Spindle cell tumor putatively derived from prostatic specialized stroma.
Includes pure stromal and mixed epithelial-stromal(phyllodes) tumor.
Classified into prostatic stromal tumors of uncertain malignant potential (STUMP) and prostatic stromal sarcoma (PSS).
Phyllodes tumor is classified into STUMP or PSS depending on the stroma (sarcomatous or not).
Mean age 58 years (range 27-83 years), and include younger patients (>50% with PSS are < 50 years old).
Majority presents with lower urinary tract or obstructive symptoms.
Usually occurs as solitary mass.
STUMP has 4 patterns: degenerative atypia(most common), hypercellular stroma, myxoid, and phyllodes type(stroma + epithelial).
STUMP lacks significant cellular atypia (except degenerate atypia), mitotic activity, necrosis, or extraprostatic growth (image A) & (image B).
PSS is more cellular, pleomorphic, mitotically active and has necrosis (image C) & (image D).
Immunohistochemistry: stromal cells are CD34+ and PR+.
Epithelial component (only in phyllodes tumor) is PSA+ and PSAP+.
STUMP has good prognosis since it's mostly confined to prostate, but may recur.
PSS has high recurrence rate, locally aggressive and can metastasize.
DDX: other prostatic spindle cell lesions such as BPH (closest mimic of STUMP but nodules multiple with spindle cells following small vessels), sarcomatoid carcinoma (keratin+ since its epithelial), smooth muscle tumors (actin+), GIST (c-kit+), and solitary fibrous tumor.