This paper describes how a specific genetic is the hallmark of a rare thyroid nodular disease that mimics papillary thyroid cancer and in the past resulted in surgery to make the diagnosis. Many patients suffered the loss of their thyroid thinking it was housing a papillary thyroid cancer. They were reassured by the surgeon they were lucky not to have cancer. Were they lucky? They lost the whole thyroid gland because of the cancer diagnosis or half. Both have a high risk of needing thyroid hormone for life and suffering complications of the surgery as well. The unusual cytology, and ultrasound should be a clue to get molecular markers. A chromosomal rearrangement that lead to a fusion between the PAX8 gene and either the GLIS3 or the GLIS1 gene is hallmark of a HTA. Even today a quick diagnosis of PTC by the pathologist can still lead to surgery. We recommend molecular markers to be sure it is not a benign condidtion called GTA. The case below is an example how close the patient came to surgery without studying the genetic markers for HTA.
- A second opinion request from a young man with a FNA diagnosis of PTC, who refused surgery until he had more information.
- Hyalinizing trabecular tumor (HTT) is a thyroid neoplasm with peculiar morphologic features that overlap with those of papillary thyroid carcinoma (PTC).
- The presence of enlarged oval nuclei, nuclear grooves, and intranuclear pseudoinclusions, and hyaline like material makes precise cytopathologic diagnosis challenging compare3d to PTC.
- A Class IV or V cytology would commonly result in a total thyroidectomy.
- The recent discovery of the strong association between GLIS fusions and HTT sheds light on its pathogenesis and offers a pathway for its presurgical identification.
- With a bizarre shaped nodule on ultrasound suggesting Reidel’s thyroiditis, trabecular tissue fragments and many nuclear inclusions the potential for a HTT needs to be considered.
- The main point of this case is to press the need for more opinions before surgery even when the surgeon says the nodule is cancer.
- Everyday new developments can alter the need for surgery.
- The presence of a GLIS fusion can change the diagnosis suggested on cytology to HTA.
- This is benign and before genetic markers it needed surgery to make the diagnosis.
- The patient with a HTA nodule today can continue to follow by ultrasound or ablate with RFA.
- With RFA the thryoid gland will be spared and there is no need for surgery or thyroid hormone repalcement for life.
GLIS rearrangements in thyroid nodules: A key to preoperative diagnosis of hyalinizing trabecular tumor
Hyalinizing trabecular tumor (HTT) is a rare thyroid neoplasm with peculiar morphologic features that overlap with those of papillary thyroid carcinoma (PTC). Specifically, the presence of enlarged oval nuclei, nuclear grooves, and intranuclear pseudoinclusions makes precise cytopathologic diagnosis challenging. If the cytopathologic diagnosis is suspicious for malignancy (Bethesda V) or is malignant (Bethesda VI), a total thyroidectomy, which would be considered an overtreatment, may follow. The recent discovery of the strong association between GLIS fusions and HTT sheds light on its pathogenesis and offers a pathway for its presurgical identification. Although the number of cases analyzed is limited, the recent landmark study shows that GLIS fusions are highly specific for HTT and that lobectomy is the likely appropriate surgical treatment, because these neoplasms, which lack invasion, are benign. For overall success, cytopathologic recognition of the subtle features is important to avoid false-positive diagnoses and directing potential HTT cases toward indeterminate cytopathologic diagnoses, which would trigger further molecular testing. Additional studies are needed to determine whether a malignant counterpart of GLIS fusion-positive HTT exists and if more conservative approaches may be taken.
