Ubiquitin Immunostaining in Thyroid Neoplasms Marks True Intranuclear Cytoplasmic Pseudoinclusions and May Help Differentiate Papillary Carcinoma from NIFTP

Papillary thyroid carcinoma (PTC) is defined by an invasive growth pattern and classic nuclear features: enlarged, grooved, overlapping nuclei with chromatin clearing and intranuclear cytoplasmic pseudoinclusions (INCP). True INCPs are characteristic of PTC, but may infrequently be seen in noninvasive follicular thyroid neoplasms with papillary-like nuclear features (NIFTP). Nuclear abnormalities that mimic INCP (“pseudo-pseudoinclusions”) are common in a variety of thyroid lesions. H&E and ubiquitin-stained whole tissue sections of classic PTC (n = 25) and NIFTP (n = 35) were evaluated. On H&E, true INCPs were present in all (100%) PTCs and absent in all NIFTPs (0%). Pseudo-pseudoinclusions were present in 13 (37%) NIFTPs. In 24 (96%) PTCs, ubiquitin was strongly expressed within INCPs. In NIFTPs, optically clear nuclei or pseudo-pseudoinclusions did not express ubiquitin (0/35). Occasionally, nuclear vacuoles in NIFTP demonstrated a marginated staining pattern, in which strong ubiquitin expression was seen at the periphery of the nucleus, but the central pale area was negative. In addition, 2 NIFTPs demonstrated intrafollicular psammomatoid calcifications which were strongly ubiquitin-positive. Psammoma bodies in PTC were ubiquitin-negative in the majority of cases. We report a previously undescribed finding: strong ubiquitin expression in true INCPs in PTC, absence of true INCPs in NIFTP, and absence of ubiquitin expression in pseudo-pseudoinclusions in NIFTP. This finding supports the difference between true INCPs (found only in PTC) and pseudo-pseudoinclusions (found in NIFTP). Using strict histologic criteria and ubiquitin immunostaining, the presence of true pseudoinclusions may exclude a diagnosis of NIFTP. Caution should be exercised when interpreting nuclear vacuoles or pseudo-pseudoinclusions.