Tumours in Iceland. 11. Malignant tumours of the thyroid gland. A histological classification and epidemiological considerations

All malignant primary tumours of the thyroid gland submitted for histological diagnosis in Iceland during the 30 years 1955-1984, and available for review, were typed histologically according to the World Health Organization classification but also taking into account the more recent well recognized follicular variant of papillary carcinoma. A total of 480 thyroid tumours were classified with a female--to male ratio of 2.8 (367 females, 129 males). The age distribution is much what would be expected, the anaplastic type of carcinomas occurring in the elderly while papillary and follicular tumours occur over a much wider age range. The incidence of thyroid carcinomas in Iceland is about 2-3 times higher than in the other Nordic countries. This is largely due to an unusually high incidence of the papillary type of carcinoma. Overall, the papillary carcinoma accounted for 80% of thyroid malignancies. The tumours diagnosed incidentally at autopsy were about 20% of the entire material, and these tumours were only of the differentiated types of thyroid carcinoma. Even if the incidentally diagnosed tumours are excluded, the percentage of papillary tumours is 77% which is unusually high. The papillary type of carcinomas occasionally occurred in familial clusters in Iceland but not sufficiently to account for the unusually high incidence. Some of the possible etiological factors are discussed.     

CD57 (leu-7) Expression is helpful in diagnosis of the follicular variant of papillary thyroid carcinoma

 CD57 (HNK-1) is a oligosaccharide antigen that is expressed by cells of several lineages. It is present on multipotential neuroepithelial cells during embryogenesis, and tumours of epithelial, neuroectodermal and nerve sheath origin also express CD57. Its role in the diagnosis of thyroid tumours is controversial. We have studied CD57 expression by immunohistochemistry to determine its utility in the classification of thyroid follicular lesions. Study material included 114 normal thyroid sections, 77 benign thyroid lesions (29 colloid nodules, 22 follicular adenomas, 20 cases of Hashimoto’s thyroiditis and 6 of Grave’s disease) and 83 thyroid carcinomas, including 31 follicular variants of papillary carcinoma. We observed CD57 positivity in 95% of thyroid carcinomas, 27% of follicular adenomas and 10% of colloid nodules. It was not expressed in the normal thyroid. CD57 expression in thyroid carcinomas was significantly different from that in normal and benign thyroid lesions (P < 0.0001). The follicular variant of papillary thyroid carcinoma also showed significantly higher CD57 expression than colloid nodules (P < 0.0009) or follicular adenomas (P < 0.0009). No significant difference was seen between colloid nodules and follicular adenomas. We conclude that CD57 immunohistochemistry is valuable in the classification of thyroid follicular lesions into benign and malignant groups and is also helpful in the diagnosis of the follicular variant of papillary thyroid carcinoma. Received: 26 August 1997 / Accepted: 14 October 1997     

Papillary thyroid carcinoma in Kanazawa, Japan: prognostic significance of histological subtypes

A retrospective analysis of 183 papillary thyroid carcinomas was made in order to assess the prognostic factors related to survival. The following factors were found to adversely affect the prognosis: trabecular subtype of papillary carcinoma, the extent of the primary tumour, regional lymph node involvement, the presence of distant metastases, old age, male sex and the extent of the neck dissection. The presence of the follicular variant of papillary carcinoma and the extent of the thyroidectomy did not influence the prognosis. The trabecular subtype of papillary carcinoma is characterized by a trabecular or solid arrangement of follicular cells with nuclei of ground-glass appearance. Univariate and multivariate analysis indicated that patients with this type of thyroid tumour had a poorer prognosis than those with the well-differentiated or follicular variants of papillary carcinoma. In our opinion the trabecular subtype of papillary carcinoma should be included as a separate entity in the WHO classification of thyroid tumours.     

High molecular weight caldesmon positive stromal cells in the capsule of thyroid follicular tumours and tumour-like lesions

AIMS: To investigate the smooth muscle nature of the spindle stromal cells in the capsule of thyroid tumours and tumour-like lesions. METHODS: Immunostaining for high molecular weight caldesmon (HCD), a highly specific marker for smooth muscle differentiation, was performed in 70 primary thyroid tumours and tumour-like lesions (21 hyperplastic nodules, 29 follicular adenomas, five minimally invasive follicular carcinomas, six widely invasive follicular carcinomas, and nine encapsulated papillary carcinomas). RESULTS: HCD positive stromal cells (HCD+ cells) were detected in the capsule of 20 of the 21 hyperplastic nodules, and all of the 29 follicular adenomas and five minimally invasive follicular carcinomas, whereas HCD+ cells were seen in the capsule of only four of the six widely invasive follicular carcinomas and no HCD+ cells were seen in the capsule of the nine encapsulated papillary carcinomas examined. CONCLUSIONS: The presence of HCD+ cells in the capsule is characteristic of thyroid follicular tumours and tumour-like lesions. The stromal cells in the capsule of thyroid follicular tumours and tumour-like lesions are different from those of encapsulated papillary carcinoma.     