Hyalinizing trabecular tumor (HTT) is a rare type of thyroid neoplasm with peculiar morphology, and there is long-standing controversy with respect to its relationship to papillary carcinoma and clinical behavior. A recent study has described a unique molecular profile of these tumors, ending this controversy and paving the way for efficient preoperative detection of these tumors and guiding patients toward optimal management.1
History and Controversy Surrounding HTT
HTT was first described in 1987 by Carney and colleagues, who studied a series of 11 tumors with a prominent trabecular pattern, abundant intratrabecular hyalinized stroma, and nuclear features of papillary carcinoma. The tumors were well circumscribed or encapsulated, had no invasion, and demonstrated no recurrence or metastasis on a 10-year mean follow-up.2, 3 Based on the indolent behavior, these were named “hyalinizing trabecular adenomas.”2 Subsequently, several studies reported cases of similar tumors with trabecular appearance and hyalinization that were associated with invasion and distant metastasis.4–6 Together with the prominent nuclear features of papillary carcinoma seen in the tumor cells, these reports argued against the benign nature of hyalinizing trabecular adenoma. As the controversy persisted, the 2004 World Health Organization classification placed this neoplasm in a separate category of tumors with low malignant potential and designated it as “hyalinizing trabecular tumor.”7 The same approach was followed by the most recent 2017 World Health Organization Classification of Tumors of Endocrine Organs.8
Until recently, the genetic mechanisms of HTT were unknown and were a subject of controversy with respect to the relationship between HTT and papillary carcinoma. In 2000, 2 publications reported that HTT frequently harbors RET/PTC rearrangement, a characteristic genetic feature of papillary carcinoma.9, 10 Subsequent studies did not confirm such findings and also showed that HTT does not have the BRAF or RAS mutations that are highly prevalent in classic type and follicular variant papillary carcinomas.11–17 Nevertheless, the existing reports on RET/PTC prevalence and the finding that HTT cells exhibit pronounced nuclear features of papillary carcinoma and are frequently diagnosed as papillary carcinoma or suspicious for malignancy in fine-needle aspiration (FNA) cytology samples18 fueled the diagnostic controversy further. Consequently, over the last 15 years, HTT has been considered a borderline tumor with low malignant potential rather than a benign adenoma. This is despite the finding that no adverse outcome has ever been documented in a tumor with such morphology that lacked invasive features at presentation.19
GLIS Fusion is a Genetic Hallmark of HTT
A recent study by Nikiforova and colleagues studied 14 tumors with fully developed histological features of HTT and found that all of them harbored a chromosomal rearrangement that lead to a fusion between the PAX8 gene and either the GLIS3 or the GLIS1 gene (Fig. 1). The most common fusion type detected in 93% of cases was PAX8–GLIS3, the result of an interchromosomal rearrangement between PAX8 located on chromosome 2q14.1 and GLIS3 on chromosome 9p24.2 (Fig 1A). PAX8–GLIS1 fusion, which was found in 7% of HTT samples, was the result of an interchromosomal rearrangement between PAX8 and GLIS1 (1p32.3) (Fig. 1B).
All 14 GLIS-rearranged tumors had the typical histopathological appearance of HTT with prominent trabecular architecture and abundant intratrabecular hyalinization (Fig. 2).1 Cytoplasmic yellow bodies were found in most cases. The nuclei of the tumor cells showed nuclear features overlapping with those of papillary thyroid carcinoma (PTC), as characteristically seen in HTT, including irregular nuclear contours and abundant intranuclear grooves and pseudoinclusions. All tumors were well delineated or encapsulated and showed no invasion.
The study also showed that GLIS-rearranged HTT carried no other genetic alterations, including BRAF and RAS point mutations, RET/PTC, or other gene fusions characteristic of PTC and all of its variants.1 In contrast, papillary carcinomas co-occurring in the same thyroid gland with HTT carried BRAF V600E and other alterations characteristic of papillary carcinoma and had no GLIS fusions. Furthermore, none of the 220 PTCs tested by Nikiforova et al1 and none of the 484 PTCs in the TCGA study20 were positive for PAX8–GLIS3 fusion (Table 1). Their findings demonstrated sharply different genetic profiles between HTT and PTC, arguing strongly against biological similarity between these 2 tumor types. Table 1. Prevalence of PAX8–GLIS3 and PAX8–GLIS1 Fusions in Hyalinizing Trabecular Tumors and Papillary Thyroid Carcinomas Reported in the Study by Nikiforova et al and in the Cancer Genome Atlas Network Cohort
|Nikiforova 2019,1 n = 14||TCGA 2014,20 n = 484||Nikiforova 2019,1 Unselected PTC, n = 111||Nikiforova 20191: Aggressive PTC, n = 109|
|PAX8–GLIS3||13/14 (93%)||0/484||0/111||0/109||0/704 (0.0%)|
|PAX8–GLIS1||1/14 (7%)||0/484||1/111||0/109||1/704 (0.1%)|
- Abbreviations: HTT, hyalinizing trabecular tumor; PTC, papillary thyroid carcinoma; TCGA, The Cancer Genome Atlas. Copyright permission granted by Mary Ann Liebert, Inc.
