Chapters_____________________
- Pathology of Thyroid Tumors
- Guidelines in the Management of Differentiated Thyroid Cancer
- Guidelines in the Management of Medullary and Anaplastic Thyroid Cancers
- Principles of Thyroid Surgery
- Role of Nuclear Medicine in Adjuvant Therapy and Follow-up of Differentiated Thyroid Cancer
- Medical Management of Differentiated Thyroid Cancer
INTRODUCTION
Thyroid tumors account to 1% of all malignancies in developed countries and 0.2% of cancer deaths. They are the most common malignancies of the endocrine system and pose a significant challenge to pathologists, surgeons, and oncologists. Most of the carcinomas affect young and middle-aged adults and are indolent malignancies with a 10-year survival that exceeds 90%. There has been an increase in the incidence rate of these tumors worldwide which can be largely attributed to more sophisticated diagnostic methods and a change in diagnostic practices with an increasing number of smaller tumors being detected of late. Thyroid tumor pathology is an area replete with diagnostic challenges. Though there are typical morphological patterns described, overlaps with non-neoplastic entities pose diagnostic difficulties. Updates in this field include ancillary and research aiming at techniques that can further narrow down our diagnosis from the different “indeterminate/gray zone” lesions detected on screening.
CLASSIFICATION
The conventional classification based on morphology and clinical features is largely supported by molecular data currently available. Genetic profiles of four main categories appear distinctly different from each other with a few areas of overlap.
The classification of thyroid tumors modified from WHO classification (2004) is as follows:
Tumors of Follicular Epithelium
- Follicular adenoma (including Hurthle cell adenoma)
- Hyalinizing trabecular adenoma
- Follicular carcinoma (including Hurthle cell carcinoma)
- Minimally invasive
- Widely invasive
- Papillary carcinomas
- Poorly differentiated carcinoma
- Anaplastic carcinoma
- Squamous cell carcinoma
- Mucoepidermoid carcinoma
- Sclerosing mucoepidermoid carcinoma with eosinophilia
- Mucinous carcinoma.
Tumors with C Cell Differentiation
Medullary carcinoma.
Tumors with Mixed Differentiation
- Collision tumor—follicular/papillary or follicular/medullary
Tumors Showing Thymic or Related Branchial Pouch Differentiation
- Ectopic thymoma
- Spindle epithelial tumor with thymus-like element (SETTLE)
- Carcinoma showing thymus-like element (CASTLE).
Tumors of Lymphoid Cells
- Malignant lymphoma
- Plasmacytoma.
Mesenchymal Tumors
- Smooth muscle tumors
- Peripheral nerve sheath tumors
- Paragangliomas
- Solitary fibrous tumors
- Follicular dendritic cell tumors
- LCH
- Angiosarcoma.
Teratomas
Secondaries
FINE NEEDLE ASPIRATION CYTOLOGY (FNAC)
Thyroid FNAC is a minimally invasive low-risk procedure commonly performed in a euthyroid patient with a clinically relevant thyroid nodule. Ultrasound guidance is generally recommended for thyroid nodule FNA. For FNA, 22 to 26-gauge needles can be used; most commonly used is a 24-gauge needle. As the name indicates, the biopsy technique uses aspiration to obtain cells or fluid from a mass. Another technique, fine needle nonaspiration (FNNA) biopsy, avoids aspiration but still permits cytologic review of thyroid masses.
The slides for wet fixation should be placed immediately in 95% alcohol for staining with the Papanicolaou stain. For Giemsa staining, air-dried smears are necessary, and prepared slides are left unfixed and transported to the laboratory. Usually, 3–6 aspirations are made. For cystic lesions, the fluid should be completely aspirated and FNA attempted on residual tissue. Sonological localization helps to see solid areas. Aspirated fluid should be placed in a plastic cup and saved for cytologic evaluation. Cell block preparation is occasionally done.
Bethesda system is used to categorize the results of cytology. Six categories as given in Table 1 are possible with increasing chance of malignancy. 2017 update of the system gives two columns of the risk of malignancy considering noninvasive follicular thyroid neoplasm with papillary like nuclear features (NIFTP) as malignancy and not.
