Distinguishing Leiomyosarcomas from Benign Fibroids
The correct answer is B: Fibroid has homogenous cellular structure. Benign leiomyomas represent clonal expansions of smooth muscle cells with histology that is virtually indistinguishable from normal myometrium and relatively uniform cellularity, whereas leiomyosarcomas demonstrate cellular pleomorphism and heterogeneous cellular characteristics 1.
Key Distinguishing Features
Cellular and Histologic Characteristics
Benign fibroids demonstrate homogenous cellular structure with histology virtually indistinguishable from normal myometrium, though cellularity can be variable with areas of fibrosis and calcification interspersed 1. In contrast, leiomyosarcomas show significant cellular pleomorphism, increased mitotic figures (typically >10 per 10 high-power fields), and coagulative tumor necrosis 1, 2, 3.
- Pseudosarcomatous leiomyomas (benign variants with atypical features) show significant histological diversity both between tumors and within the same tumor, with areas indistinguishable from conventional leiomyoma often present 4
- True leiomyosarcomas are composed of rather uniform long spindle cells similar to soft tissue leiomyosarcomas, lacking the histological diversity seen in benign variants 4
Clonal Origin and Multiplicity
Regarding option A (proximity to other fibroids): While leiomyomas are indeed clonal in origin and multiple fibroids are common (up to 70% of women have identifiable leiomyomata at menopause), this is not a reliable distinguishing feature from leiomyosarcoma 1. Leiomyosarcomas are typically solitary masses.
Regarding option C (polyclonal origin): This is incorrect. Leiomyomas represent benign sex steroid-responsive clonal expansions of individual smooth muscle cells originating from myometrial cells, not polyclonal 1, 5. They grow in a spherical nodular fashion with distinct demarcation from surrounding myometrium, reflecting their clonal origin 1.
Imaging Characteristics (Option D)
Regarding distinct MRI appearance: While MRI is superior for identifying and mapping fibroids, conventional MRI cannot accurately differentiate fibroids from sarcomas 1. However, advanced MRI with diffusion-weighted imaging and apparent diffusion coefficient (ADC) values shows promise:
- A diagnostic algorithm incorporating enlarged lymph nodes, peritoneal implants, high diffusion signal, and low ADC values achieved 98% sensitivity and 96% specificity in training sets 1, 2
- Benign leiomyomas typically show low T2 signal intensity and well-defined borders, while leiomyosarcomas present as large masses with irregular outlines and intermediate T2 signal intensity 6
- The challenge is that atypical fibroids with degeneration can mimic suspicious features 6
Clinical Risk Stratification
Age and menopausal status are critical risk factors 2:
- Leiomyosarcomas comprise only 0.1% of all uterine tumors overall 1
- However, they represent 1.7% of women undergoing hysterectomy for presumed fibroids in their seventh decade of life 1, 2
- For practical purposes, these tumors should be considered benign in premenopausal women 1
Diagnostic Approach
The gold standard remains histopathologic examination with permanent sections, as frozen section cannot reliably identify mitoses to distinguish benign from malignant 1, 2. Immunohistochemistry shows that benign leiomyomas (including pseudosarcomatous variants) express progesterone receptor intensely and frequently express estrogen receptor, while true leiomyosarcomas completely lack these receptors 4.