MRI Appearance of Prostatic Stromal Sarcoma in a Young Adult

Prostatic stromal sarcoma (PSS) is quite rare, comprising only 0.1-0.2% of all malignant prostate tumors (1). In 1998, Gaudin et al. (2) classified sarcoma-related proliferative lesions of the specialized prostatic stroma, including prostatic phyllodes tumors, into two subtypes: prostatic stromal proliferation of uncertain malignant potential and PSS (2, 3). Herein, we present a case of PSS in a young man. To the best of our knowledge, only three reports of PSS with findings from computed tomography (CT) have been reported in the English literature (1, 4, 5). However, none of those reports described findings on magnetic resonance imaging (MRI). This represents the first report in the radiological literature on the MRI appearance of PSS.

A 26-year-old man presented with a history of dysuria and hematuria for about one month. A digital rectal examination revealed a huge hard mass with an irregular surface. Serum levels of prostate-specific antigen (PSA) were not elevated (0.44 ng/mL). Various other tumor markers were also within normal limits. MRI was performed using a 1.5-T Signa Excite scanner (GE Medical Systems, Milwaukee, WI) and a phased-array torso coil. T2-weighted fast spin-echo (FSE) imaging in the transverse and coronal planes, T2-weighted echo-planar imaging (EPI) in the transverse plane, T1-weighted FSE imaging in the transverse plane, diffusion-weighted imaging (DWI) (using b-factors of 0 and 800 s/mm²) in the transverse plane, dynamic contrast-enhanced MRI (DCE-MRI) in the transverse plane, and contrast-enhanced T1-weighted FSE imaging in the transverse plane were performed. Data acquisition for DCE-MRI began simultaneously with the initiation of intravenous injection of gadopentetate dimeglumine (Magnevist; Bayer Schering Pharma, Osaka, Japan) at 0.1 mmol/kg body weight within 10 s through a peripheral intravenous cannula. This was followed by a 40-mL flush with saline. Multiphase DCE images (6 phases) were obtained every 30 s for 150 s without breath-holding.

MRI revealed an uneven multinodular mass located mainly within the central zone of the prostate and extending to the right peripheral gland at the bottom level of the central gland mass. The mass showed homogeneous low signal intensity on T1-weighted imaging (Fig. 1A) and heterogeneous high signal intensity with pseudocapsule on T2-weighted EPI (Fig. 1B). In DCE-MRI, the central gland portion of the mass showed a weak gradual enhancement containing cystic areas, whereas the right peripheral gland showed a moderate gradual enhancement (Fig. 1C-E). DWI showed the prostatic mass as an area of marked high signal intensity (Fig. 1F). Coronal T2-weighted imaging demonstrated enlarged internal iliac lymph nodes (Fig. 1G).

Fig. 1
Prostatic stromal sarcoma in 26-year-old man.

A. Axial T1-weighted image showing mass located centrally and to right side of prostate with homogeneous slight low signal intensity in comparison with left side of prostate (arrows). B. Axial T2-weighted echo-planar image showing multinodular mass (arrows) with irregular margins associated with nodular lesions (small arrows) in posterior part of mass, and presenting heterogeneous high signal intensity. Right peripheral gland (arrowheads) is compressed by central glandular mass. C-E. Axial dynamic contrast-enhanced MR images at 0 s (C), 30 s (D) and 150 s (E) after contrast injection, showing weakly enhanced mass containing non-enhanced cystic areas in central gland (arrows). As right peripheral gland shows gradual moderate enhancement (arrowheads), which differs from enhancement in left peripheral gland, tumor invasion to right peripheral gland is suggested. F. Axial diffusion-weighted image showing central gland mass (arrows) and right peripheral zone mass (arrowheads) as areas of marked high signal intensity. G. Coronal T2-weighted image showing markedly enlarged lymph nodes near seminal vesicle and separated from prostatic mass (arrow). H. Histological findings from transurethral resection of prostate. Low-power views show leaf-like projection of proliferating stroma. Cellular stroma contains round and spindle-shaped cells with atypia and mitosis (Hematoxylin & Eosin staining, × 100).

