4. Cytology/Pathology – Thyroid Cancer

The WHO periodically updates the system of histological classification of thyroid tumors. The 4th edition [252] is still authoritative for this guideline; changes to the 5th edition were not adopted until 2022 [253] and therefore no studies or clinical experience with it are yet available. The following table compares the classification of the 4th edition with the 5th edition with regard to the most important points on thyroid carcinomas of the follicular epithelial cells.

Medullary thyroid carcinoma is dealt with separately in Chapter Medullary thyroid carcinoma (MTC).

The histological type or subtype of a thyroid tumor influences the prognosis, the further therapeutic procedure (e.g. radioiodine therapy), and the design of tumor follow-up care.

Benign neoplasms

While only follicular adenoma and oncocytic adenoma (Hürthle cell adenoma) of the thyroid gland were listed in the 4th edition of the WHO classification, the 5th edition now introduces two further entities, follicular adenoma with papillary architecture and follicular nodular disease (FND).

Follicular adenoma with papillary architecture represents a benign, encapsulated neoplasm without PTC-like nuclei with a characteristic centripetal structure of pseudopapillae. These adenomas are often associated with autonomic hyperfunction.

One innovation is the classification of nodular goiter (struma colloides nodosa, SCN) as a true neoplasia. This takes into account the fact that many nodular goiters have true neoplastic parts in addition to inflammatory-reactive hyperplastic nodule formation. These often do not show a complete capsule, but classic driver mutations of a neoplasia are detectable. The authors of the 5th edition of the WHO classification refer to this lesion as "thyroid follicular nodular disease, FND".

Low-risk neoplasms

In both the 4th and 5th editions of the WHO classification, thyroid neoplasms with unclear malignant potential, non-invasive follicular neoplasia with PTC-like nuclear features (NIFTP), and hyalinizing trabecular tumors (HTT) are classified in this group. NIFTP was newly introduced in the 4th edition in order to spare patients with a biologically indolent subgroup of the follicular variant of papillary carcinoma both overtreatment and a psychologically stressful cancer diagnosis; the establishment of NIFTP necessitated a redefinition of the diagnostic criteria for PTC and has implications for the evaluation of fine needle aspiration of the thyroid gland [254].

Malignant neoplasms

Follicular thyroid carcinoma (FTC)

The WHO classification distinguishes three clinically relevant subgroups of FTC:

• Minimally invasive FTC is an invasively growing encapsulated tumor whose diagnosis is based solely on evidence of one or more complete capsular ruptures. Vascular invasion must be explicitly excluded histologically in minimally invasive FTC, as vascular invasion represents a relevant negative prognostic marker in contrast to isolated capsular breakthroughs. A capsular rupture alone represents a very low prognostic risk factor [255], [256], [257], [258], [259], [172].
• Encapsulated angioinvasive FTC is diagnosed on the basis of vascular invasion detectable in the area of the tumor capsule or beyond the tumor capsule; capsular breakthroughs may occur. Vascular invasion must be reliably differentiated from the frequently occurring artificial spread of tumor components during surgery and further processing in pathology. Encapsulated angioinvasive FTCs are divided into 2 groups. Those with limited vascular invasion of < 4 vascular invasions and tumors with ≥ 4 vascular invasions. The number of ≥ 4 vascular invasions alone does not qualify for the diagnosis of a broadly invasive FTC.
• Broadly invasive FTC is defined by varying degrees of invasive growth of the tumor into adjacent non-neoplastic tissue ("normal" thyroid tissue and/or extrathyroidal soft tissue). Vascular invasions are often additionally detectable, but are generally not necessary for the diagnosis

Encapsulated FTCs must be differentiated from adenomas (vascular and capsular invasion), encapsulated (follicular) PTCs and NIFTP (core criteria), as well as encapsulated PDTCs (Turin criteria). Broadly invasive FTC must first and foremost be distinguished from PDTC and differentiated high-grade thyroid carcinoma (DHGTC).

There are currently no specific molecular markers for the diagnosis of FTC or for differentiation from adenomas (see also the section "FNA of the thyroid gland").

Papillary thyroid carcinoma (PTC)

Due to the introduction of NIFTP (non-invasive follicular neoplasia with PTC-equivalent core features) in the WHO classification in 2017, the definition of PTC had to be changed. In addition to the characteristic nuclear features that are mandatory for PTC, invasion and/or well-developed papillae or other surrogate markers such as true psammoma corpuscles must now be present [254].

