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Year : 2019  |  Volume : 62  |  Issue : 4  |  Page : 544-548
Risk of malignancy in Thyroid “Atypia of undetermined significance/Follicular lesion of undetermined significance” and its subcategories – A 5-year experience

1 Department of Pathology, Medanta-The Medicity Hospital, Gurgaon, India
2 Department of Head and Neck Oncology, Medanta-The Medicity Hospital, Gurgaon, India

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Date of Web Publication14-Oct-2019


Context: Atypia of undetermined significance/Follicular lesion of undetermined significance [AUS/FLUS] is a heterogeneous category with a wide range of risk of malignancy [ROM] reported in the literature. The Bethesda system for reporting thyroid cytopathology [TBSRTC], 2017 has recommended subcategorization of AUS/FLUS. Aims: To evaluate the ROM in thyroid nodules categorized as AUS/FLUS, as well as separate ROM for each of the five subcategories. Settings and Design: Retrospective analytic study. Methods and Materials: A retrospective audit was conducted for all thyroid fine-needle aspiration cytology (FNAC) from January 2013 to December 2017. Slides for cases with follow-up histopathology were reviewed, classified into the five recommended subcategories, and differential ROM was calculated. Statistical Analysis Used: z test for comparison of proportions was done to evaluate the difference in ROM among different subcategories of AUS/FLUS. The P value of less than 0.05 was taken as statistically significant. Results: Total number of thyroid FNACs reported was 1,630, of which 122 were AUS/FLUS (7.5%). Histopathology was available in 49 cases, out of which 18 were malignant (ROM = 36.7%). The risk of malignancy (ROM) for nodules with architectural and cytologic atypia was higher (43.8%) than ROM for nodules with only architectural atypia (16.7%). Conclusions: The sub-classification of AUS/FLUS into subcategories as recommended by TBSRTC, 2017 may better stratify the malignancy risk and guide future management guidelines.

Keywords: Atypia of undetermined significance, risk of malignancy, subcategory, fine-needle aspiration, thyroid cytopathology

How to cite this article:
Thakur A, Sarin H, Kaur D, Sarin D. Risk of malignancy in Thyroid “Atypia of undetermined significance/Follicular lesion of undetermined significance” and its subcategories – A 5-year experience. Indian J Pathol Microbiol 2019;62:544-8

How to cite this URL:
Thakur A, Sarin H, Kaur D, Sarin D. Risk of malignancy in Thyroid “Atypia of undetermined significance/Follicular lesion of undetermined significance” and its subcategories – A 5-year experience. Indian J Pathol Microbiol [serial online] 2019 [cited 2023 Dec 1];62:544-8. Available from:

   Introduction Top

Thyroid nodules are commonly encountered in clinical practice [1] and fine-needle aspiration cytology (FNAC) is believed to be the first and most (both cost and time) effective screening test for its evaluation and risk stratification.[2] Thyroid cancer constitutes 0.1% to 0.2% of all cancers in India with age-adjusted incidence of 1/100,000 male and 1.8/100,000 female.[3]

The Bethesda system for reporting thyroid cytopathology (TBSRTC) has standardized the reporting and introduced uniform terminology to facilitate communication [4] between pathologists and clinicians, thus providing more clinically useful inputs. Inspite of numerous advantages of TBSRTC, limitations do exist, especially in the interpretation of Atypia of undetermined significance (AUS)/Follicular lesion of undetermined significance (FLUS). AUS/FLUS is a heterogeneous category, which includes eight [1] different diagnostic criterias ranging from follicular cell atypia limited by compromised specimen to atypia owing to underlying malignancy. This heterogeneity is reflected in the wide range of risk of malignancy (6–48%) reported in recent publications.[5]

Repeat FNAC, molecular testing, or lobectomy is the usual management approach for AUS/FLUS.[6] Molecular testing has been advocated as a means to avoid unnecessary surgery in a significant proportion of these patients whose nodules turn out to be benign on surgical lobectomy.[7] However, some authors have found gene expression classifier (GEC) test less cost-effective than diagnostic lobectomy when long-term follow-up is considered.[8]

TBSRTC, 2017 has recommended sub-classification of the atypia,[6] although it has not been widely used. This study was undertaken to reclassify AUS/FLUS cases into five subcategories and then correlate with histologic follow-up in resected cases with the aim of calculating the differential risk of malignancy (ROM), so as to triage patients for different management approaches.