The diagnostic accuracy of thyroid FNA for technically satisfactory specimens is greater than 95%, with positive predictive values of 89–98% and negative predictive values of 94–99%. Sensitivities for thyroid FNA range from 43% to 98%, and specificities range from 72% to 100%. For the evaluation of cystic thyroid lesions, FNA is reported to have a low sensitivity (40%).
Limitations
False-negative rates generally vary from 1.5% to 11.5% (average, <5%). The false-negative rate is defined as the percentage of patients with benign cytology in whom malignant lesions are later confirmed on thyroidectomy. False-positive rates vary from 0% to 8% (average, 3%). A false-positive diagnosis indicates that a patient with a malignant FNA result was found on histologic examination to have benign lesions. Interpretive or sampling errors account for false diagnoses. Hashimoto thyroiditis is probably the most common cause of false-positive cytology. Inadequate specimens are labeled nondiagnostic or unsatisfactory and account for 2–20% of specimens (average, 10%).5
Several factors influence nondiagnostic rates for FNA results, including the skill of the operator, vascularity of the nodule, criteria used to judge adequacy of the specimen, and the cystic component of the nodule. It is not possible to diagnose follicular carcinoma by FNAC as the full specimen needs to be studied to see the capsular and vascular invasion.
PAPILLARY CARCINOMA
This is the most common malignant tumor of the thyroid gland and comprises 80–85% of all malignancies. It is common in countries having iodine sufficient or iodine excess diets. They tend to be biologically indolent and have an excellent prognosis. Papillary carcinoma can occur at any age, but most of them are diagnosed in third and fifth decades of life. Women are more frequently affected (2–4: 1). Multifocality is common. Most important etiological role is that of radiation. It was frequently diagnosed in patients who are treated with low dose radiation to head and neck for benign diseases. It was also recognized in survivors of atomic bomb explosion in Japan. Survivors of other cancers who were treated with radiation were also found to develop papillary carcinomas as second primaries. Dietary iodine concentration appears to influence incidence and in some cases, the morphology of papillary carcinomas (Figs. 1 and 2).
Pathology
Papillary carcinoma shows varied gross features. Majority of the cases present as a solid irregular and firm gray white growth with granular cut surface. Scarring may be very prominent in some of the cases. Calcification is a common finding. It can also present as a small scar in the subcapsular location. Some lesions may be completely cystic with a solid mural nodule attached.
Papillary thyroid carcinoma (PTC) is characterized by unique nuclear features which are diagnostic of this entity. Typical cytological findings include cells in papillae with anatomical borders and monolayered sheets showing swirling. They have ovoid, overlapping nuclei with grooves, and intranuclear inclusions. Studies have shown that presence of a combination of papillae, intranuclear inclusions, and metaplastic squamoid cytoplasm is 98% predictive of papillary carcinoma in cytology material.6
Nuclear grooves are also seen in thyroiditis, HTA, and adenomatous hyperplasias while intranuclear inclusions are encountered in medullary carcinomas, HTAs, and paragangliomas. Histological sections show overlapping and clearing of nuclei typically described as Orphan Annie nuclei.
Depending on different patterns, cell types, and clinical features, different variants have been described. Except for oncocytic and hobnail variants, all others should have more than 50% of the tumors showing their unique patterns along with nuclear features for their diagnosis. An oncocytic variant shows oncocytic morphology in ≥75%; hobnail variant shows hobnailed nuclei in ≥30%.
Fig. 2: Papillary carcinoma with overlapping clear “Orphan Annie nuclei” with nuclear grooves and inclusions (H & E, 40X).
Among the variants, specific prognostic significance is connoted to the following types:
- Tall cell variant
- Cells should be 3 times taller than their width
- Accounts for 10% of PTC cases
- More seen in elderly
- Usually large and show extrathyroid extension and recurrences more frequently
- 10-year survival rate is 70%
- Less sensitive to radioactive iodine (RAI) therapy
- High prevalence of BRAF mutation
- Diffuse sclerosing variant
- More seen in children and young adults
- Clinically aggressive
- All cases are associated with lymph node metastasis at the time of presentation
- Diffusely infiltrating tumor with sclerotic stroma showing squamous morules, psammoma bodies, and associated thyroiditis
- Lung metastasis is also more common at presentation (25%)
- Columnar cell variant
- Very rare
- High columnar cells with pseudostratification, supra and subnuclear vacuolations reminiscent of early secretory endometrium
- Clinically aggressive
- Cribriform morular variant
- Seen typically in patients of familial adenomatous polyposis (FAP) and Gardner's syndrome (APC mutations)
- Shows cribriform features, solid and spindle areas with squamoid morphology
- Usually multifocal
- Clear cell variant
- Recognition of this variant at metastatic sites can be problematic without immunostains
- Hobnail variant
- Recently described entity
- Clinically aggressive
- Oncocytic variant
- Resistant to RAI therapy
- Clinically aggressive.
