## Distinguishing Adenomyosis from Uterine Fibroids ### Key Discriminating Feature **Key Point:** Junctional zone (JZ) abnormalities on MRI are the gold standard for diagnosing adenomyosis and represent the single most specific feature that distinguishes it from fibroids. ### Comparative Table | Feature | Adenomyosis | Uterine Fibroids | |---------|-------------|------------------| | **Junctional Zone on MRI** | Thickened (>12 mm), irregular, poorly defined | Normal or preserved | | **Uterine Enlargement** | Diffuse, symmetrical | Irregular, nodular, focal | | **Ultrasound Appearance** | Heterogeneous myometrium, no discrete mass | Discrete, well-circumscribed lesions | | **Endometrial-Myometrial Interface** | Disrupted, indistinct | Preserved | | **Bleeding Pattern** | Heavy, dysmenorrheic | Heavy (variable) | | **Age of Onset** | Typically >40 years | Can occur at any age | ### Why MRI Junctional Zone is the Discriminator **High-Yield:** Adenomyosis involves ectopic endometrial glands and stroma invading the myometrium, causing: 1. Junctional zone thickening (>12 mm) 2. Irregular, ill-defined JZ borders 3. Hyperintense foci on T2-weighted imaging (representing cystic dilatation of basalis endometrium) Fibroids, being benign smooth muscle tumors, do NOT disrupt the junctional zone — they displace it. **Clinical Pearl:** While both conditions cause heavy menstrual bleeding and uterine enlargement, **only adenomyosis shows JZ pathology**. This is why MRI is the imaging modality of choice when distinguishing these two entities in equivocal cases. ### Why Other Features Are Non-Discriminatory - **Irregular nodular enlargement:** Both conditions can present this way - **Heavy menstrual bleeding:** Both present with menorrhagia - **Hypoechoic lesions on ultrasound:** Fibroids are hypoechoic; adenomyosis shows heterogeneous myometrium but not discrete masses [cite:Park 26e Ch 10]
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