Imaging of Prostate Cancer

Prostate cancer is the most common cancer and one of the leading causes of cancer death in American men. The American Cancer Society estimates that in 2006, 234,460 new cases of prostate cancer will be diagnosed and 27,350 men will die from this disease in the United States [1]. The management of prostate cancer is challenging because the disease has variable clinical and pathologic behavior. The choice of treatment should be patient specific and risk adjusted, aimed at improving cancer control while reducing the risks of treatment-related complications. There is a growing demand for further individualization of treatment plans, which requires the accurate characterization of the location and extent of cancer. This characterization necessitates the optimal use of imaging methods that play an integral role in prostate cancer management. Risk factors for developing prostate cancer are advanced age, ethnicity, and family history of the disease. More than 65% of all prostate cancers occur in men older than 65 years. African American men

have the highest incidence of prostate cancer in the world.Familial predisposition is seen in 5% to 10% of prostate cancers.A diet high in saturated fat may also play a role.
Measurement of prostate-specific antigen (PSA) in blood and digital rectal examination (DRE) are offered for early detection of the disease for men at average risk beginning at age 50 years and for men at high risk beginning at age 45 years.
Treatment options for prostate cancer vary depending on age, disease stage, potential side effects of the treatment, and other medical conditions of the patient.Surgery, external beam radiation therapy, and brachytherapy can be used for treatment of early-stage prostate cancer.Hormonal therapy, chemotherapy, radiation therapy, or a combination of these can be used to treat metastatic disease or as supplemental therapies in early-stage disease.Watchful waiting without immediate treatment can be offered in some older patients who have limited life expectancy or less-aggressive tumors.Follow-up after radical prostatectomy Follow-up after radiation therapy -Summary -Acknowledgments -References Today, more than 90% of prostate cancers are diagnosed during early stages.Over the past 20 years, the 5-year survival rate for all stages increased from 67% to 100% [1].This improvement in 5-year survival rate is mainly due to early diagnosis.
This article reviews the role of imaging in the diagnosis and management of prostate cancer.Transrectal ultrasonography (TRUS), which can be used to guide biopsy, is the most frequently used imaging technique in cancer detection.For determining the extent of disease, CT and MR imaging are the most commonly used modalities; bone scintigraphy and positron emission tomography (PET) have roles only in advanced disease.Currently, the role of imaging in prostate cancer is evolving to improve disease detection and staging, to determine the aggressiveness of disease, and to predict outcomes in different patient populations.

Screening
Prostate cancer screening is performed with DRE and measurement of serum PSA level.Since the advent of PSA screening, the incidence of prostate cancer has increased, but most prostate cancers are now diagnosed at an early stage.
There are certain limitations to PSA screening.PSA is not specific for prostate cancer and can be elevated in other conditions including benign prostatic hyperplasia, inflammation, trauma, and urinary retention.Although cancerous prostate tissue produces far more PSA in the serum than hyperplastic tissue, benign prostatic hyperplasia is the most common cause of elevated serum PSA concentration [2].
Patients who have abnormal DRE findings or elevated PSA levels are further evaluated with prostate biopsy.Establishing a threshold at which prostate biopsy should be performed in asymptomatic patients is very difficult.Using a cutoff value that is too low may cause unnecessary biopsies or detection of clinically insignificant cancers, whereas using a cutoff value that is too high may prevent detection of early-stage but aggressive tumors.A PSA level of 4.0 ng/mL is generally accepted as the lower limit for biopsy consideration [3].
Although PSA screening is a valuable tool in the early detection of prostate cancer, it is only one of the factors used to assess the likelihood that a patient has prostate cancer.Evaluation of other risk factors and DRE results may necessitate prostate biopsy in some patients who have normal PSA levels.

