British Journal of Radiology (2007) 80, e180-e183
© 2007 British Institute of Radiology
doi: 10.1259/bjr/57293350
Primitive neuroectodermal tumour of the prostate gland: ultrasound and MRI findings
N Thete, MBBS, DMRD
D Rastogi, MBBS, DMRD
S Arya, MBBS, DNB, MD
A Singh, MBBS, DMRD
P Rao, MBBS, DMRD
A Chandge, MBBS
and
M Ramadwar, MBBS, MD
Tata Memorial Hospital, Ernest Borges Road, Parel, Mumbai-400012, Maharashtra, India
Correspondence: Dr Nilesh Thete, Block 15, Makarand Society, Plot-152, Sector 3, RDP-7, Charkop, Mumbai-67, Maharashtra, India. E-mail: nilesh.thete{at}gmail.com
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Abstract
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We report a rare case of primitive neuroectodermal tumour (PNET) of the prostate occurring in a 26-year-old man with a mass replacing the prostate gland in the absence of any lesion involving the bone and soft tissues. To our knowledge, there is no radiological literature that has described the imaging findings in a case of PNET of the prostate. Imaging findings in cases of PNET of the kidney are described in the literature. On ultrasound, the mass appeared multilobulated and multinodular with intratumoural heterogeneity. No brightly reflective echoes with posterior acoustic shadowing to suggest calcification were noted. MRI revealed a heterogeneous lobulated mass with irregular septae within. The mass showed low signal intensity on T1 weighted images and appeared heterogeneous on T2 weighted images. Haemorrhage and necrosis were also seen in the tumour.
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Introduction
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Primitive neuroectodermal tumours (PNETs) are composed of small malignant undifferentiated cells and occur mainly in children and young adults. These tumours show predilection for bones and soft tissues in the paraspinal region and lower extremities [1]. Ewing's sarcoma and PNETs share the same chromosomal translocation, resulting in fusion of the EWS gene with a number of erythroblast transformation-specific domain family genes, and are now considered as the same entities. A greater degree of diagnostic reliability is offered by molecular analysis, as PNETs are characterized by a non-random translocation, which results in fusion of the EWS gene on chromosome 22q12 to FLI1 in 11q24, ERG in 21q22 and, rarely, ETV1 in 7p22, E1AF in 17q12 and FEV in 2q33. Both EWS–FLI1 and EWS–ERG rearrangements show different combinations of exons encoding different fusion transcripts and producing different chimeric proteins [2]. Primitive peripheral neuroectodermal tumour has been described in a variety of primary visceral sites but, to the best of our knowledge, this is the third case being reported in the prostate gland. The two initial cases have been reported by histopathology, immunohistochemistry and cytogenetics [3, 4]. We report a rare case of PNET of the prostate occurring in a 26-year-old man with a mass replacing the prostate gland in the absence of any lesion involving bone and soft tissues, and also describe the MRI findings. Histopathology and immunohistochemistry confirmed the diagnosis.
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Case report
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A 26-year-old man presented with symptoms of dysuria and pelvic discomfort. There was no history suggesting bone pain, pathological fractures or anaemia. The patient had no manifestation of paraneoplastic syndrome.
On clinical examination, a palpable mass was felt in the suprapubic region. On digital rectal examination, a firm pelvic mass was felt. There were no palpable lymph nodes.
The serum prostate-specific antigen level was 0.3 ng ml–1. The serum alkaline phosphates, liver function tests and renal function tests were normal. The urine examination and the haemogram showed no abnormality.
Abdominal and transrectal ultrasound was performed with a 3.5 MHz and a 5 MHz intracavitatory electronic focused array transducer, which revealed a large mass in the pelvis, posterior to the urinary bladder. The mass was seen replacing the prostate gland and the seminal vesicles.
The mass appeared multilobulated and multinodular with intratumoural heterogeneity. No brightly reflective echoes with posterior acoustic shadowing to suggest calcification were noted.
Multiplanar MRI was performed on a General Electric (Signa) 1.5 Tesla magnet using:
- Axial, sagittal and coronal FSE (fast spin echo) T1 weighted (T1W) sequence (repetition time (TR) 320/echo time (TE) 12.6);
- Sagittal FRFSE (fast recovery fast spin echo) T2 weighted (T2W) sequence (TR 4420/TE 103), axial FRFSE (fast recovery fast spin echo) T2W fat saturation (TR 3460/TE 104);
- Coronal STIR sequences (TR 3200/TE 24.6/TI 150) with a torso coil.
MRI revealed a large mass replacing the prostate and extending superiorly to occupy most of the true pelvis. The mass showed areas of haemorrhage and necrosis (Figure 1
, black arrows). The seminal vesicles could not be identified separately. The urinary bladder with a Foley's catheter in situ was displaced anteriorly and compressed (Figure 2, large white arrow). The posterior vesical wall was in close contact with the mass with possible adherence. The rectum was displaced posteriorly against the sacrum, but was not infiltrated (Figure 2, large black arrow).
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Figure 1. Coronal and sagittalT1 weighted images showing the pelvic tumour with areas of haemorrhage and necrosis (black arrows).
