VALIDITY OF FINE NEEDLE ASPIRATION CYTOLOGY IN THE DIAGNOSIS OF THYROID DISEASES

Introduction: Fine needle aspiration cytology (FNAC) is regarded as an important first line investigation for evaluation of thyroid swellings. It has a diagnostic accuracy of over 90% in terms of specificity, sensitivity and predictive values. Objective: The aim of this study was to determine the validity (specificity, sensitivity positive/negative predictive values) of FNAC in the diagnosis of thyroid diseases. Methods: FNAC reports and histopathology reports of patients with thyroid diseases, who have undergone surgery, were retrospectively retrieved from data banks in the Department of Pathology, University of Sri Jayewardenepura, from August 2004 to August 2006. In addition a prospective study in patients with thyroid diseases who came to this unit for FNAC from June to September 2006 was carried out. In all the cases the histological diagnosis was compared with the cytological diagnosis and the results were documented and analyzed by using standard statistical methods. Results: FNAC has an acceptable validity in the diagnosis of thyroid diseases with high specificity (86.74%), sensitivity (84.05%), positive predictive value (84.05%) and negative predictive value (86.74%).


Introduction
Thyroid diseases are a common clinical problem worldwide.The incidence of thyroid cancers is approximately 122,000 new cases per year worldwide. 1In Sri Lanka, thyroid cancers are responsible for 4.6% of all cancers and account for 6.9% of cancers in females. 2 Fine Needle Aspiration Cytology (FNAC) is widely accepted a simple and minimally invasive investigation in the assessment of nodular thyroid disease and has shown to have a high sensitivity in diagnosing malignancies as well as in the evaluation of thyroid nodules. 3t present, in Sri Lanka, FNAC is used as a routine first line diagnostic method to assess thyroid diseases.As it is a simple technique which can be carried out in the out-patient department, it can be readily repeated if necessary, and has good patient compliance.FNAC is therefore used as the key investigation in combination with radiological investigations in many recognized centers to assess discrete thyroid swellings to diagnose or exclude a malignancy.FNAC has also reduced the need of isotope scans & the necessity for surgery.Thyroid nodules that are considered for FNAC include firm, palpable, solitary nodules, nodules associated with suspicious clinical or ultrasonographic features, dominant nodules in a multinodular goiter, recurrent cystic nodules, and a nodules associated with palpable lymph nodes. 4everal studies based on different methodologies have reported a high degree of sensitivity, specificity, positive and negative predictive values of FNAC for thyroid. 5It has a diagnostic accuracy of over 90% in terms of predictive value, sensitivity and specificity in the diagnosis of malignancy.Accuracy of the diagnosis has been shown to increase with the experience of the pathologist. 5,6he procedure involves aspiration of the thyroid by a 10ml syringe attached to a fine needle (23-25 gauged).Tissue sludge, tissue fragments and blood are aspirated and smeared on glass slides.One slide is usually air dried and stained with May-Grunwald-Giemsa stain and the other smear is fixed in alcohol and stained by Haematoxylin and Eosin stain or Papanicolaou stain.][9] Although FNAC is considered as a highly sensitive investigation, it is always reproducible.The false positive rate is low with respect to malignancy, but there is definitely a higher false negative rate especially in follicular neoplasm. 9,10here can be high rate of unsatisfactory aspirates particularly in cystic swellings due to a low cellular yield.It can result in non representative samples especially in dominant nodules of a multinodular goiter unless the surgeon marks the dominant nodule.The use of ultrasound guided aspirations has helped to overcome this problem in obtaining representative samples.Although FNAC entails some difficulties in small thyroid enlargements, and sampling errors in inexperienced hands it can be a rapid and useful diagnostic investigation.
The current study was carried out to assess the sensitivity, specificity, & negative and positive predictive values of FNAC.Sensitivity: the proportion of the true positives among those having the disease.Specificity: the proportion of true negatives among those not having the disease.Positive predictive value: the probability of the person having the disease when the test is positive.Negative predictive value: the probability of the person not having the disease when the test is negative.