Diagnosis
Needle biopsy, which is often guided by TRUS, continues to be the ''gold standard'' for the diagnosis of prostate cancer.TRUS provides reasonably goodquality images of the prostate and adjacent structures and facilitates needle placement and tissue sampling.
The fact that prostate cancer is often a multifocal and heterogeneous disease makes diagnosis by biopsy difficult.Only a small amount of tissue is obtained with needle biopsy.Thus, sampling errors are common.Initial TRUS-guided biopsy detects prostate cancer in only 22% to 34% of the cases [4][5][6].Thus, many patients require repeat biopsy.In patients who have initial negative results from TRUS-guided prostate biopsy, prostate cancer is detected in 10% to 19% on the second, in 5% to 14% on the third, and in 4% to 11% on the fourth repeat biopsy [4][5][6].The traditional sextant biopsy schema, in which six parallel core samples are obtained, is now considered inadequate.Newer prostate biopsy strategies include higher numbers of biopsy samples from different regions of the prostate to improve cancer detection and risk assessment [7,8].

Tumor detection and staging
The TNM staging system is widely used to stage prostate cancer (Table 1) [9].Although imaging techniques are sometimes useful in the detection of prostate cancer, their main use is in the staging of the disease.A combination of the currently available imaging modalities is usually necessary to help determine appropriate treatment strategies.

Transrectal ultrasonography
In addition to its role in biopsy guidance, TRUS is a commonly used imaging method for the detection and local staging of prostate cancer because of its widespread availability and ease of use.
TRUS provides good-quality images of the prostate gland because a high-frequency (5-to 7.5-MHz) probe can be placed in the rectum close to the prostate.Prostate cancer is most often seen as a hypoechoic area within the peripheral zone.Up to 40% of prostate cancers, however, are isoechoic, limiting their detection with TRUS [10,11].The finding of a hypoechoic area within the peripheral zone is not specific for prostate carcinoma and can also be seen in benign processes such as prostatitis and focal atrophy [11].Therefore, TRUS has a limited role in the detection of prostate cancer [12].
Capsular bulging and irregularity and the obliteration of the fat plane posterior to the prostate and of the rectoprostatic angle are findings suggestive of extracapsular extension on TRUS.In addition, seminal vesicle invasion by the tumor can be observed on TRUS.The accuracy of TRUS in the prediction of extracapsular extension of prostate cancer varies widely, with sensitivities ranging from 50% to 92% and specificities ranging from 58% to 86% [13][14][15].For the diagnosis of seminal vesicle invasion, reported sensitivities range from 22% to 60%, and the specificity is about 88% [14,16].
The major limitation of TRUS is its limited soft tissue resolution.Color Doppler and power Doppler, which can show vascular changes in tissues, can be added to improve the detection of prostate cancer on TRUS [17,18].Even with these techniques, however, accuracy of TRUS in the local staging of prostate cancer remains limited.Contrast-enhanced TRUS is a new technique that is under investigation for the assessment of prostate cancer [19].A study showed that contrast-enhanced TRUS improved the sensitivity of TRUS in tumor detection from 38% to 65%, with no significant change in its specificity, which was about 80% [20].

CT
CT for local staging of prostate cancer is of little value because even with contrast enhancement, CT lacks the soft tissue resolution necessary for the detection of prostate cancer within normal prostate.
When there is marked extracapsular extension, soft tissue extending into the periprostatic fat and adjacent structures can be diagnosed with CT (Fig. 1).Unilateral enlargement of a seminal vesicle by soft tissue-density tumor with obliteration of the fat plane between the seminal vesicle and prostatic base is suggestive of seminal vesicle invasion.With its accuracy of about 65% to 67%, however, CT is of limited clinical use for the local staging of prostate cancer [21,22].CT may be helpful in the evaluation of patients who have advanced disease with adjacent organ invasion and distant lymphadenopathy [23], although patients presenting  Recently developed multidetector CT technology allows estimation of prostate perfusion and localization of prostate cancer.One report indicated that this technique was able to detect only localized, high-volume, poorly differentiated prostate cancers [24].Further research is needed to define the role of multidetector CT in the evaluation of prostate cancer.