Keratin subsets in papillary and follicular thyroid lesions

 Previous studies indicate that keratins 7, 8 and 18 are present in all thyroid papillary and follicular lesions, but the distribution of other keratins has been incompletely characterized. The profile of individual keratin (K) polypeptides was evaluated immunohistochemically in over 200 non-neoplastic and neoplastic thyroid papillary and follicular lesions. Monoclonal antibodies to K19, K17, K16, K5/6 and K10 were applied in paraffin sections of formaldehyde-fixed tissue. K19 was present variably, often only focally in goitres, and was present only sporadically in papillary hyperplasia. However, K19 was strongly and uniformly expressed in virtually all papillary carcinomas, indicating differential diagnostic usefulness in differentiating papillary hyperplasia and papillary carcinoma. About half of the follicular carcinomas (defined as tumours strictly excluding the follicular variant of papillary carcinoma) were also strongly K19-positive, suggesting that K19 patterns are not reliable in differentiating papillary and follicular carcinoma. K17 and K5/6 were present in cysts and squamous metaplasia of goitres, and focally in papillary but only exceptionally in follicular carcinoma in areas of squamous differentiation and tumour cells in desmoplastic stroma. K16 in turn was present only focally in well-developed squamous metaplasia in goitres but was not found in differentiated thyroid carcinomas. K10, a high-molecular-weight keratin typical of epidermal differentiation, was identified neither in non-neoplastic nor in neoplastic differentiated thyroid lesions, including squamous metaplasia. These results indicate that papillary carcinomas differ from other differentiated thyroid tumours in their varying, usually focal, expression of stratified epithelial keratins that are partly but not exclusively related to squamous differentiation in such lesions. However, papillary carcinomas do not express truly epidermally restricted keratins; their previously described reactivity with polyclonal ”epidermal keratin” antibodies most probably results from the reactivity of such antibodies with K19. Received: 14 April 1997 / Accepted: 28 May 1997     

Expression of Met protein and urokinase-type plasminogen activator receptor (uPA-R) in papillary carcinoma of the thyroid

Met protein encoded by MET oncogene is the high affinity receptor for hepatocyte growth factor (HGF)/scatter factor (SF). HGF/SF has to be cleaved in its heterodimeric form by the urokinase-type plasminogen activator (uPA) to become active as a ligand for Met receptor. The expression of Met protein and of the high affinity receptor for uPA (uPA-R) was investigated in 39 samples of papillary carcinoma using immunohistochemistry. Reactivity for Met protein was present in 33 of 34 tumours, mostly with a diffuse pattern of staining. Reactivity for uPA-R was present in 78 per cent of papillary tumours and exhibited a pattern of staining similar to that of Met protein. Staining for uPA-R was present in 23 of 25 cases (92 per cent) of papillary carcinoma with prominent sclerosis, and in only 1 of 7 cases (14 per cent) without sclerosis. Peritumoural normal thyroid, follicular adenomas, and follicular carcinomas were negative for Met protein and for uPA-R. Hyperfunctioning tall thyroid cells showed weak membrane reactivity for uPA-R and for Met protein. The findings of immunohistochemistry were confirmed at the mRNA level using in situ hybridization, since the signal for uPA-R and Met RNAs was detected in most tumour cells of five cases of papillary carcinoma.     