- Follicular variant
- Unique variant with different genetic profile
- Often diagnosed as follicular neoplasms in FNA due to microfollicular arrangement and equivocal nuclear features
- Percentage of tumor to be involved by this pattern is still debated
- Subcategorized into unencapsulated, encapsulated/well-demarcated and diffuse/multinodular variants
- Encapsulated—akin to follicular neoplasms, better prognosis
- Diffuse type more aggressive.
Noninvasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features
Noninvasive encapsulated follicular variant of PTC has been established in the literature as a clinically indolent tumor. Despite this, it was traditionally treated like all other PTCs. In an attempt to reduce overtreatment of this entity, a panel of experts reclassified this entity as NIFTP. It is a low-grade tumor with an indolent clinical course. The removal of NIFTP from the carcinoma category has significantly lowered the rates of malignancy in thyroid fine-needle aspiration diagnostic categories and rates of adverse oncologic events in PTC. NIFTP is characterized as a low-to-intermediate suspicion nodule on ultrasound, often shows an indeterminate preoperative cytology diagnosis and demonstrates molecular alterations characteristic of low-grade thyroid follicular tumors. The set of criteria to diagnose this entity is follicular growth with less than 1% papillae, nuclear score of 2–3, absence of psammoma bodies and necrosis, less than 3 mitoses per 10 high per fields and less than 30% solid growth. Management recommendations for NIFTP are very similar to low-risk PTC measuring less than 4 cm. Most studies agree that nodules with nuclear features suggestive of NIFTP or small PTC should get lobectomy without radioactive iodine.
Papillary Microcarcinoma
It is not a specific variant but includes all papillary carcinomas that measure 1 cm or less in dimension (stage IA).
Clinical Markers for Potential Aggressiveness
- Nonincidental presentation
- Positive preoperative FNA
- Lymph node metastasis
- Positive family history
- Nodules in contralateral lobes
- Male gender.
Histological Markers for Aggressiveness
- Multifocal/bilateral
- Size ≥6 mm
- Desmoplastic fibrosis
- Presence of poorly differentiated compo-nents
- Lymphovascular emboli.
Prognosis
Prognosis of papillary carcinoma is excellent. Ten year survival rate is over 90% and for young patients, over 98%. Tall cell and columnar cell variants have a less favorable prognosis than conventional papillary carcinomas.
FOLLICULAR CARCINOMA
It is a malignant epithelial tumor showing follicular cell differentiation and lacking diagnostic nuclear features of papillary carcinoma. It accounts for 10–15% of thyroid malignancies. It is more common in women in the fifth decade. Incidence is higher in iodine deficient areas. Follicular carcinomas most commonly present as large asymptomatic thyroid nodules which are typically cold on scintigraphy. Distant metastasis is seen in up to 20% during presentation. Oncocytic variants typically occur 10 years later than the conventional types and show greater propensity for recurrence and local invasion. Follicular carcinomas are usually encapsulated with gray tan to brown bulging cut surface. Widely invasive carcinomas may show extensive permeation of the capsule. Rarely thyroid veins and superior vena cava may be involved. Multifocality is uncommon. Distal metastasis to the lung and bones are common (Figs. 3 and 4).
Cytology
Aspirates will be hypercellular and show repetitive microfollicles and scant colloid. Atypical nuclear features do not denote malignancy. Demonstration of capsular or vascular invasion is needed for the diagnosis of follicular carcinoma.