MR imaging and MR spectroscopic imaging
MR imaging and proton MR spectroscopic imaging are rapidly evolving as the most sensitive tools for the noninvasive, anatomic, and metabolic evaluation of prostate cancer [25,26].Therefore, this article places special emphasis on these techniques.
MR imaging demonstrates the zonal anatomy of the prostate with excellent soft tissue resolution and allows assessment of local extent of disease.The addition of MR spectroscopy can improve prostate cancer detection and localization.Furthermore, MR spectroscopy provides metabolic information correlating with pathologic Gleason grade and thus may offer a noninvasive means to better predict prostate cancer aggressiveness [25,26].
A magnet strength of at least 1.5 T is required for high-quality MR imaging and MR spectroscopic imaging study of the prostate.The combined use of an endorectal coil with a pelvic phased-array coil markedly improves image quality.In general, T1-weighted axial images of the entire pelvis are obtained for the detection of nodal disease.Thinsection (3-mm) T2-weighted images with a small field of view (w14 cm) in the transverse, sagittal, and coronal planes are used for tumor detection, localization, and staging.The use of a dynamic contrast-enhanced MR sequence is optional and may aid in tumor detection.Postbiopsy hemorrhage may cause under-or overestimation of the tumor presence and local extent.Therefore, MR imaging must be delayed for at least 4 to 8 weeks after prostate biopsy.
The MR spectroscopic imaging techniques that are commercially available include chemical shift imaging with point resolved spectroscopy (PRESS) voxel excitation and band selective inversion with gradient dephasing for water and lipid suppression.The PRESS technique generates a cubic or rectangular voxel by the acquisition of three orthogonal slice selective pulses (ie, a 90 pulse followed by two 180 pulses).Currently, three-dimensional proton MR spectroscopic mapping of the entire prostate is possible with a resolution of 0.24 mL or smaller, depending on the parameters used.The setup for spectroscopic imaging is the same as for morphologic imaging, and both datasets are usually acquired in the same examination to overlay metabolic information directly on the corresponding anatomic display (Fig. 2).
On MR imaging, prostate cancer is most easily seen on T2-weighted images as a focus of decreased signal intensity (Fig. 3).Low signal intensity can also be seen in several other conditions such as hemorrhage, prostatitis, atrophy, benign prostatic hyperplasia nodules, or sequelae resulting from radiation therapy or hormonal treatment.
MR spectroscopy provides metabolic information about prostatic tissue by displaying the relative concentrations of citrate, creatine, choline, and polyamines within contiguous voxels.Normal prostate tissue contains high levels of citrate-higher in the peripheral zone than in the central and transition zones.Glandular hyperplastic nodules, however, can demonstrate citrate levels as high as those observed in the peripheral zone.In the presence of prostate cancer, the citrate level is diminished or not detectable because of a conversion from citrate-producing to citrate-oxidating metabolism.The choline is elevated due to a high phospholipid cell membrane turnover in the proliferating malignant tissue.Therefore, voxels containing prostate cancer depict an increased choline-to-citrate ratio (Fig. 4).Because the creatine peak is very close to the choline peak in the spectral trace, the two may be inseparable; therefore, for practical purposes, the ratio of choline plus creatine to citrate ([Cho 1 Cr]/Cit) is used for the spectral analysis in the clinical setting.With the latest spectroscopic sequences, polyamine peaks can also be resolved.The polyamine peak decreases in the presence of prostate cancer.
The classification system described by Kurhanewicz and colleagues [27] is often used for spectral interpretation.A voxel is classified as normal, as suspicious for cancer, or as very suspicious for cancer.Furthermore, a voxel may contain nondiagnostic levels of metabolites or artifacts that obscure the metabolite frequency range.Voxels are considered suspicious for cancer when (Cho 1 Cr)/Cit is at least 2 SD above the average ratio for the normal peripheral zone, and voxels are considered very suspicious for cancer when (Cho 1 Cr)/Cit is more than 3 SD above the average ratio [28].Voxels considered nondiagnostic contain no metabolites with signalto-noise ratios greater than 5.In voxels in which only one metabolite is detectable, the other metabolites are assigned a value equivalent to the noise SD.
It has been shown that the (Cho 1 Cr)/Cit ratio in a lesion correlates with the Gleason grade [29].Thus, a potential advantage of MR spectroscopy is that it may allow noninvasive assessment of prostate cancer aggressiveness.
One study found that in prostate cancer detection and tumor localization, MR imaging had 61% and 77% sensitivity, respectively, and 46% and 81% specificity, respectively, with moderate inter-reader agreement; MR spectroscopy had significantly higher specificity (75%, P<.05) but lower sensitivity (63%, P<.05).The investigators reported high specificity (91%) when combined MR imaging and MR spectroscopy indicated a positive result, and high sensitivity (95%) when either test alone did so [30].A recent study comparing DRE, TRUS-guided biopsy, and MR imaging in the detection and localization of prostate cancer showed that MR imaging significantly increased the accuracy of prostate cancer localization compared with DRE or TRUSguided biopsy (P<.0001 for each).The area under the receiver operating characteristic (ROC) curve for tumor localization was higher for MR imaging than for DRE at the prostatic apex (0.72 versus 0.66), the midgland (0.80 versus 0.69), and the base (0.83 versus 0.69); it was also higher for MR imaging than for TRUS-biopsy at the midgland (0.75 versus 0.68) and the base (0.81 versus 0.61) but not the apex (0.67 versus 0.70) [31].
Most MR imaging studies focus on tumor detection in the peripheral zone of the prostate, where most cancers originate.The transition zone, however, harbors cancer in up to 25% of radical prostatectomy specimens [32].A recent study showed that MR imaging can be used to assess transition zone prostate cancers.In detecting the location of transition zone cancer, the areas under the ROC curves of two readers were 0.75 and 0.73.Both readers' accuracy in detecting transition zone cancer foci increased significantly (P 5 .001)as tumor volume increased [33].
Dynamic contrast-enhanced MR imaging has been proposed as a means of achieving higher accuracy in prostate cancer localization and staging than can be obtained with conventional T2-weighted MR imaging [34].It has been postulated that on dynamic contrast-enhanced imaging, increased microvascular density in prostate cancer results in different contrast enhancement than that seen in normal prostate.Numerous contrast enhancement parameters can be used to differentiate cancerous from benign tissue, including onset time, time to peak enhancement, peak enhancement, relative peak enhancement, and washout time.A recent study suggested that the peak enhancement of cancer relative to that of surrounding benign tissue is the most accurate parameter for cancer localization [35].Another suggested approach to cancer detection is the identification of areas of enhancement on early postcontrast images (within the first 30-60 seconds after contrast material injection) [36].The challenge in dynamic contrast-enhanced MR imaging is to provide an optimal balance between temporal and spatial resolution.More studies are necessary to optimize the technology and define the clinical value of this technique.
