In recent years, the high-grade follicular cell-derived non-anaplastic (HGFC-NA) thyroid carcinomas attract attention of experts in endocrine disorders. According to the WHO classification (2022), this group is classified as a separate category combining aggressive neoplasms with the thyroid differentiation, high mitotic activity, necrotic foci, and poor prognosis. HGFC-NA tumors occupy an intermediate position between differentiated and anaplastic carcinomas [1].

Two subtypes are distinguished in the structure of HGFCNA tumors: poorly differentiated thyroid carcinoma (PDTC) and differentiated high-grade thyroid carcinoma (DHGTC). The PDTC diagnostic criteria fixed in the Turin Proposal (2006) include solid, trabecular or insular growth pattern, absence of nuclear features of papillary carcinoma, and ≥ 3 mitoses per 2 mm² or necrotic foci [2] (fig. 1).

It is important to distinguish the HGFC-NA tumors to clarify the diagnosis and choose the tactics that also includes targeted approaches. These tumors reflect a transitional biological range and require the comprehensive assessment of morphological, immunohistochemical, and molecular features. The HGFCNA tumors are characterized by solid, trabecular, follicular, and sclerosing structures accompanied by cellular atypia, high Ki-67 index (> 20–30%), and the signs of neoangiogenesis [3–5].

Mutations in the TERT promoter, TP53 и BRAF are considered to be the key molecular alterations. These are associated with the poor prognosis, radioiodine-resistance, and metastasis. DHGTC can have any morphological structure, including papillary or follicular, but it is diagnosed with ≥ 5 mitoses per 2 mm² and/or necrosis, regardless of differentiation [6, 7].

A clinical case of DHGTC with morphological assessment, immunohistochemistry, and molecular testing is provided below.

Clinical case

The female patient K. aged 62 years presented with complaints of the neck enlargement, moderate difficulty swallowing and hoarseness escalating within six months. She had a history of hyperthyroidism during therapy with antithyroid drugs and stage II hypertension. The patient had no family history of cancer. Examination revealed a dense mass sized up to 3.5 cm in the right lobe of the thyroid gland, which moved when swallowing.

Instrumental methods

Ultrasonography revealed a hypoechoic nodule sized 3.5 × 2.8 cm with uneven contours, hypervascularization, and microcalcifications. The mass TI-RADS score was 5. CT of the neck revealed no invasion of the surrounding tissues and regional lymph nodes. The fine-needle aspiration biopsy was classified as Bethesda V (“suspect for malignancy”).

Surgical treatment

The right-sided hemithyroidectomy was performed. There were no complications in the postoperative period.

Macroscopic examination

The tumor had a grey-white color, irregular lobular pattern, foci of necrosis and microcalcifications. Tumor size: 3.4 × 2.8 × 2.5 cm. The section showed alternating solid zones and areas of coagulative necrosis.

Microscopic examination

The tumor showed pronounced architectonic heterogeneity: solid, trabecular, and pseudofollicular structures surrounded by thin fibrous septa. Cell nuclei were hyperchromic, roundoval, with moderate atypia and clearly visible nucleoli. Mitotic activity was high: 8–10 mitoses per 10 fields of view at ×400, which was above the diagnostic threshold for DHGTC. Furthermore, foci of coagulative necrosis and microvascular proliferation with signs of vascular invasion were revealed. There were no typical nuclear features of papillary carcinoma.

To demonstrate the differential diagnosis features, an example of anaplastic thyroid carcinoma (АТС) is provided. АТС can resemble high-grade tumors in terms of morphology, but it is distinguished by higher cellular pleomorphism, the presence of giant multinucleated cells, and a larger number of atypical mitoses (fig. 2).

The combination of features identified is in this case typical for the solid/trabecular and pseudofollicular structure (fig. 3А); foci of necrosis and high mitotic activity (fig. 3B) make it possible to classify the tumor as a highly probable differentiated high-grade follicular thyroid carcinoma (DHGTC) belonging to the HGFC-NA group. Final verification required the use of additional histological assessment, extended immunohistochemistry (TTF-1, PAX8, CK19, p53, Ki-67), and molecular testing methods. The latter included assessment of mutations in the genes TERT, TP53, and BRAF using the NGS-panel method with confirmation by Sanger sequencing.

Immunohistochemistry analysis

Tumor cells expressed TTF-1 and PAX8, which confirmed their origin in follicular cells. The Ki-67 index reached 35%. Galectin-3 expression and focal HBME-reaction were reported; no calcitonin and thyroglobulin were detected.

