Clinical applications of image-based radiation therapy for the study of prostate cancer have expanded significantly over the past years. The results of recent studies of magnetic resonance imaging (MRI) combined with magnetic
Magnetic resonance spectroscopic imaging (MRSI) is in manyways appealing for the evaluation of prostate cancer. This technique can becombined with standard magnetic resonance imaging (MRI), and spectroscopicenergy combines anatomic resolution with the ability to determine metabolicactivity of the tissue. In the past, intrarectal coils have demonstratedexcellent anatomic resolution but have not been precise enough to demonstratemicroscopic extension postbiopsy in all patients. Previous biopsies and bleedingcan make interpretation even more difficult.
On the other hand, intraprostatic metabolic evaluation can behelpful in the detection of prostate cancer, and MRSI can identify changes inthe ratios of choline to citrate that help distinguish areas of cancer.Elevation of choline is associated with changes in the neoplastic cellularmembrane, and reduction in citrate is associated with changes in the neoplasticcell metabolism. Polyamines such as spermine may also be reduced in cancer. As aresult, MRSI also can improve cancer detection in patients with postbiopsybleeding.
It appears that MRSI can significantly improve the accuracyof cancer diagnosis, making it feasible to intensify radiotherapy dosimetry in aspecific area within the prostate. Theoretically, this strategy will improvetreatment efficacy and reduce toxicity. Given the multicentricity that typicallyoccurs with prostate cancer, however, it is not clear if all areas of cancerwould be identified.
In addition, MRI/MRSI may prove beneficial in the follow-upof patients undergoing radiotherapy. Obviously, brachytherapy performed withneedles in place may make posttreatment evaluation of the prostate moredifficult. Hormonal therapy also makes interpretation of results more uncertainby masking tumor that later becomes more viable and, therefore, detectable.Conversely, the answer to early cancer detection may be facilitated by these newtechniques.
The authors describe some of the limitations of the techniqueincluding better evaluation of the peripheral zones than of the transition zone.The transition zone may also normally have reduced citrate levels. Small regionsof 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 transitionzone or peripheral zone. Lastly, it may be more difficult to evaluate large andirregular glands.
In spite of these potential limitations, magnetic resonancespectroscopy, like positron-emission tomography, offers an alternative to thestandard radiologic evaluation of the prostate cancer patient and provides foran in vivo metabolic assessment that is likely to be increasingly helpfulin the management of patients. Addressing both the advantages and thedisadvantages of MRSI, Dr. Roach and colleagues have provided an excellentsummary of a subject that will probably continue to grow in importance.