Tarik M. Elsheikh, MD
Director of Cytology
PA Labs, LLC
Ball Memorial Hospital
Muncie, IN.
Follicular Lesions of the Thyroid: Classification and Criteria
Introduction
Fine needle aspiration cytology (FNA) of the thyroid gland is primarily a “screening
test”, but is also diagnostic in many conditions. Its main priority is not to miss too many
cancers. High sensitivity, therefore, coupled with low false negative results is what most
pathologists and clinicians stride towards achieving. Surgery for benign disease is well
accepted by clinicians, as they understand that a major role of FNA is to provide a
relative risk of malignancy, for the clinician and patient, on which they can base their
management decision upon, i.e. surgery vs. watching.1 I will discuss, in this presentation,
the differential diagnosis of follicular lesions, cytologic criteria, terminology recently
suggested by the Papanicolaou society, classification, and clinical implications of various
diagnostic entities.
Follicular lesions
Follicular lesions of the thyroid represent the most problematic area of thyroid FNA. The
major entities included in the differential diagnosis are hyperplastic /adenomatoid nodule,
follicular neoplasm (adenoma and carcinoma), and follicular variant of PTC.
Table 1: Differential diagnosis of Follicular lesions
• Hyperplastic/adenomatoid nodule
• Follicular Neoplasm
– Follicular adenoma
– Follicular carcinoma
• Follicular variant of Papillary carcinoma
In general, smears containing abundant colloid and few cells are more likely to be benign
(zone I), whereas markedly cellular aspirates with scant or absent colloid are more likely
to be neoplastic (zone II) [figure modified from Demay. Art & Science of Cytopathology,
1996].
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Nodular
Goitre
Cellular Nodule
Follicular
Neoplasm
II
I
Colloid
III
Cells
Some aspirates, however will show features that fall in the middle, sometimes refered to
as the grey zone (zone II), i.e. moderate amount of colloid and moderate cellularity, and
may be difficult to classify as either benign or neoplastic.
Interobserver variability
Several studies have examined the agreement in distinguishing follicular neoplasm (FN)
from cellular hyperplastic nodule.2-4 Areas of greatest debate and confusion included the
terminology and criteria employed in diagnosing FN. Differences in terminology involve
mainly the use of 2 diagnostic categories (i.e. follicular lesion and FN) versus one
category. Criteria for the diagnosis of FN varied from strict to none other than cellularity.
For example, the proportion of microfollicles needed to establish a FN diagnosis varied
from none to predominant. There was no clear definition of how cellular an aspirate
needed to be in order to be classified as “hypercellular”. There were also major
disagreements in recognizing colloid, especially when it had a watery-thin appearance.
Hyper plastic nodule
Hyper plastic nodule (HN) is characterized by the presence of abundant colloid and
variable number of follicular cells. Often there is evidence of oncocytic metaplasia and
degenerative changes including macrophages and old blood. The follicular cells are
predominately arranged in flat sheets with a honeycomb configuration. The presence of
few microfollicular structures is accepted. Occasionally, large balls and microtissue
fragments are present, especially when larger gauge needles are used. The nuclei are
uniform in appearance and approximate the size of RBC’s. They show finely granular
chromatin with rare small nucleoli. There is minimal nuclear overlapping and crowding.
Colloid, when dense, is easy to recognize. It has a dark blue-violet-magenta appearance
on Diff Quik stain, while stains dark green-orange with Papanicolaou (figure). Thin
colloid has a blue-violet appearance on DQ, and light green-orange appearance on Pap
stains. It often shows cracks and folds imparting a “thin membrane” or a “crazy
pavement” look. Thin colloid, however, maybe difficult to recognize in Papanicolaou
stained specimens and bloody specimens, where it can be easily confused with serum. It
may also disappear completely in thin-layer preparations.5
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Follicular neoplasm
Using specific cytologic criteria, Kini et al. reported a 75% accuracy rate in the diagnosis
of follicular carcinoma (FC)6 Most other studies, however, could not reproduce such
accuracy.7 In our opinion, and those of most experts in the cytology field, FNA can not
distinguish between follicular adenoma and follicular carcinoma. Histologic confirmation
is needed in such cases in order to demonstrate the presence of capsular and /or vascular
space invasion. There are, however, several cytologic features reported to be associated
with increased cancer risk (40-60% cancer risk). These features include an increased
nuclear size (at least twice the size of RBC), marked nuclear atypia including significant
nuclear pleomorphism and irregularity, significant nuclear overlapping, and
predominance of microfollicular structures (involving > 75% of thyroid clusters).4, 8-11 It
is important to emphasize, however, that the mere presence of microfollicles is not
equated with neoplasia. In fact, studies have shown that microfollicles associated with no
atypia had a low cancer risk (6%), and that microfollicles lacking nuclear overlap and
mixed with abundant colloid had a 0% chance of harboring cancer.