   Materials and Methods Top

This retrospective study was approved by Medanta Institutional Review Board, thus waiving the requirement of informed consent [Reference no. 966/2019 (Academic)].

The database of the Department of Pathology was searched for all thyroid FNAC's between January 1, 2013 and December 31, 2017. All specimens were May Grumwald Giemsa and Papanicolaou stained conventional smears and reported according to TBSRTC. FNAC smears of cases with surgical resection were reviewed by two cytopathologists who were blinded for final diagnosis.

Cytologic criteria

AUS/FLUS cases were evaluated individually for these morphologic features- nuclear enlargement, crowding/overlapping, anisonucleosis, nuclear grooves, fine chromatin, clearing, inclusions, microfollicles, and distinct anatomic edge.

On the basis of these features, cases were reclassified into five distinct subcategories as recommended by TBSRTC, 2017. The detailed criteria mentioned by Bongiovanni et al.[4] was considered.

  1. Cytologic atypia

    1. Aspirates with low cellularity showing only rare follicular cells showing nuclear atypia suggestive of papillary thyroid carcinoma [PTC]
    2. Rare atypical cells with nuclear atypia in a background of an otherwise benign nodule
    3. Diffuse but mild nuclear change.

  2. Architectural atypia

    1. Hypocellular aspirate with few microfollicles and scant/absent colloid
    2. Few smears showing predominantly microfollicles, while others showing mixed micro and macrofollicles
    3. Occasional cluster with distinct anatomic edge defined as cellular clusters with well-defined row of cuboidal or columnar cells.

  3. Cytologic and architectural atypia

  4. Smears, where both these features coexist.

  5. Hurthle cell atypia

    1. Paucicellular aspirate with virtually exclusive Hurthle cell population
    2. Cellular aspirate with a nearly exclusive population of Hurthle cell in a clinical setting of Hashimoto's thyroiditis or multi-nodular goiter.

  6. Atypia – not otherwise specified (NOS)

Atypical lymphocytes that lack sufficient atypia to be interpreted as suspicious/malignant.

Histologic follow-up

Only patients who underwent surgical resection were included in sub-classification study. The aspirated nodules were matched with resected nodules by comparing site and size of nodules on radiology and histopathology. Incidental papillary microcarcinomas were excluded from the analysis.


SPSS, version 24.0 software was used. z test for comparison of proportions was done to evaluate the difference in ROM among different subcategories of AUS/FLUS. The P value of less than 0.05 was taken as statistically significant.

   Results Top

Rate of AUS/FLUS

A total of 1,630 FNAC's were performed during this 5-year study period (January 2013 to December 2017). Of these, 122 (7.5%) were diagnosed as AUS/FLUS, while 1,508 (92.5%) were placed in other definitive categories of TBSRTC. Thus, AUS/FLUS was reported in 7.5% of all thyroid FNAC.

Surgical follow-up

Of 122 cases diagnosed as AUS/FLUS, 49 patients had undergone surgical resections. The remaining 73 patients had a repeat FNAC, followed-up clinically or were lost to follow-up. Eighteen cases out of 49 had malignant outcomes, four were non-invasive follicular tumor with papillary like nuclear features (NIFTP) and 27 were diagnosed as benign nodules.