Material and methods
The study was a descriptive study carried out on patients who were admitted to Colombo South Teaching Hospital (CSTH).FNAC reports and histopathology reports of patients with thyroid diseases, who have undergone surgery, were retrospectively retrieved from data banks in the department of pathology, USJP, for a period of three years from August 2004 to September 2006.In all the cases, the histological and cytological diagnoses were documented, compared and analyzed using standard statistical formulae (Non-probability sampling).Sample size was calculated using standard formula with the minimum sample size estimated as 50.

Study instruments Data extraction sheets. (FNAC reports & histological reports). 3. Data collection proper
Data from the FNAC report of each patient and the corresponding histology report was collected and tabulated for analysis.
(2) Data Analysis Data analysis was carried out manually according to the standard analytical methods.

Sensitivity = a
Positive predictive value = a a+c a+b

Results
Data from 158 patients who underwent FNAC and who had subsequent histology results were collected and of the above 6 were excluded due to inadequate information given for a conclusive diagnosis.Histological and cytological analysis showed 83 (54.60%) non-neoplastic conditions and 69 (45.39%) neoplastic conditions.At present in Sri Lanka, FNAC is used as a routine diagnostic method for thyroid disease.The current study is the first in Sri Lanka to ascertain the specificity, sensitivity and the positive and negative predictive values of FNAC.
The FNAC diagnoses included 13/152 multinodular goiters, 8/152 thyroiditis, 62/152 colloid nodules, 22/152 papillary carcinomas, 46/152 follicular neoplasm, 1/152 Hurthle cell neoplasm.The most common non-neoplastic thyroid condition was colloid goiter, which accounted for 78% of all non-neoplastic conditions and is similar to the published data. 8Out of the neoplastic conditions, follicular neoplasms are the commonest and it includes both follicular adenoma and follicular carcinoma.Other researchers have found papillary carcinoma as the commonest.
In the current study, the sensitivity, specificity, positive predictive value and negative predictive value were 84.05%, 86.74%, 84.05%, and 86.76% respectively.These values are comparable with published data and comparison is given in Table 5.According to the above results of the Asian, English and European studies the sensitivity, specificity, positive and negative predictive values of FNAC in detecting thyroid dise ases ranges from 84-98%, 67-100%, 34-92%, and 65-94% respectively.The determinant factors for such a wide range could be the methodological differences, the technique of aspiration, preparation of smears, the nature of the nodule, condition of the sample and experience of the reporting pathologist.In the current study, all the smears were fixed in alcohol and stained with Haematoxylin and Eosin.Aspiration and interpretation were both done by histopathologists.In some comparable studies, air-dried smears also prepared and cytological interpretation was carried out by cytopathologists, who are specialized in cytodiagnosis.

