## Discriminating Features Between PTC and FTC ### Nuclear Morphology — The Key Distinguisher **Key Point:** Papillary thyroid carcinoma (PTC) is defined by its distinctive nuclear features, NOT by growth pattern. The hallmark nuclei are: - Nuclear grooves (longitudinal folds) - Pseudoinclusions (cytoplasmic invaginations into nucleus) - Pale, finely dispersed chromatin ("Orphan Annie eye" appearance) These features are present in PTC regardless of whether the tumor grows in papillary or follicular architecture. ### Comparison Table: PTC vs FTC | Feature | PTC | FTC | |---------|-----|-----| | **Nuclear grooves** | Present (diagnostic) | Absent | | **Pseudoinclusions** | Present | Absent | | **Chromatin pattern** | Pale, fine (Orphan Annie) | Coarse, darker | | **Capsular invasion** | Not required for diagnosis | Required for diagnosis | | **Vascular invasion** | Not required for diagnosis | Required for diagnosis | | **Follicular architecture** | May be present | Always present | | **RAS/BRAF mutations** | BRAF V600E common | RAS mutations common | ### Clinical Pearl **High-Yield:** The diagnosis of PTC depends on **nuclear features**, not growth pattern. A tumor with papillary nuclear features but follicular architecture is still PTC ("follicular variant of PTC"). Conversely, a tumor with follicular architecture and normal nuclei but capsular/vascular invasion is FTC. ### Why Capsular/Vascular Invasion Is Not the Discriminator Capsular and vascular invasion are **required** for FTC diagnosis but are **not present** in most PTC cases. However, they do not distinguish PTC from FTC — rather, their presence in a follicular-patterned tumor with normal nuclei defines FTC. They are not a feature of PTC itself. [cite:Robbins 10e Ch 24] 
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