As mentioned before, biopsy remains the gold standard for the diagnosis of prostate cancer.In patients who have elevated PSA levels or clinical findings suggestive of prostate cancer and negative TRUS-guided biopsy results, MR imaging and MR spectroscopy can be used to localize areas that may harbor prostate cancer and can help direct targeted biopsies and limit multiple repeat biopsies [37,38].MR imaging-guided transrectal prostate biopsy is technically possible; however, the uses of and indications for MR imaging in prostate biopsy and other types of prostate interventions such as brachytherapy seed placement are under investigation [39][40][41].
MR imaging criteria for extracapsular extension include a contour deformity with a step-off or angulated margin, an irregular capsular bulge or retraction, a breach of the capsule with evidence of direct tumor extension, obliteration of the rectoprostatic angle, and asymmetry of the neurovascular bundles.MR imaging criteria for seminal vesicle invasion include contiguous low-signal intensity tumor extension from the base of the gland into the seminal vesicles, disruption or loss of the normal architecture of the seminal vesicle and decreased conspicuity of the seminal vesicle wall, tumor extension along the ejaculatory duct (nonvisualization of the ejaculatory duct), asymmetric decrease in the signal intensity of the seminal vesicles with mass effect, and obliteration of the angle between the prostate and the seminal vesicle on sagittal images.MR imaging is also helpful for diagnosing the invasion of adjacent organs (eg, the urinary bladder and rectum).Combined transverse, coronal, and sagittal planes of section facilitate evaluation of extracapsular extension, seminal vesicle invasion, and adjacent organ invasion [25,26] (Figs.5-8).
The accuracy of MR imaging in the local staging of prostate cancer varies widely (from 50% to 92%) [34].MR imaging has been reported to have 13% to 95% sensitivity and 49% to 97% specificity for the detection of extracapsular extension and 23% to 80% sensitivity and 81% to 99% specificity for the detection of seminal vesicle invasion [42][43][44][45][46][47][48][49][50].A recent study showed that for two readers, the areas under the ROC curves were 0.93 and 0.81 for the detection of seminal vesicle invasion at MR imaging; the features that had the highest sensitivity and specificity were low signal intensity within the seminal vesicle and lack of preservation of seminal vesicle architecture.Tumor at the prostate base that extended beyond the capsule and low signal intensity within a seminal vesicle that had lost its normal architecture were highly predictive of seminal vesicle invasion [51].
MR spectroscopy may have a role in reducing the wide variation in the accuracy of MR imaging for local staging of prostate cancer, which may be attributed to the lack of standardized diagnostic criteria and interobserver variability in image interpretation.In a study on the detection of extracapsular extension by two independent readers, the addition of MR spectroscopy to MR imaging reduced interobserver variability and significantly improved accuracy for the less experienced reader, whose area under the ROC curve increased from 0.62 to 0.75 (P<.05); for the more experienced reader, the addition of MR spectroscopy also improved accuracy, but not significantly (the area under the ROC curve increased from 0.78 to 0.86) [52].
A study has shown that for the prediction of extracapsular extension, MR imaging findings contribute significant incremental value to clinical variables (areas under the ROC curves for detection of extracapsular extension with and without endorectal MR imaging findings were 0.838 and 0.772, respectively, P 5 .022)[53].A related study that analyzed the same data demonstrated that the incremental value of MR imaging in predicting extracapsular extension was only significant when interpretation was performed by genitourinary Prostate Cancer Imaging 213 radiologists with experience in endorectal MR imaging [54].In another study, MR imaging and combined MR imaging and MR spectroscopic imaging contributed significant incremental value (P%.02) to the staging nomograms in predicting organ-confined prostate cancer; the contribution of MR findings was significant in all risk groups but was greatest in the intermediate-and high-risk groups (P<.01 for both) [55].
For the assessment of lymph node metastases, MR imaging, like CT, has low sensitivity (0%-69%) [47,[56][57][58][59][60][61].The low sensitivity of MR imaging and CT is mainly due to the inability of cross-sectional imaging to detect metastases in normal-sized nodes.High-resolution MR imaging with lymphotropic superparamagnetic nanoparticles, however, is a promising technique in the detection of occult lymph node metastases because it allows detection of metastases in normal-sized lymph nodes.In one study, MR imaging with lymphotropic superparamagnetic nanoparticles had a significantly higher sensitivity than conventional MR imaging in the detection of metastasis on a node by node basis (90.5% versus 35.4%, P<.001); the new technique also had a sensitivity of 100% and a specificity of 95.7 in detecting nodal metastasis on a per patient basis [62].
Although these results are promising, the low incidence of lymph node metastasis in patients who have prostate cancer does not warrant the routine use of lymphotropic superparamagnetic nanoparticles.A recent study showed that incorporation of the Partin nomogram results and standard MR imaging findings regarding extracapsular extension and seminal vesicle invasion improves the prediction of lymph node metastasis on MR imaging in patients who have prostate cancer [63].Only 22 (5%) of 411 patients in the study had lymph node metastases at surgical pathology, and MR imaging was an independent, statistically significant predictor of lymph node metastasis (P 5 .002),with positive and negative predictive values of 50% and 96.36%, respectively.On multivariate analysis, prediction of lymph node status using a model that included all MR imaging variables (extracapsular extension, seminal vesicle invasion, and lymph node metastases) along with the Partin nomogram results had a significantly greater area under the ROC curve than the univariate model that included only MR imaging lymph node metastasis findings (areas under the ROC curves were 0.892 and 0.633, respectively, P<.01).The investigators suggested that because MR imaging offers high negative predictive value for lymph node metastasis in addition to anatomic information useful for  treatment planning, MR imaging and the Partin nomogram could be used together to determine whether imaging with lymphotropic superparamagnetic nanoparticles is warranted.

