes imaged in vivo and the nodes examined pathologically from the axillary node dissection surgical specimen is rarely—if ever—possible, which may lead to errors in the reporting of an imaging modality's diagnostic accuracy. A satisfactory solution would be to perform an image-guided needle biopsy of any abnormal node with placement of a metallic marker for subsequent identification during the pathologic examination of the surgical specimen from axillary node dissection.
- Imaging techniques that rely on blood perfusion cannot be used for ex vivo examination of surgical specimens from axillary node dissection.
- Currently, no imaging modality can detect micrometastases (< 2 mm in diameter), the significance of which remains controversial. Although micrometastases possibly affect long-term survival, there is debate about whether their presence should alter patient management.
A few common-sense tips are useful in the evaluation of nodal basins:
- Multiple mildly abnormal nodes in the same nodal basin are probably benign.
- If similar mildly abnormal nodes are found in the contralateral basin, then the indeterminate nodes in question are probably benign (with the exception of lymphoma or leukemia).
- If only one or a few nodes are abnormal and other adjacent nodes appear completely normal, then these nodes are suspicious for metastasis until proven otherwise, usually via US-guided fine-needle aspiration (FNA).
Recent advances in US equipment used for small body parts include very-high-frequency and multiarray transducers that operate at peak frequencies of up to 17 MHz and provide exquisite spatial resolution. Such transducers allow visualization of lymph node metastases as small as a few millimeters.
Among recent image-processing techniques, real-time compound scanning, which was initially predicted to provide higher-quality images than those attainable with conventional US, has not proved as beneficial as hoped. In fact, in our experience, the significant blurring associated with this technique has a negative effect on image quality.
Tissue harmonic imaging slightly increases spatial resolution and boosts contrast. In our experience, though, it does not provide any substantial benefit in the US evaluation of nodal metastases.