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Smooth Muscle Tumors

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Leiomyoma (LM)

  • May arise from periglandular tissues or prostatic capsule.
  • Rare, most occur in 60s presenting with obstructive symptoms.
  • May be mistaken clinically as BPH and encountered often in TUR.
  • Typically, solitary 2-7 cm, but can grow considerably large (>10 cm).
  • Gross: solitary, well circumscribed, white-tan with smooth outer surface.
  • Traditionally described in 2 forms, as solitary tumors and in background of BPH. (Those in BPH were probably "leiomyomatous" type hyperplasia).
  • Histology: fascicles of benign spindle cells with cigar-shaped nuclei.
  • Immunohistochemistry: smooth muscle marker actin+ and desmin+.
  • Rarely, exhibits polygonal or round cell morphology (leiomyoblastoma).
  • DDX: leiomyomatous BPH (multiple) and other spindle cell tumors.

Leiomyosarcoma (LMS)

  • Infrequent (<1%), but LMS together with rhabdomyosarcoma are the most predominant types of prostate sarcomas in adult.
  • Occur in 40s to 70s, but may occur in younger and pediatric patients.
  • Most presents with obstructive symptoms; also hematuria and weight loss.
  • LMS may grow large (mean 9 cm) and replace entire prostate; often is difficult to completely resect the tumor.
  • Histology:
    • Dense fascicles of spindle cells with pleomorphic cigar-shaped nuclei, prominent nucleoli and abundant mitosis; higher grade tumors, fasciculations become vague.
    • Necrosis is common and tumors may undergo cystic degeneration
  • Immunohistochemistry: diagnosis confirmed by smooth muscle markers, such as actin+ and desmin+.
  • Aggressive course with 1-year survival of 55%.
  • DDX: other spindle cell tumors such as sarcomatoid carcinoma (keratin+), stromal sarcoma
    (CD34+, actin-), GIST (c-kit+) and SFT (CD34+, actin-).