Prostate Cancer

Accurate staging of prostate cancer is essential for the prognosis and treatment planning. In particular, it is crucial to determine the local extent of prostate cancer (extracapsular extension and seminal vesicle infiltration) and the presence of metastatic disease (lymphatic or hematogenous), because radical prostatectomy is the preferred method of treatment for patients with disease confined to the capsule. Patients with disease outside the prostate are generally not surgical candidates and may be offered an alternative therapy. Clinical staging based on digital rectal examination, transrectal ultrasound, Gleason score, sextant biopsy, and prostate-specific antigen (PSA) has limited accuracy and may underestimate the extent of disease. Previous studies indicated that digital rectal examination underestimates the local extent of cancer in 40% to 60% of the cases.74 The purpose of preoper-

ative imaging evaluation is to increase the accuracy of the assigned clinical stage.49

The current major clinical role of MRI is to detect disease outside the capsule, including extracapsular extension, seminal vesicle infiltration, nodal metastasis, and bone marrow metastasis once cancer has been diagnosed (Figure 31.11).74 Detection of such extracapsular disease eliminates unnecessary surgical procedures.49,74 A decision analysis model suggested that preoperative MRI was cost-effective for men with moderate or high probability of extracapsular disease.75

On T2-weighted images, prostate cancer is typically seen as an area of decreased signal intensity within the normally high-signal-intensity peripheral zone. The presence of decreased T2 signal intensity in the peripheral zone is of limited sensitivity as some tumors are isointense and of limited specificity because other causes, such as hemorrhage, prostatitis, scarring, radiotherapy, cryosurgery, and hormonal therapy can cause low T2 signal intensity. Overall, the sensitivity of MRI for detecting prostate cancer is approximately 60%.76 A more-recent study indicated that in patients with an elevated PSA and negative transurethral sonography-guided quadrant or sextant biopsy results, MRI had a sensitivity of 83% and a positive predictive value of 50% for detection of prostate cancer.76 Extracapsular tumor extension is seen as asymmetry or invasion of the neurovascular bundle, obliteration of the rectoprostatic angle, and bulging of the prostate capsule, which may be irregular with a square or rectangular edge or, less commonly, a smooth curvilinear bulge. Accuracy of staging extracapsular extension with MRI ranges

figure 31.11. Prostate cancer. Axial T1-weighted gradient echo image with fat suppression obtained after gadolinium contrast agent injection. There is a large heterogeneously enhancing mass involving the prostate gland, with involvement of the bladder wall and rectal wall. Pathologically, it was poorly differentiated carcinoma with small cell neuroendocrine features.

figure 31.11. Prostate cancer. Axial T1-weighted gradient echo image with fat suppression obtained after gadolinium contrast agent injection. There is a large heterogeneously enhancing mass involving the prostate gland, with involvement of the bladder wall and rectal wall. Pathologically, it was poorly differentiated carcinoma with small cell neuroendocrine features.

from 51% to 92%. It remains impossible to detect microscopic extracapsular extension.74 Seminal vesicle invasion is seen as enlargement of one seminal vesicle with abnormal asymmetric low signal intensity within the lumen on T2-weighted images, although low signal intensity of the seminal vesicles can be caused by other reasons including hemorrhage, radiation, hormonal therapy, and amyloid deposits.49 The accuracy of detection of seminal vesicle invasion with endorectal coil ranges between 54% and 96%.77-80 A meta-analysis suggested that turbo spin echo, endorectal coil, and multiple imaging planes improve staging performance by MRI.81

By combining diagnostic variables (age, PSA level, and Gleason tumor grade) with MRI findings, the accuracy of staging can be increased.82 It has been shown that the addition of MRI improves the prediction of seminal vesicle invasion and extracapsular extension in patients with intermediate risk, as indicated by PSA levels of 10 to 20ng/mL and Gleason scores of 5 to 7.83

CT is not recommended for local staging because of its inability to differentiate among normal, hyperplastic, and cancerous glands.49 Sensitivity in detection of extracapsular extension of prostate carcinoma is low, especially in early clinical stages. However, it provides useful information for clinically suspected advanced disease with apparent extra-capsular extension and extrapelvic involvement.49 Guidelines for the use of CT in patients with PSA greater than 20 ng/mL have been reported and are in clinical use.84

CT and MRI are useful to detect nodal metastasis in a select group of patients with high risk for nodal metastasis, predicted by digital rectal examination, serum PSA, and biopsy Gleason score.74 A study showed that identification of enlarged lymph nodes with subsequent biopsy using CT guidance was shown to identify lymph nodes metastases and thus prevent unnecessary surgery in more than 10% of patients.85 In this study, nodes 6 mm or greater in cross-sectional diameter were considered pathologic and were biopsied. CT-guided aspiration biopsy improved the specificity and accuracy of CT in diagnosing lymph node metastases from 96.7% and 93.7% to 100% and 96.5%, respectively. The overall accuracy of CT in detecting pelvic lymph node metastases from prostate cancer is in the range of 67% to 93%.49 MRI with a three-dimensional technique has revealed an accuracy of 90% in the detection of nodal metastasis in bladder and prostate cancer.51 Use of ultrasmall superparamagnetic iron oxide particles (USPIO) was investigated to detect clinically occult lymph node metastases in prostate cancer and significantly improved the detection of small and otherwise undetectable lymph node metastasis compared to conventional MRI. It increased sensitivity and specificity of nodal metastasis from 35.4% and 90.4% to 90.5% and 97.8%, respectively.86

MR spectroscopic imaging (1H-MRSI) is a method that demonstrates normal and altered tissue metabolism. It has been shown that prostate cancer is characterized at MRSI by increased choline and/or decreased levels of citrate.87 The addition of MRSI to MRI has been shown to increase staging accuracy88 and have potential for more accurate tumor localization.89 MRSI, interventional MRI-guided biopsy, and therapy are currently under investigation90,91 and not considered to be routine.

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