Peripheral Versus Intraparenchymal Papillary Thyroid Microcarcinoma: Different Morphologies and PD-L1 Expression

Area of Science:

• Oncology
• Pathology
• Immunohistochemistry

Background:

• Peripheral localisation of papillary thyroid microcarcinoma (PTMC) is associated with aggressive clinicopathological features, including lymph node metastasis (LNM).
• Tumor cell PD-L1 expression is linked to increased tumor survival, progression, and potentially aggressive clinical course.

Purpose of the Study:

• To evaluate the relationship between clinicopathological features of PTMC, including tumor localization, and PD-L1 immunoexpression.
• To compare the characteristics of peripherally localized PTMC (p-PTMC) with PD-L1 expression against intraparenchymal PTMC (i-PTMC) without PD-L1 expression.

Main Methods:

• Retrospective analysis of 99 patients diagnosed with PTMC (≥5 mm).
• PD-L1 protein expression assessed by immunohistochemistry.
• PTMCs categorized into four groups based on localization (peripheral vs. intraparenchymal) and PD-L1 expression (positive vs. negative).

Main Results:

• The most frequent group was peripherally localized PTMC with PD-L1 expression (G1, 46.5%).
• G1 showed significant associations with classical morphology, invasive growth, lymphatic invasion (LI), vascular invasion (VI), psammoma bodies, intratumoral fibrosis, PD-L1 positive tumor-infiltrating lymphocytes, and multinuclear giant cells (MGCs).
• Lymph node metastases (LNMs) were significantly more frequent in G1 compared to other groups (p=0.000).

Conclusions:

• The morphology and tumor microenvironment of p-PTMC with PD-L1 expression differ significantly from i-PTMC without PD-L1 expression.
• These differences encompass clinicopathological features related to biological aggressiveness, such as the occurrence of LNM.