Pathology-based thermal ablation safety margin for follicular adenoma: a digital whole-slide study of follicular carcinoma invasion.

BACKGROUND

Follicular adenoma (FA) and follicular thyroid carcinoma (FC) are difficult to differentiate before surgery, raising concerns that thermal ablation (TA) of FA might treat an occult minimally invasive FC. Therefore, the microscopic extent of FC invasion was quantified to propose a pathology-informed ablation safety margin.

METHODS

A retrospective analysis was conducted on 43 surgically resected FCs treated between 2016 and 2023. Two readers independently reviewed the pathological images (×40 magnification). The capsule infiltration distance (the maximum depth of infiltration beyond the capsule) and the minimally invasive infiltration distance (the ultrasound-undetectable occult infiltration depth) were measured. These distances were compared by tumor stage, FC pathological classification, and lymph node metastasis (LNM).

RESULTS

The capsule infiltration distance ranged from 0.14 to 6.10 mm (mean, 1.49 ± 1.22). The minimally invasive infiltration distance ranged from 0.09 to 1.36 mm (mean, 0.56 ± 0.37). The distances did not differ by tumor stage, FC pathological classification, sex, or lymph node metastasis (LNM) (all P > 0.05). Age was associated with a larger capsule infiltration distance (P = 0.040). However, age was not associated with the minimally invasive infiltration distance (P = 0.367).

CONCLUSION

The minimally invasive infiltration in FC did not exceed 1.36mm in this cohort. Since the actual ablation area achieved by clinical TA exceeds the pathological margin, this supports the extended ablation safety margin of 2mm beyond the sonographic tumor boundary. This TA margin could cover occult minimally invasive FC.