REVIEW
Best practice in thyroid pathology
C E Anderson, K M McLaren
Department of Pathology, Royal Infirmary of
Edinburgh, 51 Little France Crescent, Edinburgh EH16 4SA,
UK. Correspondence to:Dr C E Anderson,
Department of Pathology, Royal Infirmary of Edinburgh, 51 Little
France Crescent, Edinburgh EH16 4SA, UK;
Catriona.Anderson@ed.ac.uk
15 January 2002
ABSTRACT
Thyroid
pathology is a specialist area but is often encountered by the
general pathologist in a variety of forms including cytology,
frozen sections, and resection specimens. In the thyroid gland, as
for other endocrine organs, many aspects of diagnosis are unique to
this area of histopathology; thus, the aims of this paper are to
set out best practice guidelines which, although not entirely
comprehensive, will be of practical use.
Keywords: thyroid; papillary carcinoma;
follicular neoplasm
Abbreviations: CK19, cytokeratin
19; FNA, fine needle aspiration
Most pathology departments receive relatively
small numbers of thyroid specimens compared with those from other
organ systems, but they are common enough to be dealt with fairly
frequently by the general pathologist. Therefore, every pathologist
who reports thyroid specimens should have an understanding of the
diagnostic peculiarities and pitfalls of thyroid cytology and
histopathology.
THE
REQUEST FORM
As for any specimen, the request form should be completed in full
with patient details and clinical information. The information
given should include the results of related biochemical and
radiological investigations if applicable, such as thyroid
stimulating hormone concentrations, anti-thyroid antibody titres,
and ultrasound scan findings.
FINE
NEEDLE ASPIRATION
The use of fine needle aspiration (FNA) in the evaluation of a
thyroid nodule is a relatively non-invasive technique that can
often be diagnostic and may prevent unwarranted surgery. The method
of preparation used can give varying cytological appearances and
each has advantages and disadvantages. Recently, newer techniques
have been developed—for example liquid based cytology, which allow
lysis of blood and the preparation of further samples or a cell
block for immunocytochemistry. However, the cytological appearances
with liquid based cytology are somewhat different to those on
conventional smears and further experience of the technique is
required. Indeed, one recent study has suggested that liquid based
thin layer methods are not ideal for use in thyroid
aspirates.
1
"The use of fine needle aspiration in
the evaluation of a thyroid nodule is a relatively non-invasive
technique that can often be diagnostic and may prevent unwarranted
surgery"
Regardless of the technique used, an aspirate
should only be regarded as adequate if at least six epithelial
groups are present. It should be categorised into one of the
following five diagnostic groups, as recommended in the forthcoming
thyroid cancer minimum data set.
- Thy1: non-diagnostic (either because of
inadequate cellularity or if technical problems preclude
diagnosis).
- Thy2: non-neoplastic (features consistent with a
multinodular goitre or thyroiditis).
- Thy3: follicular lesions, including those where
neoplasia cannot be definitely excluded. In particular, a cellular
dominant nodule is indistinguishable from a follicular neoplasm and
the lesion should be managed as such.
- Thy4: suspicious of malignancy.
- Thy5: diagnostic of malignancy with unequivocal
features of papillary (fig 1
 ), medullary, or anaplastic
carcinoma, or of lymphoma or metastasis.
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Figure 1 Fine
needle aspirate of a papillary carcinoma showing nuclear grooves
and optical clarity (Papanicolaou
stain). | |
In the case of an aspirate that is predominantly composed of
lymphocytes, a diagnosis of Hashimoto’s thyroiditis is supported by
the presence of three components; namely, lymphocytes, plasma
cells, and oncocytic epithelium, in conjunction with an appropriate
clinical history.
THYROID
FROZEN SECTIONS
There is still debate concerning the best operative procedure for
differentiated thyroid carcinoma. In several centres, lobectomy is
chosen for the dominant nodule in multinodular goitre, follicular
adenoma, and minimally invasive follicular carcinoma, with total
thyroidectomy reserved for widely invasive follicular carcinoma and
papillary carcinoma. In some centres, all differentiated thyroid
carcinomas are managed by total thyroidectomy. Conversely, a
microfocus of papillary carcinoma or an intracystic papillary
carcinoma may simply justify long term follow up rather than
immediate radical surgery. Disagreement also exists over the use of
frozen section in the intraoperative assessment of thyroid
nodules.
