Correlation of Ultrasonography Guided Fine Needle Aspiration Cytology with Postoperative Histopathology in Diagnosis of Thyroid Nodule

Background: Fine-needle aspiration cytology (FNAC) is recommended as a decisive diagnostic step in the workup of patients with nodular thyroid disease. Unfortunately, FNAC can miss malignancies in smaller and deeper nodule. Ultrasound guided FNAC (US-FNAC) can reduce this error in suspicious thyroid nodule. Objectives: To find out the correlation of USG guided FNAC with postoperative histopathology in diagnosis of thyroid nodule. Methods: After obtaining clearance and approval from Institutional Review Board, all 45 patients of thyroid nodule who were admitted in the Department of Otolaryngology – Head & Neck surgery of Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka from April 2017 to August 2018 and had fulfilled the inclusion and exclusion criteria were selected for the study. Each patient was assessed before surgery by USG guided FNAC and post operatively


Introduction:
Thyroid nodules are common in adults, which may be detected by palpation in 3-7% of patients. The prevalence may be raise as high as70% or more, if sensitive imaging such as ultrasonography is being employed. Most of these thyroid nodules are benign in nature. To avoid unnecessary surgery ultrasono-graphy and FNAC is being used as a diagnostic tools to differentiate between malignant and benign lesions. Although accurate diagnosis between follicular carcinoma and follicular adenoma is difficult by FNAC 1 .
In patients with nodular disease FNAC is widely recommended as an initial and crucial test to select those patients who require excision of the lesion and subsequent histologic diagnosis. If FNAC proves to be either suspicious or malignant, surgery is indicated. The optimal diagnostic strategy is aiming to avoid surgery in patients with benign thyroid disease, while at the same time performing prompt surgical treatment of patients with thyroid carcinoma. To achieve this, FNAC must score high on test characteristics. This can be achieved by USG guided aspiration. 2 5-38% of clinically detectable thyroid nodules are malignant. Of the initial screening tests for patients with thyroid nodules, FNAC is widely used. It is used to differentiate benign from malignant thyroid nodules and helps preoperatively in selecting patients for surgery. But USG guided FNAC may increase its accuracy & will reduce unnecessary thyroid surgeries 3 .
Historically, the diagnostic criteria and reporting nomenclature of FNAC is varied internationally. These are epitomizedby the North American National Cancer Institute(Bethesda) terminology (2007), Italian (2007,2014), Australian (2014), Japanese (2013) and British Thyroid Association/Royal College of Pathologists guidelines (2002, 2007, and 2016). The terminology for non-diagnostic, benign, malignant and suspect for malignancy is similar across each of these classifications with minor differences inemphasis for the equivocal/indeterminate category. Numerical categories increase accuracy, aid local audit, allow comparison with other centers including internationally and can guide discussion on further management 4 .
The aim of this study was to identify the correlation of USG guided FNAC with postoperative histopathology in thyroid nodule. Sensitivity, specificity, positive predictive value, negative predictive value and accuracy of USG guided FNAC was also evaluated after the surgical removal and histopathological diagnosis. FNAC was carried out under ultrasound guidance so that the needle position can be controlled and samples can be regarded as representative, even within very small nodules. A written "informed consent" was taken. Experienced radiologist performed an ultrasonography guided fine needle aspiration with the use of 10 ml disposable syringe with 23-gauge needle by using a perpendicular puncture. When the needle tip reached the target nodule, the needle was observed as a small echogenic spot within the nodule on the US monitor. After placing needle tip in the appropriate area of the target nodule, sampling was commenced. After obtaining a sample, the specimen was mounted immediately onto a glass slide. Specimens were fixed with 95% ethanol and were sent for cytological evaluation. Reporting was done by "the Bethesda system for reporting Thyroid Cytopathology" (TBS-RTC). Then all other relevant investigations were done &informed written consent was taken from all patients for surgery except Bethesda class I. Patients with non-neoplastic FNAC diagnoses underwent surgery because of pressure symptoms or unwillingness to carry out follow-up. All patients, reported as Bethesda class II -VI were operated under general anesthesia. Postoperative histopathology report was obtained & correlation was done with preoperative USG guided FNAC. Data were collected by interview and by laboratory investigation using structured data collection sheet.

