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OG34.3 | Gestational Trophoblastic Disease — SDL Guide (Part 2)

Investigations and Serial β-hCG Monitoring

The defining investigation in GTD is serum β-hCG measurement, which functions simultaneously as a diagnostic biomarker, a treatment response monitor, and a surveillance tool for recurrence. No other solid tumour has a tumour marker as reliable, quantitative, and sensitive as β-hCG is in GTD — it is secreted by virtually all trophoblastic tissue and falls to zero when all GTD cells are eradicated. This makes β-hCG the prototype of the ideal tumour marker: it is 100% sensitive for residual disease, it changes quantitatively and proportionally to tumour cell numbers, and it can be monitored serially with inexpensive blood tests. Understanding the expected post-molar regression curve — and recognising abnormal deviations from it — is a core clinical skill that can save a patient from progressing silently to metastatic choriocarcinoma. The critical insight is that β-hCG has a half-life of approximately 24–36 hours, so a sustained plateau or rise in weekly measurements cannot be explained by physiological lag — it signals active trophoblastic cell secretion, indicating either retained viable molar tissue or malignant transformation to GTN.

The post-molar β-hCG monitoring protocol (standard practice after suction evacuation) uses weekly serum β-hCG measurements. Normal regression after CHM evacuation: β-hCG typically normalises within 8–12 weeks. After PHM, normalisation is usually faster (within 5–8 weeks). Three abnormal patterns require oncological evaluation:
1. Plateau: <10% rise or fall over three consecutive weekly measurements
2. Rise: ≥10% rise over two consecutive weekly measurements
3. Elevated β-hCG beyond 8 weeks (CHM) or 6 weeks (PHM) after evacuation without a clear downward trajectory

The diagnostic criteria for post-molar GTN (requiring treatment) per FIGO/RCOG are: (1) plateau of β-hCG for ≥3 weeks (days 1, 7, 14); (2) rise of β-hCG for ≥2 consecutive weeks (days 1, 7); (3) elevated β-hCG ≥6 months after evacuation; (4) histological diagnosis of choriocarcinoma regardless of β-hCG.

Beyond β-hCG, the investigation of suspected GTN includes: Pelvic ultrasound (transvaginal) to assess uterine dimensions, identify myometrial invasion (heterogeneous vascular lesion within myometrium), and detect theca lutein cysts. Chest X-ray is the first-line investigation for pulmonary metastases (present in ~4–5% of moles at time of evacuation, and in ~80% of choriocarcinoma). CT chest/abdomen/pelvis is the gold-standard staging investigation once GTN is diagnosed — it quantifies the number and site of metastases for WHO/FIGO scoring. MRI brain is performed if CNS metastases are suspected (headache, focal neurological signs, or ultra-high β-hCG >50,000 mIU/mL in high-risk GTN). FDG-PET has a limited role in GTN staging but may detect disease where CT and MRI are negative in selected cases.

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Differential Diagnosis

The differential diagnosis of GTD involves two related challenges: distinguishing a molar pregnancy from other causes of first-trimester complications, and — in the context of a proven molar pregnancy — distinguishing post-molar GTN from normal β-hCG regression or from retained products of conception.

First-trimester differential (before or around diagnosis): A partial mole must be distinguished from an incomplete or missed abortion — both can present with similar early-pregnancy bleeding, normal-sized uterus, and only modest β-hCG elevation. The distinction requires histological examination of evacuation tissue; ultrasound is not reliably specific for PHM (sensitivity ~40%). A complete mole is more clinically distinctive (uterus large for dates, hyperemesis, 'snowstorm' ultrasound), but early CHM before hydrops is well established can be mistaken for a complete abortion or even a normal early intrauterine pregnancy on ultrasound. Ectopic pregnancy is also in the differential of a positive pregnancy test with bleeding; β-hCG is typically much lower and rises more slowly. Twin pregnancy (normal + molar) is a rare entity — one normal foetus co-exists with molar tissue; this requires specialist management.

