In this study, we examined the cytologic characteristics, the distribution of the TBS categories, and the histologic correlates of a series of eight ALK-rearranged PTCs. In our series, the original FNA cytology diagnosis yielded a notable false-negative result (TBS II) in three out of eight patients. Following the review of seven cases with available FNA slides, we excluded one TBS II case with missed sampling for the final analysis. Two cases initially designated as TBS II were reclassified to TBS III due to the presence of scant microfollicles. Another case originally classified as TBS III remained the diagnosis of TBS III after review based on rare nuclear grooves and pseudoinclusions. The high rate of TBS III interpretation (50%, 3/6) in our series likely resulted from subtle nuclear features and the presence of only microfollicular pattern. Two cases (33.3%, 2/6) were diagnosed as TBS V based on reduced nuclear elongation and chromatin alterations as well as focal grooves and pseudoinclusions. Only one case (16.7%, 1/6) met the criteria for TBS VI. In a case report, Jurkiewicz et al. illustrated a STRN::ALK PTC with preoperative FNA diagnosis of TBS III, displaying focal cytological atypia with mild nuclear enlargement and occasional nuclear grooves [13]. Panebianco et al. also observed attenuated nuclear features in FNA cytology of ALK-rearranged PTC and a high rate of indeterminate diagnoses (TBS III: 66%, TBS IV: 10%, TBS V: 16%) [12]. The corresponding pathological features of ALK-rearranged PTCs in this study showed predominant follicular growth and rare papillary growth in a subset of cases. Limited nuclear elongation and reduced chromatin alterations were also observed. Nuclear grooves and pseudoinclusions were infrequently identified and were obvious in only three of eight cases (37.5%). The findings between cytology and pathology overlapped with good correlation in our study.

The terms “BRAF-like” and “RAS-like” was first applied in The Cancer Genome Atlas (TCGA) study to describe gene expression pattern in PTCs [2]. TCGA study identified that ALK-rearranged PTCs had a neutral gene expression profile which was intermediate between those of BRAF p.V600E mutant tumors (“BRAF-like”) and RAS mutant tumors (“RAS-like”) based on their BRAF-RAS scoring system. Recently, a few studies and expert reviews have adopted “BRAF-like” and “RAS-like” designations to describe the cytomorphologic features that are most commonly associated with tumors with the BRAF p.V600E mutation (or tumors with a similar gene expression profile) as “BRAF-like” whereas tumors with a RAS mutation (or tumors with a similar gene expression profile to tumors with a RAS mutation) show “RAS-like” nuclear atypia [16,17,18]. While “BRAF-like” nuclear atypia is characterized by classical chromatin margination (nuclear clearing) and heterogenous nuclear membrane irregularities resulting in intranuclear pseudoinclusions, “RAS-like” tumors are characterized by attenuated chromatin margination and reduced nuclear irregularity. The data from our study showed that ALK-rearranged tumors usually demonstrate “RAS-like” nuclear atypia rather than “BRAF-like” atypia. Interestingly, Chou et al. also described that the majority of ALK-rearranged PTCs (eight out of 14 cases) showed a predominance of follicular growth and variable nuclear atypia, which would be considered morphologically “RAS-like” [8].

To date, approximately 100 cases of ALK-rearranged PTCs have been reported. The clinicopathologic features of ALK-rearranged PTCs are summarized in Table 4. ALK-rearranged PTCs constitute an overall 2.4% of PTCs, with a female predominance (female to male ratio of 3.56) and a younger age at onset compared to non-ALK-rearranged PTC patients [7,8,9,10,11, 19,20,21,22,23,24,25,26,27,28,29,30,31,32]. The majority of ALK-rearranged PTCs were PTCs with a mixed papillary and follicular growth (53%), followed by classic (22.9%), solid/trabecular (9.7%), diffuse sclerosing (9.7%), tall cell (2.4%), oncocytic (1.2%), and Warthin-like (1.2%) subtypes [7,8,9,10,11,12,13, 19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34]. Our findings aligned with the descriptions in previous literature of a predominance of female patients and various histological subtypes/variants of ALK-rearranged PTCs.

The majority of ALK-rearranged thyroid carcinomas were PTCs (87.3%), followed by poorly differentiated thyroid carcinomas (11%) and anaplastic thyroid carcinomas (1.7%) [7,8,9,10,11,12,13, 20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. The most frequent ALK fusion variants were STRN::ALK (45.8%) and EML4::ALK (31.4%). Other less common fusion partner genes included CTSB, GTF2IRD1, PPP1R21, TFG, CCDC149, and ITSN2 [12, 20, 28, 36]. Similarly, STRN::ALK and EML4::ALK fusions comprised the majority of our cases, including three (50%) with EML4::ALK and two (33.3%) with STRN::ALK fusion among the six cases tested. We also identified a rare ITSN2::ALK fusion gene (case 1), previously reported in only one FNA cytology case [12]. Interestingly, the FNA cytology of ITSN2::ALK PTC in our study exhibited the most pronounced PTC nuclear features, consistent with the diagnostic criteria of TBS VI (Fig. 4). The histological examination revealed more frequent PTC nuclear features than EML4::ALK and STRN::ALK fusion–associated cases.

The prognosis of ALK-rearranged thyroid carcinomas remains uncertain due to its rarity. In our study, all eight cases displayed an absence of high-grade features, and seven showed no signs of recurrence during the follow-up period. Most previously reported PTC cases also exhibited no evidence of disease during the follow-up period [7, 8, 12, 20, 21, 23, 25, 34]. There was only one classic subtype PTC that persisted with disease for 5 years and another tall cell subtype PTC that manifested with recurrence 3 years after thyroidectomy [22, 33]. Although rare, similar to other molecular alterations, ALK rearrangement may also occur in aggressive thyroid cancers, such as poorly differentiated or anaplastic thyroid carcinoma [12, 26, 34,35,36,37]. Since 2011, several ALK tyrosine kinase inhibitors have been approved for the treatment of non-small cell lung carcinoma [38]. Godbert et al. reported a case of anaplastic thyroid carcinoma showing focal follicular variant PTC component with tumor regression after off-label use of crizotinib [35]. Thus, ALK inhibitors also appear to be a promising therapeutic agent for aggressive thyroid cancer with ALK fusions.