PTC are either unencapsulated and thus basically invasively growing, or encapsulated. Encapsulated PTCs require invasion in the capsular region (or beyond) and/or papillae for diagnosis; encapsulated PTCs with a completely or predominantly follicular structure are usually differentiated from NIFTP on the basis of invasion in the capsular region and/or immunohistochemical/molecular pathological evidence of a BRAF V600E mutation in addition to other surrogate markers such as true psammoma corpuscles.

Completely encapsulated PTC of the follicular subtype, which cannot be classified as NIFTP based on histomorphological criteria and do not have a BRAF V600E mutation, are classified according to the WHO classification (5th edition) as a separate entity in the sense of an infiltrative encapsulated follicular variant of PTC (IEFVPTC). According to current understanding, these tumors behave biologically more like FTC and are assigned to the group of RAS-like thyroid tumors in terms of molecular biology [253].

According to current understanding, PTC can be diagnosed molecularly on the basis of a BRAF V600E mutation, RET fusion or NTRK fusion (so-called diagnostic markers of PTC), whereby the absence of these mutations does not rule out PTC. These genetic alterations do not occur in adenomas or in FTC, but they can also be detected in poorly differentiated carcinoma (PDTC, also known as differentiated high-grade thyroid carcinoma DHGTC) and anaplastic carcinoma (ATC). Therefore, a morphological differentiation of PTC as a well-differentiated carcinoma from PDTC and DHGTC is mandatory, particularly in the case of a solid/trabecular growth pattern, necrosis and/or ≥3 mitoses/2mm² (PDTC) or classic architecture with ≥ 5 mitoses/2mm², Ki67 10-30 %, necrosis (DHGTC).

PTC subtypes

Numerous subtypes of PTC are listed in both the 4th and 5th editions of the WHO classification. It is advisable to list the subtypes with prognostic significance in the pathology report. The large cell variant, columnar cell variant, and hobnail variant are considered aggressive subtypes. These usually occur at an advanced age and are often associated with vascular invasion, increased mitotic activity, and an advanced pathological stage. A DHGTC should be considered in the differential diagnosis. The indication of an aggressive subtype (especially the hobnail variant) requires the exclusion of regressive processes in classic PTC, which can also show hobnail-like changes. The aggressive subtypes can also appear as clinically non-aggressive and encapsulated, especially in younger patients. In these cases, pathological signs such as extrathyroidal extension, lymphatic and venous infiltration and lymph node metastases are absent [253], [260].

Cases with aggressive clinical courses have also been described for the diffuse sclerosing and solid or trabecular subtypes. It remains to be seen whether a prognostic significance can be confirmed in future studies following the introduction of the PDTC and DHGTC entities as a possible differential diagnosis [253], [260].

Papillary thyroid microcarcinoma (PTMC) will no longer be listed as a subtype in the 5th edition of the WHO classification, as it is not a histomorphological subtype and will only be named as a PTC with a corresponding size indication (corresponds to TNM stage pT1a). PTC with a diameter ≤ 1 cm can also become clinically conspicuous primarily with lymph node metastases [253], [260]. These tumors are therefore obviously biologically different from incidences of small PTC without prior clinical suspicion of malignancy. The prognostic significance of histological subtypes of PTC in TNM stage pT1a should therefore be critically questioned, especially in the case of incidental tumors.

Oncocytic thyroid carcinoma (OTC) / Hürthle cell carcinoma

Tumors with a proportion of >75% of oxyphilic differentiated cells are referred to as Hürthle cell neoplasms if their structure basically corresponds to an adenoma or an FTC. In the WHO classification (4th edition, 2017), Hürthle cell neoplasms are listed as a separate entity; until then, these tumors were classified as adenoma, oxyphilic variant, or FTC, oxyphilic variant. In the 5th edition of the WHO classification, the name was changed to oncocytic thyroid carcinoma.

Oncocytically differentiated PTC, PDTC, and MTC are still classified as oncocytic variants of the respective tumor type.

Oncocytic thyroid carcinoma / Hürthle cell carcinoma is differentiated from oncocytic adenomas / Hürthle cell adenomas based on the diagnostic criteria of the FTC; oncocytic carcinomas / Hürthle cell carcinomas are also subclassified into minimally invasive, encapsulated angioinvasive and broadly invasive. Broadly invasive oncocytic thyroid carcinomas / Hürthle cell carcinomas must be differentiated from the oncocytic variant of PDTC.

Poorly differentiated carcinoma (PDTC)

PDTC is morphologically and biologically located between the differentiated (FTC, PTC) and the ATC. The diagnostic criteria were defined in 2007 in the so-called Turin Proposal [261].

