Trucut/Core Biopsy versus FNAC: Who Wins the Match? Thyroid Lesions and Salivary Gland Lesions: An Overview.

Thyroid and salivary gland are amongst the most common sites subjected to fine needle aspiration cytology (FNAC) due to easy accessibility of these sites along with high diagnostic accuracy of FNAC. The performance of FNAC can be increased with the use of ultrasonographic guidance and rapid on-site evaluation. Cell block along with immunochemistry and other ancillary techniques further helps in clinching the correct diagnosis in certain diagnostically challenging cases. Core needle biopsy (CNB) is proposed to have a better diagnostic accuracy in certain situations. However, CNB has no definite role as an upfront first-line diagnostic technique for thyroid or salivary gland lesions. We describe role of FNAC and CNB in thyroid and salivary gland lesions with detailed discussion of advantages and disadvantages of both these techniques.

THYROID LESIONS

The reported prevalence of thyroid nodules is approximately 4–7% in the general population, and these include a wide variety of non-neoplastic, benign, and malignant lesions.[1] Early detection and distinction between benign and malignant thyroid lesions is important to guide the clinical treatment and appropriate surgical modalities. Although radiological investigations, such as ultrasonography (USG), can suggest malignancy in a thyroid lesion, these are not confirmatory. Peculiar signs on USG that suggest malignancy include taller than wider lesion, irregular nodule, hypoechogenic lesion, and microcalcification.[2] Tissue diagnosis by fine needle aspiration cytology (FNAC), core needle biopsy (CNB), or excision biopsy and histopathology is required in these cases to establish a definite diagnosis. Many of these lesions have overlapping morphological features, and at times, multiple pathologies can coexist, which pose a diagnostic challenge to the pathologist.

Role of fine needle aspiration cytology

FNAC is the first line diagnostic technique for thyroid nodules since decades. It is simple, safe, inexpensive, and has high diagnostic accuracy (97%). The sensitivity for detecting malignancy has been reported between 83% and 98% and specificity ranges between 70% and 100%. Diagnostic accuracy can be increased by rapid on-site evaluation (ROSE) of the aspirate, proper triaging of the aspirated material, and use of ancillary techniques. Importance of a skilled cytopathologist cannot be over-emphasized in the interpretation of thyroid aspirates. The inconclusive/nondiagnostic rates vary from 5% to 40%. Some of the important causes for nondiagnostic FNAC include faulty aspiration technique, faulty slide preparation, interpretation errors by inexperienced cytopathologists, and overlapping cytomorphological features. In addition, the false-positives range from 1% to 7.7%.[3]

Role of core needle biopsy in thyroid nodules

CNB is a recently popularized diagnostic technique for thyroid lesions. Some studies comparing CNB and FNAC have reported lower inconclusive/nondiagnostic rates for CNB compared to FNAC. USG-guided CNB (USCNB) further increases the diagnostic yield. However, an experienced radiologist, anesthetist, and histopathologist are required to achieve an accurate diagnosis. The complication rate varies from 0.4% to 1% with complications such as infection, nerve damage causing dysphonia, arteriovenous fistula formation, hemoptysis, and vasovagal reaction. The complications can be minimized if the procedure is performed by experienced radiologists with dedicated training who are familiar with the radiological features of various anatomical structures in the cervical region. A major limitation in the use of CNB is that, unlike the Bethesda System (TBS) of reporting of thyroid FNAC, there is no standardized system available for reporting thyroid lesions on CNBs. Only recently, a Task Force Committee of the Korean Society of Thyroid Radiology (KSThR) formulated some recommendations regarding the role of CNB in thyroid nodules based on the evidence from current literature.[4] These recommendations specify the use of CNB in different clinical circumstances.

1. First inconclusive/nondiagnostic aspirate

It has been observed that inconclusive/nondiagnostic FNAC results show a wide range of risk of malignancy rates (6.6–39.5%) on subsequent histopathology. The current KSThR guidelines recommend a repeat FNA under USG guidance in these circumstances. However, CNB can be considered as an adjunctive diagnostic means, especially in cases with repeated nondiagnostic results, as it can achieve a correct diagnosis in 86–98.9% of such cases.[5] The reported nondiagnostic rates of CNB vary from 1.1% to 40%. A combination of repeat FNA and CNB, achieving significantly better results than either technique alone, has also been suggested by some authors.[6]

2. Atypia of undetermined significance/follicular lesion of undetermined significance lesion

The category 3 lesion by the Bethesda System (TBS), i.e. atypia of undetermined significance/follicular lesion of undetermined significance (AUS/FLUS) is reported in approximately 10–20% of the cytology aspirates. The current guidelines recommend a repeat FNA for these nodules. However, even on repeating the FNAC, 1–7% cases can be nondiagnostic and a repeat AUS/FLUS report is seen in another 3.8–31.0% cases. CNB can be considered as a subsequent alternative in such cases to obtain a definitive diagnosis.[7]

3. Follicular neoplasm

FNA cannot distinguish follicular carcinoma from follicular adenoma as the differentiation is based on the histological evaluation of surgical specimens for capsular and/or vascular invasion. CNB can act as a complementary technique in follicular lesions and can differentiate encapsulated follicular neoplasms from non-neoplastic nodules such as follicular hyperplasia. However, CNB cannot always differentiate a follicular carcinoma from a follicular adenoma.

