Gestational trophoblastic tumors (GTTs) and neoplasias (GTNs) may be classified as follows:
- Hydatidiform mole (HM).
- Invasive mole (chorioadenoma destruens).
- Placental-site trophoblastic tumor (PSTT; very rare).
- Epithelioid trophoblastic tumor (ETT; even more rare).
HM is defined as products of conception that show gross cyst-like swellings of the chorionic villi that are caused by an accumulation of fluid. There is disintegration and loss of blood vessels in the villous core.
A complete mole occurs when an ovum that has extruded its maternal nucleus is fertilized by either a single sperm, with subsequent chromosome duplication, or two sperm, resulting in either case in a diploid karyotype. The former case always yields a mole with a karyotype of 46 XX, since at least one X chromosome is required for viability and a karyotype of 46 YY is rapidly lethal to the ovum. The latter case may yield a karyotype of 46 XX or 46 XY. About 90% of complete HMs are 46 XX. On ultrasound examination, complete moles rarely reveal a fetus or amniotic fluid.
A partial mole occurs when the ovum retains its nucleus but is fertilized by a single sperm, with subsequent chromosome duplication, or is fertilized by two sperm; the possible resulting triploid karyotypes are 69 XXY, 69 XXX, or 69 XYY. Therefore, in contrast to a complete mole, the partial mole chromosomes of a partial mole are only two-thirds paternal in origin. In contrast to complete moles, partial moles usually show a fetus, which may even be viable, and amniotic fluid is visible.
Complete HMs have a 15% to 25% risk of developing into GTNs, but transformation to malignancy is much more rare (<5%) in the case of partial moles.
Invasive moles (chorioadenoma destruens) are locally invasive, rarely metastatic lesions characterized microscopically by trophoblastic invasion of the myometrium with identifiable villous structures. They are usually diploid in karyotype, but may be aneuploid. Microscopically, these lesions are characterized by hyperplasia of cytotrophoblastic and syncytial elements and persistence of villous structures. They may resemble choriocarcinoma in histologic appearance. Invasive moles have more aggressive behavior than either complete or partial HMs, and they are treated similarly to choriocarcinoma (i.e., with chemotherapy). However, unlike choriocarcinoma, they may regress spontaneously.
Gestational Trophoblastic Neoplasias
Choriocarcinoma is a malignant tumor of the trophoblastic epithelium. Uterine muscle and blood vessels are invaded with areas of hemorrhage and necrosis. Columns and sheets of trophoblastic tissue invade normal tissues and spread to distant sites, the most common of which are lungs, brain, liver, pelvis, vagina, spleen, intestines, and kidney. Most choriocarcinomas have an aneuploid karyotype, and about three-quarters contain a Y chromosome. Most follow an HM pregnancy, spontaneous abortion, or ectopic pregnancy; but about one-quarter are preceded by a full-term pregnancy. Nearly all GTNs that are preceded by nonmolar pregnancies are choriocarcinomas; the rare exceptions generally are (PSTTs).