CT and MR imaging are reported to be the most accurate non-invasive methods of detecting pelvic lymph node metastases. Scheidler et al.  concluded that CT and two-dimensional MR imaging perform similarly for the detection of lymph node metastasis, with a trend towards an improved accuracy for MR imaging. Therefore, both MR imaging and CT are recommended, because unlike lymphangiography (LAG), they are non-invasive. A recent study using MR imaging with a three-dimensional technique has revealed a specificity of 98% and a positive predictive value of 94% in the detection of nodal metastasis in bladder and prostate cancer . This is clinically relevant because this can facilitate the indication for (MR-guided) biopsy , which can avoid an invasive pelvic lymph node dissection in the case of a positive biopsy. The multi-planar reconstructions obtained with this technique allow the evaluation of not only nodal size, but also nodal shape. This is important because the cut-off point between normal and metastatic nodes differ for round and oval nodes. The smallest lymph node diameter that can be detected by this method is 2 mm. Different sensitivities and specificities are acquired depending on the selection of cut-off size [31, 33, 34].
An important limitation of CT and MR imaging in the evaluation of nodal metastasis is that both imaging methods depend on enlargement of lymph nodes as a criterion for metastasis. The problem is that metastasis may also be present in normal sized nodes, thus causing low sensitivities (36-60%) . Due to their high cost, CT and MR imaging in detection of nodal metastasis should only be performed in a selected group of patients with high risk for nodal metastases, which can be predicted by DRE, PSA and biopsy Gleason score [10, 31, 35].
Hematogenous metastases are most common in the axial skeleton. Currently, the mainstay for the detection of bone metastases is a radionuclide bone scan. However, it is well known that bone scans can yield false-negative findings, especially in cases of very aggressive metastases. Furthermore, the technique has a high false-positive rate, mainly due to degenerative disease, healing fractures and various metabolic disorders and their complications (e.g., osteoporosis and osteomalacia). It has been demonstrated that bone scintigraphy is unnecessary in the evaluation of newly diagnosed, untreated prostate cancer with no clinical signs of bone pathology and serum PSA levels of less than 10 ng/ml . In patients with an elevated PSA (> 10 ng/ml) or with locally advanced tumours, bone scans are considered worthwhile for detecting both asymptomatic and symptomatic metastasis. MR imaging is more sensitive in detecting bone marrow metastases compared to ra-dionuclide bone scanning . Therefore, MR imaging can be useful in the evaluation of patients suspected of having vertebral metastases with equivocal or negative bone scans. Thanks to its high spatial resolution, MR imaging may also guide needle biopsy procedures. Plain radiographs are the least sensitive in evaluating the axial skeleton for metastases. 50% of bone mineral content must be altered before evidence of metastasis is visible.
In summary, MR imaging has a major role in detecting nodal and bone marrow metastases in patients with bladder or prostate cancer with high risk for metastatic disease.
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