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Figure 8 Utricle in a 70-year-old man. (A) Axial and (B) coronal T2-weighted images demonstrate a midline, benign utricular cyst of high signal intensity (white arrow).

diagnosed in childhood because of its association with hypospadias, cryptorchidism, and ipsilateral renal agenesis (69-71).

Mullerian duct cysts arise from the Mullerian duct remnants, which should regress in utero. These are a common cause of obstruction of the ejaculatory duct. They are spherical in shape and, if large in size, can lie superior to the prostate. They are connected to the verumontanum but do not communicate with the urethra. Mullerian duct cysts can contain hemorrhage and calculi and are associated with an increased risk of prostate carcinoma. Surgical correction of the cysts can relieve genital duct obstruction in men with infertility (69-71).

Seminal vesicle or Wolffian duct cysts result from congenital atresia of the ejaculatory duct and are often associated with ipsilateral renal agenesis. The cysts are unilateral, located laterally in the seminal vesicle, commonly protrude into the bladder, and may present with symptoms of hematospermia, hematuria, and epid-idymitis. Wall irregularity or a mass associated with the cyst is suggestive of underlying adenocarcinoma. If large enough, seminal vesicle cysts can cause extrinsic compression on the ejaculatory ductal system and contribute to male infertility (69,70,72,73).

Prostate Malignancies

MR Appearance of Prostate Cancer

Adenocarcinoma of the prostate is most commonly located in the peripheral gland and may be unifocal or multifocal. Lesions are of low signal intensity on T1-and T2-weighted imaging due to an increase in cell density and replacement of prostatic ducts (Fig. 9A and B) (4). However, it is the low T2-signal intensity that contributes to the decreased specificity in detection of cancerous lesions. Other etiologies of low T2-signal intensity include hemorrhage, prostatitis, posttherapy changes, and scar (51,52,65,66). On T1-weighted images, identification of high signal intensity blood surrounding low signal intensity tumor (halo sign) can help distinguish which components are related to hemorrhage and which to tumor, because these two abnormalities are sometimes identical in appearance on T2-weighted imaging (55).

If intravenous contrast is administered, prostate cancer shows early and rapid enhancement relative to surrounding peripheral gland and rapid wash-out on delayed images (74,75). Therefore, early dynamic images provide the best contrast between tumor and normal tissue (25,28,33).

The detection of central gland carcinoma, which is also of low T1- and T2-signal intensity, is difficult in the presence of nodular-appearing benign prostatic hyperplasia. Most central gland tumors are found incidentally during TURP of the prostate, but aggressive cancers can spread beyond the prostate. Endorectal MR can occasionally depict central tumors not detectable by digital rectal exam (DRE) or TRUS and may provide an alternative to random biopsies in patients with an elevated prostate specific antigen (PSA) and negative palpation and TRUS.

More unusual tumors of the prostate gland have unique imaging features on MRI. Mucinous adenocarcinoma of the prostate is a rare cancer that accounts for 2% of prostate neoplasms and requires that at least 25% of the tumor consist of pools of extracellular mucin (76-78). The mucin results in a different MR appearance than that of typical adenocarcinoma (76). Mucinous tumors show T2-signal intensity equal to or greater than that of normal peripheral gland and can therefore be

Figure 9 Sixty-two-year-old man with carcinoma confined to the prostate. (A) Axial T1-weighted and (B) axial T2-weighted images demonstrate a focal low T1-and low T2-signal intensity mass (black arrow) in the right posterior peripheral gland.

confused with normal prostate, dilated periprostatic veins, or other conditions, such as cysts or abscesses (76,77).

Prostate sarcomas account for less than 0.1% of prostate malignancies and carry a poor prognosis due to their aggressive natural history (79). Leiomyosarcoma is the most common cell type in adults, whereas rhabdomyosarcoma is more common in children (80,81). Prostate sarcomas tend to be large and lobulated at presentation and often invade the bladder, rectum, and other pelvic soft tissues (82).

Staging of Prostate Carcinoma

Staging assists in treatment planning because stage T1-T2 disease can be treated with radical prostatectomy, brachytherapy, or radiation therapy, whereas patients with diseases that are stage T3 or more advanced are not surgical candidates and may be more appropriately treated with hormonal therapy or palliative radiation (83). Clinical staging of prostate cancer using DRE, serum PSA level, and biopsy specimen tumor grade (Gleason score) is the most universally available method and is fairly specific for advanced disease (stage T3-T4) (83-85). However, the realization that this method understages early extracapsular spread of tumor has led to the development of such imaging techniques as endorectal MRI (83-85).