From review of the literature, our proposed cytologic criteria for a specific diagnosis of
follicular neoplasm include high cellularity and scant colloid. In addition, there is
prominent microfollicular and/or syncytial arrangement, involving greater than 50-75%
of the cellular groups. There is prominent nuclear overlapping and crowding of the
follicular cells with noticeable uniform appearance. Significant nuclear atypia may or
may not be present, and includes nuclear enlargement that is greater than twice the size of
RBC, coarse and clumped chromatin and prominent enlarged nucleoli.
Challenges in the diagnosis of Hyperplastic/Adenomatoid nodule (HN)
Clearly, one of the most difficult problems in thyroid cytology is distinguishing HN with
little colloid from FN with some colloid.12 As previously mentioned, the mere presence
of microfollicles is not diagnostic of FN, as microfollicles may be focally seen in 5-10%
of HN. Up to 30% of HN’s are highly cellular, while 15-20% of cases show scant colloid.
Although degenerative changes are often associated with HN, they may be found in up to
30% of FN. A definitive diagnosis of HN should not be made in the absence of colloid.1,
12-14
Challenges in the diagnosis of Follicular Neoplasm
Low cellularity may be encountered in aspirates of FN due to poor biopsy technique, or
due to macrofollicular architecture, yielding prominent colloid and scant follicular cells.
Some FN may also be highly vascular, yielding abundant blood with rare follicular
groups showing prominent nuclear overlapping and/or microfollicular structures.4
PSC Approach to Grey Zone and Terminology
Although the terms “Follicular Lesion” and “Follicular Neoplasm” are used
interchangeably by some authors, we do not consider them synonymous. According to
literature review, lesions categorized as “indeterminate” account for 5 to 42% of FNA
diagnoses. We do not recommend the use of “Indeterminate” as a stand alone diagnosis,
as its meaning has not been standardized, and may be interpreted in different ways.
“Indeterminate” has been used by different authors and institutions to refer to a variety of
diagnoses including FN, follicular lesion, suspicious for malignancy, and atypia not
otherwise specified. Redman et al. surveyed 133 clinicians (Endocrinologists, Surgeons,
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Thyroid specialists), in order to determine the implications of FNA diagnoses on
management options.15 Clinicians opted for repeat FNA in 98% of the responses, when
the cytologic terminology was “Non-diagnostic”. “Suspicious” diagnoses elicited a 96%
surgical excision response. Clinicians, on the other hand, chose repeat FNA (58%) and
surgery (32%) for “Indeterminate” diagnoses. They selected repeat FNA (37%) and
surgery (52%) for “Atypical” designations.15 This study clearly demonstrated that
confusion arose with “Atypical” and “Indeterminate” diagnoses. “Indeterminate” was
confused with “non-diagnostic” in some cases, and “Atypical” was too ambiguous and
treated as “Suspicious” in many cases. The majority of clinicians, on the other hand,
correctly interpreted “Non-diagnostic” and “Suspicious” diagnoses.
Follicular Lesions and Terminology
Two European studies found no malignancies on followup of FNA’s diagnosed as FL and
FN. 16 17 The authors advocated a less aggressive approach to management, i.e. clinical
followup. FN, however, was loosely defined in these studies as hypercellular smears
associated with scant colloid, and presence of microfollicles. There was no mention of
percentage of microfollicle formation, nuclear features of follicular cells, or other
architectural patterns. Permissiveness in applying strict criteria to the diagnosis of FN can
lead to significant reduction of malignancy rate on followup. Cytologic features such as
architecture and nuclear atypia, in addition to colloid and cellularity, should be
incorporated into our criteria, in order to better define and classify those lesions falling in
the grey zone. The utilization of more strict criteria can help “shrink” the grey zone
further, resulting in less number of cases classified as “indeterminate”.
Cellular lesion can not rule out FN
In USCAP 2006, the Papanicolaou Society of cytopathology introduced the terminology
of “Cellular lesion can not rule out FN”, in dealing with lesions falling in the grey zone.
This terminology was chosen in order to stay away from the confusing terms of follicular
lesion, indeterminate, atypical, etc. This designation is employed when the major
differential diagnosis is hyperplasic nodule vs. FN. These aspirates are often highly
cellular with scant colloid. There is admixture of flat sheets and microfollicles/syncytial
fragments. Minimal nuclear overlapping and crowding may be present. This diagnosis is
also rendered when smears from different passes show mixed cytologic findings ranging
from “benign” to “possible FN”. Bloody specimens of low cellularity, but containing
microfollicles and prominent nuclear overlap (highly vascular lesions) would also be
included in this category.
Follicular variant of papillary carcinoma (FVPC)
These aspirates mainly display branching monolayered sheets, which is considered to be
a significant low power discriminating feature from follicular neoplasms.18 Occasionally,
there is predominance of microfollicles similar to follicular neoplasm, but appreciation of
the nuclear features of PTC will usually establish the diagnosis of FVPC in most cases.