Statistical Analysis:
Data were processed and analyzed using Microsoft Excel 2016(Microsoft office professional plus 2016). Correlation of USG guided FNAC with histopathology was done by Pearson's correlation coefficient.

Results:
The mean age of the respondents was 33.33 years with SD±10.84. Most of them were female, house-wife& living in rural areas. Highest number of respondents were diagnosed as Bethesda class II in USG guided FNAC.   On histopathology 27/45 (60%) specimens were benign lesions.

Discussion:
USG guided FNAC is a least invasive, simple and most accurate method to evaluate thyroid nodule. Ultrasound guidance allows continuous visualization of the needle during insertion and sampling with resulting in pinpoint accuracy with a high level of safety.
In the present study, age of the patients ranged from 18-65 years with a mean age of 33.33years with SD±10.84 years. These findings are in conformity with Irfanet al. 2014. They reported mean age 33.35 ± 11.77 years 5 . This finding may be due to prevalence of endemic goiter is more in younger age group.
In our study most of the respondents were female. This finding is similar to the finding of Haberal et al.2008.where 83.8% respondents were female and 16.2% respondents were male 6 . Goiter is predominantly present in female as there is estrogen receptor in thyroid which stimulates goitrogenesis.
We found that occupation of the most of the respondents were housewife. This finding has concordance with Kocak et al. 2013. where Goiter prevalence was highest in agricultural workers (35.3 %) and housewives (32.2 %), and lowest in the unemployed (3.5 %). They concluded that there was significant difference between occupational groups and goiter prevalence 7 . Our most of respondents were female and goiter is more common among them. This may explain why goiter is common in housewives.
In our study most of respondents were from rural area. It defer from study of Irfan et al. 2014. where 68.75 % respondents were from urban area and 31.25% respondents were from rural area 5 . Lack of health education, more consumption of goitrogen may be the cause of higher goitrogenesis in rural areas.  A review of studies that evaluated the role of US-FNAC in the detection of thyroid cancer revealed a sensitivity of 76%-98%, specificity of 71%-100%, false-negative rate of 0%-5%, false-positive rate of 0-5.7%, and overall accuracy of 69%-97%. 12 Our results have shown Pearson's correlation coefficient was 0.85. Which reflects USG guided FNAC has very strong positive relationship with post operative histopathology& in our study sensitivity and specificity of US-FNAC was 94% and 93% respectively. The PPV 88%, NPV 96% and the diagnostic accuracy was 93%.
In our study most of the malignant lesion was papillary carcinoma (9), rest are follicular carcinoma (6) and medullary carcinoma (1%). (Table 2&4) Some of the potential advantages of US-FNAC in the evaluation of nodular thyroid disease is guiding the needle to take samples from non-palpable nodules. The major benefit of this technique is accurate sampling of small or multiple nodules. Certain sonologic features of thyroid lesions are predictive of malignancy and hence such lesions can be accurately aspirated 14 . Another crucial benefit of using US-FNAC is significant reduction in the number of inadequate aspirates 11 . It is possible that reduced number of inadequate aspirates may potentially lead to increase in the yield of cancer and the diagnostic accuracy of FNAC. The rate of inadequate specimens in our study was 4.4 % (2 out of 45 samples), which is comparable to the above quoted studies. (Table I & IV) We categorized cytological results into six classes according to Bethesda description for thyroid cytology. Such categorization of FNAC smears results is necessary to allow clinicians to use cytology results to guide patient management with specific reference to the need for thyroidectomy. Our study showed that use of better reporting system (Bethesda methods) there was relatively low rate of non diagnostic or inadequate smears, allowing it to be useful method for determining the treatment plan for non palpable smaller thyroid nodules. In our study the incidence of adequate specimens was 43/45 (96%) with US-FNAC as compared to the Kim et al.
2009 who found it 81% in his series of 201patients 15 .

Conclusion:
USG guided FNAC isnearly as accurate as histopathology in diagnosis of thyroid nodule.

Recommendation:
Small and suspicious thyroid nodule on USG should undergo an USG guided FNAC.
Inadequate smears from free hand FNAC yields accurate result by USG guided FNAC.