Additional information was generated on the functional consequences of PAX8–GLIS fusions. PAX8 is a paired-box transcription factor that is highly expressed in differentiated thyroid follicular cells and is required for normal thyroid development and function.21 GLIS1 and GLIS3 belong to a family of the GLI-similar zinc finger transcription factors that can act as either activators or repressors of gene transcription; and, in the thyroid, GLIS3 is known to be an important regulator of thyroid hormone biosynthesis.22, 23 The study by Nikiforova and colleagues demonstrated that PAX8–GLIS fusions lead to strong overexpression of the 3′-portions of the GLIS genes containing the intact DNA-binding zinc finger domain of these transcription factors.1 Consequently, the PAX8–GLIS3 fusion-positive tumors showed consistent upregulation of expression of extracellular matrix-related genes, including several collagen genes, such as COL4A1, coding for the α-1 subunit of collagen IV. This is likely responsible for the excessive production and deposition of collagen IV and other collagens in HTT, resulting in extensive hyalinization, which is a characteristic microscopic feature of these tumors.
Proposal for Reclassification of HTT
Based on study results demonstrating the occurrence of GLIS fusions in all HTTs and clear biological separation between these tumors and PTCs, Nikiforova and colleagues proposed to reclassify HTT as “GLIS-rearranged hyalinizing trabecular adenoma.”1 This proposal was supported by the findings that none of the 9 patients with GLIS-rearranged HTTs had a tumor recurrence after a mean follow-up of 65 months. Similarly, among 118 tumors reported in the largest study to date,19 tumors with histopathologic features of HTT that lacked invasion did not have regional or distant metastases at presentation, and patients experienced no tumor recurrence or any other adverse effects on follow-up. This suggests that, in the absence of invasion, GLIS fusion-positive thyroid neoplasms currently known as HTTs are benign, as initially suggested by Carney and colleagues.2
Cytologic Challenges Resulting from HTT
Since the initial description of HTT by Carney et al in 1987,2 many pathologists and clinicians have recognized the challenges involving the accurate identification of this entity in presurgical fine-needle aspiration (FNA) cytology specimens. The earliest publication on the cytologic features of HTT from 1989 had already recognized the potential pitfalls for malignant overcalls.3 The challenges to solving the problem of presurgical diagnostics are 3-fold: First, some of the key cytologic features of PTC and medullary thyroid carcinoma, such as enlarged oval nuclei, nuclear grooves, and intranuclear pseudoinclusions, are typically seen in HTT (Fig. 3).1, 3, 18, 24–48 Tabulation of the cytology diagnoses standardized into the Bethesda System for Reporting Thyroid Cytopathology showed that 42.1% and 26.4% of HTT cases were diagnosed on FNA as suspicious for malignancy (Bethesda V) and malignant (Bethesda VI), respectively (Table 2), which often triggered total thyroidectomy.
