Stromal tumor of uncertain malignant potential (STUMP) is a rare stromal tumor of the prostate with a broad spectrum of histologic patterns and variable clinical behavior.

Terminology

Stromal tumor of uncertain malignant potential has been also known as atypical stromal hyperplasia, cystic epithelial-stromal tumor, cystosarcoma phyllodes or phyllodes variant ¹.

Epidemiology

A stromal tumor of uncertain malignant potential is a rare entity with only a few large case series published. Patients are apparently most commonly affected in their 6th and 7th decade of life ^1-4. 

Clinical presentation

Most common symptoms include urinary frequency and/or urgency, voiding difficulties, poor flow or urinary retention as well as hematuria or hematospermia.  Digital rectal examination (DRE) might reveal prostatic enlargement ^1-3. 

Prostate specific antigen (PSA) might be elevated ¹.

Complications

If left untreated stromal tumor of uncertain malignant might lead to urinary retention.

Pathology

Stromal tumors of uncertain malignant potential are considered prostatic stromal lesions, which cannot be classified as sarcomas and comprise a broad spectrum of histological patterns ¹.

Macroscopic appearance

Macroscopic appearance might vary. Different colors range from white over tan to yellow and consistency might be solid or firm, partially cystic or multiloculated. Cysts may vary considerably in size and content might be serous, mucinous or bloody ¹. Both the transitional and peripheral zone might be involved.

Microscopic appearance

The microscopic appearance will vary, but all of them will feature an expansion of the prostatic stroma. Areas of necrosis are rare and there is usually only very minor mitotic activity ¹.

There are four different patterns of hypercellular stroma associated with ^1,2,5:

1. cytological atypia or degenerative atypia 

2. bland fusiform cells looking alike benign prostatic hyperplasia

3. benign glands showing a leaf-like pr phyllodes-like growth

4. bland stromal cells and myxoid pattern

Immunophenotype

Immunohistochemistry stains are usually positive for CD34, vimentin and variably with smooth muscle actin. Progesterone receptor will be positive ¹.

Radiographic features

Imaging is done to depict the local extent of the tumor and might help in the diagnosis.

Ultrasound

Transrectal ultrasound might show multiple hypoechoic, cystic lesions.

CT

CT might show a mass with multiple hypodense lesions and enhancing septa after contrast ⁴.

MRI

MRI might show an expansive well-defined mass with multiple leaf-like cystic changes interspersed by bands or septae or a diffusely heterogenous lesion ^3,4,6.

• T1: homogeneously low signal

• T2: high signal intensity cystic areas interspersed by low signal intensity bands

• T1 C+ (Gd): enhancement of the septae

Radiology report

The radiological report should include a description of the following:

• tumor size and extent

• localization within the prostate gland

• signs of extracapsular extension

• lymph node involvement

Treatment and prognosis

Due to the variable and unpredictable behavior of this entity patient management is considered a significant challenge even though many display an indolent course. Therefore a strict follow up is required and surgical management should be considered especially in younger patients ¹.

History and etymology

The term stromal tumor of uncertain malignant potential (STUMP) was used after a study by Gaudin et al. in 1998 ^1-3.

Differential diagnosis

Conditions that can mimic the presentation and/or the appearance of a leiomyosarcoma of the prostate gland include:

• benign prostatic hyperplasia

• stromal sarcoma (especially low grade)

• cystadenoma of the prostate