Histology
Follicular carcinomas show variable morphology with cells arranged in follicles, solid or trabecular patterns. They are divided into two major categories—minimally invasive and widely invasive. While conceptually simple, there is no consensus as to the definition of capsular invasion. Some authorities require complete transgression of the capsule, while other authorities do not require complete transgression of the capsule. Minimally invasive carcinomas have limited capsular and/or vascular invasion.
9Widely invasive carcinomas have widespread invasion of thyroid tissue and/or blood vessels. The probability of aggressive behavior increases with the extent of vascular invasion. The term “grossly encapsulated angioinvasive follicular carcinoma” has been suggested for those tumors that demonstrate vascular invasion only.
Variants
- Oncocytic
- Clear cell
- Mucinous.
Prognosis
Minimally invasive follicular carcinomas have very low long-term mortality (3–5%). Widely invasive carcinomas have 50% mortality. Oncocytic carcinoma behaves more aggressively than conventional types with higher frequencies of extrathyroidal extension, local recurrence, and nodal metastasis. Adverse prognostic factors include age more than 45 years, oncocytic tumor type, extrathyroidal extension, tumor size greater than 4 cm, and presence of distant metastasis.
POORLY DIFFERENTIATED CARCINOMA
They are defined as follicular cell neoplasms that show limited evidence of follicular cell differentiation and occupy both morphologically and behaviorally an intermediate position between differentiated and undifferentiated carcinomas. Turin proposal (2006) remains so far the most accepted criteria for diagnosing this entity. According to this proposal poorly differentiated carcinomas (PDTCs) are defined by: (1) presence of trabecular/insular/solid (TIS) architecture, with (2) at least one of the following features—convoluted nuclei, mitotic figures of >3/hpf, or coagulative necrosis; (3) absence of conventional nuclear features of papillary carcinoma. PTDCs can be seen as component of well-differentiated carcinomas and as little as 10% is sufficient to confer an aggressive biological behavior. Most tumors present as cold nodules with or without enlarged lymphadenopathy. Lung and bone metastases are also relatively frequent at the time of diagnosis. Extrathyroidal extension is less commonly seen than in anaplastic carcinomas.
Focal TP53 positivity, increased ki67 index (10–30%), and absence of E-cadherin membrane expression are important immunohistochemical features. They show reactivity for both thyroglobulin and TTF1, although thyroglobulin positivity may be focal.
These patients respond poorly to radioiodine therapy. The prognosis depends primarily on TNM staging, completeness of surgery, and response to radioactive iodine therapy.
ANAPLASTIC (UNDIFFERENTIATED) CARCINOMA
Anaplastic carcinomas are highly malignant tumors that histologically appear wholly or partially composed of undifferentiated cells that exhibit immunohistochemical or ultra-structural features indicative of epithelial differentiation. It affects mainly the elderly age group with higher incidence reported in endemic goiter regions. It has a high mortality rate (90%) with a median survival rate of up to 6 months after diagnosis. Patients typically present with rapidly expanding neck mass with pressure symptoms like hoarseness and dysphagia. Tumors are hard and fixed, and frequently invade the surrounding structures. Lymph node involvement as well as distant metastasis is seen in up to 40% of cases. All anaplastic carcinomas are staged as T4 (T4a—intrathyroidal, T4b—extrathyroidal extension) (Fig. 5).
Cytologically, smears are cellular with highly pleomorphic cells seen singly and in clusters. Three types of cells are observed—spindle, giant cell, and squamoid.10
There is increased mitosis and necrosis with the background characteristically showing polymorphonuclear leukocytes. There may be an associated differentiated component in some of the cases. Variants described are: (1) osteoclastic, (2) carcinosarcoma, (3) paucicellular, and (4) lymphoepithelioma and the likes.
These tumors are negative for thyroglobulin and TTF1, and show variable positivity with epithelial markers. Immunohistochemistry is used to differentiate them from other mesenchymal tumors, melanomas, and lymphomas.
Prognostic factors are related primarily to the extent of disease at presentation. Prognosis depends on the size of the undifferentiated component and the efficacy of eradicative surgery. Five-year survival rate ranges from 0% to 14%.
