Prostatic stromal tumour of uncertain malignant potential

A 58-year-old patient presented with lower urinary tract symptoms and a prostate-specific antigen (PSA) level of 4.6 ng/ml. On digital rectal examination, an enlarged prostate with a palpable hard mass on the left side was detected. Multiparametric magnetic resonance imaging (mpMRI) of the prostate was ordered for further evaluation.

The mpMRI of the prostate revealed a large, well-defined exophytic prostatic mass originating from the left midgland posterior peripheral zone. It was heterogeneously hypointense on T2-weighted (T2W) images, with a hypointense pseudocapsule. Minimal cystic areas, some with high signal intensity on T1-weighted (T1W) images, were observed. The mass demonstrated heterogeneous progressive enhancement and no restriction diffusion. The mass measured 56 x 78 x 94 mm (T x AP x L) and extensively extended through the periprostatic fat from the pelvic inlet superiorly to the urogenital diaphragm inferiorly and laterally to the levator ani. The margins were regular, and no invasion of adjacent structures was observed. The mass displaced the bladder, prostate, membranous urethra, seminal vesicles, vas deferens, and rectum. No pelvic lymphadenopathy or ascites were observed.

A mesenchymal neoplasm of the prostate was suspected. CT-guided core biopsy confirmed morphological and immunohistochemical features consistent with prostatic stromal tumour of uncertain malignant potential (STUMP). The sample showed stromal hyperplasia with atypia, without nuclear atypia, mitoses, or necrosis, and no malignancy criteria were present. Immunohistochemistry analysis revealed positive staining for CD34, SMA, desmin, androgen receptors, and progesterone receptors, while S100, CD117, DOG1, and Bcl2 showed negative staining. The nuclear proliferation index (Ki67) was low (<2%).

Background

Prostatic STUMP is a rare mesenchymal tumour characterized by atypical proliferation of the hormonally responsive prostatic stroma [1]. The aetiology and pathogenesis of prostatic STUMPs remain unknown, with no identified risk factors [2]. These tumours can occur throughout adulthood but are typically seen in the sixth decade of life [3-5]. They present as solitary masses, primarily in the peripheral zone, although they can also be found in the transition zone [3].

Clinical and Imaging Perspective

Prostatic STUMPs are typically large at presentation and most patients present with nonspecific lower urinary tract symptoms, abnormal findings on digital rectal examination, and normal PSA levels [1].

MRI is the preferred imaging modality for evaluating prostatic masses. MpMRI provides essential information on lesion location, size, characteristics, and relation to adjacent structures, which may suggest a specific diagnosis and guides patient management [1, 5]. Prostatic STUMPs typically appear as well-defined solid masses originating from the peripheral zone and displacing adjacent structures [1, 6, 7]. The signal intensity on T1W and T2W images depends on the stroma type, and the relative amount and content of the dilated cystic glands [6-8]. A T2-hypointense pseudocapsule may also be seen [1]. The solid component of the tumour typically shows heterogeneous enhancement and moderately restricted diffusion [1].

Although imaging findings can suggest the diagnosis, accurate differentiation from other mesenchymal tumours, such as prostate stromal sarcomas (PSS), requires histopathological analysis [1]. Specific histological and immunohistochemical features distinguish STUMPs from other conditions [1, 9].

Outcome

Treatment options range from conservative management to radical resection, considering patient age, comorbidities, tumour size, growth pattern, and invasion extent [3, 5, 9]. Radical prostatectomy offers a potential cure in early-stage cases, minimizing the risk of underdiagnosing PSS by biopsy sampling error [5]. Watchful waiting may be an option for slow-progressing STUMPs or patients refusing surgery, but disease progression risk should be considered [5].

Although most prostatic STUMPs are indolent and may be cured with surgical resection, their uncertain malignant potential manifests as unpredictable behaviour, including local invasion, high local recurrence rates and association with PSS or adenocarcinomas [3, 9-11]. Rare metastases to lymph nodes, lungs, and bones have been reported [4, 11].

Close follow-up is crucial regardless of the chosen treatment [5]. Imaging plays a critical role in post-resection surveillance for local recurrence and in monitoring tumour growth and changes in expectant management [6, 7].

Written informed patient consent for publication has been obtained.