Epithelial markers in thyroid carcinoma: an immunoperoxidase study

Ten cases each of papillary, follicular, anaplastic and medullary carcinoma of the thyroid were stained for thyroglobulin, calcitonin, epithelial membrane antigen (EMA), carcinoembryonic antigen (CEA) and cytokeratin (CAM 5.2). Monoclonal or affinity purified polyclonal antibodies, and an indirect immunoperoxidase technique were used. All the papillary and follicular tumours, 5/10 anaplastic and 3/10 medullary carcinomas contained thyroglobulin. Only the 10 medullary carcinomas stained positively for calcitonin. Three out of 10 papillary, 1/10 follicular, 0/10 anaplastic and 10/10 medullary carcinomas were positive for CEA. Nine out of ten papillary, 7/10 follicular, 2/10 anaplastic and 3/10 medullary carcinomas were positive for EMA. Ten out of 10 papillary, 10/10 follicular, 5/10 anaplastic and 10/10 medullary carcinomas were positive for cytokeratin. The presence of calcitonin and CEA is of value in the diagnosis of medullary carcinoma, and enable its distinction from anaplastic thyroid carcinoma. Thyroglobulin is a useful marker in thyroid carcinomas.     

The RET proto-oncogene in medullary and papillary thyroid carcinoma

 The evolution of cancer is a multistep phenomenon, and multiple cellular genetic lesions are involved in the emergence of the malignant neoplasm. Several early events have been implicated in the neoplastic transformation of thyrocytes, and recent reports have described the involvement of specific genetic alterations in different types of thyroid neoplasms: ras point mutations are frequently observed in tumours with follicular histology, gsp – the mutated form of the alpha subunit of the Gs-protein – is encountered in up to 73% of papillary or follicular thyroid carcinomas, and a high prevalence of p53 point mutations has been found in anaplastic thyroid carcinomas but not in differentiated follicular tumours. More recent studies revealed that the RET proto-oncogene is involved in the oncogenesis of medullary thyroid carcinoma (MTC) and papillary thyroid carcinoma (PTC) by activation of its tyrosine kinase either by point mutation or rearrangement. In this review the most important recently published data on alterations of the RET proto-oncogene in heritable and sporadic MTCs and in PTCs will be summarized. Emphasis will be directed to the pathophysiological mechanisms of tumour initiation, the indications and limitations of DNA testing, and the clinical implications of identified RET defects in thyroid lesions. Received: 9 January 1997 / Accepted: 17 February 1997     

Immune characterization of thyroid neoplasm's and its variants using immunohistochemical markers: CK-19, Galectin-3 and Hector Battifora mesothelial-1

BackgroundNodular lesions of the thyroid are amongst the common palpable lesions that are encountered by the pathologists in the fine needle aspiration clinics and not only aspiration smears, but even biopsy sections pose significant challenges in their characterization and further classification. Neoplastic lesions of the thyroid have shown a steady rise worldwide and are diagnosed at age younger than most other cancers. Histopathology remains the gold standard in diagnosis and classification of thyroid neoplasms, with variable sensitivity and specificity of immunohistochemical markers, also attributed to variation in the inclusion criteria. We classified the thyroid neoplasms based on WHO Classification (2017) and aimed to study the diagnostic utility of immunohistochemical markers - CK-19, Galectin-3 and Hector Battifora mesothelial-1 performed on manual tissue microarray sections to differentiate various variants of papillary carcinoma from its mimickers, specifically follicular patterned papillary neoplasms from other follicular patterned lesions.MethodProspective study of neoplastic lesions of thyroid from July 2018 to August 2020. Authors describe the clinico-radiological, cytological, histo-morphological and immunohistochemical features of neoplastic nodular lesions of the thyroid.ResultsProspective analysis of nodular thyroid lesions yielded 76 cases, of which 38 were neoplastic. Cytology showed discordance in 10/24 cases, amongst the discordant cases, 70% were confirmed as papillary carcinoma. CK-19 showed high expression in all variants of papillary carcinomas (24/24), low expression in well differentiated tumor of uncertain malignant potential (WD-TUMP) and medullary carcinoma. It was negative in follicular and Hurthle cell neoplasms. Galectin-3 showed 100% specificity and HBME-1 showed 100% sensitivity in diagnosis of papillary carcinoma and its variants. Adenomatoid nodules did not express Gal-3 which helped in their differentiation from FVPTC.ConclusionsGal-3 in combination either with CK-19 or HBME-1 improves the sensitivity and specificity of detection of papillary carcinoma, its variants and its differentiation from follicular patterned lesions to 100% with a significant p value.     