Molecular testing

The TP53 mutation with the mutational p53 expression pattern was identified in the tumor, while no TERT and BRAF V600E mutations were reported. The profile in combination with morphology and immunohistochemistry assessment confirmed the diagnosis of DHGTC belonging to the HGFC-NA group.

Clinical case discussion

The HGFC-NA tumors represent a recently distinguished category showing high diagnostic complexity. These occupy an intermediate position between differentiated and anaplastic carcinomas combining thyroid differentiation with aggressive biological behavior [8, 9].

The pronounced architectonic heterogeneity is the key feature of HGFC-NA tumors. Solid, trabecular, pseudofollicular, and sclerosing structures can be combined in the same tumor, which hampers the diagnosis, especially in small biopsies [10]. According to the WHO classification (2022), the decisive criteria are the presence of ≥ 5 mitoses per 2 mm², focal necrosis, and Ki-67 index > 20% with the preserved thyroid differentiation. In the case provided, the Ki-67 index reached 35%, and the TP53 mutation identified suggested poor prognosis [11].

From a molecular perspective, HGFC-NA tumors are characterized by genomic instability. The most significant are mutations in the TERT promoter (35–40% of cases) associated with radioiodine resistance, TP53 alterations (20–25%) reflecting genomic instability, and the less frequent BRAF V600E mutations that are found mainly in DHGTC, which can determine sensitivity to the MAPK cascade inhibitors [12].

In this case, molecular testing revealed the TP53 mutation, the presence of which confirmed poor prognosis, while there were no TERT and BRAF V600E mutations. These findings are consistent with the literature data emphasizing that the presence of the combination of TERT and TP53 mutations significantly worsens the prognosis, and an isolated TP53 mutation also reflects high genomic instability. A negative test for BRAF precludes the possibility of using the MAPK cascade inhibitors, which emphasizes the need to search for other therapeutic targets.

For clinical practice it is important to distinguish PDTC and DHGTC, since the diagnostic criteria of those partially overlap. Comparative analysis of morphological, immunohistochemical, and molecular features is provided in :tab_1.

From the prognostic point of view, the HGFC-NA tumors are characterized by aggressive course: the five-year survival rate is only 40–60%, it decreases considerably when there are TERT and TP53 mutations. The main risk factors are high Ki-67 levels (>20–30%), micro- and macrovascular invasion, tumor extent at the time of diagnosis. Early detection of these features is of fundamental importance for personalized treatment selection, including targeted drugs, and patient enrollment in clinical trials [13–15].

A practical algorithm to diagnose PDTC and DHGTC can be presented as follows:

1. Morphological analysis: detection of the architectonic heterogeneity, necrosis, and mitotic activity.
2. Quantitative criteria: the number of mitoses ≥ 3 per 2 mm² suggests PDTC, the number of mitoses ≥ 5 suggests DHGTC.
3. Immunohistochemistry analysis: determination of the Ki-67, p53, TTF-1, PAX8 expression. The high Ki-67 index (>20–30%) and mutational p53 pattern are the signs of high tumor grade.
4. Molecular testing: determination of the TERT, TP53, BRAF The combination of those determines the prognosis and the targeted therapy options.
5. Data integration: the final classification is based on the combination of morphology, quantitative characteristics, and molecular profile.

Targeted therapy prospects

Identification of the BRAF V600E mutations opens the prospects for the use of MAPK cascade inhibitors (dabrafenib, trametinib). The use of appropriate targeted agents can be considered when the PI3K/AKT/mTOR pathway is activated. Participation in clinical trials of new drugs remains a promising area for patients with the combination of TERT and TP53 mutations. Thus, the HGFC-NA tumor molecular profiling is not only of prognostic, but also of therapeutic value, since it allows one to select personalized treatment strategies.

CONCLUSION

High-grade non-anaplastic (HGFC-NA) thyroid tumors represent a rare and clinically significant category requiring a specific diagnostic approach. The case of differentiated high-grade carcinoma (DHGTC) provided showed the features typical for this group: morphological heterogeneity, focal necrosis, high mitotic activity, and the Ki-67 index above 30%, as well as the mutational p53 expression pattern with no TERT and BRAF alterations. The combination of morphological, immunohistochemichal, and molecular data made it possible to confirm the diagnosis and estimate poor prognosis. This observation emphasizes the need for comprehensive assessment of HGFC-NA tumors involving mandatory consideration of the quantitative criteria (mitotic activity, Ki-67), as well as the TERT and TP53 status. Disregard for these features can result in underestimation of the grade and selection of suboptimal treatment tactics. In the Russian context, the case presented demonstrates the importance of the molecular profiling introduction into routine practice of pathology diagnosis, which will make it possible to improve verification accuracy, enable timely detection of aggressive form and patient management strategy optimization.