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Difficulty: HardPathology and Biopsy Interpretation

A 54-year-old woman with a history of hypothyroidism secondary to Hashimoto thyroiditis undergoes a right thyroid lobectomy for a 3.5-cm solid nodule. Preoperative fine-needle aspiration was reported as Bethesda Category IV (suspicious for a follicular neoplasm). The surgical pathology report of the resected lobe describes a well-circumscribed, entirely encapsulated tumor composed exclusively of closely packed follicles. Microscopic examination of the follicular epithelial cells reveals nuclear enlargement, overlapping nuclei, prominent longitudinal nuclear grooves, and central chromatin clearing. Two microscopic psammoma bodies are identified within the lesion. Extensive sampling shows no evidence of capsular penetration or vascular invasion by the tumor cells.

Based on these histopathologic findings, which of the following is the most accurate diagnostic interpretation?

  1. Papillary thyroid carcinoma, as the characteristic nuclear features are diagnostic even in the absence of invasionAnswer
  2. B
    Follicular adenoma, because the tumor is completely encapsulated and lacks capsular or vascular invasion
  3. C
    Non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP), given the strict encapsulation and lack of invasion
  4. D
    Follicular thyroid carcinoma, because the dense architectural crowding of the follicles overrides the lack of capsular invasion
  5. E
    Primary thyroid lymphoma, which is indicated by the rapidly enlarging nodule in the setting of chronic lymphocytic thyroiditis

Answer

Papillary thyroid carcinoma, as the characteristic nuclear features are diagnostic even in the absence of invasion
The pathology report describes classic nuclear features of papillary thyroid carcinoma (PTC): enlarged, overlapping nuclei with longitudinal grooves and chromatin clearing ('Orphan Annie eyes'). In thyroid pathology, the presence of these nuclear features is the gold standard for diagnosing PTC, even when the tumor has an exclusively follicular growth pattern (follicular variant of PTC) and lacks any signs of capsular or vascular invasion. The finding of psammoma bodies further cements the diagnosis of PTC and explicitly rules out more indolent classifications such as NIFTP (Non-invasive follicular thyroid neoplasm with papillary-like nuclear features), which strictly requires the absence of psammoma bodies.

Step-by-Step Solution

1
Analyze the architectural and cytologic features described in the pathology report.
The tumor is encapsulated with a follicular architecture, but displays nuclear enlargement, overlapping, grooves, and chromatin clearing (Orphan Annie eyes), along with psammoma bodies.
Identifying these specific cytologic findings is the critical first step in classifying thyroid neoplasms.
2
Determine if the nuclear features override the tumor's architecture and lack of invasion.
The presence of classical PTC nuclear features dictates a diagnosis in the papillary carcinoma family, regardless of the strictly follicular architecture or the absence of capsular/vascular invasion.
Diagnostic criteria for PTC are driven by cytology, whereas follicular carcinoma/adenoma differentiation is driven by invasion.
3
Evaluate the possibility of a non-invasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP).
Although the tumor is non-invasive and encapsulated, the presence of psammoma bodies is an absolute exclusion criterion for NIFTP.
NIFTP has strict inclusion and exclusion criteria to prevent over-treating indolent lesions, and psammoma bodies confirm classic PTC biology.

Key Concept

The diagnosis of papillary thyroid carcinoma (PTC) relies on characteristic nuclear features (grooves, pseudoinclusions, chromatin clearing) and is independent of capsular or vascular invasion, which are the criteria used to differentiate follicular adenoma from follicular carcinoma.
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Topics

Pathology and Biopsy Interpretation

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