IMAGING

Prostate cancer: identification with combined diffusion-weighted MR imaging and 3D 1H MR spectroscopic imaging—correlation with pathologic findings
Mazaheri Y, Shukla-Dave A, Hricak H, Fine SW, Zhang J, Inurrigarro G, Moskowitz CS, Ishill NM, Reuter VE, Touijer K, Zakian KL, Koutcher JA
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY, USA
Radiology. 2008; 246: 480-8

• Purpose: To retrospectively measure the mean apparent diffusion coefficient (ADC) with diffusion-weighted magnetic resonance (MR) imaging and the mean metabolic ratio (MET) with three-dimensional (3D) hydrogen 1 ((1)H) MR spectroscopic imaging in regions of interest (ROIs) drawn over benign and malignant peripheral zone (PZ) prostatic tissue and to assess ADC, MET, and combined ADC and MET for identifying malignant ROIs, with whole-mount histopathologic examination as the reference standard.
• Materials and Methods: The institutional review board approved this HIPAA-compliant retrospective study and issued a waiver of informed consent. From among 61 consecutive patients with prostate cancer, 38 men (median age, 61 years; range, 42-72 years) who underwent 1.5-T endorectal MR imaging before radical prostatectomy and who fulfilled all inclusion criteria of no prior hormonal or radiation treatment and at least one PZ lesion (volume, > 0.1 cm(3)) at whole-mount pathologic examination were included. ADC maps were generated from diffusion-weighted MR imaging data, and MET maps of (choline plus polyamine plus creatine)/citrate were calculated from 3D (1)H MR spectroscopic imaging data. ROIs in the PZ identified by matching pathologic slides with T2-weighted images were overlaid on MET and ADC maps. Areas under the receiver operating characteristic curves (AUCs) were used to evaluate accuracy.
• Results: The mean ADC +/- standard deviation, (1.39 +/- 0.23) x 10(-3) mm(2)/sec, and mean MET (0.92 +/- 0.32) for malignant ROIs differed significantly from the mean ADC, (1.69 +/- 0.24) x 10(-3) mm(2)/sec, and mean MET (0.73 +/- 0.18) for benign ROIs (P < .001 for both). In distinguishing malignant ROIs, combined ADC and MET (AUC = 0.85) performed significantly better than MET alone (AUC = 0.74; P = .005) and was also better than ADC alone (AUC = 0.81), although the difference was not statistically significant (P = .09).
• Conclusion: The combination of ADC and MET performs significantly better than MET for differentiating between benign and malignant ROIs in the PZ. (c) RSNA, 2008.
  
  
• Editorial Comment
  Magnetic resonance imaging (MRI) combined to 3D-magnetic resonance spectroscopic imaging (MRSI) is the only non-invasive technique with the potential to provide useful information regarding the detection, localization, staging and prognosis of prostate cancer. The combination of these techniques (MRI+MRSI) has improved the diagnostic assessment of prostate cancer beyond the morphologic information provided by conventional MR imaging. To further improve the specificity and sensitivity of MRI+MRSI, other complimentary techniques such as dynamic contrast enhanced MR imaging and diffusion-weighted MR imaging (DWI) has been used in last years. DWI is used to detect the state of molecular translational motion of water in the tissue. In prostate cancer, densely packed malignant epithelial cells, causes restricted diffusion of water relative to that of normal tissue. Since apparent diffusion coefficient (ADC) reflects primarily diffusion coefficient of extra-cellular water, ADC values tend to be lower for tumors compared to normal tissue (benign tissue: ADC values > 1.3 cm2/s; cancer tissue: ADC values < 1.3 cm2/s). Contrary to cancer in BPH, extra-cellular space volume is higher, thus ADC values are higher as well. The authors of this retrospective study, nicely shows that the association of the measurements of the mean ADC values (DWI) with the mean metabolic ratio (MRSI) performs significantly better than the mean metabolic ratio alone for the discrimination of normal and malignant prostatic tissue of the peripheral zone. Since 2004, we have been using in our institution MRI+MRSI, DWI and dynamic contrast enhancement for all patients evaluated for detection or staging prostate cancer. We agree with the authors’ statement that MRSI+DWI are much better than the isolated use of any of these techniques. Unfortunately this combination does not prevent false positive results found particularly in patients with focal prostatic atrophy (1).

Reference
1. Prando A, Billis A: Focal prostatic atrophy mimicking prostatic cancer: an entity not known by the radiologist. Scientific poster presented at the RSNA 2007.

Dr. Adilson Prando
Chief, Department of Radiology and
Diagnostic Imaging
Vera Cruz Hospital
Campinas, São Paulo, Brazil