The tumor-node-metastasis (TNM) staging classification for prostate cancer includes evaluation of both local and distant extent of disease (Table 1) (86). The role

Table 1 TNM Staging Classification of Prostate Carcinoma

Stage Description

Primary tumor (T)

Tx Tumor cannot be assessed

T0 No evidence of primary tumor

T1 Tumor not clinically palpable or visible by imaging, but

T1a found incidentally during surgery, in 5% or less of tissue

T1b found incidentally during surgery, in 5% or more of tissue

T1c identified by needle biopsy performed because of elevated PSA

T2 Tumor confined within the prostate, involving

T2a less than half a lobe of the prostate

T2b half a lobe of the prostate, but not both lobes

T2c both lobes of the prostate

T3 Tumor extending through the prostate capsule

T3a Extracapsular extension through one lobe

T3b Extracapsular extension through both lobes

T3c Extracapsular extension into the seminal vesicles

T4 Tumor fixed, invading structures other than the seminal vesicles

T4a Invasion of bladder neck, external sphincter, or rectum

T4b Invasion of muscles and/or pelvic wall Regional lymph nodes (N)

Nx Nodes cannot be assessed

N0 No regional node metastasis

N1 Single node metastasis, <2 cm in greatest dimension

N2 Single node metastasis; 2-5 cm in greatest dimension or multiple nodes, none larger than 5 cm

N3 Metastasis larger than 5 cm in any node Distant metastasis (M)

Mx Presence of metastasis cannot be assessed

M0 No distant metastasis

M1 Distant metastasis

M1a Nonregional lymph nodes involved

M1b Bone(s) involved

M1c Other site(s) involved

Abbreviations: PSA, prostate specific antigen; TNM, tumor-node-metastasis. Source: From Ref. 86.

of MRI in prostate cancer is to differentiate patients with organ-confined disease (stage T1-T2) from patients with locally invasive disease that has extended beyond the capsule into the periprostatic fat, lymphatics, and seminal vesicles (stage T3), or into adjacent organs (stage T4).

The common pathways of prostate cancer spread, including capsular penetration, neurovascular bundle invasion, invasion of adjacent organs, regional lymph node metastases, and osseous metastases can all be assessed with MRI. When evaluating MR examinations for all these features, it must be remembered that high specificity (minimization of false-positive diagnoses) for the diagnosis of stage T3 disease is strongly desirable. This is because a false-positive diagnosis of tumor spread could deprive a patient of potentially curative surgery (87). For this reason, some radiologists classify a patient as suffering from stage T3 disease only if there is gross extracapsular tumor beyond the capsule or into the seminal vesicles (83).

Pathways of Spread

Capsular Penetration. Stage T1-T2 disease (cancer confined to the prostate) is established on MRI if one of the following three criteria is met: (i) if normal high T2-signal intensity peripheral gland is seen between the tumor and the prostatic capsule, (ii) if there is clear delineation of the capsule despite broad contact between the tumor and capsule, or (iii) if tumor causes only a smooth capsular bulge (9,43).

Five imaging features that are suspicious for capsular penetration include (i) irregularity of the gland contour, (ii) an irregular bulge of the capsule, (iii) frank disruption of the capsule, (iv) hypointense focal thickening of the capsule, or (v) retraction of the capsule adjacent to the tumor (Fig. 10) (9,15,44).

Figure 10 Sixty-two-year-old man with carcinoma of the prostate. Axial T2-weighted image demonstrate low signal intensity tumor (*) in the mid-peripheral gland of the prostate. Irregularity of the gland contour and disruption of the thin black low signal intensity capsule (thin black arrow) reflects capsular penetration.

Figure 10 Sixty-two-year-old man with carcinoma of the prostate. Axial T2-weighted image demonstrate low signal intensity tumor (*) in the mid-peripheral gland of the prostate. Irregularity of the gland contour and disruption of the thin black low signal intensity capsule (thin black arrow) reflects capsular penetration.