The guidelines of the British Society of
Endocrine Surgeons state that a preoperative FNA is desirable in
the assessment of every solitary thyroid nodule,
2 and if this is diagnostic of
papillary carcinoma, then it is generally agreed that a frozen
section is unnecessary.
3 If FNA was not performed or the
result was inadequate or only suspicious of papillary carcinoma
then a request for a frozen section may result.
The main arguments against performing frozen
section are first the cytological appearances, with potentially
misleading nuclear optical clarity and difficulty in discerning
nuclear grooves and intranuclear protrusions. Second, dissection of
the soft, freshly removed gland inevitably causes architectural
disruption that can make evaluation of the capsule difficult.
Third, a diagnosis of minimally invasive follicular carcinoma is
unlikely to be made on a frozen section because this would require
the identification of focal capsular or vascular invasion, which
may not be present in the section taken. However, in cases where no
preoperative diagnosis is available, many surgeons will request a
frozen section and each department should come to an agreement on
this matter with the local surgical team, perhaps after determining
local reliability of FNA and frozen sections.
4
In our department, we perform frozen sections on
solitary thyroid nodules in some circumstances and we follow a
protocol that seeks to minimise the problems listed above (fig
2). After routine measurements, the
thyroid capsule is inspected and any palpable lesion measured,
before taking one complete transection through the lesion. This
enables the assessment of encapsulation, solidity, and the degree
of cystic change. Although not widely used, a dab imprint of the
cut surface is diagnostically valuable,
5 and the cytological information
so obtained is helpful in distinguishing a papillary carcinoma from
a follicular neoplasm. If it suggests papillary carcinoma, then one
confirmatory frozen section is performed. If it suggests a
follicular neoplasm, then the specimen should be fixed for
subsequent dissection, as described below. In this way, a papillary
carcinoma may be diagnosed with confidence and a widely invasive
follicular carcinoma distinguished from the benign or minimally
invasive counterparts purely on macroscopic inspection accompanied
by cytological affirmation that it is a follicular
lesion.
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Figure 2
Possible protocol for dealing with thyroid specimens. FNA, fine
needle aspiration; FS, frozen
section. | |
GROSS
EXAMINATION
Thyroid
tumours
The specimen should be described as a total thyroidectomy, left
lobectomy, or right lobectomy (± isthmus). The specimen should be
weighed, measured, and then the external appearance described,
paying particular attention to whether the capsule is intact. For a
total thyroidectomy specimen each lobe should be measured
separately.
In most cases, the specimen should then be
sectioned horizontally (fig 3). In the case of a probable
follicular neoplasm, one author has put forward an alternative way
of sectioning in an effort to optimise sampling of the
capsule.
6 In this method, follicular
neoplasms are bisected in the centre and then each half incised at
intervals of 2–3 mm in the capsular plane. Each cut should not be
completed until the whole lesion has been incised and then the cuts
can be continued to free each slice in turn. This method avoids
tangential cutting of the capsule and the difficulty encountered
when only a small piece of tissue is left to cut.
The site of the tumour should be noted, along with the distance to
the nearest excision margin. The size of the lesion must be noted
for the pT aspect of pathological staging, taking the size of the
largest lesion if more than one is identified. Extension of the
tumour beyond the thyroid gland should be noted.
"In papillary carcinoma, the tumour type
should be specifically mentioned, because the tall cell, oncocytic,
solid, and diffuse sclerosing variants are said to have a worse
prognosis than papillary carcinoma of the classic
type"
Representative blocks should be selected based on
the probable histological nature of the lesion. It is well
recognised that papillary carcinoma can be multifocal
7 and, therefore, if this diagnosis
is suspected, the adjacent and opposite lobe should be sampled and
any pale areas processed. In a follicular lesion that is not
grossly invasive, the interface between the tumour and the capsule
and the tumour and the adjacent gland should be extensively sampled
to look for capsular and vascular invasion. In large lesions, at
least 10 blocks should be taken and small lesions should be
processed in their entirety. In medullary carcinoma, the upper part
of the lobes should be sampled because the proportion of C cells is
greater in this area, and therefore C cell hyperplasia is more
likely to be detected.