Capromab pendetide immunoscintigraphy
Capromab pendetide immunoscintigraphy is a murine monoclonal antibody that reacts with prostate membrane-specific antigen, which is highly expressed in prostate cancer.Immunoscintigraphy is accomplished by labeling the antibody with indium 111.After infusion of the antibody, whole-body planar and single-photon emission CT images are obtained.Capromab pendetide immunoscintigraphy can be used for the detecting lymph node metastases, the site of relapse in a patient who has a detectable PSA after prostatectomy, and occult metastasis before primary therapy [64].One study found that capromab pendetide immunoscintigraphy scanning had 67% to 94% sensitivity and 42% to 80% specificity for the detection of lymph node metastases [65,66].Another study on the use of capromab pendetide immunoscintigraphy scanning to evaluate patients who had recurrent prostate cancer showed that the technique had a sensitivity of 89%, a specificity of 67%, and an overall accuracy of 89% [67].Coregistration of capromab pendetide immunoscintigraphy images with MR imaging or CT could improve the specificity of the examination [68]; however, there are many reasons for false-positive uptake of this antibody, and the image quality is often suboptimal.In the era of PET, the imaging of prostate cancer with capromab pendetide immunoscintigraphy should no longer be encouraged.

Radionuclide bone scintigraphy
Radionuclide bone scan is a sensitive imaging method used to detect bone metastases in patients who have prostate cancer (Fig. 9) [69].Bone scans are commonly obtained even for patients in lowand intermediate-risk categories [70]; however, studies have shown that patients who have PSA levels of 20 ng/mL or less and a Gleason score lower than 8 have a 1% to 13% rate of positive bone scans [71,72].Other studies have confirmed that in patients who have low PSA levels (<10 ng/mL) and no skeletal symptoms, the yield of bone scanning is too low to warrant its routine use unless the patient has stage T3 or T4 disease or a high Gleason score [73][74][75].Therefore, a PSA level of greater than 15 to 20 ng/mL is usually used as the cutoff point for obtaining a bone scan, but patients who have skeletal symptoms and those who have a high Gleason score or advanced stage should also be assessed with bone scans.

