Effect of MR Lymphography on the Probability for Lymph Node Involvement in Patients with Prostate Cancer

CLINICAL RELEVANCE/APPLICATION: In prostate cancer patients, the presence of lymph node metastasis is an important factor when evaluating treatment options. MRL may help reduce the need of invasive staging by PLND. PURPOSE: To investigate the effect of the results of MR Lymphography (MRL), compared to using the Nodal Risk Formula (NRF), on the probability of lymph node metastases. METHOD AND MATERIALS: Our database contained 375 consecutive patients with prostate cancer, included retrospectively, who had a serum PSA level >10 ng/ml or a Gleason Sum (GS) > 6. All patients had been assessed by MR Lymphography (MRL) between 4/8/2003 and 4/19/2005, and had undergone pelvic lymph node dissection (PLND) or fine-needle aspiration biopsy. A contrast agent based on ultrasmall superparamagnetic particles of iron oxide (USPIO) was used to dicriminate lymph node metastases from normal lymphatic tissue. All MRL images were analysed by a radiologist. The risk of lymph node metastasis (Rm) was estimated using the Roach Nodal Risk Formula (NRF): Rm=(2/3)PSA+[(GS-6)10], where an outcome >15 is considered high risk. Two logistic regression models were constructed. In both, the result of histopathology of the lymph nodes was the dependent variable and the NRF result was taken as independent variable. In one of the models, the result of MRL was added as a second independent variable. The impact of interactive terms was also estimated. RESULTS: Positive lymph nodes were detected by histology in in 61 out of 375 patients (16%). A total of 21 out of 312 patients with a negative MRL result had metastatic lymph nodes. Of the 63 patients with a positive MRL result, 40 had metastatic lymph nodes. A total of 13 out of 131 with low risk according to NRF had metastatic lymph nodes. Of the 244 patients with a high risk according to NRF, only 48 had metastatic lymph nodes. In the regression analysis, interactive terms were not statistically significant and were excluded from the model. After including the result of MRL in the logistic regression model, the outcome of the NRF was no longer significant. The result of MRL was significant in the model (p<0.001). CONCLUSION: MRL has a significant effect on the probability of lymph node involvement. As soon as the result of MRL is known, computing the nodal risk with the Nodal Risk Formula is no longer appropriate.