Prostate cancer is second only to nonmelanoma skin cancer in incidence, and is the second leading cause of cancer death in men in the United States. Despite advances in early detection and treatment, more than 230,000 new cases and nearly 30,000 deaths from prostate cancer were expected in the United States alone in 2004. The only established risk factors for prostate cancer are age, race, and family history. Current methods of prostate cancer evaluation by digital rectal examination, transrectal ultrasound, histologic Gleason score (obtained with sextant biopsy) and serum prostatic specific antigen (PSA) assay can generally predict whether tumors will exhibit indolent or aggressive behavior. However, the overwhelming majority of prostate cancers fall between these extremes, in the range where these techniques are of limited accuracy.
Prostatic magnetic resonance imaging (MRI) and magnetic resonance spectroscopic imaging (MRSI) promise to correct these shortcomings. Currently the primary indication for prostate MRI and MRSI is in the evaluation of men with newly diagnosed prostate cancer with a moderate or high risk of extraprostatic disease, a clinical scenario in which it is often uncertain whether surgery or radiotherapy is the optimal treatment. Data critical to this decision is the size and level of aggressiveness of the tumor, as reflected in its metabolic activity, and the presence and degree of extra-glandular extension of tumor. The comprehensive and detailed evaluation obtained with MRI is not limited to the prostate gland. This imaging technique also allows detection of periprostatic and pelvic pathology. While MRI provides anatomic information, MRSI provides functional information that differentiates normal tissue from tumor and reflects the degree of tumor metabolism. Additional potential roles for MRI and MRSI include tumor detection and treatment follow-up, as detailed below.
Metabolic peaks relevant to prostate MRSI are choline (Cho), creatine (Cr), and citrate (Ci), occurring at downward frequency shift relative to water of approximately 3.2, 3.0 and 2.6, respectively, expressed in parts per million (ppm).
MRI and MRSI can be performed as a single combined examination in approximately 1 hour. An endorectal coil is required for MRSI and has been shown to improve MRI results as well, providing improved accuracy in tumor staging. Specialized software currently available from all major MRI vendors is necessary to perform MRSI. The metabolic peaks relevant to prostate MRSI are choline (Cho), creatine (Cr), and citrate (Ci) (Fig. 1). Several studies have shown that prostate cancer is characterized at MRSI by increased Cho (a normal cell membrane constituent elevated in many tumors) and reduced Ci (a constituent of normal prostatic tissue) (Fig. 2). Cr is included with Cho because the spectral peaks of these 2 compounds are often inseparable. In addition, Cr is a stable compound in normal and cancerous prostatic tissue. Ratios of Cho + Cr / Ci in normal prostatic tissue have been established to be lower than 0.5, while the ratio is usually > 0.5–0.8 in cancer, as established by researchers at the University of California San Francisco.
Metabolic changes that may be seen in prostate cancer include increased Cho, and decreased Ci.
Role of MRI and MRSI for detection of prostate cancer:
Prostate cancer is characterized at MRI as a low T2 signal intensity focus within the normally high T2 signal intensity tissue in the peripheral zone of the gland. However, this sign is of limited sensitivity because some tumors are isointense with the normal peripheral zone tissue. This feature is also of limited specificity because there are multiple other causes of low T2 signal intensity in the peripheral zone, such as hemorrhage, prostatitis, scarring, and post-treatment changes (such as radiotherapy, cryosurgery and hormonal therapy). MRSI, with its capability for biochemical characterization of both normal and abnormal tissue, offers a potential incremental benefit in this setting.
Role of MRI and MRSI for staging of prostate cancer:
MRI and MRSI are essential to determine inoperability by detecting extracapsular extension and local invasion of the seminal vesicles, as well as metastases to the lymph nodes and bone (Fig. 3). Extracapsular extension is suggested when one or more of the following signs are present: focal irregular capsular bulge, invasion of the neurovascular bundles (which constitute the nerves and vessels going to the prostate gland), and obliteration of the rectoprostatic angle (Fig. 4).
Multiple studies have shown that MRI possesses a high staging accuracy of 86% to 88% when using surface phased-array coils. The addition of MRSI to MRI has been shown to increase staging accuracy for less experienced readers and reduce interobserver variability. However, it must be recognized that microscopic and early macroscopic invasion cannot be reliably shown using current technology.
Role of MRI and MRSI in treatment planning of prostate cancer
Several possible roles for MRI and MRSI have been suggested in radiation treatment planning. Dose-volume planning with MRI can decrease radiation dose to the bladder, rectum and femoral heads when compared with CT. MRI and MRSI can also be potentially useful in planning other targeting therapies currently being developed, such as brachytherapy, cryoablation and radiofrequency ablation.
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