MEDULLARY CARCINOMA
Medullary thyroid carcinoma is a malignant tumor showing C cell differentiation. It constitutes 5–10% of all thyroid malignancies. Up to 25% cases are heritable, caused by germ-line mutations in RET proto-oncogene. Mean age at presentation is 50 years for sporadic cases. MEN II B patients present in infancy or early childhood while MEN IIA associated tumors occur in late adolescence or early adulthood. Patients with FMTC present at an age of 50 years. Tumors typically are located in the middle third of the lobes. They present as cold nodules with more patients presenting with nodal metastasis (50%) and up to 15% with distant metastasis. Virtually all MTCs produce calcitonin, and serum levels are typically increased. Paraneoplastic syndromes may occur due to production of other peptides and amines. Cytologically smears show loosely cohesive cells with polygonal, bipolar, or spindle shapes. Plasmacytoid cells are common and multinucleated giant cell morphology is occasionally encountered. MGG stains show characteristic red cytoplasmic granules. Amyloid may be found in 50–70% of the cases (Figs. 6 to 8).
Histologically, cells with salt and pepper chromatin are arranged in sheets, nests or trabeculae in an organoid fashion. Necrosis is infrequent. Variants include spindle cell, small cell, giant cell, oncocytic, clear cell, etc. (12 variants are described). Occasional cases may show more pleomorphic features.
Cells are positive for calcitonin, CEA, chromogranin, synaptophysin, TTF1, and low molecular weight keratin. C cell hyperplasia can be seen in surrounding thyroid tissue adjacent to the invasive tumor and also in prophylactic thyroidectomies in hereditary cases. “Neoplastic C cell hyperplasia” is a precursor lesion for heritable medullary carcinomas and is composed of groups of intrafollicular atypical C cells while “reactive C cell hyperplasias” seen in variety of other pathophysiological conditions is characterized by an increased number of normal appearing C cells.
Five and ten-year survival rates are 83.2% and 73.7%, respectively. Older age, male gender, and extent of local tumor invasion are associated with reduced survival.11
Presence of distant metastasis is also an independent predictor of poor prognosis. Children with MEN IIB present at an earlier age and have a higher risk for aggressive forms when compared with MEN IIA cases. Presence of necrosis, squamous metaplasia, <50% of calcitonin reactive cells or CEA reactive cells in the absence of calcitonin has also been considered as a poor prognostic feature.
MOLECULAR GENETICS
Alterations of follicular cells that lead to carcinogenesis are caused by unopposed activation of either the mitogen-activated protein (MAP) kinase pathway or the phosphatidylinositol-3-kinase (PI3K)/AKT pathway. Specifically, the MAP kinase pathway (encompassed by the MEK and ERK kinase cascade) is regulated by the RET, RAS, and BRAF genes. Point mutations in the BRAF and RAS genes or RET/PTC translocation can lead to unopposed cellular proliferation and to a carcinogenic environment via the MAP kinase pathway.
BRAF gene alterations are present in about 30–45% of patients with PTCs and, on average, in 15% (range, 12–47%) of patients with PDTCs. The BRAF gene is a marker of adverse prognostic factors, including disease aggressiveness, decreased radioiodine trapping, tumor recurrence, lymph node or distant metastatic disease, and extrathyroidal extension. RET/PTC rearrangements seen in 30% of PTC ultimately result in the unopposed activation of the MAP kinase pathway.
PAX8: PPARγ rearrangements are almost always associated only with follicular carcinomas. Such rearrangements are almost never expressed in patients with PDTCs.
RAS gene alterations are present in about 40–50% of patients with FTCs and, on average, in 1235% (range, 20–50%) of patients with PDTCs and ATCs. It is a marker of tumor dedifferentiation and adverse prognostic outcome. TP53 gene alterations are rarely associated with WDTCs; however, they are highly prevalent in patients with PDTCs (about 28%; range, 17–38%) and in patients with ATCs (64%; range, 20–88%). Unlike the RAS and BRAF gene alterations, p53 mutations possess an exclusive function in triggering tumor dedifferentiation and evolution to PDTC and ATC.
The molecular genetics of medullary carcinoma is well-established, showing mutations in the RET proto-oncogene. Germline RET mutations are associated with hereditary medullary carcinomas, including familial medullary carcinoma (familial MTC) and the multiple endocrine neoplasia syndromes (MEN2a and 2b). However, it must also be noted that sporadic tumors may also harbor RET mutations (30–66%). Sporadic tumors may also harbor HRAS or KRAS mutations as well (up to 25%).