Any lymph nodes submitted with the specimen
should be dissected in the usual way, noting whether they are
ipsilateral or contralateral to the tumour, or in the
midline.
Non-neoplastic
thyroids
These specimens should be described, weighed, and sectioned as
above. The slices should be carefully examined for the presence of
pale foci because incidental carcinomas can be found in Hashimoto’s
thyroiditis, multinodular goitre, and Graves’
disease.
8–
10
MICROSCOPIC
EXAMINATION
Thyroid
tumours
Treatment and prognosis are closely related to histological tumour
type and accurate staging,
11 and therefore a thorough
microscopic examination is essential. For malignant tumours, the
information listed in table 1 should be noted and the TNM staging
should be recorded.
The diagnosis of papillary carcinoma is based on the identification
of true papillary architecture in the classic type, in addition to
several well recognised nuclear features (table 2). In particular, care should be
taken to assess the nuclear morphology of apparently follicular
lesions so that the follicular variant of papillary carcinoma is
not overlooked. Clues helpful in identifying the follicular variant
(fig 4) include intensely staining colloid
with a scalloped outline,
12 and multinucleate giant cells
within the follicles. A recent study has described further criteria
including the presence of cells with dense, cerebriform nuclei and
overall distortion of the follicular architecture.
13 In papillary carcinoma, the
tumour type should be specifically mentioned, because the tall
cell, solid, and diffuse sclerosing variants are said to have a
worse prognosis than papillary carcinoma of the classic
type.
14,
15
Malignant follicular neoplasms should be described as either widely
or minimally invasive. A widely invasive follicular carcinoma shows
macroscopic invasion or extensive infiltration of the surrounding
thyroid parenchyma and vessels microscopically. In contrast, a
minimally invasive follicular neoplasm, which has an excellent long
term prognosis, is defined as that which shows capsular invasion or
invasion of vessels within or adjacent to the
capsule.
16 In this regard, the presence of
capsular thickening, even focally, is a suspicious feature that
should prompt a careful search for capsular or vascular invasion.
Capsular invasion is defined as complete penetration of the capsule
by an invasive tongue of tumour (fig 5) and should not be confused with
entrapment of follicles or pseudoinvasion as a result of previous
FNA. This last situation is usually associated with inflammation
and haemosiderin deposition.
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Figure 5
Minimally invasive follicular carcinoma—the invasive tongue of
tumour has completely penetrated the capsule of the neoplasm
(haematoxylin and eosin
stain). | |
In a case of medullary carcinoma, it is conventional to describe
the cellular pattern of the lesion, although this has no known
prognostic relevance. The presence of amyloid should be noted
because this is thought to be associated with a better
prognosis.
17 Confirmation of the nature of
the carcinoma by immunocytochemistry for calcitonin and
carcinoembryonic antigen is usual, particularly in poorly
differentiated lesions, although immunoreactivity for calcitonin
may be lost in these cases. All newly diagnosed medullary
carcinomas should now be offered genetic testing because of the
association with multiple endocrine neoplasia syndromes.
If a tumour is undifferentiated,
immunocytochemistry for thyroglobulin and calcitonin should be
performed in an attempt to identify a differentiated component. In
cases lacking such hormonal expression, cytokeratin markers may at
least permit distinction from a sarcoma, and crucially in a lesion
of small cell type, from a lymphoma. If lymphoma involving the
thyroid is suspected then the appropriate immunocytochemical
markers should be selected.
Immunocytochemistry in solitary
thyroid nodules
The role of immunocytochemistry in the diagnosis of solitary
thyroid nodules has developed considerably over the past few years.
Several studies have shown that cytokeratin 19 (CK19) is intensely
expressed by almost all examples of papillary
carcinoma.
18–
20 Its value lies in the
distinction between papillary carcinoma and papillary
hyperplasia—for example, in a follicular adenoma—although this is
compounded somewhat by the finding of focal weak CK19 positivity in
some follicular adenomas.
21 Immunocytochemistry for S100 can
also be of use because it is typically expressed by papillary
carcinomas but not by areas of papillary hyperplasia.