PDTC show a solid, trabecular and/or insular structure (so-called STI pattern), usually two or even all three patterns are found side by side in varying degrees. In addition, at least one of the three criteria necrosis, mitoses (≥3/2mm²), and convoluted nuclei must be detectable for the diagnosis of PDTC. In the majority of cases, there are also clear signs of invasion (vascular invasion, broadly invasive growth), but in individual cases the diagnosis is made solely on the basis of the STI pattern and the detection of tumor necrosis and/or mitoses (≥3/2mm²).

To date, no molecular genetic alterations specific to PDTC are known.

Differentiated high-grade thyroid carcinoma (DHGTC)

Despite the often clinically indolent course of differentiated thyroid carcinomas (PTC and FTC), some patients experience distant metastasis and recurrence and fail radioiodine therapy. In addition to molecular risk factors (see Chapter Risk factors for the development of thyroid cancer), histomorphological criteria for differentiated carcinomas with unfavorable disease progression have been identified in recent years and the 5th edition of the WHO classification has introduced the tumor entity of differentiated high-grade thyroid carcinoma (DHGTC). Due to the lack of insular, solid or trabecular architecture, these tumors could not be classified as poorly differentiated thyroid carcinomas (PDTC). Similar to PDTC and high-grade MTC, the division activity of the tumor cells is also an important criterion here. The detection of five or more mitoses per 2 mm² supports the diagnosis of a DHGTC. An alternative/additional criterion is the detection of true tumor necrosis, which must be differentiated from focal ischemic necrosis.

Convoluted nuclei are an additional criterion for PDTC in accordance with the Turin criteria. In practice, however, these are a rather uncertain criterion due to the high interobserver variability. These nuclear changes play no role in the diagnosis of DHGTC [261].

Anaplastic thyroid carcinoma (ATC)

Anaplastic thyroid carcinoma (ATC) - or previously undifferentiated thyroid carcinoma (UTC) - is rare compared to differentiated thyroid carcinomas, but contributes significantly to the mortality of thyroid carcinomas due to the often extremely poor prognosis. At the time of diagnosis, the tumor is often at an advanced stage due to its rapid growth. Microscopically, ATC is defined by a widespread absence of histomorphologic and immunohistochemical characteristics typical of thyroid carcinomas, so that it is primarily a diagnosis of exclusion based on the clinical-radiologic findings and the patient's medical history. In many cases, pan-cytokeratin and PAX8 positivity can be detected. Markers such as TTF1 and thyroglobulin are usually negative. ATC must always be differentiated immunohistochemically from angiosarcoma, lymphoma, and medullary thyroid carcinoma. The paucicellular variant of ATC can be difficult to differentiate from Riedel's goiter, particularly in biopsies, due to its clinical appearance and extreme paucity of cells. In some cases, more differentiated parts (FTC, PTC, PDTC, DHGTC) may be present in the ATC. Depending on the differentiation, numerous molecular aberrations can be detected in ATC. All the mutations typical of differentiated thyroid carcinomas can also occur. In special cases (e.g. BRAF V600E, RET fusion, NTRK fusion), this results in a treatment option using molecularly targeted therapeutics approved for ATC or the possibility of an individual treatment attempt. Depending on the clinical context, genome-wide parallel sequencing using a large gene panel and subsequent discussion in an interdisciplinary molecular tumor board is therefore advisable.

Frequency of the various entities

There is little recent, reliable data on the distribution of histological types of thyroid cancer in Germany. In a representative population study from Lower Franconia, in which the changes from 1981-2015 were analyzed in 5-year intervals [10], there was an increase in the incidence of thyroid cancer from 4.5 to 8.7/100,000/year in women and from 1.7 to 4.1/100.000/year in men, which is mainly attributable to papillary thyroid carcinomas, while the incidence of anaplastic thyroid carcinomas decreased with a largely constant proportion of follicular thyroid carcinomas. In the last analyzed 5-year interval 2011-2015, the incidence of papillary and follicular and anaplastic thyroid carcinomas was 7.60 and 1.06 and 0.08/100,000/year in women and 3.41 and 0.63 and 0.04 /100,000/year in men, respectively. A similar trend was also described in a population-based study from 25 countries [20].

As far as the TNM stage is concerned, the study from Lower Franconia showed a significant increase in T1 stages with a simultaneous decrease in T4 and M1 stages in the analyzed period from 1981 to 2015, but also a significant increase in T3 stages and lymph node involvement, which argues against overdiagnosis as the sole cause of the observed increase in the incidence of thyroid cancer [10].