Estimation of ROM

The upper limit estimate of malignancy was calculated as 18/49 (36.73%) in AUS/FLUS category patients (which underwent surgical resection). The lower limit estimate of malignancy was speculated as 18/122 (14.7%) in AUS/FLUS category (assuming that all patients without surgical follow-up likely had benign thyroid lesions).

Estimation of ROM in subcategories of AUS/FLUS.

Sub-categorization of AUS/FLUS was done only in cases with surgical follow-up.

Approximately, 65.3% (32/49) cases were diagnosed as AUS/FLUS-both cytologic and architectural atypia. When NIFTP was not taken as equivalent to carcinoma, 14 had a malignant histopathologic outcome. Thus, ROM was 43.8% (14/32) in this subcategory [Table 1]. There were three cases of classical PTC, ten cases of FVPTC, and one case of follicular carcinoma [Figure 1]. We also calculated ROM when NIFTP was taken as carcinoma. ROM was 56.3% (18/32) as all cases of NIFTP were placed in subcategory III of AUS/FLUS [Table 1].
Table 1: Risk of malignancy for AUS/FLUS subcategories and types of carcinoma on follow-up resection

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Figure 1: FVPTC (a and b). (a) Follicular cells arranged in microfollicular pattern with nuclear crowding and enlargement (MGG, ×600). (b) Microfollicular pattern with nuclear clearing (H and E, ×400). Classical PTC (c and d). (c) Clusters with distinct anatomic edge, nuclear crowding and enlargement (MGG, ×400). (d) Papillae lined by cuboidal cells with nuclear clearing (H and E, ×200)

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In total, 24.5% (12/49) cases were diagnosed as architectural atypia. Out of these, two were malignant [one case each of classical PTC and follicular variant of papillary thyroid carcinoma (FVPTC)]; thus, ROM was 16.7% (2/12) in this subcategory [Table 1]. Rest of the cases were adenomatoid nodule [Figure 2]a and [Figure 2]b and follicular adenoma.
Figure 2: Adenomatoid nodule (a and b). (a) Repetitive microfollicular pattern (MGG ×400) (b) Closely packed microfollicles (H and E, ×400). (c) Hurthle cell variant of PTC showing single population of hurthle cells (Papanicolaou, ×400). (d) Marginal zone NHL showing monotonous population of lymphoid cells (MGG, ×400)

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Four cases were placed in AUS/FLUS – Hurthle cell atypia [Figure 2]c, out of which one was Hurtle cell variant of PTC on histologic follow-up.

Single case placed in AUS/FLUS- (NOS) (lymphoid) [Figure 2]d was confirmed as marginal zone non-Hodgkin lymphoma histologically.

None of the cases in our study was placed in AUS/FLUS–cytologic atypia alone subcategory.

   Discussion Top

TBSRTC has initially recommended that the use of AUS/FLUS should be limited to approximately ≤7% of all FNAC's.[6] However, this figure was according to initial minimally published data [2],[4] and was believed to prevent excessive use of this category. Current data suggest a higher rate for AUS/FLUS with 2017 TBSRTC stating that 10% might be a more realistic limit.[6] AUS/FLUS rate of 7.5% in this study is compatible with recent TBSRTC guidelines.

Rate of malignancy in AUS/FLUS was estimated to be 5–15% initially,[9] but this range had also seen an upward shift with a wide range reported in the literature after TBSRTC was introduced.[2],[10] Gan et al.[10] and Kim et al.[2] found this range to be 15.7 - 81% and 6–48%, respectively.

Present study suggests an estimated malignancy rate of 14.7–36.7%. The lower limit estimate of ROM depending on the total number of AUS/FLUS underestimates ROM owing to verification bias, while the upper limit estimate according to histologic follow-up of resected cases suffers from selection bias. Thus, calculation of actual ROM is difficult and value probably falls between these two estimates.[6]

According to Gan et al.,[10] average malignancy risk for all studies considering surgical resection was 34% and is quite similar to our upper limit estimate of ROM for AUS/FLUS.

