Thyroid
12 curated traps and look-alikes for this organ system.
Incomplete capsule sampling → missed invasion
Sample entire capsule circumference. For tumors >3 cm: minimum 1 section per cm of greatest dimension plus any area of capsule thickening, irregularity, or hemorrhage.
Post-FNA pseudoinvasion
Fissure-like (not mushroom-like) shape; adjacent hemorrhage; stromal repair; prior FNA history. True invasion = full-thickness rupture with mushroom-like expansion.
Dyshormonogenetic goiter overcalled as carcinoma
Bizarre hyperchromatic nuclei are BETWEEN nodules (not within) in dyshormonogenetic goiter. Require true capsular/vascular invasion for carcinoma.
Hashimoto reactive nuclei mimicking PTC
True PTC = full nuclear feature set + psammoma bodies + HBME-1/Galectin-3/CK19+ + CD56 LOST. Reactive: CD56 retained, HBME-1/Galectin-3 negative.
NIFTP vs. encapsulated follicular variant PTC
Any capsular/vascular invasion OR any psammoma body = encapsulated FV-PTC, NOT NIFTP. Mandate complete capsule sampling.
LGCCC vs. SDC analog: LGCCC of thyroid not applicable; but oncocytic medullary vs. Hürthle Ca
Cytoplasm: amphophilic = medullary; brightly eosinophilic = Hürthle. Calcitonin+/CEA+/Tg− = medullary. ALWAYS check calcitonin in solid/trabecular oncocytic thyroid tumors.
Vascular invasion artifact (Masson lesion)
True vascular invasion = tumor cells in vessel lumen attached to wall, covered by endothelium or fibrin thrombus. Masson lesion = papillary endothelial hyperplasia without intraluminal tumor.
Paucicellular anaplastic Ca vs. Riedel thyroiditis
Even rare atypical cells in dense fibrosis with necrosis + vascular permeation → anaplastic Ca (Keratin+/PAX8+). IgG4+ plasma cells + storiform fibrosis without atypia → Riedel.
Multifocal sclerosing thyroiditis vs. papillary microcarcinoma
Sclerosing thyroiditis has reactive nuclear change at periphery, no nuclear pseudoinclusions, no psammoma bodies, BRAF-negative.
Metastatic RCC vs. primary clear cell thyroid tumor
PAX8 is positive in both — run full panel: thyroglobulin / TTF-1 (positive in thyroid primary; negative in RCC) and CD10 / RCC marker (positive in RCC).
Hyalinizing trabecular tumor pseudo-PTC nuclei
MIB-1 (Ki-67) shows membranous staining at room temperature in HTT; NO BRAF / RAS; abundant intratrabecular hyaline material.
Cribriform-morular variant in young woman = screen FAP
Nuclear β-catenin + APC/CTNNB1 mutation; refer family for FAP screening; absent BRAF/RAS.