Detection of occult lymph node (LN) metastasis can alter treatment selection and patient outcome. An exciting new technique developed over the past decade is reviewed in the epub version of Abdominal Imaging with regard to technique and outcomes in prostate and bladder cancer.

Dr. Saokar and associates at the Massachusetts General Hospital report on lymphotrophic nanoparticle enhanced MR (LNMRI). The technique utilizes ultra-small superparamagnetic iron oxide particles coated with low molecular weight dextran. The nanoparticles range from 30-50nm and have a prolonged blood half-life. The particles cross the capillary walls following intravenous injection and are transported to LNs. This transport process takes 24-36 hours and MR imaging is then performed.

LNMRI indirectly evaluates macrophage function in the LN. Normal or benign nodes phagocytose the nanoparticles resulting in a drop in the MR signal intensity due to the effect of the iron oxide. These nodes appear dark on T2 weighted images. Malignant LNs are replaced with tumor cells and therefore have fewer or no macrophages and less iron oxide accumulation. These appear bright on T2 weighted images. The iron oxide particles are biodegraded, enter the plasma iron pool and incorporated into hemoglobin. Side effects from the iron oxide injection occur in 28% of patients, are usually mild and consist of headaches, uticaria, back pain and vasodilatation.

MR imaging is performed before and after particle injection, so that comparison can be made. Quantitative methods are now used to accurately assess degree of uptake and thus reduce variability of interpretation. False-positive results can occur due to granulomatous disease, infection, focal nodal necrosis or a prominent fatty LN hilum.

In a group of 30 patients with genitourinary malignancy, the sensitivity and specificity were 100% and 80%, respectively (Radiology 1998; 207:799-808). In a recent trial of prostate cancer patients, the detection of metastatic LN increased from 35% with MR alone to 91% using LNMRI (N Engl J Med 2003; 348:2491-2499). The specificity also increased from 90% to 98%. In 58 bladder cancer patients, a sensitivity and specificity of 76% and 99% were reported (Radiology 2004; 203:449-456).

This technique is superior to traditional imaging for the detection of LN metastasis as it adds a functional component to LN size and shape to improve the diagnostic capabilities.

By Christopher P. Evans, M.D.

Abdom Imaging 2006 DOI 10.1007/s00261-006-9119-7 Link here.

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