The US diagnosis of a lymph node metastasis is based on the enlargement and/or focal deformity (bulge) at the periphery of the node and—at least as important—on the marked decrease in echogenicity exhibited by an intranodal metastatic deposit. Because the lymph circulates from the periphery to the hilum of the node, early metastatic deposits develop preferentially at the periphery.
Minute (measuring at least 4 or 5 mm) metastatic foci can be detected at the periphery of a totally echogenic node or if they produce a focal hypoechoic bulge on the surface of the node (Fig. 34-2). Even when the central fat is hypoechoic, metastatic deposits appear darker than the hypoechoic fat. Lymph nodes that are massively involved with metastatic tumor are easily recognized on US as rounded (when small) or irregularly shaped (when large) masses with little or no residual central echogenic fat (Fig. 34-3).
Sonogram of an early axillary lymph node metastasis. The metastatic deposit (arrows), which measures about 0.7 × 0.4 cm, creates a bulge at the surface of an otherwise normal-appearing, fat-replaced node.
Massive metastatic involvement of axillary nodes. Sonogram shows complete replacement of 3 nodes (N) by markedly hypoechoic tumor.
If the primary tumor contains microcalcifications, identification of microcalcifications within a node is synonymous with metastatic involvement (Fig. 34-4).
Metastatic axillary node with microcalcifications. Sonogram shows numerous punctate echoes within the node reflecting the presence of microcalcifications.
On PDUS, the Doppler signals associated with metastatic nodes range from absent to numerous and disorganized. This wide range of PDUS appearances of malignant nodes considerably limits the role of PDUS in the diagnosis of nodal metastases—at least of small ones. On the other hand, the demonstration on PDUS of a dense harmonious vascular network covering the thickened cortex of a moderately enlarged node in a fashion similar to the cortical perfusion of a kidney (ie, with fine, parallel, hair-like vessels nearly reaching the capsule) correlates well with a diagnosis of BRH (Fig. 34-5).
Benign reactive hyperplasia. Power Doppler sonogram shows a dense harmonious vascular network covering the thickened hypoechoic cortex.
Although US with state-of-the-art equipment can reliably detect lymph node metastases larger than 7 or 8 mm, it cannot, like other "nonfunctional" imaging modalities, demonstrate metastases that are only a few millimeters in size.
Because of the paucity of the cellular component of the metastatic deposit, nodal metastases from invasive lobular carcinomas, like the primary tumors from which they derive, can also have a deceptive US appearance and be very difficult to recognize. It is not unusual for such metastatic nodes to appear with an evenly thickened cortex and residual central fat, suggesting a benign node (Fig. 34-6). On cytology, only a few scattered cell groups are seen, and cytokeratin stain is often required for confirmation of the metastatic involvement.
Metastatic axillary node from infiltrating lobular carcinoma. Sonogram shows grossly even thickening of the hypoechoic cortex. This appearance can be seen with benign reactive hyperplasia.
The internal mammary chains constitute the second pathway for lymphatic drainage of the breast. US examination of the parasternal region is a simple, fast, and effective method of detecting internal mammary lymphadenopathy.5 Because normal internal mammary nodes are too small to be visible on US, any hypoechoic mass seen along the internal mammary chains in a patient with breast cancer should be viewed as a potential metastasis (Fig. 34-7).
Internal mammary metastatic node. Longitudinal extended-field-of-view grayscale sonogram along the left internal mammary chains shows a small metastatic node (arrow). (C, sternocostal cartilage.)
Metastatic internal mammary nodes are classified as N2 (in the absence of clinically evident axillary lymph node metastasis) or N3b (in the presence of clinically evident axillary lymph node metastasis). Detection of an internal mammary nodal metastasis (in addition to axillary metastases) therefore qualifies the disease as stage IIIC. (See Chapter 13 for cancer staging tables.)
When they are involved with metastatic disease, intramammary nodes are coded as axillary nodes. Any suspicious intramammary node in a cancer-containing breast should be sampled with FNA. However, the remote possibility of metastatic axillary nodes coexisting with a benign intramammary node in the vicinity of a cancer should be kept in mind. Therefore, a benign result of the FNA biopsy of an intramammary node should prompt the verification of any additional indeterminate axillary node.
In general and in simpler terms, a node that is completely replaced by echogenic fat (a "white node") is benign. A node that is enlarged, deformed, and completely hypoechoic (a "black node") is metastatic until proven otherwise. A hypoechoic node seen in an area where nodes are not normally seen (eg, internal mammary chains, supraclavicular fossa) is suspicious until proven otherwise.
Affected infraclavicular nodes are important to detect and confirm by FNA because their adverse prognostic significance is worse than that of axillary nodes.6
The presence of metastatic infra (N3a) or supraclavicular nodes (N3c) qualifies the disease as stage IIIC.
Metastasis to any other lymph node, including the cervical or contralateral internal mammary lymph nodes, is coded as a distant metastasis (M1).