4. Calcified Thyroid Nodules

Calcified nodules of the thyroid gland are frequently encountered and are an important cause of non-diagnostic aspirates. Some authors have suggested use of CNB as an alternative diagnostic procedure for calcified thyroid nodules as it has a lower technical failure and nondiagnostic rate (1.1%; 0.7–7.7) in such cases.[8]

5. Thyroid lesions in children

Thyroid nodules are rarely seen in pediatric population, and it has been observed that a majority (67.3%) of these are cysts. A study for the role of CNB in pediatric thyroid nodules has shown relatively high nondiagnostic (13%) and inconclusive (30%) rates.[9]

6. CNB as first-line diagnostic technique

None of the available guidelines recommend using CNB as a first-line alternative to FNAC. At best, the current evidence suggests CNB as an effective diagnostic tool for thyroid nodules with suspicious USG features and a negative FNA report. More evidence from large well-designed studies is necessary before CNB can be considered a first-line diagnostic tool.

The previous studies suggest a definite complementary role of CNB in certain specific thyroid pathologies such as lymphoma, anaplastic carcinoma, and medullary carcinoma, as it has significantly higher sensitivity and positive predictive value than FNA and reduces the rate of unnecessary diagnostic surgery.[51011] Further, there is a suggested role of CNB in the preoperative diagnosis of hyalinizing trabecular tumor (HTT), as immunohistochemistry for Ki67 and cytokeratin 19 can further aid in differentiating HTT from PTC.[12] FNAC with an adequate cell block can replace CNB in such situations. In addition, CNB also significantly decreases the rate of misinterpretation of parathyroid lesions as follicular neoplasms of the thyroid.[5]

CONCLUSION

Till date, CNB has no definite role as an upfront first-line diagnostic tool for thyroid lesions; FNAC should still be used as a first-line diagnostic modality with CNB being reserved as a complementary modality for selective cases with nondiagnostic/inconclusive results on aspiration. ROSE and radiologically-guided FNAC significantly increases the diagnostic accuracy of FNA.

SALIVARY GLAND LESIONS

Traditional diagnosis in salivary gland lesions used to be achieved by excision or open biopsy, and these procedures were not free of complications such as facial nerve injury, infection, fistula, sialocele, and tumor spillage and recurrence.[13]

FNAC is being used as a first-line diagnostic technique since 1980s, as it is safe, quick, relatively noninvasive, and inexpensive technique of obtaining diagnostic material. FNA obviates the need of surgery in up to 40% of the cases and has a high specificity (94–100%) and diagnostic accuracy (86–98%).[14] FNAC without radiological guidance has relatively poor sensitivity (70–80%). Nondiagnostic/false-negative rates in FNAC vary from 0 to 37%.[15] The wide variation in accuracy rates of FNAC reported in the English literature are due to crossovers between the morphologies of benign and malignant lesions, interpretation errors in basaloid and oncocytic neoplasms, presence of rare subtypes, and double pathologies in salivary gland lesions. Positive predictive value of FNAC is less than 60% in cystic lesions. Post-FNAC glandular responses resemble those following salivary infarction with presence of squamous metaplasia, necrosis, “ghost” architecture, inflammation, and a myofibroblastic reaction, which can lead to interpretation errors both for cytopathologists as well as histopathologists. Image guidance with ROSE to assess the adequacy of sample, reporting by dedicated cytopathologists and use of ancillary techniques, especially fluorescence in-situ hybridization (FISH) and flow-cytometry, can help in optimizing FNAC results.[16]

Role of core needle biopsy

CNB was initially applied in a series of parotid lesions in the year 1999. CNB requires an operator trained in ultrasound and biopsy techniques with the use of 18 to 20-gauge needles (thicker than FNAC) with usually with 2–4 needle passes. Therefore, CNB is associated with increased cost of the initial diagnostic procedure along with delay in diagnosis compared to FNA. It is more invasive, requires local anesthesia, and is associated with more complications such as hemorrhage, hematoma, facial nerve injury, and tumor seeding along the needle tract.[17] However, advantages of CNB over FNAC include better sampling with more diagnostic material having preserved architecture, better tumor typing and grading with capsular assessment, and possibility of performing immunohistochemistry. Sensitivity of CNB in salivary gland lesions is 75–98% and specificity varies from 75 to 100% with <3% unsatisfactory rates.[18]

Therefore, CNB is reserved for mainly nondiagnostic cases after initial FNAC, especially if the patient is not fit for surgery. Otherwise, FNAC combined with ultrasonography and ROSE remains the first choice of investigation in salivary gland lesions.

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Conflicts of interest

There are no conflicts of interest.