Delayed Cancer Diagnosis in Thyroid Nodules Initially Treated as Benign With Radiofrequency Ablation: Ultrasound Characteristics and Predictors for Cancer

Abstract

Objective: Regrowth after radiofrequency ablation (RFA) of symptomatic large thyroid nodules, initially treated as benign, sometimes turns out to be malignancies. This study aimed to assess the ultrasound (US) characteristics of thyroid nodules initially treated as benign with RFA and later diagnosed as cancers, predictive factors for cancers masquerading as benign, and methods to avoid RFA in these cancers.

Materials and methods: We reviewed the medical records of 134 consecutive patients with 148 nodules who underwent RFA between February 2008 and November 2016 for the debulking of symptomatic thyroid nodules diagnosed as benign using US-guided biopsy. We investigated the pre-RFA characteristics of the thyroid nodules, changes at follow-up after RFA, and the final surgical pathology.

Results: Nodule regrowth after RFA was observed in 36 (24.3%) of the 148 benign nodules. Twenty-two of the 36 nodules were surgically removed, and malignancies were confirmed in seven (19.4% of 36). Of the 22 nodules removed surgically, pre-RFA median volume (range) was significantly larger for malignant nodules than for benign nodules: 22.4 (13.9-84.5) vs. 13.4 (7.3-16.8) mL (P = 0.04). There was no significant difference in the regrowth interval between benign and malignant nodules (P = 0.49). The median volume reduction rate (range) at 12 months was significantly lower for malignant nodules than for benign nodules (51.4% [0-57.8] vs. 83.8% [47.9-89.6]) (P = 0.01). The pre-RFA benignity of all seven malignant nodules was confirmed using two US-guided fine-needle aspirations (FNAs), except for one nodule, which was confirmed using US-guided core-needle biopsy (CNB). Regrown malignant nodules were diagnosed as suspicious follicular neoplasms by CNB. Histological examination of the malignant nodules revealed follicular thyroid carcinomas, except for one follicular variant, a papillary thyroid carcinoma.

Conclusion: Symptomatic large benign thyroid nodules showing regrowth or suboptimal reduction after RFA may have malignant potential. The confirmation of these nodules is better with CNB than with FNA.

Keywords: Follicular thyroid carcinoma; Radiofrequency ablation; Thyroid nodule; Ultrasonography.

Fig. 1. A 21-year-old female patient with minimally invasive follicular thyroid carcinoma (FTC) in surgery after radiofrequency ablation (RFA). The patient had a 4.6-cm sized symptomatic large thyroid nodule (A: transverse ultrasound [US] image, B: longitudinal US image) located in the right thyroid gland, classified as low suspicion on US and confirmed as benign using two fine-needle aspirations and treated with RFA (C). One year after RFA, the nodule size increased to 5.0 cm, appeared hypoechoic, and revealed hypervascularity (D). Pathology report after right lobectomy revealed minimally invasive FTC.

Fig. 2. A 47-year-old female patient with encapsulated angioinvasive follicular thyroid carcinoma (FTC) in surgery after radiofrequency ablation (RFA). Ultrasonography shows a 3.7 cm-sized predominantly solid isoechoic nodule with an initial volume of 12.8 mL (A: transverse, B: longitudinal ultrasound [US] images). This nodule was classified as low suspicion at US and confirmed as benign by two fine-needle aspirations. Six months after RFA, the mass reduced to 3.2 cm. C: Eight years after RFA, the nodule increased to a 4.9-cm predominantly solid hypervascular nodule (D, E: Doppler US image) with a volume of 26.5 mL. The core-needle biopsy results suggested follicular neoplasm, and a final pathology report after right lobectomy confirmed encapsulated angioinvasive FTC.