Discussion
There were 11(7.2%)false positives in the current study (Table 5, Figure 3).This rate is higher than the reported false positive rates ranging from 0-4%.The factors attributing to these could be poor aspiration technique and smear preparation technique, poorly localized lesions, inadequate clinical history and investigation findings, coexisting dual pathology, noncompliance of the patients, and differences in interpretation by the pathologists.The majority of false positive cases (10/11) were due to misinterpretation of thyroiditis and colloid goiters as follicular neoplasms.This is a diagnostic pitfall where long-standing multinodular goiter or thyroiditis, exhibit areas of follicular hyperplasia, if sampled, can mimic a follicular neoplasm.There was one (1/11) false positive FNAC diagnosis of papillary carcinoma for autoimmune thyroiditis.This may be due to the overlap of cytological features observed in thyroiditis and papillary carcinoma.False negative rate was 11(7.2%) in the current study, which is slightly higher than the highest reported incidence of 6%.The majority of these were follicular neoplasms (8/11) which were misinterpreted as colloid nodules.There were three (3/11) follicular carcinomas diagnosed as colloid nodules.This is expected with small thyroid nodules where the aspiration, which is a blind procedure, may not be representative of the lesion and may contain material from the surrounding non-neoplastic colloid nodules.This problem can be overcome by using ultrasound guided needle aspiration, which will be more representative of the lesion.Cytological diagnosis of follicular adenoma versus carcinoma is not possible with fine needle aspiration and diagnosis is dependent on histological assessment for capsular or vascular invasion.In some studies follicular lesions were excluded from the calculation where this group was included in the neoplastic group.In this study, both follicular adenoma and follicular carcinoma were included in the follicular neoplasm category with a sensitivity of 68.18% and a positive predictive value of 65.21%.
More than 85% of specificity has been shown in diagnosing colloid goiters, thyroiditis, multinodular goiter, follicular neoplasm, and papillary carcinoma.
Positive predictive value for multinodular goiter (50%) and follicular neoplasm (65.21%) are quite low compared with other thyroid conditions.colloid goiter there are small and large follicles so when only small follicles are aspirated it shows a vague follicular configuration mimicking follicular neoplasm.Low positive predictive value of multinodular goiter may be due to low prevalence of the disease in the sample.Negative predictive values for colloid goiter, thyroiditis, multinodular goiter, follicular neoplasm, and papillary carcinoma are more than 85%.

Limitations
There were 11 false negative cases, the majority of cases being due to failure to differentiate colloid goiter from follicular neoplasm.This is expected with small thyroid nodules where the aspiration, which is a blind procedure, may not be representative of the lesion and may contain material from the surrounding non-neoplastic areas.This problem can be overcome by using ultrasound guided needle aspiration, which will have a more representative cell yield.There were 11 false positive cases in our study.This was mainly due to misinterpretation of thyroiditis and colloid goiter as a follicular neoplasm.Cytological diagnosis of follicular adenoma versus carcinoma is not possible with fine needle aspiration and diagnosis is dependent on histological assessment for capsular or vascular invasion.In some studies follicular lesions were excluded from the calculation where this group was included in the neoplastic group.In our research we included both follicular adenoma and follicular carcinoma in the neoplastic group.

Conclusion and Recommendations
FNAC has an acceptable validity in the diagnosis of thyroid diseases by means of high specificity (86.74%), sensitivity (84.05%), positive predictive value (84.05%) and negative predictive value (86.74%).Colloid goiter was found to be the commonest non-neoplastic condition in the current study.Out of the neoplastic conditions, follicular neoplasms were the commonest followed by papillary carcinoma.
From the current study, the following recommendations can be made to improve the sensitivity, specificity, negative and positive predictive values that determine the validity of the FNAC in the diagnosis of a thyroid disease in Sri Lanka.
 Adherence to recommended techniques. Ultrasound guided aspiration for sample collection in small, ill-defined nodules and in a dominant nodule of a multi nodular goiter.
 In instances, where the FNAC results are not satisfactory in the diagnosis of a particular diseases condition, use of other supplementary investigations such as radiology and thyroid autoantibody levels.
 In all instances, a proper clinical history and investigation findings should be provided to the reporting pathologist.
 Training courses and workshops in cytopathology to enhance knowledge and expertise of the pathologists and improve reproducibility, sensitivity and specificity of FNAC.
Figure 1b.Histology of the colloid nodule -Colloid filled follicles lined by flattened thyroid epithelial cells (Haematoxylin and Eosin x400)

Table 1 .
The numbers of each neoplastic and non-neoplastic disease on FNAC and histology.

Table 2 .
Comparison of FNAC diagnosis with the corresponding histological diagnosis.

Table 3 .
Validity of FNAC in the diagnosis of each thyroid disease.

Table 4 .
Results of false positives and false negatives.
Example of a true negative-Diagnosis of a colloid nodule (Figures 1a & 1b

5 .
Specificity, sensitivity, positive (PPV) and negative (NPV) predictive values in previous studies compared with this study.