Table 2. Distribution of Cytopathologic Diagnoses of Histologically Proven Hyalinizing Trabecular Tumor Cases (1989-2019)
|Cytology Diagnosis||Bethesda Category||No. of HTT Cases (%)|
|Nondiagnostic||Bethesda I||4 (2.9)|
|Benign||Bethesda II||2 (1.4)|
|Atypia of undetermined significance/follicular lesion of undetermined significance||Bethesda III||17 (12.1)|
|Follicular neoplasm/suspicious for follicular neoplasm||Bethesda IV||21 (15.0)|
|Suspicious for malignancy||Bethesda V||59 (42.1)|
|Malignant||Bethesda VI||37 (24.6)|
Over the last 30 years, numerous studies have investigated features that may help cytopathologists separate potential HTT cases from other thyroid neoplasms. Of these, cohesive clusters of neoplastic cells with wispy cytoplasm (Fig. 4A), nuclear palisading (Fig. 4B), purple-red stromal deposits (basement membrane-like material on Romanowsky-type stains) (Fig. 4C), and cytoplasmic yellow bodies (Fig. 4D) were recognized as characteristic alterations present in the majority of HTT cases.27, 31, 34, 46 However, these features were not entirely specific for HTT and were found in some cases of papillary carcinoma, follicular carcinoma, and Hurtle cell adenoma.34, 46, 49 Therefore, a definite diagnosis of HTT by cytomorphologic evaluation alone was not considered to be realistic. In the largest cytologic series of HTT, Casey et al found that bloody background, neoplastic cells with abundant cytoplasm arranged in cohesive aggregates or single cells, polygonal nuclei with very frequent grooves and pseudoinclusions, and hyaline material were suggestive of HTT (Table 3).28
Table 3. Cytologic Features of Hyalinizing Trabecular Tumors
|Relatively specific features||Neoplastic cells in cohesive aggregates with wispy cytoplasm Nuclear palisading Metachromatic (purple-red) stromal deposits (Diff-Quik or Romanowsky stain) Cytoplasmic yellow bodies|
|Characteristic but not specific features||Epithelioid or spindle cells Abundant, pale cytoplasm Enlarged, oval nuclei Nuclear grooves (often numerous) Intranuclear pseudoinclusions (often numerous)|
The second major challenge regarding the cytologic diagnosis of HTT is the inconsistency in the quality and quantity of the diagnostic cytologic elements displayed on the slides. When comparing 29 cases of HTT with cases of papillary carcinoma and medullary carcinoma, Casey et al found that the cellularity in HTT was “variable,” in contrast to “hypercellular” for carcinomas.28 These results suggest that the lesional nature of HTT, perhaps because of the degree of hyalinization in a given case, may be contributing to the lack of specimen cellularity or display of characteristic cytologic features. Such cases most likely are diagnosed as atypia of undetermined significance/follicular lesion of undetermined significance (Bethesda III), which comprises approximately 10% of HTT FNA cases.
Third, the relative rarity of HTT has limited the opportunity for general cytopathologists to become proficient in recognizing the key cytologic features this entity. Although the exact incidence is not known, the finding that 11 of 26 reports (42%) documenting the cytology of HTT are individual case reports is likely a reflection of its rarity.1, 3, 16, 18, 25–27, 29–36, 38–42, 44–48, 50, 51 Therefore, heightened awareness of the subtle cytologic features of HTT and the availability of novel, specific molecular markers are important in the presurgical identification of this neoplasm.
Preoperative Detection of GLIS Fusions in Thyroid Nodules
The recent discovery of GLIS fusions has remarkable potential for improving the clinical management of affected patients because these fusions, particularly PAX8–GLIS3, are highly specific and sensitive for HTT. Indeed, the study by Nikiforova et al demonstrated that GLIS fusions can be detected preoperatively in thyroid FNA samples using a targeted next-generation sequencing test (ThyroSeq v3 GC; University of Pittsburgh Medical Center, Molecular and Genomic Pathology Laboratory).1 Prospective analysis of greater than 10,000 consecutive FNA samples from thyroid nodules with indeterminate (primarily Bethesda III and IV) cytology identified PAX8–GLIS3 in 8 nodules (approximately 0.1%). Among those, surgical follow-up was available for 5 patients, and all of them were diagnosed as HTT on surgical pathology.
These results establish the feasibility of GLIS fusion detection in thyroid FNA samples and demonstrate that it could serve as a useful tool for the diagnosis of HTT. This would be particularly important for nodules with a suspicious for malignancy (Bethesda V) cytology diagnosis, which may trigger thyroidectomy.46 The cytomorphologic recognition of the subtle features of HTT (Table 3) by the cytopathologist becomes a key in selecting FNA samples for molecular testing for GLIS fusions to identify potential HTT cases, avoid a false-positive overcalls, and guide patients toward appropriate management. Although the number of cases with prospectively accumulated follow-up studied by Nikiforova et al was low and further studies are required to establish the probability of malignancy in nodules carrying PAX8–GLIS3 and PAX8–GLIS1 fusions reliably, the available data suggest that thyroid lobectomy is likely to be an appropriate surgical approach for such nodules. Future studies will determine whether a malignant counterpart to the GLIS-rearranged hyalinizing trabecular adenoma exists and the management of patients with GLIS fusion-positive nodules can be deescalated even further with delay or complete avoidance of surgery.