Reduced HBME-1 immunoreactivity of papillary thyroid carcinoma and papillary thyroid carcinoma-related neoplastic lesions with Hürthle cell and/or apocrine-like changes

BACKGROUND: We have recently observed that Hürthle cell tumours and papillary thyroid carcinoma with tumour cells showing decapitation of luminal portion of the cytoplasm (apocrine-like changes) display negative or decreased immunoreactivity for HBME. The purpose of this study is to correlate papillary thyroid carcinoma with positive and negative immunoreactivity for HBME with the histopathological features. METHODS AND RESULTS: Two hundred and five thyroid neoplasms including carcinoma and adenomas were grouped into Hürthle cell tumours, tumours with or without some features of Hürthle cells, tumours with apocrine-like changes and adenomas with or without limited nuclear features of papillary thyroid carcinoma but not diagnostic for papillary thyroid carcinoma. All neoplasms were submitted for immunostaining with cytokeratin 19 (CK19) and HBME. Papillary thyroid carcinoma, follicular carcinoma and follicular adenoma that have areas of limited nuclear features but not diagnostic for papillary thyroid carcinoma showed stronger immunostaining for HBME than their respective counterparts with Hürthle cell changes. All Hürthle cell tumours showed negative to focal reactivity. This decrease of reactivity for HBME was proportional to the levels of Hürthle cell changes. In addition, focal to extensive apocrine-like changes were seen in most Hürthle cell neoplasms and rarely seen in non-Hürthle cell neoplasms. Apocrine-like changes abolished or decreased HBME immunoreactivity of papillary thyroid carcinoma and tumours with limited nuclear features. Immunostaining for cytokeratin AE3 was not affected by Hürthle cell or apocrine-like changes. CONCLUSIONS: All papillary thyroid carcinomas without Hürthle cell or apocrine-like differentiation are reactive for HBME. Hürthle cell tumours and tumours with Hürthle cell or apocrine-like changes show negative or focal reactivity for HBME. Except for this limitation, HBME is a sensitive marker for papillary thyroid carcinoma and tumours with limited nuclear features.     

Der hyalinisierende trabekul?re Tumor der Schilddrüse

Hyalinizing trabecular tumours of the thyroid represent a rare entity of follicular cell derived tumours and are characterized by a marked intratrabecular hyalinisation. These tumours share architectural similarities with medullary thyroid carcinomas and exhibit nuclear features such as nuclear pseudoinclusions resembling papillary thyroid carcinoma. However, the clinical behaviour remains unclear. On the basis of their inconspicuous appearance and absence of invasion or recurrence during follow-up, the tumour was initially classified as an adenoma. Subsequently, molecular findings such as the detection of RET?/?PTC rearrangements in some hyalinizing trabecular tumours favoured the designation as a variant of papillary thyroid carcinoma. However, miRNA profiling of hyalinizing trabecular tumours compared with benign thyroid lesions and papillary thyroid carcinoma failed to demonstrate the characteristic up-regulation found in papillary thyroid carcinoma. This article summarizes conventional diagnostic criteria with supplementary information regarding molecular pathogenesis of hyalinizing trabecular tumours of the thyroid.     

Can CD10 be used as a diagnostic marker in thyroid pathology?

CD10–common acute lymphoblastic leukemia antigen is a membrane-bound zinc metalloproteinase that is expressed by different hematopoietic cell types at unique stages of lymphoid and myeloid differentiation. It was reported to be expressed in various nonlymphoid cells and tissue, as well as in various types of neoplasms. Recently, it has been found to be useful in the differential diagnosis of benign and malignant follicular-patterned lesions of the thyroid. In the present study, we evaluated the staining pattern of CD10 in various thyroid lesions, including 14 benign and 61 malignant cases, as well as in adjacent thyroid tissue. CD10 was negative in normal thyroid tissue, adenomatous nodules, minimally invasive follicular carcinoma, and well-differentiated carcinoma. It was expressed in nine of 14 (64.2%) conventional papillary carcinomas, four of 24 (16.6%) follicular variant of papillary carcinomas, three of six (50%) papillary microcarcinomas, one of nine (11.1%) widely invasive follicular carcinomas, and three of ten (30%) follicular adenomas. In contrast to results of previous studies, CD10 is not useful in the classification of thyroid follicular lesions as benign or malignant, but it shows strong positivity in conventional papillary carcinoma.     

