Primary retroperitoneal neoplasms are a rare but diverse group of benign and malignant tumors that arise within the retroperitoneal space but outside the major organs in this space. Although computed tomography and magnetic resonance imaging can demonstrate important characteristics of these tumors, diagnosis is often challenging for radiologists. Diagnostic challenges include precise localization of the lesion, determination of the extent of invasion, and characterization of the specific pathologic type. The first step is to determine whether the tumor is located within the retroperitoneal space. Displacement of normal anatomic structures of the retroperitoneum is helpful in this regard. For tumors that are located within the retroperitoneum, the next step is to identify the organ of origin. Specific signs, including the "beak sign," the "embedded organ sign," and the "phantom (invisible) organ sign," are useful for this purpose. When there is no definite sign that suggests the organ of origin, the diagnosis of a primary retroperitoneal tumor becomes likely. Awareness of specific patterns of spread, specific tumor components, and tumor vascularity help in further narrowing the differential diagnosis. Attention to these diagnostic clues is essential in making an accurate radiologic diagnosis of primary retroperitoneal tumors and in obtaining clinically significant information.

Purpose:To investigate the utility of apparent diffusion coefficient (ADC) values for discriminating tumor in patients with prostate cancer from normal prostatic tissues in healthy adult men, and to identify correlations between ADC and histologic grade of prostate cancer. Materials and Methods:A total of 125 healthy male volunteers (mean age, 60 years; range, 50 -86 years) and 90 prostate cancer patients (mean age, 71 years; range, 51-88 years) underwent diffusion-weighted imaging (DWI) of the prostate with a single-shot echo-planar imaging sequence using b-factors of 0 and 800 sec/mm 2 . ADC was measured from two locations in the peripheral zone (PZ) and two locations in the central gland (CG) in normal subjects, and tumor locations of PZ or transition zone (TZ) in patients with prostate cancer. Results:Mean ADC values of tumor regions in both PZ (1.02 Ϯ 0.25 ϫ 10 Ϫ3 mm 2 /sec) and TZ (0.94 Ϯ 0.21 ϫ 10 Ϫ3 mm 2 /sec) were significantly lower than those in the corresponding normal regions (1.80 Ϯ 0.27 ϫ 10 Ϫ3 mm 2 /sec and 1.34 Ϯ 0.14 ϫ 10 Ϫ3 mm 2 /sec, respectively) (P Ͻ 0.0001 each). Furthermore, a significant negative correlation was identified between ADC in PZ cancer and tumor Gleason score ( ϭ Ϫ0.497, P Ͻ 0.0001). Conclusion:ADC values appear to provide acceptable diagnostic accuracy in both PZ and TZ.

Interpretation of combined T2-weighted, dynamic contrast-enhanced, and DW MR image findings can yield reasonable diagnostic accuracy in both the PZ (80% [191 of 240 regions]) and the TZ (74% [59 of 80 regions]).

Purpose:To evaluate the differences in enhancement of the abdominal solid organ and the major vessel on dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) obtained with gadolinium ethoxybenzyldiethylenetriamine pentaacetic acid (Gd-EOB-DTPA: EOB) and gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) in the same patients. Materials and Methods:A total of 13 healthy volunteers underwent repeat assessments of abdominal MR examinations with DCE-MRI using either Gd-DTPA at a dose of 0.1 mmol/kg body weight or EOB at a dose of 0.025 mmol/kg body weight. DCE images were obtained at precontrast injection and in the arterial phase (AP: 25 seconds), portal phase (PP: 70 seconds), and equilibrium phase (EP: 3 minutes). The signal intensities (SIs) of liver at AP, PP, and EP; the SIs of spleen, renal cortex, renal medulla, pancreas, adrenal gland, aorta at AP; and the SIs of portal vein and inferior vena cava (IVC) at PP were defined using region-ofinterest measurements, and were used for calculation of signal intensity ratio (SIR). There was no significant difference in mean SIR of liver at PP between EOB (0.529 Ϯ 0.124) and Gd-DTPA (0.564 Ϯ 0.139). Conversely, the mean SIR of liver at EP was significantly higher with EOB (0.576 Ϯ 0.167) than with Gd-DTPA (0.396 Ϯ 0.093) (P Ͻ 0.001). Results Conclusion:Lower arterial vascular and parenchymal enhancement with Gd-EOB, as compared with Gd-DTPA, may require reassessment of its dose, despite the higher late venous phase liver parenchymal enhancement.

Combined T2-weighted imaging, DWI, and DCE-MRI findings appear to be potentially useful for detecting and managing prostate cancer, even when performed for patients with gray-zone PSA levels.