Positron emission tomography
The role of PET is still under investigation in the staging workup of patients who have prostate cancer (see Fig. 9) [76][77][78][79][80]. Fluorodeoxyglucose (FDG), the most commonly used PET tracer, was reported to be ineffective for the initial staging of prostate cancer because most primary prostate cancer lesions were not detected by FDG-PET [81].FDG-PET, however, could have a role in the detection of local recurrence or distant metastases with increasing PSA after initial treatment failure [55,76,80,82].

Treatment planning
The therapeutic options for patients who have prostate cancer vary widely and include watchful waiting, androgen ablation (chemical or surgical castration), hormone therapy, radical surgery, and various forms of radiation therapy (brachytherapy, external beam irradiation).The choice of optimal treatment strategy in patients who have prostate cancer is patient specific and risk adjusted.The therapeutic goal is to maximize cancer control while minimizing the risks of complications.The optimal treatment option for prostate cancer is chosen based on clinical TNM stage, Gleason grade, and the level of PSA.Other factors such as patient age, associated medical illnesses, and the patient's personal preferences also have an effect on the treatment planning process.The findings from imaging studies assist in this patient-specific treatment planning approach.Imaging may also have a role in the guidance and assessment of emerging local prostate cancer therapies.

Post-treatment follow-up
After treatment, patients who have prostate cancer are followed with periodic measurement of PSA levels and DRE.Imaging is necessary after treatment for clinically localized prostate cancer only if there are suspicious findings on DRE, PSA is elevated, or the patient has symptoms such as bone pain.

Follow-up after radical prostatectomy
Radical prostatectomy includes resection of the prostate, the seminal vesicles, and the pelvic lymph nodes.After radical prostatectomy, PSA decreases to undetectable levels (<0.1 ng/mL) within a few weeks of surgery and should remain undetectable thereafter [87,88].Detectable levels of PSA in patients who have undergone radical prostatectomy indicate that there is residual prostate tissue because PSA is specific to the prostate.A rise in a previously undetectable or stable PSA level after surgery is suggestive of residual, recurrent, or metastatic disease.In these cases, the role of imaging is to help distinguish locally recurrent disease (which can be managed with local therapy) from distant metastatic disease (which requires systemic therapy).The type of recurrence is difficult to determine clinically because an increasing PSA level is rarely associated with symptoms or findings at physical examination.