22,
23 The difficulty of an
encapsulated nodule showing a follicular growth pattern and focal
areas with some but not all of the cytological features of
papillary carcinoma is well recognised, and CK19 positivity in
these cases lends weight to a diagnosis of the follicular variant
of papillary carcinoma
24,
25 (fig 6).
Galectin 3 is a carbohydrate binding lectin that appears to play a
role in diverse processes such as embryogenesis, inflammation,
apoptosis, and neoplasia.
26 It is expressed in a high
proportion of thyroid papillary and follicular carcinomas but is
seldom expressed, other than in scattered cells, in follicular
adenomas or dominant nodules of multinodular goitre.
20,
27,
28 If the various reports
describing this pattern are borne out, this antibody may be of
particular value in distinguishing a minimally invasive carcinoma
from a cellular follicular adenoma, because galectin 3 positivity
would prompt further sampling of the lesion to identify foci of
capsular invasion.
The use of CK19 and galectin 3 could potentially
be extended to augment the interpretation of a preoperative FNA.
The cytological features of a papillary carcinoma may on occasion
be focal or subtle, and the expression of CK19 with or without
galectin 3 would add weight to the impression (fig
7
). The absence or presence of
galectin 3 may supplement the cytological features of a follicular
neoplasm, with a negative result supporting a benign
conclusion.
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Figure 7
Immunohistochemistry for cytokeratin 19 in a fine needle aspirate
of papillary carcinoma (Giemsa preparation with
immunohistochemistry performed after removal of the
coverslip). | |
Obviously, a diagnosis of papillary carcinoma requires the correct
nuclear criteria and minimally invasive follicular carcinoma, the
correct architectural features. The pattern of expression of CK19
and galectin 3 should therefore be used to supplement the
histological impression. A further caveat with the use of these
antibodies is that both can be positive in non-neoplastic thyroid
epithelium in the context of inflammation.
29 Therefore, it remains to be seen
how useful these markers really become in routine practice.
"All newly diagnosed medullary
carcinomas should now be offered genetic testing because of the
association with multiple endocrine neoplasia
syndromes"
In the assessment of papillary lesions,
immunohistochemistry for the ret protooncogene product can also be
helpful in identifying papillary carcinomas, although not all
papillary carcinomas will be positive and findings vary between the
histological types.
30
Non-neoplastic thyroid
disease
In the microscopic assessment of non-neoplastic thyroid disease, in
addition to correctly diagnosing the non-neoplastic process
involving the gland, care must be taken to exclude any foci of
incidental malignancy. Obviously, any areas that were
macroscopically suspicious should be examined carefully. In
Hashimoto’s thyroiditis, the diagnosis of an associated lymphoma
can be particularly troublesome. Areas showing complete filling and
expansion of follicles by lymphocytes or complete loss of
epithelium are worrying, and immunocytochemistry or in situ
hybridisation for light chain restriction may be helpful. However,
the use of the polymerase chain reaction to detect a monoclonal
gene rearrangement is the preferred method for confirmation of
subtle changes in lesions lacking sufficient histological evidence
of lymphoma.
Take home messages
- Thyroid fine needle aspiration is an effective diagnostic tool
and should be performed preoperatively in all solitary
nodules
- The use of thyroid frozen section is disputed among both
pathologists and surgeons; as yet, there is no consensus and this
issue should be discussed with the multidisciplinary team at a
local level
- In a follicular neoplasm, the capsule should be extensively
sampled to identify any foci of capsular or vascular
microinvasion
- In the microscopic examination of follicular lesions, the
follicular variant of papillary carcinoma should be
considered
- Several new immunohistochemical markers are reported to be
helpful in the diagnosis of solitary nodules, but their use in
routine practice is still being
evaluated
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CONCLUSION
The pathology of the thyroid gland presents the pathologist with a
particular set of diagnostic problems. If best practice and the
minimum data set guidelines are adhered to, the correct diagnosis
should be reached in most cases. Newer techniques such as
immunocytochemistry can certainly be helpful in more difficult
cases but, as in all areas of pathology, histological features take
precedence and good communication with the relevant clinical
colleagues is paramount.
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