MRI Can Identify Metastatic Lymph Nodes

November 29, 2011 (Chicago, IL) – Diffusion-weighted magnetic resonance imaging (DWI) can be used to detect pelvic lymph node metastases even in normal-sized lymph nodes. DWI is noninvasive, has a high negative predictive value, and might improve the staging of bladder and prostate cancer.

The research was presented during a prostate imaging session here at the Radiological Society of North America 97th Scientific Assembly and Annual Meeting.

Evis Sala, MD, PhD, from the University of Cambridge, United Kingdom, who moderated the session, described the work on pelvic lymph nodes as "the best talk of the session."

Dr. Harriet Thoeny

Harriet C. Thoeny, MD, from the University Hospital Inselspital in Bern, Switzerland, presented the results of the prospective study and discussed the implications with Medscape Medical News. She explained that if bladder lymph nodes are larger than 1 cm and prostate lymph nodes are larger than 8 cm, they are typically flagged for biopsy. However, her team found that approximately 25% of patients with normal-sized lymph nodes have micrometastases.

The study involved 87 patients who underwent conventional magnetic resonance imaging and DWI. Forty lymph nodes were removed from each patient, for a total of 3533 lymph nodes. As Dr. Thoeny explained, "that is the really huge and really the sexy part of this presentation…. Our surgeons took out all lymph nodes."

Image analysis was performed prospectively by 3 independent readers, and the results of the DWI analysis were compared with biopsy results. On a per-patient basis, the 3 readers yielded sensitivities of 64%, 80%, and 84% and specificities of 80.6%, 91.9%, and 90.3%. This resulted in positive predictive values of 57.1%, 80%, and 77.8% and negative predictive values of 84.7%, 91.9%, and 93.3%.

Diagnostic accuracies were 75.9%, 88.5%, and 88.5% for the 3 independent readers. The kappa value between readers 1 and 2 was 0.54, between readers 1 and 3 was 0.65, and between readers 2 and 3 was 0.67.

DWI has been used for many years to image the brain, particularly for the detection of acute stroke. Only recently have investigators directed the technique at the more movement-prone torso.

DWI can be quantified using apparent diffusion coefficient (ADC) values. Previous studies have shown that tumors have low ADC values, but only recently have ADC values been used to identify lymph node metastases. Studies to date have focused on enlarged lymph nodes, and have found that they have low ADC values when they have metastatic cancer. This is the first study to show that some normal-sized lymph nodes have low ADC values and are positive for metastases.

All of the metastatic lymph nodes identified in this study were smaller than 5 mm, and many were smaller than 3 mm, suggesting that they would have been missed with conventional techniques.

Dr. Thoeny and Dr. Sala have disclosed no relevant financial relationships.

Radiological Society of North America (RSNA) 97th Scientific Assembly and Annual Meeting: Abstract SSC06-06. Presented November 28, 2011.