The TNM classification and the staging based on it - including age - is the most commonly used criterion worldwide for risk stratification and estimating the prognosis of thyroid malignancies [262], [263], [264], [265], [266], [267], [268], [269], [270], [271], [272], [273]. The changes implemented in the 8th edition compared to the 7th edition lead to an overall "down-staging", which is intended to avoid unjustified overtreatment [274]. However, criticism of the 8th edition was also voiced [275], for example the fact that the minimal extrathyroidal tumor extension - with regard to the prognostic significance of which there is no consensus - is no longer included in the 8th edition; at the same time, supplementary proposals were made to compensate for the criticized deficits.

Compared to the previous editions, the current 8th edition of the TNM classification does not include the prognostic classification (higher grouping) of microscopic and thus initially organ capsule-exceeding growth. The reason for this is the often unclear capsule situation in the dorsal parts of the thyroid gland with a sometimes difficult to delimit organ capsule and retracting fatty/connective tissue parts with a high inter-individual variation in the assessment by the individual pathologist [276].

The application of strict histomorphologic criteria for microscopic extrathyroidal spread has been shown to have a clear impact on the postoperative management of patients [258]. The extranodal spread of local lymph node metastases is also described as a significant prognostic marker [277], [278], [279], which is why this should be indicated in the TNM classification as extracapsular extension (ece+) in the pN category.

In order to establish comparability with earlier TNM classifications, it may be useful to also report the older TNM classification (7th edition) in the findings. Alternatively, the TNM classification generally provides the option of specifying an extended classification for the respective organ system. For stages T1 to T3, this was published in a publication by Schmid and Wittekind et al. [280] in a proposal for extension, including the microscopic extrathyroidal and nodal extracapsular spread (N1a and b) (Table TNM classification of thyroid carcinomas according to the 8th edition of the UICC with proposals for the addition of prognostically relevant criteria).

General prognostic histomorphological markers such as infiltration of lymphatic (L) and blood vessels (V) as well as perineural sheaths (Pn) are often only seen in the area of the tumor in advanced tumors with extrathyroidal extension. Angioinvasive follicular thyroid carcinomas and some histomorphologic subtypes of PTC are exceptions. An immunohistochemical work-up to confirm infiltration of lymphatic vessels and/or veins can help in individual cases, but is generally not helpful, as the thyroid gland is highly vascularized and interpretation of the antibody staining is therefore difficult.

The synchronous detection of local lymph node metastases, even in material from central or lateral compartments that may have been sent in at the same time, is a sign of infiltration of the afferent/efferent lymphatic pathways and should therefore be classified as L1 (see TNM classification, Supplement).

Grading information is not provided for thyroid carcinoma, as it is included in the type diagnosis of the malignancy.

A statement on removal in healthy / not in healthy tissue of the primary tumor (R classification) is mandatory for thyroid resectates, lobectomies, and thyroidectomies. If the specimens are sent in fragmented, it is often not possible to determine the R classification. The R classification (R0/1/2) is not an integral part of the TNM classification but should be listed both in the text of the findings and in a structured manner in association with the TNM formula.

As of the 6th TNM edition (UICC 2002), the localization of any lymph node metastases is also included in the N-stage to a more precise degree than before.

As of the 6th edition (UICC 2002), tumor freedom with at least 6 resected lymph nodes was no longer a mandatory requirement for the classification pN0. In the surgical guidelines for the surgical treatment of malignant thyroid diseases of the DGAV since 2012 [AWMF 088/002; 2012], prophylactic compartmental lymph node resection without clinical evidence of lymph node filiae is no longer mandatory in the primary surgical procedure when weighing up the benefits and risks. One consequence of this is that, on the one hand, pathological findings are more frequently classified as "pNX" and, on the other hand, even if only a small number of lymph nodes have been examined and are tumor-free, they are classified as "pN0", with correction of the N stage then more frequently required in the course of the procedure.

As a rule, the M stage cannot be determined during primary surgical treatment, but only during the course of the disease, primarily on the basis of the imaging diagnostics used, in particular radioiodine scintigraphy. Only in rare cases of cytologically or histopathologically confirmed distant metastasis is the stage indicated with the suffix "p" as pM1.

The pTNM stage can only be supplemented with the suffix "c" in a narrow time window of 3-6 months after surgery (e.g. from pNX MX to cN1b cM1). If changes are only detected after 3-6 months, the additional suffix "r" is used (e.g. rpT4a or rcT4a).

Some tumors, especially histological subtypes, are sometimes difficult to classify in the pathological assessment (e.g. minimally invasive follicular carcinoma, broadly invasive follicular carcinoma, or the "non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP)", differentiation of tumors with "uncertain malignant potential (UMP)"). However, the actual histological (sub)type often has a fundamental influence on the prognosis and the therapeutic approach. In case of doubt, the expertise of a reference pathologist should therefore be consulted.