Radionuclide bone scintigraphy
Radionuclide bone scan is often the first examination obtained.If the bone scan is negative or inconclusive, then further imaging studies are performed.The probability of a positive bone scan in patients who have biochemical failure following radical prostatectomy is very low until PSA levels increase above 30 to 40 ng/mL [89].Therefore, bone scans are recommended only when the patient has symptoms of bone pain, a rapid rise in PSA, or markedly elevated PSA [90].
Transrectal TRUS is the most commonly used imaging modality in the detection of local recurrence following prostatectomy.TRUS is also used for biopsy guidance of the vesicourethral anastomosis and prostatic fossa to document local recurrence.A negative result on TRUS-guided biopsy, however, does not definitely rule out recurrent disease due to possible sampling errors.Only about 25% of men who have prostatectomy and PSA levels lower than 1 ng/mL have positive biopsy results [91].The yield for detection of locally recurrent tumor with TRUS-guided biopsy, however, rises significantly with increasing PSA levels [91,92].

CT
CT is not effective for detecting early recurrent tumor in the surgical bed.A study showed that CT detected only 36% of recurrences, and all of these were larger than 2 cm [93].CT can be useful in the evaluation of nodal recurrence (Fig. 10); however, CT relies only on size criteria for the detection of positive lymph nodes.CT can also be useful in detecting bone and visceral metastases, although bone scan and MR imaging are superior in the diagnosis and follow-up of bone metastases [94].

MR imaging
Due to its excellent soft tissue resolution, MR imaging is superior to TRUS and CT in the detection of clinically evident locally recurrent disease after radical prostatectomy (Fig. 11).Two studies reported that MR imaging had 95% to 100% sensitivity and 100% specificity for detecting local recurrence [ 95,96].In one of these studies, local recurrences seen on MR imaging were perianastomotic in 29% of patients and retrovesical in 40%, within retained seminal vesicles in 22%, and at anterior or lateral surgical margins in 9%; the mean diameter of tumors was 1.4 cm (range, 0.8-4.5 cm), and PSA levels ranged from undetectable to 10 ng/mL (mean, 2.18 ng/mL) [95].
MR imaging can also be used in the detection of nodal disease; however, the accuracy of MR imaging for staging pelvic lymph nodes by size criteria is similar to that of CT.MR imaging is more sensitive and specific in the diagnosis of bone metastases compared with bone scan; however, it is more feasible to use bone scan to cover the entire skeleton.In addition, bone scan is more cost-effective than MR imaging.Therefore, MR imaging should be used when other imaging modality findings are indeterminate.

Positron emission tomography
The role of PET in detecting recurrence, assessing prognosis, monitoring therapy, and studying the biology of prostate cancer is still under investigation.Prostate Cancer Imaging 217 FDG-PET could have a role in the detection of local recurrence or distant metastases with increasing PSA after initial treatment failure [76,80,82].A study showed that FDG-PET detected local or systemic disease in 31% of 91 patients who had PSA relapse following prostatectomy.In this study, the probability for disease detection increased with PSA level [82].

Follow-up after radiation therapy
Following radiation therapy, the PSA level decreases in most patients during the first year.Recurrence following radiation therapy is defined as three consecutive rises in PSA after a postradiation PSA nadir [97].Detection of local recurrence after failed radiation therapy is a clinical challenge because an increase in the PSA level is not a reliable variable for differentiating local from distant recurrence.After radiation therapy, the prostate becomes atrophic and fibrotic, making detection of local recurrent disease within the irradiated prostate difficult by DRE.If bone scan is positive, then further imaging is not necessary.If the bone scan is negative, then TRUS-directed biopsy of the prostate can be performed.Lymph nodes can be evaluated by CT or by MR imaging.
It is commonly believed that assessment of location and extent of prostate cancer on MR imaging is hindered by tissue changes related to radiation therapy (Fig. 12).A recent study showed, however, that MR imaging and MR spectroscopy could be more sensitive than sextant biopsy and DRE for sextant localization of cancer recurrence after external beam radiation therapy [98].In this study, MR imaging and MR spectroscopy had sensitivities of 68% and 77%, respectively, whereas the sensitivities of biopsy and DRE were 48% and 16%, respectively.MR spectroscopy appears to be less specific (78%) than the other three tests, which each had a specificity higher than 90%.Another recent study also showed that in patients who underwent salvage prostatectomy after failed radiation therapy, MR imaging could help identify tumor sites and depict local extent of disease.In this study, areas under the ROC curves for two readers were 0.61 and 0.75 for tumor detection, 0.76 and 0.87 for prediction of extracapsular extension, and 0.70 and 0.76 for prediction of seminal vesicle invasion [99].

