## Malignant Transformation of Gestational Trophoblastic Disease **Key Point:** Choriocarcinoma is the most common malignant form that develops as a sequela of molar pregnancy, arising in 5–10% of patients after evacuation of a complete mole. ### Spectrum of Malignant GTD | Type | Origin | Frequency After Mole | Malignant Potential | Key Features | |------|--------|----------------------|---------------------|---------------| | **Invasive mole** | Molar trophoblast invades myometrium | 10–15% | 15–20% become malignant | Chorionic villi present; β-hCG elevated | | **Choriocarcinoma** | Malignant trophoblast (no villi) | 5–10% | ~100% malignant | No chorionic villi; very high β-hCG; rapid dissemination | | **PSTT** | Intermediate trophoblast | Rare after mole | Variable | Low β-hCG; late presentation; uterine mass | | **ETT** | Intermediate trophoblast | Extremely rare | Variable | Very rare; recent pregnancy history | **High-Yield:** Choriocarcinoma is the most common **malignant** GTD that develops after molar pregnancy. It arises in 5–10% of complete mole evacuations and in 1–5% of partial mole cases. ### Pathophysiology of Choriocarcinoma Development 1. Molar pregnancy contains abnormal trophoblastic tissue 2. After evacuation, residual trophoblastic cells may persist in the uterus 3. These cells undergo malignant transformation → invasive growth into myometrium and blood vessels 4. Rapid dissemination via haematogenous route (lungs most common site) 5. Markedly elevated β-hCG due to high tumour burden ### Clinical Features of Post-Molar Choriocarcinoma - **Timing:** Usually within 6–12 months after molar evacuation - **Presentation:** Persistent vaginal bleeding, abdominal pain, or symptoms from metastases - **β-hCG:** Persistently elevated or rising after molar evacuation (diagnostic criterion) - **Metastases:** Lungs (80%), vagina (30%), brain (10%), liver (10%) - **Prognosis:** Good with chemotherapy (>95% cure rate with early detection and treatment) **Mnemonic:** **CHORIO** — **C**horoid/CNS metastases, **H**aemorrhage, **O**varian enlargement (theca lutein cysts), **R**apid dissemination, **I**nvasive growth, **O**utstanding response to chemotherapy (if caught early) ### Why Other Options Are Less Common as Malignant Sequelae **Invasive mole** (10–15% of post-mole cases): - While more frequent than choriocarcinoma as a direct complication of mole evacuation, only 15–20% of invasive moles become malignant - Does not represent the most common malignant transformation - Presence of chorionic villi distinguishes it from choriocarcinoma **Placental site trophoblastic tumour** (rare): - Arises from intermediate trophoblast, not syncytiotrophoblast - Extremely uncommon after molar pregnancy - Typically follows normal pregnancy - Low β-hCG is characteristic (unlike choriocarcinoma) **Epithelioid trophoblastic tumour** (extremely rare): - Rarest form of GTD - Arises from intermediate trophoblast - Almost never develops as a sequela of molar pregnancy **Clinical Pearl:** The WHO classification (2015) recognizes four types of GTD: complete mole, partial mole, invasive mole, and choriocarcinoma. Placental site and epithelioid trophoblastic tumours are classified separately as **gestational trophoblastic neoplasia (GTN)**. When a question asks about "malignant transformation" of molar pregnancy, choriocarcinoma is the answer. **Warning:** Do not confuse **frequency of occurrence after mole** (invasive mole is more common) with **most common malignant form** (choriocarcinoma is the answer). The question specifically asks for malignant transformation, which points to choriocarcinoma.
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