Magnetic resonance spectroscopic imaging (MRSI) is in many ways appealing for the evaluation of prostate cancer. This technique can be combined with standard magnetic resonance imaging (MRI), and spectroscopic energy combines anatomic resolution with the ability to determine metabolic activity of the tissue. In the past, intrarectal coils have demonstrated excellent anatomic resolution but have not been precise enough to demonstrate microscopic extension postbiopsy in all patients. Previous biopsies and bleeding can make interpretation even more difficult.
On the other hand, intraprostatic metabolic evaluation can be helpful in the detection of prostate cancer, and MRSI can identify changes in the ratios of choline to citrate that help distinguish areas of cancer. Elevation of choline is associated with changes in the neoplastic cellular membrane, and reduction in citrate is associated with changes in the neoplastic cell metabolism. Polyamines such as spermine may also be reduced in cancer. As a result, MRSI also can improve cancer detection in patients with postbiopsy bleeding.Future Applications
It appears that MRSI can significantly improve the accuracy of cancer diagnosis, making it feasible to intensify radiotherapy dosimetry in a specific area within the prostate. Theoretically, this strategy will improve treatment efficacy and reduce toxicity. Given the multicentricity that typically occurs with prostate cancer, however, it is not clear if all areas of cancer would be identified.
In addition, MRI/MRSI may prove beneficial in the follow-up of patients undergoing radiotherapy. Obviously, brachytherapy performed with needles in place may make posttreatment evaluation of the prostate more difficult. Hormonal therapy also makes interpretation of results more uncertain by masking tumor that later becomes more viable and, therefore, detectable. Conversely, the answer to early cancer detection may be facilitated by these new techniques.Potential Limitations
The authors describe some of the limitations of the technique including better evaluation of the peripheral zones than of the transition zone. The transition zone may also normally have reduced citrate levels. Small regions of cancer with high ratios of choline to citrate may be masked by normal tissue. There may be difficulties in deciding whether there is tumor in the transition zone or peripheral zone. Lastly, it may be more difficult to evaluate large and irregular glands.
In spite of these potential limitations, magnetic resonance spectroscopy, like positron-emission tomography, offers an alternative to the standard radiologic evaluation of the prostate cancer patient and provides for an in vivo metabolic assessment that is likely to be increasingly helpful in the management of patients. Addressing both the advantages and the disadvantages of MRSI, Dr. Roach and colleagues have provided an excellent summary of a subject that will probably continue to grow in importance.