Summary
Imaging modalities are rapidly evolving to provide improved evaluation of prostate cancer.Our understanding of imaging criteria and experience in image interpretation are also growing.In addition to the traditional roles of imaging in prostate cancer (ie, localization and staging), extensive research is being done on metabolic imaging to predict cancer aggressiveness.Future directions in prostate cancer imaging include more precise patient stratification for different management options and new methods for guidance and assessment of emerging local prostate cancer therapies.

Fig. 1 .
Fig. 1.Contrast-enhanced CT images of a 69-year-old man who had Gleason grade 4 1 5 prostate cancer.(A) Image shows a large prostatic mass (arrow) invading the bladder (B) and extending to the right pelvic wall.(B) Image shows hydronephrosis (arrow) in the right kidney due to obstruction of the right ureter by the mass.

Fig. 2 .
Fig. 2. Gleason grade 4 1 3 prostate cancer in a 65-year-old man.Transverse T2-weighted MR image (A) and corresponding MR spectroscopic data (B) superimposed on the anatomic image show the tumor (arrow) on the left side.

Fig. 3 .
Fig. 3. Gleason grade 4 1 3 prostate cancer in a 66-year-old man.Transverse (A), coronal (B), and sagittal (C) T2weighted MR images show a low-signal intensity focus consistent with tumor (arrow) in the peripheral zone of the prostate extending from left midgland to apex.

Fig. 4 .
Fig. 4. Gleason grade 5 1 4 prostate cancer in a 59year-old man.MR spectroscopy shows normal spectra in the healthy (H) right peripheral zone and suspicious spectra with elevated choline and reduced citrate in the left peripheral zone tumor (T).

Fig. 5 .Fig. 6 .
Fig. 5. Gleason grade 4 1 4 prostate cancer in a 54year-old man.This tumor was clinically staged as T1c; however, a transverse T2-weighted MR image showed a small tumor (T) with gross extracapsular extension (arrows) and the tumor was staged as T3a, which was confirmed at pathologic examination after prostatectomy.

Fig. 8 .
Fig. 8. Gleason grade 4 1 5 prostate cancer in the transition zone in a 68-year-old man.This anterior tumor involving the entire transition zone was clinically staged as T1c; however, a transverse T2-weighted MR image showed a large transition zone tumor (T) with gross anterior extraprostatic extension (arrows) and the tumor was staged as T3a, which was confirmed at pathologic examination after prostatectomy.Although the tumor was large, its anterior location far from the rectum made clinical evaluation difficult.PZ, peripheral zone.

Fig. 9 .
Fig. 9. Gleason grade 5 1 4 prostate cancer in a 76-year-old man.Bone scan (A) and PET (B) show a metastatic focus in the left femur (large arrows).Metastatic lymph nodes (small arrows) are also seen on PET (B).

Fig. 11 .
Fig. 11.Recurrent prostate cancer after radical prostatectomy in a 76-year-old man.Transverse (A), sagittal (B), and coronal (C) T2-weighted MR images demonstrate nodular intermediate-signal intensity tumor (arrows) consistent with local recurrence in the bladder neck.

Fig. 12 .
Fig. 12. Gleason grade 4 1 5 prostate cancer in a 66-year-old man.Transverse T2-weighted MR image (A) shows a large tumor (T) with gross extracapsular extension (arrows).After radiation treatment, transverse T2-weighted image (B) shows that the tumor (T) and extracapsular extension markedly decreased (arrows).Note that the rest of the prostate demonstrates diffuse low signal intensity consistent with radiation changes (B).

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