MRI feature analysis of uncommon prostatic malignant tumors

Dear Editor, Most prostatic neoplasms are epithelial in origin. Non-epithelial prostatic neoplasms are quite rare, but they cover a broad array of types that include neuroendocrine tumors, mesenchymal tumors, hematolymphoid tumors, miscellaneous tumors, etc.12 Unlike prostate cancer, there is no specific serum marker for non-epithelial prostatic neoplasms at present, and the majority of patients present with a large pelvic mass and compressive symptoms of dysuria or abdominal pain. Imaging plays an important role in the investigation of prostate masses, although transrectal ultrasound-guided prostate biopsy remains the gold standard for diagnosis. Magnetic resonance imaging (MRI) contributes to determining the site of origin of the tumor, local extent, and signal characteristics owing to its high soft-tissue contrast resolution. Many studies have demonstrated that MRI characteristics can help predict histological types of tumors, which can guide clinical treatment.345 As there is a low morbidity rate for uncommon prostatic neoplasms, MRI features have rarely been described in the literature and they are mainly reported as case reports.678 In this letter, we retrospectively reviewed the MRI features of 15 cases of uncommon prostatic malignant tumors, which, to our knowledge, is the largest number of these tumors reported to assess MRI features. As accurate diagnosis is critical for appropriate clinical workup and management, the aim of this study was to explore some salient MRI characteristics of uncommon prostatic malignant tumors to improve understanding and facilitate the differential diagnosis. This retrospective study was approved by Tongji Hospital, Tongji Medical College, Huzhong University of Science and Technology Institutional Review Board. We collected the clinical data of 15 cases of uncommon prostatic tumor patients from January 1, 2011, to December 31, 2015, in Tongji Hospital. And informed consents were obtained from all patients. The age range was from 22 years to 69 years, with a median age of 46 years. Nine cases presented with dysuria, two cases had lower abdominal pain, two cases were accidentally discovered on routine physical examination, one case had dyschezia as well as fresh blood in the feces, and one case had waist pain as well as perineal pain. The median value of the serum prostate-specific antigen (PSA) was 1.412 (range: 0.435-126.634) ng ml−1. The pathological study was performed via transrectal ultrasound-guided biopsy in 11 patients, transurethral resection of the prostate in one patient, and open surgery in three patients. The final pathological diagnoses were embryonal rhabdomyosarcoma (RMS) in two patients, prostatic stromal sarcoma (PSS) in four, mesenchymal tumors not-otherwise-specified in four, small cell carcinoma (SCC) in three, and lymphoma in two. All the patients underwent conventional MRI including T1-weighted imaging (T1WI) and T2-weighted imaging (T2WI), nine patients underwent diffusion-weighted imaging (DWI), and ten patients underwent dynamic contrast-enhanced (DCE) MRI. The clinical characteristics and MRI findings are shown in Table 1. The detailed clinical, immunohistochemical, and MRI information are shown in Supplementary Table 1–Supplementary Table 3. Tumors were round (n = 3), lobular (n = 6), and irregular (n = 6). Eleven tumors occupied the entire prostate, with loss of the normal zonal anatomy on T2-weighted images. Four tumors occupied the majority of the prostate. Among the nine patients who had DWI examinations, all tumors showed high signal and the apparent diffusion coefficient value was relatively low. A typical case of embryonal rhabdomyosarcoma is shown in Figure 1. The tumor presented with a well-defined T2 low signal pseudocapsule with internal cystic degeneration, bleeding, large necrotic areas and exerted a significant mass effect on bladder and rectum.Table 1: The clinical characteristics and magnetic resonance imaging findingsClinicopathologic characteristics Detailed immunohistochemistry characteri Magnetic resonance imaging findingsFigure 1: A 43-year-old man with embryonal rhabdomyosarcoma. (a) A large well-defined and heterogeneous mass invaded the entire prostate with compression of the urinary bladder and rectum on sagittal T2WI. (b) The tumor revealed heterogeneous signal intensity with internal cystic degeneration, bleeding, and large necrotic areas on axial T2WI. (c) The tumor revealed isointense signals, containing areas of high signal compatible with hemorrhage on T1WI. (d) DWI with a b value (diffusion-sensitized gradient) of 1000 s mm−2, (e) the solid part of the tumors demonstrated restricted diffusion and a low apparent diffusion coefficient value. (f) Light microscopic examination for biopsy (HE staining magnification, 200×). Immunohistochemical studies showed that the neoplastic cells were positive for (g) Myogenin, (h) MyoD1 and (i) Ki-67 LI 60%. Scale bars = 100 μm. T2WI: T2-weighted