In familial setting, prophylactic total thyroidectomy is performed for family members based on positive mutational analysis.
Table 2 summarizes the prevalence of genetic alteration in patients with various thyroid carcinomas.
Molecular testing in indeterminate thyroid nodule: Molecular testing for thyroid nodules helps improve the diagnostic accuracy of thyroid cytology for indeterminate cases and potentially to guide the extent of surgery as initial therapy for suspected thyroid malignancies. Diagnostic test accuracy must also be considered, with evaluation of test sensitivity, specificity, and the underlying disease prevalence in the population. Diagnostic tests with high sensitivity and high negative predictive value (NPV) are good tests to “rule out” the presence of disease, depending on the disease prevalence within the population, suggesting that a negative test result has high accuracy (approximately 95%) to reassure patients that cancer is not present in the thyroid nodule evaluated. Diagnostic tests with a high specificity and high positive predictive value (PPV) are good to “rule in” disease, suggesting that a positive test result has high accuracy to confirm that a nodule is indeed cancerous,
The “rule in” tests assess for the presence of single gene point mutations (BRAF or RAS) or gene rearrangements (RRET/PPTC, PAX8//PPPARγ) which have been shown to increase the ability to predict cancer, while the “rule out” test (Afirma Gene Expression Classifier) utilizes a proprietary gene expression classifier (RRNA expression) specifically designed to maximize the ability to define a process as benign. None of the presently available tests is associated with a 100% negative or positive predictive value (NNPV or PPV). Thus, no currently available molecular test identifies the absence or presence of malignancy in all indeterminate nodules.
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ANCILLARY TESTING
Ancillary testing can be used for diagnostic, prognostic, and, to some extent, therapeutic purposes in thyroid cancer. While several markers are now commonly used in patient management, they are not yet a “universal standard of care.”
A number of immunohistochemical markers has been proposed to confirm the diagnosis of papillary carcinoma, allowing for distinction from other lesions/tumors in the differential diagnosis. These markers include HBME-1, galectin 3, CITED-1, HMWCK, CD56, and cytokeratin 19. The literature has demonstrated a high sensitivity and specificity with various combinations of these markers for the diagnosis of papillary carcinoma and is particularly useful in resolving the diagnosis for follicular patterned lesions. However, these panels are not infallible as there are false-positives and false-negatives. With regard to the rare but important cribriform morular variant of papillary carcinoma, nuclear beta catenin accumulation is essentially a defining feature that is diagnostically invaluable.
FURTHER READING
- College of American Pathologists (CAP). (2014). CAP guidelines for thyroid cancer reporting. [online] Available from: http://webapps.cap.org/apps/docs/committees/cancer/cancer_protocols/2014/Thyroid_14Protocol_3100.pdf. [Accessed Jan., 2019].
- Crippa S, Mazzucchelli L, Cibas ES, et al. The Bethesda system for reporting thyroid fine-needle aspiration specimens. Am J Clin Pathol. 2010;134(2):343–4.
- Demellawy DE, Nasr A, Alowami S. Application of CD56, P63 and CK19 immunohistochemistry in the diagnosis of papillary carcinoma of the thyroid. Diagn Pathol. 2008;3:5.
- Fischer S, Asa SL. Application of Immunohistochemistry to thyroid neoplasms. Arch Pathol Lab Med. 2008;132(3):359–72.
- Hannallah J, Rose J, Guerrero MA. Comprehensive literature review: Recent advances in diagnosing and managing patients with poorly differentiated thyroid carcinoma. Int J Endocrinol. 2013;2013:317487.
- Legakis I, Syrigos K. Recent advances in molecular diagnosis of thyroid cancer. J Thyroid Res. 2011;2011:384213.
- LiVolsi A. Papillary thyroid carcinoma: An update. Mod Pathol. 2011;24(Suppl 2):S1–9.
- RC Path guidelines for thyroid cancer reporting, 2014.
- WHO Bluebooks. Endocrine organs, 3rd edition. WHO Bluebooks; 2004.
- Xing M, Haugen BR, Schlumberger M. Progress in molecular-based management of differentiated thyroid cancer. Lancet. 2013;381(9871):1058–69.