Breast Cancer Staging: PET Scans, Ultrasounds, and Biopsies

Breast cancer continues to be a major health problem throughout the world and requires accurate staging to determine the most appropriate treatment and to estimate prognosis. Cancer staging describes the extent or severity of the disease. One way that cancer staging is determined is to assess if the cancer has metastasized (spread) to the lymph nodes. Lymph nodes are small egg shaped structures in the body that help fight against infection. The first lymph node (or group of nodes) that cancer spreads to is known as the sentinel lymph node.

There are multiple ways to assess for lymph node metastasis. The most definitive way is to perform a biopsy of the lymph node(s) where cancer spread is the most likely. A biopsy is the process of removing living tissue or cells from organs or other body parts of patients for examination under a microscope or in a culture to help make a diagnosis, follow the course of a disease, or estimate a prognosis. In breast cancer, the lymph node(s) biopsied are one or more (usually one to three) of the axillary lymph nodes, which are located under the arm pits. A more intensive surgical process is an axillary lymph node dissection, which involves removing at least six of the lymph nodes and having them analyzed at a laboratory to be tested for cancer. In most cases, if the sentinel lymph node biopsy shows metastasis, then a lymph node dissection follows.

Due to the invasiveness of these surgical procedures, there has been increased interest in developing non-invasive ways to detect breast cancer spread to the lymph nodes. One method is to use ultrasound scanning. Ultrasound scanning is a procedure that uses high-frequency sound waves to produce images of internal body structures. Another technique is a positron emission tomography (PET) scan combined with computed tomography (CT). When combined, this is known as PET/CT and can be used to scan the entire body. A PET scan involves injecting the patient with a small radioactive chemical and being placed in a machine that detects and records energy given off by the substance. The computer translates the energy into 3D pictures which provides information about how cells in the body are functioning because normal cells react differently to the chemical than healthy cells. CT scanning is an advanced imaging technique that uses x-rays and computer technology to produce more clear and detailed pictures than a traditional x-ray.

In a recent study published in the medical journal, Acta Radiologica, researchers compared the diagnostic utility of PET/CT scans to ultrasound scanning in detecting axillary lymph node metastases. The authors found that PET/CT scans were more accurate than ultrasounds in evaluating axillary lymph node metastases. The PET/CT scan also detected lymph node metastases in seven out of 91 patients (8%) that had not been detected by another imaging modality. However, because the PET/CT scan still has a low sensitivity (54%) for detecting axillary lymph node metastasis, it was noted that it cannot serve as a substitute for sentinel lymph node biopsy.

Suggested reading: The Mayo Clinic Breast Cancer Book

Reference: Riegger C, Koeninger A, Hartung V, Otterbach F, Kimmig R, Forsting M, Bockisch A, Antoch G, Heusner TA. Acta Radiol. (2012) Comparison of the diagnostic value of FDG-PET/CT and axillary ultrasound for the detection of lymph node metastases in breast cancer patients. Acta Radiologica, 53(10): 1092-8.