Not infrequently, FVPC may show abundant colloid or paucity of nuclear features of
PTC, leading to a false negative diagnosis of benign thyroid disease or follicular
neoplasm. As a matter of fact, FVPC is only second to sampling error as the most
common cause of false negative diagnoses in thyroid FNA. Wu et al. reported 11 false
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negative cases of FVPC, where 6 cases were attributed to sampling error (micropapillary
carcinoma), and 5 cases showed focal atypia in a background of abundant colloid and
cystic change.19
Suspicious for PTC
We issue a diagnosis of “suspicious for PTC” when focal nuclear features of PTC, such
as focal nuclear grooves associated with nuclear enlargement and powdery chromatin, are
appreciated in an aspirate. The combination of flat syncytial sheets, nuclear enlargement,
and fine powdery chromatin, was found to be the most sensitive, whereas the
combination of nuclear enlargement, fine chromatin and nuclear grooves was found to be
the most specific, in establishing the diagnosis of FVPC.20 Intranuclear psuedoinclusions
are seen in less than half of FVPC cases.20 It is important to recognize “suspicious for
PTC” as a distinct category, and not to lump it with other” indeterminate” or “follicular
neoplasm” diagnoses, because of its substantially greater association with malignancy on
surgical followup. Logani et al. and Wu et al. reported cancer followup rates of 77% and
75%, respectively, when rendering such diagnoses.20, 21 This is in contrast to the cancer
followup rate of 10-30% typically associated with indeterminate or follicular neoplasm
diagnoses. With such an increased risk of malignancy, clinicians and patients may
consider total thyroidectomy as another option, in place of lobectomy. Another
management choice includes lobectomy with intra-operative consult, which has been
shown to be helpful in additional 30% of cases.22
Suspicious for malignancy
We use this category when the cytologic features are suggestive of a specific malignancy,
but a definitive diagnosis can not be rendered. A definitive diagnosis of malignancy is
often not rendered in these cases due to quantitative reasons ( i.e. malignant appearing
cells, but limited cellularity) or qualitative reasons (i.e. focal or less than well developed
features of malignancy, or an atypical lymphoid population). The most commonly
encountered example of this diagnostic category is “suspicious for PTC”.
Diagnostic categories (as proposed by PSC)
1. Unsatisfactory
2. Benign, non-neoplastic
3. Cellular lesion, can not rule out follicular neoplasm
4. Follicular Neoplasm
5. Suspicious for malignancy
6. Malignant.
Clinical implications
The majority (70-80%) of FNA’s classified as FN, are neoplastic on histologic followup.
Using strict cytologic criteria, diagnoses of “FN” and “cellular nodule can not rule out
FN” show cancer on histologic followup in 30% and 10% of cases, respectively. Using
permissive criteria, however, and when FN and indeterminate diagnoses are combined,
cancer is found in 20% of the cases. It is important, therefore, to separate FN as a distinct
category from cellular hyperplastic nodule or other indeterminate follicular lesions. Most
clinicians will recommend excision for FN, and accept the fact that the cytologic
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diagnosis is probabilistic and may be benign on followup. 22-25 Clinical followup or repeat
FNA is recommended for those cases classified as “cellular nodule can not rule out FN”.
Assessment of probability of finding cancer on followup thyroidectomies
Wu et al Examined 401 FNA’s with followup surgical excision, and calculated the cancer
rates and risks associated with various cytologic diagnoses. Providing this data to
clinicians and patients may be extremely helpful in deciding on management options.
FNA Diagnosis
Cancer Rate Cancer Risk*
Benign non-neoplastic
3%
-Cellular lesion, R/O FN
14 %
5X
Follicular neoplasm
33 %
11 X
Suspicious
56 %
20 X
Malignant
100 %
Inadequate/unsatisfactory
12 %
Cancer risk is compared to benign NN diagnosis
Thyroid FNA Diagnosis and management options
Management options
Diagnosis
Followup
Benign
Cellular lesion, R/O FN
Followup
or
repeat FNA
Neoplasm
Excision / Lobectomy
Suspicious
Excision
Malignant
Excision
Unsatisfactory
Repeat FNA (US)
Summary
Thyroid FNA is primarily a screening tool, therefore, a conclusive diagnosis is not
always required. A cytologist’s role is to minimize the number of indeterminate
diagnoses without yielding unacceptable false negative and false positive rates. FNA can
assign diagnostic probabilities that can help guide patient management in many cases.
Although the utilization of these “diagnostic categories” is encouraged, they should not
be used alone. Diagnoses should be qualified, when applicable, with an appropriate
differential diagnosis. Individual centers should monitor their own diagnostic accuracy
and cancer risks, and provide these data to their clinicians, to help guide their
management. Recommendations for followup may be included in the report, if acceptable
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to clinicians. The use of the term “Atypical” or “Indeterminate” as a stand alone
diagnosis is not recommended. Their meanings are not standardized and may be
interpreted in different ways. Close cooperation between pathologists and clinicians is
essential, so that the terminology used in the report and its clinical implications are
clearly defined.
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