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Prostatic Stromal Tumors

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  • 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.

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  • Histology:
    • 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.