MR Spectroscopy Improves MRI Evaluation of Prostate Cancer
MR Spectroscopy Improves MRI Evaluation of Prostate Cancer
Abstract & Commentary
Synopsis: Three-dimensional proton MRSI increases the specificity of findings at standard MRI of prostate cancer. Although currently performed in a limited number of facilities, MRSI is poised to enter the radiologist’s diagnostic armamentarium soon.
Source: Scheidler J, et al. Prostate cancer: Localization with three-dimensional proton MR spectroscopic imaging—clinicopathologic study. Radiology 1999;213:473-480.
Researchers at the university of california at San Francisco are developing a new method for improving the noninvasive evaluation of prostate cancer. The technique is based on the fact that cancerous prostate tissue contains less citrate and more choline than does normal or hypertrophic prostate tissue. The researchers performed both standard magnetic resonance imaging (MRI) of the prostate and 3D proton spectroscopic imaging of the prostate in 53 patients with biopsy-proven prostate cancer. At T2-weighted MRI, foci of decreased signal in the peripheral zone of the prostate were considered to be suspicious for cancer. At magnetic resonance spectroscopic imaging (MRSI), areas with a choline plus creatine to citrate ratio > 2 SD above normal were considered to possibly represent cancer, and areas > 3 SD above normal were considered to represent cancer; such areas were color-coded in red and overlaid on the standard MR images. Results of MRI and of MRSI, alone and in combination, were then correlated with findings at histopathologic examination of the radical prostatectomy specimen in each patient.
If MRI and MRSI were both considered positive for cancer, the specificity was 87-91% for the two readers. The sensitivity was 89-95% when one of the two tests was considered suspicious or positive for cancer. Scheidler and colleagues conclude that the combination of MRI and MRSI is better than MRI alone for detecting and localizing foci of cancer in the prostate.
Comment by David M. Panicek, MD
Prostate MRI, typically performed with an endorectal coil, has become a relatively common examination in many academic centers and community-based imaging facilities. Despite the considerable information that can be gleaned from those anatomic images, the finding of a focus of abnormally low-signal intensity within the peripheral zone of the prostate on T2-weighted images is relatively nonspecific for the diagnosis of prostate cancer. Moreover, the thin prostatic capsule is often difficult to delineate in its entirety on those images, limiting evaluation for subtle extracapsular extension of tumor. These issues are important clinically for a multitude of reasons. For example, the location and volume of intraprostatic tumor can influence placement of brachytherapy implant seeds, as well as planning of external radiation therapy. Also, the volume of intraprostatic tumor correlates with the probability of lymph node metastasis and with patient prognosis, and can be monitored in patients who elect a strategy of "watchful waiting." Demonstration of the presence of extracapsular extension of tumor generally precludes performance of potentially curative radical prostatectomy. Additionally, images showing tumor at the prostatic apex alert the surgeon to potential difficulties in obtaining a clean margin during creation of the urethral-bladder anastomosis.
Three-dimensional proton spectroscopic imaging, by visually demonstrating the altered ratio of choline plus creatine to citrate in cancerous prostate tissue, provides key metabolic information that, when combined with standard anatomic MRIs, improves the identification and localization of cancer within the prostate in an objective manner. Also, as shown in another recent article from the same institution, this information can improve the accuracy of the often difficult task of evaluating for extracapsular extension of prostate tumor, particularly for less experienced readers.1 MRSI can be implemented on many state-of-the-art MR scanners, and appears likely to become a commonly performed examination in the near future.
Reference
1. Yu KK, et al. Prostate cancer: Prediction of extracapsular extension with endorectal MR imaging and three-dimensional proton MR spectroscopic imaging. Radiology 1999;213:481-488.
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