Objective:

To quantify the microscopic extent of follicular carcinoma (FC) invasion and propose a clinically significant, pathology-informed ablation safety margin for thermal ablation of follicular adenoma (FA).

Key Findings:

• Capsule infiltration distance ranged from 0.14 to 6.10 mm (mean 1.49 ± 1.22 mm).
• Minimally invasive infiltration distance ranged from 0.09 to 1.36 mm (mean 0.56 ± 0.37 mm).
• No significant differences in infiltration distances based on tumor stage, FC classification, sex, or lymph node metastasis.
• Age was associated with a larger capsule infiltration distance (P = 0.040), indicating a potential risk factor.

Interpretation:

The minimally invasive infiltration in FC did not exceed 1.36 mm, supporting an extended ablation safety margin of 2 mm beyond the sonographic tumor boundary to encompass occult minimally invasive FC, thereby enhancing patient safety.

Limitations:

• Retrospective design may introduce selection bias.
• Limited sample size may affect generalizability.
• Inclusion criteria may exclude some relevant cases, and potential biases in image interpretation should be considered.

Conclusion:

An ablation margin of 2 mm is recommended for thermal ablation of FA to ensure coverage of potential occult FC invasion, thereby reducing the risk of misdiagnosis.