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Transrectal ultrasonography-guided prostate biopsy and transurethral resection of prostate were performed. The resected specimen predominantly comprised elongated ducts and cellular stroma that contained areas of necrosis with calcification (Fig. 1H). Hypercellular stroma without glands was specifically detected. Although epithelial components did not show malignant changes, stromal cells showed ovoid, spindle-shaped and hyperchromatic nuclei in cellular areas, and spindle-shaped nuclei in myxomatous areas. An average of two mitoses per high-powered field was counted. Epithelial cells showed positive immunostaining for pan-cytokeratin (AE1/AE3, CAM5.2). In stromal cells, positive immunostaining was detected for vimentin, CD34, and progesterone receptor; whereas, negative results were obtained for pan-cytokeratin, CD31, S-100, HHF-35, desmin, α smooth muscle actin, MyoD1, c-kit, and estrogen receptors. The MIB-1 labeling index was elevated to > 50%. Consequently, the tumor was histologically diagnosed as PSS (2).

Although the patient was treated by chemotherapy, he showed progressive disease with bone, lung, liver, and mediastinal lymph node metastases and died seven months after admission.

Primary prostate sarcomas are rare malignant tumors of the prostate gland (1, 2). Leiomyosarcoma is the most common histological subtype of prostate sarcoma seen in adults (6). Prostate stromal sarcoma is rarer with fewer than 30 documented cases (1-5, 7-16). Most patients with prostatic sarcoma, including PSS, present with symptoms of urethral obstruction (1-3, 5, 6), as in the present case. The mass effect of the tumor, mainly located in the central gland, probably caused the early onset of symptoms (17). In previous reports on PSS, age at diagnosis has ranged from 19 to 86 years (mean, 48 years) (1-5, 7-16). PSA levels in patients with PSS including the present case have been relatively low (0.1-4.5 ng/mL) in comparison with prostatic adenocarcinoma (1, 3-5). PSS is often large, with most tumors having a diameter > 4 cm (1, 3-5). In immunohistochemical studies, PSS are typically positive for vimentin and CD 34, and negative for estrogen receptor and HHF-35 (1, 2).

On MRI, a prostatic adenocarcinoma typically demonstrates an unidentified area with signal isointensity relative to background prostatic structures on T1-weighted imaging; an area in the peripheral zone with homogeneous low signal intensity with mass effect, or an area in the transition zone with homogeneous low signal intensity, ill-defined margins, and lack of a capsule, with or without a lenticular shape and invasion of anterior fibromuscular stroma on T2-weighted imaging; an area with focal early enhancement on DCE-MRI; and an area with focal high signal intensity relative to background prostatic structures on DWI (18-21). In the present case, the main tumor in the central gland showed a multinodular shape with heterogeneous high signal intensity and a low signal intensity pseudocapsule on T2-weighted imaging. Furthermore, both areas of tumor showed gradual weak enhancement on DCE-MRI. PSS may differ from a typical adenocarcinoma with respect to shape, vascularity on DCE-MRI, and signal intensity on T2-weighted imaging, thus reflecting tissue construction and cellular structure. Conversely, these PSS findings resemble those of leiomyosarcoma and rhabdomyosarcoma, which are a more common form of sarcoma involving the prostate (22, 23). Furthermore, DWI clearly demonstrated both central gland and peripheral zone parts of the tumor as showing marked high signal intensity, suggesting the usefulness of this modality for determining the local extent of the lesion. MRI features of PSS have not previously been reported in the English literature. However, two reports of MRI findings for stromal sarcoma of uterine endometrium by Koyama et al. (24) and Ueda et al. (25) demonstrated that stromal sarcoma was relatively large and showed heterogeneous signal intensity on T2-weighted imaging, heterogeneous contrast enhancement of the tumor, marginal nodules, multiple nodule formation, hemorrhage, and necrosis in the tumor. All these MRI findings except hemorrhage were also seen in our case.

MRI findings including T2-weighted imaging, DCE-MRI, and DWI in the present case seemed to reflect the pathological features of PSS, which include aggressive tumor growth, greater cellularity, and necrosis with mitotic activity.

In the present case, the tumor was mainly located in the central gland, and appeared as a signal hypointense mass on T1-weighted imaging, and as a heterogeneously signal hyperintense multinodular mass on T2-weighted imaging. DCE-MRI indicated weak to moderate gradual enhancement containing cystic areas. PSS did not resemble prostatic adenocarcinoma on MRI, which may thus play a role in differentiating PSS from prostatic adenocarcinoma. When an adult male shows MRI findings as mentioned above, although these imaging findings are not pathognomonic, PSS should be included in the differential diagnoses of a prostatic adenocarcinoma.