Extraprostatic spread of tumor is diagnosed if one of the three following imaging findings is present: (i) hypointense stranding in the periprostatic fat, (ii) obliteration of the fat plane between the posterior prostate and anterior rectum, or (iii) presence of clear-cut extracapsular tumor (9,15).

Differing sensitivities and specificities for endorectal MR diagnosis of capsular penetration (13% to 97% and 47% to 97%, respectively) have been reported (15,24). The lower sensitivities may reflect reader inexperience or use of very strict criteria for establishing the presence or absence of capsular penetration (24,87). However, reduced sensitivity may not have great clinical impact, because microscopic extracap-sular tumor extension has been shown not to affect surgical cure rates or overall patient survival (15,88-90).

Neurovascular Bundle Invasion. The neurovascular bundles create a potential pathway for tumor spread because they course along the posterolateral aspect of the gland and pierce the capsule, resulting in areas of capsular weakness, particularly at the apex (39,43). Invasion of the neurovascular bundle may be better appreciated on T1-weighted imaging and can manifest as asymmetric enlargement of the bundle or as gross extracapsular extension of tumor at the posterolateral aspect of the prostate. Tumor infiltration into the neurovascular bundle may preclude treatment with nerve-sparing prostatectomy.

Seminal Vesicle Invasion. There are three possible pathways of seminal vesicle invasion: (i) direct spread along the ejaculatory ducts, (ii) direct extension of tumor from the base of the prostate, or (iii) skip metastases (91). Five MR findings that suggest seminal vesicle invasion are (i) asymmetry of the seminal vesicles caused by abnormal low T2-signal intensity, (ii) loss of the normal fat plane between the base of the prostate and inferior aspect of the seminal vesicles, (iii) low T2-signal intensity mass, (iv) focal or diffuse wall thickening, or (v) nonvisualization of the ejaculatory ducts or walls of the seminal vesicles (Fig. 11) (8,18,83,92).

Invasion of the wall of the seminal vesicle is classified as stage T3c disease and carries a worse prognosis than do capsular penetration or invasion of the soft tissue surrounding the seminal vesicle (T3a) (93). Invasion of the seminal vesicle is associated with microscopic lymph node metastases in 80% of cases, which increases the risk of recurrence after prostatectomy (94). Although early studies had demonstrated high specificity and low sensitivity of MRI in diagnosing seminal vesicle invasion, advancements in MRI imaging techniques have resulted in improved sensitivities (59-80%) and specificities (83-93%) (15,24,95). Some potential mimickers of invasion include (i) extrinsic compression of the seminal vesicle by BPH (83), (ii) inflammatory changes (43), (iii) postbiopsy hemorrhage (Fig. 12A and B) (51,52), (iv) prior radiation or hormonal therapy (96,97), and (v) amyloid deposition (98-100).

Regional Lymph Node Metastases. The likelihood of lymph node metastases is related to grade and stage of the primary mass. For example, a patient with a well-differentiated stage T1a tumor has a 0% chance of lymph node metastasis, whereas a patient with a poorly differentiated stage T3 tumor has 68% to 93% chance of metastases (101). The prognosis of patients with bilateral lymphadenopathy is poorer than for patients with a single ipsilateral lymph node (88).

Imaging the entire pelvis is important in staging prostate cancer because the lymphatic drainage of the prostate gland involves the obturator, internal iliac, external iliac, common iliac, and presacral lymph node chains (43). Lymphatic spread appears to be a stepwise process from the pelvis to the retroperitoneum in patients with newly diagnosed prostate cancer (102). However, lymph node metastases can

Figure 11 Forty-seven-year-old man with widespread prostate carcinoma. Axial T2-weighted image depicts low signal intensity tumor (black arrow) extending from the base of the prostate gland to invade nearly the entire left seminal vesicle and vas deferens (white arrow).

"skip" pelvic lymph nodes and appear in the retroperitoneum in men who have received radiotherapy with extended pelvic fields. Therefore, it is insufficient to image only the pelvis in patients with suspected disease recurrence (102).