Some studies have also found an inverse relationship between rate of AUS/FLUS and ROM.[4] The higher AUS/FLUS and lower ROM rates were ascribed to the inclusion of cases with poor preservation. The higher ROM in our study could possibly be owing to the complete exclusion of cases with compromised specimens, and the selection bias as only cases with worrisome clinical or radiologic findings were taken up for surgery.

TBSRTC, 2017 has calculated ROM with inclusion and exclusion of NIFTP as malignancy. ROM in cases with surgical follow-up dropped from 44.9% to 36.7% with the exclusion of NIFTP in our study. Cases with NIFTP are still considered to be surgical candidates, so its inclusion still forms a prospect for future studies.

Sub-categorization of AUS/FLUS to better delineate the risk stratification in this heterogeneous category has recently been evaluated and is recommended by TBSRTC, 2017.[6]

Various subcategories have been used by different authors.[2],[11],[12],[13],[14],[15],[16],[17],[18] We have attempted to reclassify AUS/FLUS into five subcategories as recommended by TBSRTC. We found ROM (when NIFTP ≠ carcinoma) of 43.8% in subcategory III (cytologic + architectural atypia) as compared to 16.7% in subcategory II (architectural atypia alone). This was not statistically significant (P-value was 0.096). ROM (when NIFTP = Carcinoma) in subcategory III was 56.3%, as compared to 16.7% in subcategory II, the difference being statistically significant (P-value was 0.019).

None of the cases were placed in subcategory I (cytologic atypia only). According to Ahn et al.,[19] architectural + nuclear atypia subcategory has not been widely studied. Most of the studies [12],[13],[15],[17] have used cytologic atypia and architectural atypia as separate groups. Few studies [2],[18] have subcategorized AUS/FLUS similar to our study but have found ROM in cytologic atypia subcategory to be the highest. The reasons for highest ROM in cytologic + architectural atypia in our study could be because of three reasons. Most cases of malignancy in AUS/FLUS was found to be FVPTC. These cases are likely to show both architectural (microfollicular pattern) and nuclear (although minimal) atypia. Secondly, in addition to microfollicular pattern, we also included a distinct anatomic edge as one of the criteria for architectural atypia. Moreover, another reason could be low threshold for architectural atypia.

FVPTC was the most common carcinoma in AUS/FLUS group in our study. This is in concordance with other studies.[4],[18]

The management guidelines for subcategories of AUS/FLUS have not been precisely laid down and are still an area of study. Review of literature shows that ROM for nodules with nuclear atypia ranges from 32 to 76%, while it is 10–32% in nodules with only architectural atypia.[10] Our study reveals a ROM in the subcategories within this range.

A repeat FNAC or clinical and radiologic follow-up may be an adequate management plan for cases with only architectural atypia while surgery could be the best option for patients with nuclear atypia with/without architectural atypia. However, long-term prospective studies are required to determine if sub-categorization (by experienced cytopathologists) in AUS/FLUS category will help in triaging patients for surgery in resource-poor countries.

Personalized treatment approach with close and clear communication between surgeon, radiologist, and cytopathologist may further improve patient management.

   Conclusion Top

In our study, ROM for AUS/FLUS with histologic follow-up was found to be 36.7%. We found a significantly higher ROM in subcategories with nuclear + architectural atypia (43.8%) than only architectural atypia (16.7%). Thus, subcategorization of AUS/FLUS may better stratify the risk within this heterogeneous category and guide future management guidelines.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.


Authors acknowledge contribution of Mr Manish Kumar Singh in the statistical analysis of this study.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

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Agarwal S, Jain D. Thyroid cytology in India: Contemporary review and meta-analysis. J Pathol Transl Med 2017;51:533-47.  Back to cited text no. 3
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Correspondence Address:
Abha Thakur
Department of Pathology, Medanta-The Medicity Hospital, Gurgaon
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/IJPM.IJPM_319_19

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