imaging; T1WI: T1-weighted imaging; DWI: diffusion-weighted imaging; HE: hematoxylin-eosin.For the ten prostatic malignant mesenchymal tumors, tumors had low signals mixed with patchy high signals (n = 5) and were isointense (n = 5) on T1WI. Most prostate sarcomas had heterogeneous signal intensity on T2WI, which was consistent with multiple cysts (8/10), hemorrhagic necrosis (7/10) as well as marked heterogeneous enhancement (6/6) after the administration of contrast agents. Most of the prostate sarcomas (9/10) presented with a well-defined T2 low signal pseudocapsule and exerted a significant mass effect on adjacent structures, such as bladder (n = 5) and rectum (n = 5). There was evidence of adjacent invasion of the seminal vesicle (n = 8), bladder (n = 2), and rectum (n = 3). There was evidence of distant metastatic spread (lung metastasis in two cases and liver metastasis in one case). For the three small-cell carcinomas, T2WI showed a large heterogeneous prostatic mass and the prostate capsule was incomplete. The lesions were isointense on T1WI and mildly hyperintense on T2WI. The lesions showed mild or moderate heterogeneous enhancement after the administration of a contrast agent. Some cases showed invasion of adjacent structures, which showed involvement of the seminal vesicle (n = 3), bladder (n = 3), and rectum (n = 2). Pelvis lymph node metastasis was present in one case. There was evidence of distant metastasis in one (liver) case and two (bone) cases. For the two B-cell non-Hodgkin lymphomas, one case was predominantly in the prostate and one case extended to adjacent tissues. The tumors were found to be homogeneously isointense on T1WI and homogeneously isointense on T2WI. Both cases underwent dynamic contrast-enhanced scans and demonstrated moderate and homogeneous enhancement. All of the cases had nodal involvement. One case showed multiple enlarged pelvis and retroperitoneal nodes. Adjacent involvement included the seminal vesicle (n = 2), bladder (n = 2), and rectum (n = 1). There was no clinical or radiologic evidence of distant metastasis in any patients. Noninvasive multiparametric MRI provides comprehensive information that is needed for preoperative diagnosis and evaluation of prostate neoplasms. By combining the clinical finding and imaging, it is possible to primarily differentiate the histologic subtypes of prostate neoplasms. First, it is essential to distinguish uncommon prostatic malignant tumors from prostate adenocarcinoma because of their differences in management and prognoses. Prostate cancer tends to occur at an older age after 50 years, while some malignant mesenchymal tumors such as RMS are more common in children and young adults.6 Most prostate cancer cases are asymptomatic and are detected by PSA screening. However, the uncommon prostatic malignant tumors including RMS, stromal tumors, and lymphomas are presented with obstructive urinary symptoms and total serum PSA levels in the normal range. On MRI, most cases are found to have malignant enlargement of the prostate, with loss of the normal zonal anatomy, whereas prostate cancer more often shows homogeneous hypointense masses on T2WI. Second, several features may help differentiate the histologic subtypes of uncommon prostatic malignant tumors. Most of the malignant mesenchymal tumors, such as RMS and PSS, demonstrate a well-defined and low signal intensity pseudocapsule. Cystic components are visualized.9 In contrast to the mesenchymal tumors, neuroendocrine SCC is characterized by regional nodal invasion, adjacent involvement, and distant metastasis. Hemorrhage and necrosis are very common because of high malignancy and rapid growth and there is no pseudocapsule. Serum neuron-specific enolase values and serum PSA is always elevated.1011 The typical characteristics of prostate lymphoma are that tumors are homogeneously isointense on T1WI and homogeneously isointense on T2WI, and rarely have hemorrhage and necrosis. Tumors are moderately and homogeneously enhanced after gadolinium injection.12 In conclusion, we found some differences among RMS, PSS, SCC, and lymphoma, although there was some overlap in the MR imaging findings. Combined with clinical and laboratory indicators, MRI can provide a credible preoperative diagnosis, avoiding the provision of unnecessary clinical testing or therapies to patients. AUTHOR CONTRIBUTIONS ZYF and LW participated in the study conception and design, data analysis and interpretation, and manuscript drafting. XDM participated in the paper's discussion. ZK (Zan Ke) and PPZ helped collect the clinical data. LW, BSL, and ZK (Zhen Kang) reviewed and edited the manuscript. All authors read and approved the final manuscript. COMPETING INTERESTS All authors declare no competing interests. ACKNOWLEDGMENTS This study was supported by the grant from the National Natural Science Foundation of China (No. 81671656, 81171307). Supplementary information is linked to the online version of the paper on the Asian Journal of Andrology website.