Cytological Features of Preneoplastic Conditions of the Thyroid Gland

Abstract

          Thyroid nodules are one of the most common pathological conditions in clinical practice, being detected in more than 50% of the adult population [9]. Due to the expansion of ultrasound capabilities, nodules smaller than 1 cm are also increasingly found [9]. Of these nodules, 70–75% are benign, 5–10% are malignant tumors, and the remaining 20% fall into the indeterminate category [1, 9]. It is precisely the indeterminate categories that constitute the most complex part of cytological diagnosis [1, 3]. Therefore, correct cytological assessment of preneoplastic and borderline conditions of the thyroid gland is of particular importance [2, 3]. To standardize cytological conclusions, several systems have been proposed [7]. The Bethesda System (TBSRTC), introduced in 2007, is the most widely accepted system [7, 9]. It was endorsed in clinical guidelines in 2015 by the American Thyroid Association (ATA) [9]. The Bethesda System divides indeterminate lesions into groups such as AUS, FN, OFN, and SFM [1, 9]. For each category, a risk of malignancy (ROM) is defined, which plays an important role in choosing clinical management [9]. The third edition of TBSRTC (2023) is aimed at a more precise classification of indeterminate lesions [9]. Category IV of the Bethesda System — follicular neoplasm (FN) — is considered one of the most difficult diagnostic areas in thyroid cytology [1]. The diagnosis of FN is used to classify cases that may correspond to neoplasms of invasive follicular origin [1]. To make a diagnosis of FN, adequate cellularity, neoplasm-specific architecture, and follicular or oncocytic cytomorphology are required [1]. In the differential diagnosis, follicular nodular disease, parathyroid biopsy, metastatic carcinoma, NIFTP, medullary carcinoma, and lymphocytic thyroiditis should be excluded [1, 3]. Follicular adenoma and follicular carcinoma cannot be distinguished cytologically, because the difference between them is based only on histological detection of capsular invasion [1, 12]. Oncocytic (Hürthle) cell lesions pose a separate diagnostic difficulty [10]. These cells can occur in both neoplastic and nonneoplastic conditions [10]. Therefore, their differential diagnosis requires strict adherence to Bethesda criteria [1, 10]. The diagnosis of oncocytic follicular neoplasm (OFN), like FN, is not reliably distinguished without histological examination [1].