A 35-year-old nulliparous woman with a history of severe endometriosis and chronic pelvic pain undergoes transvaginal ultrasound, which reveals a 4 cm well-defined mass in the endometrium with heterogeneous echotexture. Histology from endometrial biopsy shows benign smooth muscle proliferation with disorganized arrangement. Which investigation would be most appropriate to confirm the diagnosis and assess the extent of myometrial involvement?

Explanation

## Clinical Context The patient presents with a clinical and histological picture consistent with **uterine leiomyoma (fibroid)** — a benign smooth muscle tumor arising from myometrial hyperplasia and hypertrophy. The key diagnostic challenge is determining the extent of myometrial invasion and relationship to the endometrial cavity. ## Investigation of Choice: Pelvic MRI with T1 and T2-Weighted Sequences **Key Point:** Pelvic MRI is the gold standard for fibroid assessment because it: 1. **Characterizes tissue composition** — T2-weighted imaging distinguishes fibroids (low signal) from adenomyosis (high signal with ill-defined margins) 2. **Maps myometrial involvement** — determines submucosal, intramural, and subserosal components (FIGO classification) 3. **Assesses degeneration** — identifies red degeneration, hyaline degeneration, or cystic changes 4. **Guides treatment planning** — critical for deciding between hysteroscopic resection, myomectomy, or expectant management **High-Yield:** MRI is superior to ultrasound for: - Distinguishing fibroids from adenomyosis - Determining exact location and number of fibroids - Assessing junctional zone involvement - Detecting complications (torsion, necrosis, malignant transformation) ## FIGO Classification of Fibroids (Requires MRI) | Type | Location | Submucosal Extent | Clinical Significance | |------|----------|-------------------|----------------------| | 0 | Pedunculated, intracavitary | 100% in cavity | Hysteroscopic resection possible | | 1 | Submucosal | ≥50% intracavitary | Hysteroscopic or combined approach | | 2 | Submucosal | <50% intracavitary | Myomectomy preferred | | 3 | Intramural, abutting endometrium | — | Medical or surgical management | | 4 | Intramural, away from endometrium | — | Expectant or myomectomy | | 5 | Subserosal | — | Laparoscopic myomectomy | | 6 | Parasitic/pedunculated subserosal | — | Laparoscopic resection | | 7 | Cervical | — | Specialized approach | | 8 | Extrauterine (broad ligament, etc.) | — | Surgical planning required | **Clinical Pearl:** In a woman of reproductive age with fibroids, MRI-based FIGO classification directly determines surgical approach and fertility outcomes. Ultrasound alone cannot reliably assign FIGO type. ## Why MRI > Ultrasound for Fibroid Characterization ```mermaid flowchart TD A[Pelvic mass on ultrasound]:::outcome --> B{Fibroid vs<br/>adenomyosis?}:::decision B -->|Ultrasound unclear| C[Pelvic MRI]:::action B -->|MRI obtained| D{FIGO type?}:::decision D -->|Type 0-2| E[Hysteroscopic resection<br/>± myomectomy]:::action D -->|Type 3-4| F[Myomectomy or<br/>medical management]:::action D -->|Type 5-6| G[Laparoscopic myomectomy]:::action E --> H[Fertility preserved]:::outcome F --> I[Treatment individualized]:::outcome G --> J[Minimal morbidity]:::outcome ``` **Tip:** The phrase "assess the extent of myometrial involvement" in the stem is a red flag for **MRI** — it is the only investigation that provides layer-by-layer tissue characterization and FIGO classification.