2011 Joint AAPM/COMP Meeting
Jonathan Mamou, PhD
For more information about the American Association of Physicists in Medicine, visit aapm.org/

A reliable means of detecting metastases in lymph nodes is essential for accurate staging of
cancer and effective planning of therapy. The standard means of detecting metastases in nodes is
histopathologic evaluation of surgically dissected nodes; however, because examinations tend to
be limited to thin sections derived from the center of the node, this time-consuming procedure
appears to have high false-negative rates for metastases that are 2 mm or smaller. We have been
investigating high-frequency (HF) quantitative ultrasound (QUS) methods as a more-effective
means of providing a rapid and reliable detection of metastases in dissected nodes based on their
ultrasound-scattering properties.

We acquired HF ultrasound and histological data from dissected lymph nodes of patients with
colorectal, gastric, breast, and other cancers. Freshly dissected nodes were scanned in a saline
bath using a raster pattern to acquire 3D RF echo-signal data. Scans utilized a broadband, F-2,
25.6-MHz, single-element transducer with scan vectors separated by 25 µm in X and Y
directions. Scanned nodes were color inked to provide references for subsequent orientation,
then fixed and serially-sectioned in their entire volume at 50-µm intervals. The presence of
metastatic foci was determined histologically in every section; evaluation of node status in the
center section provided a basis for comparison with conventional methods.

To date, we have analyzed the echo signals of more than 240 nodes including abdominal nodes
of colorectal and gastric cancer patients and axillary nodes of breast-cancer patients. 3D images
generated from RF data were segmented semi-automatically to select nodal tissue for analysis.
Echo signals from nodal tissue were processed to yield QUS estimates, which included spectral
parameters, scatterer-property estimates, and B-mode envelope-signal statistical features.
Different histological node architectures were observed and different QUS results were obtained
for abdominal compared to axillary nodes.

Linear discriminant analysis and ROC-curve methods were applied to assess the ability of QUS
estimates to distinguish cancerous from non-cancerous nodes. Classification performance was
assessed for individual estimates and various linear combinations of estimates. ROC results for
axillary as well as abdominal nodes showed excellent classification. For abdominal nodes, the
best area under the ROC curve exceeded 0.95. For axillary nodes, the best area approached 0.90.
Images based on QUS parameters showed an excellent ability to depict metastatic foci.

Results to date strongly suggest that HF QUS methods may provide a clinically valuable means
of detecting small metastatic cancers in dissected lymph nodes and significantly improving the
sensitivity for metastases that might not be detected using current, standard histopathology
procedures. The ability of HF QUS to reveal otherwise missed metastases will enable
pathologists to more-efficiently focus histological effort on regions of nodes that have a high
suspicion of containing cancer and minimizing wasted time evaluating actually cancer-free
nodes. Future studies will investigate the applicability of these methods to detection of nodal
metastases in situ.

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