甲状腺乳头状癌中ret癌基因的表达

目的：观察人甲状腺乳头状癌中ret癌基因的表达。方法：应用RT－PCR技术研究甲状腺乳头状癌新鲜组织中ret癌基因活化形式PTC基因的表达。结果：25例甲状腺乳头状癌中有6例（24％）PTC基因表达阳性,主要分布于Ⅱ级以上肿瘤中;而甲状腺滤泡型癌、甲状腺瘤和正常甲状腺组织吕PTC均为阴性。结论：PTC基因是ret癌基因新的活化形式,ret基因的活化仅限于甲状腺头状癌类型中,可能是乳头状癌的特殊遗传事件,而无PTC基因表达的乳头状癌,其分子发生机制可能与其他基因的遗传性改变有关。因此检测PTC基因可作为判断甲状腺乳头状癌生物学行为的有效参考指标。     

Cytokeratin 19 and galectin-3 immunohistochemistry in the differential diagnosis of solitary thyroid nodules

AIMS: The immunohistochemical expression of cytokeratin 19 (CK 19) and galectin-3 was evaluated in 69 thyroid lesions to assess their potential as markers in the diagnosis and classification of thyroid malignancy. The following were studied: 26 cases of papillary carcinoma, 12 of follicular carcinoma, 20 follicular adenomas, two medullary carcinomas, one anaplastic carcinoma and eight multinodular goitres. METHODS AND RESULTS: Formalin-fixed paraffin-embedded thyroid tissues were stained immunohistochemically for both CK 19 and galectin-3. CK 19 expression was found in all 26 papillary carcinomas, five of 12 follicular carcinomas, two of two medullary carcinomas and one case of anaplastic carcinoma. Only five of 20 follicular adenomas were positive for CK 19, and this was in a focal distribution. Two of eight multinodular goitres stained focally positive. Galectin-3 expression was found in 22 of 26 papillary carcinomas, 12 of 12 follicular carcinomas and one of two cases of medullary carcinoma. Only two of 20 follicular adenomas were positive. Three of eight multinodular goitres showed focal galectin-3 expression. CONCLUSIONS: Our findings suggest that the immunohistochemical localization of CK 19 and of galectin-3 is a useful adjunct to the histopathological diagnosis of a solitary thyroid lesion. The expression of CK 19 favours a diagnosis of papillary carcinoma in all its variant patterns. Galectin-3 may serve as a marker for the recognition of follicular carcinoma, particularly the minimally invasive form.     

Molecular profile and clinical-pathologic features of the follicular variant of papillary thyroid carcinoma. An unusually high prevalence of ras mutations

The follicular variant (FV) of papillary thyroid carcinoma is characterized by a follicular growth pattern and cytologic features of papillary carcinoma. ret/PTC rearrangements are common in classic papillary thyroid carcinoma (PTC) and PAX8-PPAR gamma and ras mutations in follicular thyroid carcinoma. Their prevalence in FV has not been established. We studied these genetic alterations and clinical-pathologic features in 30 FV cases and compared those with 46 non-FV papillary carcinomas. FV cases revealed 1 ret/PTC rearrangement (3%) and 13 ras mutations (43%). Non-FV cases harbored 13 ret/PTC (28%) (P = .006) and no ras mutations (P = .0002). No PAX8-PPAR gamma was found in either group. FV cases demonstrated a significantly higher prevalence of tumor encapsulation, angiovascular invasion, and poorly differentiated areas and a lower rate of lymph node metastases. These data indicate that the FV of papillary carcinoma has a distinct set of molecular alterations and is characterized by a high frequency of ras point mutations.     

Molecular pathology of thyroid tumours of follicular cells: a review of genetic alterations and their clinicopathological relevance

Thyroid cancer is the most common endocrine malignancy. Knowledge of the molecular pathology of thyroid tumours originating from follicular cells has greatly advanced in the past several years. Common molecular alterations, such as BRAF p.V600E, RAS point mutations, and fusion oncogenes (RET–PTC being the prototypical example), have been, respectively, associated with conventional papillary carcinoma, follicular‐patterned tumours (follicular adenoma, follicular carcinoma, and the follicular variant of papillary carcinoma/non‐invasive follicular thyroid neoplasm with papillary‐like nuclear features), and with papillary carcinomas from young patients and arising after exposure to ionising radiation, respectively. The remarkable correlation between genotype and phenotype shows how specific, mutually exclusive molecular changes can promote tumour development and initiate a multistep tumorigenic process that is characterised by aberrant activation of mitogen‐activated protein kinase and phosphoinositide 3‐kinase–PTEN–AKT signalling. Molecular alterations are becoming useful biomarkers for diagnosis and risk stratification, and as potential treatment targets for aggressive forms of thyroid carcinoma. What follows is a review of the principal genetic alterations of thyroid tumours originating from follicular cells and of their clinicopathological relevance.     

