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WHO Grading of Immature Teratomas of the Ovary: Grading is based on the number of low power fields (LPF) (4.5 mm diameter) containing immature neuroepithelium in any one slide. The same method is applied to metastatic foci. Gliomatosis peritonei (containing only mature glial tissue by definition) is considered grade 0 teratoma.

Grade I: 0-1 LPF of immature neuroepithelial tissue in any one slide; behavior is benign

Grade II: 2-3 LPF of immature neuroepithelial tissue in any one slide; behavior is malignant

Grade III: >3 LPF of immature neuroepithelial elements in any one slide; rare or no mature tissues; behavior is malignant.

One can easily see from this grading system that thorough sampling is a must for accurate grading. A two-tier grading system (where Grade I is low-grade and Grades II and III are collectively referred to as high-grade) has a better interobserver reproducibility than the three-tiered grading system.

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