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Figure 2. Axial and sagittalT2 weighted images demonstrating the multinodular and multilobulated tumour with fibrous septae (white arrows). The compressed rectum (large black arrow) and Foley's catheter balloon (large white arrow) are also seen.
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On T1W sequences, the mass was predominantly isointense to muscle, but several irregular hyperintense areas were seen interspersed within the mass (Figure 1
, black arrows). On T2W sequences, the mass appeared heterogeneous, multilobulated and multinodular. Most of the peripheral nodules appeared homogeneously hyperintense to muscle, but hypointense to urine in the adjacent bladder. The central nodules displayed variegated intensity with some isointense and some darkly hypointense areas. A few linear low signal intensity areas were seen within the mass, which represented fibrous septae (Figure 2, white arrows).
Therefore, preoperative diagnosis based on imaging alone was difficult, and histopathology and immunohistochemistry were essential for confirmation. However, imaging can be valuable in guiding biopsy as in our case. On ultrasound, the mass was multilobulated and multinodular with intratumoural heterogeneity.
A transrectal ultrasound-guided core biopsy was performed with a 21 cm long, 18G gun biopsy needle. Three 2.2 cm sized cores were obtained and sent for histopathological evaluation.
Histopathology showed a malignant small round cell tumour indicating a PNET. The tumour cells were weakly positive for Mic-2. There was no identifiable prostatic tissue. The haematoxylin and eosin (H&E) stained photomicrograph (40x magnifications) shows small round cells, and the CD99 immunostain photomicrograph (40x magnifications) shows Mic-2 positivity (Figure 3).
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Figure 3. Haematoxylin and eosin(H&E) stained photomicrograph demonstrating small round cells, and the CD99 immunostain photomicrograph showing Mic-2 positivity.
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In our case, no further surgical intervention was possible as the patient was lost to follow-up.
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Discussion
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The common acinar prostatic adenocarcinoma constitutes 95% of all malignant prostatic neoplasms. Numerous rare morphological variants of prostatic carcinoma have been identified in the last two decades. These atypical prostate cancers account for less than 5% of all prostatic malignancies. These prostatic malignancies are listed in Table 1 [5].
Extraskeletal PNET mainly occurs in the paravertebral region, the chest wall and the lower extremities, and less commonly in the pelvis, retroperitoneum or upper extremities. Exceptional sites of occurrence include several organs of the genitourinary system, such as the kidney [6], ureter [7], bladder [8], testis [9] and seminal vesicles [10], as well as many other visceral sites (ovary, pancreas, uterus, parotid gland and lungs) [11]. Visceral locations are unusual, with the kidney being the most commonly cited [12].
MRI features of PNET of the kidney have been described in the literature as a heterogeneous lobulated mass with irregular septae within. The mass shows low signal intensity on T1W images and appears heterogeneous on T2W images. Haemorrhage and necrosis are also seen in the periphery of the tumour. The septae within the mass enhance on post-contrast scans [13].
Microscopically, PNETs are characterized by primitive round cells reactive to anti-CD99 antibody (Mic-2). CD99 positivity distinguishes PNET from other rare entities such as lymphoma, rhabdomyosarcoma and other neuroendocrine cancers [10].
In some cases, cytogenetic analysis is helpful to confirm the diagnosis. More than 90% of PNETs demonstrate a translocation between the long arms of chromosomes 11 and 22, which is pathognomonic of this family of tumours [10]. Therefore, either immunohistochemical evaluation or chromosomal analysis identifying the specific translocation t(11:22)(q24:q12) may establish the diagnosis.
The main differential diagnoses considered in our case, taking into account the age of the patient and the tumour site, were embryonal rhabdomyosarcoma, malignant lymphoma and desmoplastic small round cell tumour.
Rhabdomyosarcoma is the most common sarcoma of the bladder and prostate gland. In this case, the morphological findings and the negative immunostaining for desmin and myoglobin ruled out this possibility.
Small cell carcinoma of the prostate gland, a rare and aggressive neoplasm, was ruled out, as the tumour occurs in an older age group and is negative for cytokeratin. Malignant lymphoma was easily ruled out by negativity for leukocyte common antigen (LCA). Finally, a possible secondary involvement of the prostate gland by a desmoplastic small round cell tumour was excluded by the lack of expression of cytokeratin, desmin and neuron-specific enolase (NSE).
The vast majority of PNETs express a surface antigen encoded by the Mic-2 gene, and CD99, a glycoprotein product of the Mic-2 gene, is present on the cell surface of 90% of cases, thus serving as a useful marker. In this case, CD99 immunostaining was positive, thereby confirming the diagnosis.
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Conclusion
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Our experience with MRI in this case of PNET of the prostate defined the tumour as heterogeneous, multinodular, multilobulated and multiseptate with areas of haemorrhage and necrosis. With such findings on MRI, PNET should be considered in the differential diagnosis of aggressive tumours in young adults even in unusual locations.
Received for publication August 7, 2004.
Revision received July 24, 2005.
Accepted for publication October 30, 2005.
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References
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Abstract
Introduction