Special types of thyroid carcinoma

This article reviews the small percentage of thyroid tumours that are not classified as classic papillary thyroid carcinoma, follicular thyroid carcinoma, and medullary thyroid carcinoma. It includes subtypes of papillary thyroid carcinoma, including, tall‐cell, hobnail/micropapillary, columnar cell, diffuse sclerosing and solid variants. Poorly differentiated carcinoma, high‐grade carcinoma and anaplastic thyroid carcinoma are reviewed. Also discussed are entities that are unusual but need to be recognized as primary thyroid neoplasms, i.e. mucoepidermoid carcinoma, sclerosing mucoepidermoid carcinoma with eosinophilia, and mammary analogue secretory carcinoma/secretory carcinoma. The pathological features and prognostic factors are described; a brief review of molecular correlates of these neoplasms is included.     

CD98 expression is decreased in papillary carcinoma of the thyroid and Hashimoto’s thyroiditis

Aims: CD98 is a component of the large neutral amino acid transporter (LAT), which is a cell surface amino acid transporter. CD98 also binds to and activates β₁‐integrin, promoting anchorage‐independent growth. CD98 expression is increased in a variety of carcinomas but its distribution in the normal and neoplastic thyroid gland has not been reported. The aim was to examine the immunohistochemical expression of CD98 in normal and diseased thyroid tissue. Methods and results: One hundred and forty thyroid cases were selected from the archives of the Department of Pathology, including normal controls, neoplasms (follicular adenoma, follicular carcinoma and papillary carcinoma) and non‐neoplastic conditions (multinodular goitre, Graves’ disease and Hashimoto’s thyroiditis). Immunohistochemistry for CD98 was performed and each case was scored for proportion of cells and intensity of immunoreactivity. In normal thyroid, there was moderately strong expression of CD98 in the lateral cell membranes of follicular cells. A similar pattern of expression was seen in follicular adenoma, minimally invasive follicular carcinoma, multinodular goitre and Graves’ disease. In most cases of papillary carcinoma and in the inflamed areas of Hashimoto’s thyroiditis, expression of CD98 was decreased. Conclusions: CD98 expression is down‐regulated in thyroid papillary carcinoma; this may relate to the better prognosis associated with many of these tumours.     

Focal features of papillary carcinoma of the thyroid in fine-needle aspiration material are strongly associated with papillary carcinoma at resection

The cytologic features of papillary carcinoma of the thyroid in fine-needle aspiration material have been well described. The significance of finding these features in only a small population of cells is not well characterized. I reviewed the results of 28 thyroid fine-needle aspirates processed as direct smears and cell blocks in which only a small population (<20 cells) showed features of papillary carcinoma. Papillary carcinoma was considered in 142 (8.98%) of 1,581 aspirates, and in 28 cases (1.77%), 20 cells or fewer showed features of papillary carcinoma and follow-up was available. Papillary carcinomas greater than 1 cm were identified in 11 cases (39%; 3 follicular variants), papillary carcinomas less than 1 cm were identified in 4 cases (14%), and benign lesions in the remaining 13 cases (46%). The background material (either scant or abundant benign epithelium) did not correlate significantly with the result of resection. Identifying features of papillary carcinoma in a small population of cells in either a scant or an abundant thyroid aspirate are associated with a high rate of papillary carcinomas at resection, only a minority of which represent either the follicular variant or incidental tumors.     

Definition of poorly differentiated carcinoma of the thyroid: The Japanese experience

The terminology and definitions pertaining to thyroid malignancies of follicular cell origin that are neither well-differentiated papillary or follicular carcinomas nor undifferentiated anaplastic carcinomas remain controversial. Against this background, we previously proposed that “poorly differentiated carcinoma” should be added to the classification of thyroid carcinoma arising from follicular epithelium. The histological criteria and biological characteristics of poorly differentiated carcinoma of the thyroid were described. In this discussion, we make a new proposal concerning the histological classification of thyroid cancers derived from follicular epithelium. According to this proposal, thyroid cancers can be divided into common types and special types. In the common types, the usual histology should be included. The common types are well-differentiated carcinoma, poorly differentiated carcinoma, and undifferentiated carcinoma. The specific types include columnar-cell carcinoma, tall cell carcinoma, cribriform carcinoma, and other rare carcinomas.