In recent decades, pathology reports have evolved from a rather informal, narrative, individualized form to documents with structured, synoptic sections due to various influences such as certifications, guidelines and advances in data processing. This makes it possible to provide important information not only for stage-oriented therapy planning, but also for personalized therapy. The implementation of evidence-based diagnostic, prognostic and predictive information in the pathology report is essential for a smooth transfer of knowledge to the medical institutions involved in thyroid tumor treatment.

Structured findings

The increasingly complex information that is required for the subsequent treatment of thyroid tumor patients has been proven to lead to the fact that purely narrative findings significantly more often do not contain important information, which subsequently leads to inquiries on the part of the treating physicians and thus to additional work and an overall poorer treatment success for various tumor entities [325], [326], [327], [328], [329], [330], [331]. This has also been shown for patients with thyroid tumors [332], [333], [334]. In addition to the classic narrative elements for macroscopy, microscopy, diagnosis and commentary, the pathology report should therefore also contain synoptic sections, e.g. in tabular form (tumor classification, biomarker analyses), in order to routinely display important information. Today, this can be implemented in many laboratory information systems (LIS) and is required by the relevant regulations for accredited institutes. In addition, a structured setup facilitates automated reporting to the cancer registry and increases the clarity of the findings, which is important, for example, when presenting in interdisciplinary tumor boards. Examples of purely synoptic approaches to the documentation of thyroid tumors are freely available as templates on the Internet from the College of American Pathologists (CAP) and the International Collaboration on Cancer Reporting (ICCR)(www.cap.org, www.iccr-cancer.org).

Minimum requirements

In the context of pathological diagnostics, the submitted material can be roughly divided into cytological specimens (e.g. FNAC), biopsies (e.g. TrueCut biopsy) and surgical specimens. What they all have in common is the basic structure of a pathology report with information about the sender, patient, clinical details, type of material, macroscopy, microscopy, diagnosis and, if applicable, comments. Depending on the type of material and diagnosis, further information such as specific classifications or results of additional examinations can be included. Mandatory and optional content is based in particular on the information from the current classification of the World Health Organization (WHO) and the TNM classification of the Union for International Cancer Control (UICC).

Surgical specimens

The histopathological processing of thyroid carcinomas naturally enables the most comprehensive information possible compared to biopsies and cytological aspirates. Postoperative risk stratification requires this important information to assess whether, for example, surgical completion, radioiodine therapy or other measures are necessary. Accordingly, the information required for a pathology report is extensive and is listed below based on the current literature [24], [335], [336], [337], [338].

• Localization information such as lateral, isthmus or e.g. retrosternal, multifocal
• Histological tumor type and, if applicable, subtype according to WHO
• Specific diagnostic criteria depending on the tumor type
• Maximum tumor diameter
• Local tumor spread, taking into account parameters important for T classification (e.g. extent of extrathyroidal spread)
• Number of lymph node metastases, indicating the number of lymph nodes examined (desirable: indication of whether lymph node capsule has been exceeded) and the compartments of lymph node origin (for correct indication of N-stage).
• Complete TNM classification including information on peritumoral infiltration of veins (V), lymphatic vessels (L) and perineural sheaths (Pn)
• Prognostic stage according to UICC
• R classification if possible (optional metric indication of the distance to the nearest resection margin)
• Secondary findings (e.g. stromal dysplasia in MTC).

Example of synoptic text module for integration into the findings report:
Tumor classification:
Localization: ICD-O-C 73.9
Typing: ICD-O-M-8260/3
Grading: not planned
Staging (TNM 8th Edition): pT2 pN0 (0/45) M0, L0 V0 Pn0, UICC stage I
R classification: R0 (removal locally in healthy tissue)
Diagnosis: ICD-10-C 73
Biomarker status:
NTRK status (PCR): negative
BRAF status (PCR): p.V600E (mutated)

In the context of the histomorphological findings, the statement of the pathological report naturally focuses on the tumor diagnosis in the sense of indicating the histological tumor type and, if applicable, subtype according to the current WHO classification. If, based on the structures included in the report, further information is available, e.g. on parameters such as infiltration of veins, lymphatic vessels or perineural sheaths and possible extrathyroidal spread, this should also be reported and commented on.

FNAC

In addition to the aforementioned minimum requirements of a pathological report, the classification of the findings within the framework of a currently valid, standardized cytological findings system (e.g. German 5-stage system, Bethesda system) in accordance with the specific publications is required for the reporting of cytological material from fine needle aspiration material [339], [290].