Axial T1-weighted MR images have been shown to be at least equal to that of computed tomography (CT) in accuracy for the detection of nodal metastases (Fig. 13A and B). However, even normal-sized lymph nodes (usually defined as measuring < 1 cm in short-axis diameter) can contain metastatic cancer, which limits the overall sensitivity of CT and MRI in diagnosing metastases (101,103). Recent investigations using lymphotropic superparamagnetic iron oxide particles that target the reticuloendothelial system have improved the diagnosis of metastases by detecting metastatic foci in normal-sized lymph nodes (104-107). Whereas normal lymph node tissue takes up this agent and becomes low in T2-signal intensity, malignant tissue does not, but, instead, remains of higher T2-signal intensity, thereby permitting its detection (104). This technique may also overcome the known limitations of standard surgical pelvic lymphadenectomy, which may miss skip metastases to the common and internal iliac lymph node chains (104).

Invasion into Adjacent Organs. Stage T4 carcinoma of the prostate is defined as gross extraprostatic extension of tumor into adjacent tissues, such as the bladder, rectum, and levator ani muscles. Because of its proximity to the bladder base, direct cranial extension by tumor into the bladder is not uncommon (83). Findings indicative of bladder invasion include loss of the normal fat plane between the bladder and

Figure 12 Left seminal vesicle hemorrhage in a 71-year-old man. (A) Axial T1-weighted and (B) axial T2-weighted images demonstrate high T1 and low T2-signal intensity methemoglobin (black arrow) in the left seminal vesicle, representing postbiopsy hemorrhage.

base of the prostate and discontinuity of the low signal intensity muscular bladder wall (83). Similar criteria are used to diagnose rectal wall invasion, although direct spread to the rectum is rare because Denonvilliers' fascia separates the anterior rectal wall from the posterior aspect of the prostate gland (83).

Figure 13 Forty-seven-year-old man with locoregional lymph node metastases from prostate carcinoma. Axial T1-weighted images demonstrate an enlarged periprostatic lymph node (black arrow) in (A) and an enlarged right external iliac lymph node (black arrow) in (B), representing metastatic disease.

Figure 13 Forty-seven-year-old man with locoregional lymph node metastases from prostate carcinoma. Axial T1-weighted images demonstrate an enlarged periprostatic lymph node (black arrow) in (A) and an enlarged right external iliac lymph node (black arrow) in (B), representing metastatic disease.

Osseous Metastases. Hematogenous spread of prostate carcinoma usually results in osseous metastases. The periprostatic venous plexus drains into a venous plexus anterior to the sacrum, a plexus that communicates with the veins of the spine. This anatomy may explain the frequency of osseous metastases to the lumbosacral spine (38). Focal metastatic lesions appear as low signal intensity lesions within normal, high T1-signal intensity fatty marrow (Fig. 14) (108,109) and as high signal intensity lesions on fat-suppressed T2-weighted and short tau inversion recovery images (108,110). Routine MR imaging of the entire pelvis with T1- and T2-weighted sequences during the initial staging evaluation of patients with newly diagnosed prostate cancer may by useful in detecting osseous metastases. MRI is also helpful in the diagnosis of metastatic prostate cancer in the setting of inconclusive X-rays or suspected spinal cord compression (111,112).

Efficacy of Endorectal MRI in Staging Prostate Cancer

Variable results have been reported for detection of extracapsular tumor spread by endorectal MRI, with accuracies ranging from 54% to 88% (7,15,16,18,30,32,90,113). Reader experience is felt to be a major determinant of diagnostic accuracy and can result in a 30% variation among readers (18). Accuracy and reproducibility of MR image interpretation may be improved with computer-aided analysis, MR spectro-scopy, and better training of readers (9,114-116). The role of MRI in staging prostate cancer continues to be controversial, and some argue that the diagnostic accuracy is insufficiently great to recommend its routine use (117). Studies have shown that physicians overuse MRI when staging newly diagnosed prostate cancer in patients at low risk for lymph node metastases (118,119). Other investigators

Figure 14 Ninety-eight-year-old man with osseous metastases from prostate cancer. Axial T1-weighted image demonstrates a focal low T1-signal intensity metastasis in the anterior column of the right acetabulum.

suggest that MRI is most beneficial and cost effective when reserved for patients at intermediate risk for extracapsular spread as defined by a PSA level of 10 to 20 ng/mL, biopsy Gleason score of 5 to 7, and DRE findings of stage T1-T2 (83).

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Responses

  • arttu
    Does the prostate and seminal vesicle communicate with the urinary system?
    4 days ago

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