Post-molar differential (β-hCG not normalising): Retained products of conception following evacuation can maintain modestly elevated β-hCG without the progressive pattern of GTN; a repeat ultrasound and uterine cavity assessment help distinguish. New intrauterine pregnancy — contraception must be maintained throughout post-molar surveillance precisely to avoid this diagnostic confusion. Quiescent GTD is a rare entity where β-hCG remains slightly elevated for months without rising, representing dormant trophoblastic tissue that may eventually resolve spontaneously or progress — management is under ongoing study.

Choriocarcinoma after non-molar pregnancy: A woman with irregular uterine bleeding, haemoptysis, or neurological symptoms after any recent pregnancy — including term delivery — should have β-hCG measured. The differential of persistent post-partum bleeding includes subinvolution, retained placenta, and choriocarcinoma; the latter is the diagnosis that cannot be missed because it is highly curable if treated promptly and potentially lethal if delayed.

Condition	β-hCG Level	Ultrasound	Key Distinguishing Feature
Complete mole	Very high (often >100,000)	Snowstorm, no fetus	Trophoblastic proliferation on histology
Partial mole	Moderately elevated	Fetal parts + hydropic villi	Triploid on FISH/flow cytometry
Incomplete abortion	Low-moderately elevated	Retained products, no villi	No trophoblastic hyperplasia on histology
Post-molar GTN	Plateau or rising	Vascular myometrial lesion	Meets FIGO diagnostic criteria
New pregnancy	Rising (normal doubling)	IUP or no IUP (ectopic)	LMP/history; resolves conceptually with scan
Choriocarcinoma (post-term)	Elevated (variable)	Uterine mass, metastases	Antecedent pregnancy; absent villi on biopsy

Management of Low-Risk GTN

Low-risk GTN (WHO/FIGO prognostic score ≤6) is treated with single-agent chemotherapy, achieving cure rates of 85–95% with primary treatment. The two agents used are methotrexate and actinomycin-D, with methotrexate being the preferred first-line agent in most international protocols due to its long track record, predictable toxicity profile, and lower emetic potential compared to actinomycin-D.

Methotrexate regimens in low-risk GTN: the most widely used schedule is methotrexate 50 mg/m² IM on days 1, 3, 5, 7 alternating with folinic acid (leucovorin) 15 mg oral on days 2, 4, 6, 8, repeating every 2 weeks. The folinic acid rescue prevents methotrexate toxicity (mucositis, myelosuppression) by rescuing normal host cells while allowing the anti-folate effect against the rapidly dividing trophoblast. An alternative simpler regimen (used in resource-limited settings) is methotrexate 0.4 mg/kg IM daily for 5 days, every 2 weeks. Toxicities of methotrexate include mucositis, bone marrow suppression, hepatotoxicity (liver function should be monitored), and teratogenicity (contraception is mandatory).

Response assessment: β-hCG is measured weekly throughout treatment. A satisfactory response is a fall of ≥10% in β-hCG per week. Treatment continues until β-hCG normalises, then two consolidation courses are given after the first normal β-hCG value to eliminate any residual subclinical disease — this consolidation is a critical step that prevents early relapse.

Criteria for cure (post-treatment): three consecutive normal weekly serum β-hCG values (<2 mIU/mL or <5 mIU/mL depending on assay).

Resistance to methotrexate is defined as: (1) β-hCG plateau or rise after two consecutive courses of methotrexate; or (2) development of new metastases during methotrexate. First-line switch is to actinomycin-D (500 μg IV every 2 weeks, or 1.25 mg/m² every 2 weeks). If resistance to both single agents develops, recalculate the prognostic score — if now ≥7, switch to EMA-CO multi-agent regimen.

Contraception during surveillance and treatment: oral contraceptive pill is the recommended method (does not affect β-hCG and also suppresses endogenous LH which can cross-react with some β-hCG assays). IUCDs should be avoided until β-hCG normalises (risk of uterine perforation in highly vascular GTN). Pregnancy must be delayed for at least 12 months after completing surveillance.

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