Is Thyroid Ablation Ready for Prime Time?
by Auh Whan Park, MD, and Ziv Haskal, MD
University of Virginia School of Medicine, Charlottesville, Virginia
Although thyroid ablation is a common procedure in many countries around the world, it is offered less frequently in North America. A variety of physicians can perform thyroid ablation, including radiologists, surgeons, and endocrinologists. Interventional radiologists in the United States are also well positioned to perform this procedure.
Thyroid nodules are a common clinical problem, found in 10% to 41% of the population via ultrasonography. The clinical importance of thyroid nodules is to exclude thyroid cancer, which occurs in 5% to 15% of the population. In contrast to thyroid cancer, benign thyroid nodules generally do not require treatment. However, some patients with benign nodules may require treatment for nodule-related symptoms and/or cosmetic problems. Although surgery is curative for thyroid nodules and radioiodine therapy effective for autonomously functioning thyroid nodules (AFTNs), some patients are unsuitable for surgery because of high surgical risk, whereas others refuse these treatments for various reasons. Surgery has potential complications in 2% to 10% of patients, including hypoparathyroidism and recurrent laryngeal nerve injury in addition to hypothyroidism and scarring. Furthermore, there is a small risk of hypothyroidism for radioactive iodine therapy.
Efforts have been made to offer nonsurgical treatment for benign thyroid nodules, leading to the development of chemical and thermal ablations. In 1990, Dr. Livraghi first introduced the use of percutaneous ethanol ablation (EA) for treating ATFNs. Although EA is very effective for cystic thyroid nodules, with a volume reduction of more than 85%, EA is less effective in solid nodules. Also, there are several limitations to EA: unpredictable diffusion within the tumor, pain, and capsular fibrosis by leaking, which makes future surgery challenging.
In thermal ablation, different energy sources are currently used, including laser ablation (LA), radiofrequency ablation (RFA), microwave ablation (MWA), and high-intensity focused ultrasound (HIFU). RFA of benign thyroid nodules has been indicated and used in patients with subjective symptoms and/or cosmetic problems and AFTNs. RFA for nonfunctioning benign nodules has results of 50% to 80% volume reduction at 6 months, 79% to 90% volume reduction at 2 years, and 93% volume reduction at 4 years. RFA effects on AFTNs were reported with 53% to 85% volume reduction rate and a highly variable rate (24% to 82%) of thyroid function normalization. One multicenter study in 1459 patients reported that the complication rate was 3.3%, including hematoma, skin burn, and vomiting. The major complications (1.4%) included voice changes, brachial plexus injury, tumor rupture, and permanent hypothyroidism.
Even with excellent volume reduction and improvement of symptomatic and/or cosmetic problems, recurrence rates of 5.6% and 9% have been reported after RFA and LA, respectively. Because of recurrence induced by marginal regrowth, it is important to completely treat the nodule margin. Several factors have been described that influence the nodule regrowth. It is reported that single-session ablation is effective in most thyroid nodules; however, for nodules larger than 20 mL, additional ablation may be required to achieve sufficient volume reduction. Postprocedural marginal vascularity is another influencing factor. Moreover, possible other influencing factors of the regrowth are the nature of the thyroid nodule, the maximum temperature reached during treatment, treatment modalities, and the type of energy source.
To improve efficacy and minimize complications, innovative devices and techniques have been developed. These include trans-isthmic approach, moving shot technique, thyroid-dedicated electrodes (7 cm, 18-19 G) with variable active tips (3.8, 5, 7, 10 mm), perithyroidal lidocaine injection, virtual needle tracking system, vascular ablation technique, and hydro-dissection technique. Above all, the trans-isthmic and moving shot techniques introduced by Dr. Baek are noteworthy. The trans-isthmic approach has been used for both RFA and EA. In contrast to the craniocaudal or lateral approaches that target the nodule directly, the electrode in the trans-isthmic approach is inserted from the isthmus or medial to the lateral aspect of a targeted nodule.
The trans-isthmic approach has advantages over the other two approaches. The electrode can be easily visualized and monitored on the ultrasound view during the procedure. Also, there will be minimal exposure of the heat to the danger triangle, including the recurrent laryngeal nerve and/or esophagus. This prevents a change in the position of the electrode tip during swallowing or talking and prevents leakage of hot ablated fluid outside the thyroid gland by the sufficient parenchyma on the path. As thyroid nodules are usually ellipsoid in shape, prolonged fixation of the electrode as in liver ablation is dangerous to surrounding critical structures. After dividing thyroid nodules into multiple small conceptual ablation units, RFA is performed unit-by-unit, by moving the electrode tip (moving shot technique).
LA is based on the emission of photons by excited atoms within target tissue. Percutaneous LA has been reported to cause the shrinkage of thyroid nodules, with volume reduction from 37% to 81% at 3 to 6 months, and from 13% to 82% at 1-year follow up. A comparison report showed that both RFA and LA are effective and safe nonsurgical methods for the treatment of benign thyroid nodules. In meta-analysis, the efficacy of RFA appears to be slightly superior to that of LA, and the adverse effects somewhat fewer, attributed to the moving shot technique and internal cooling system of the RF.
MWA generates heat by creating a homogeneous electromagnetic field that interacts with water dipoles and creates a homogeneous ablation zone. MWA permeates even tissues that are desiccated or evaporated. This differs from comparable thermal procedures such as RFA, which is dependent on local resistance. The advantage of MWA could be consistently higher and homogenous intratumoral temperatures, resulting in larger tumor ablation volumes, faster ablation times, and an improved convection profile. However, MWA increases the chance of damaging adjacent structures. Although efficacy and safety have been reported for MWA of thyroid nodules, there have been limitations with study design, small sample sizes, variations in technique, and short follow-up time. In a recent prospective multicenter study, the efficacy of RFA appears to be superior to that of MWA at 6-month follow up.
HIFU is being increasingly used for the treatment of prostate cancer and uterine fibroids. Considering its noninvasiveness, it is a promising tool of thermal ablation of benign thyroid nodules, but there is a lack of evidence to support its use. The extent of nodule shrinkage following treatment ranges from 48.8% to 68.8% and was greatest in the first 3 to 6 months. The best responders were patients with small (less than 10 mL) nodules.
Both ethanol and thermal ablations have been considered as possible alternatives for the treatment of recurrent thyroid cancers in patients at high risk of surgical complications or refusing repeated surgeries. Previous studies showed that RFA can be performed in recurrent thyroid cancers at the operation bed and cervical lymph nodes.
For primary thyroid cancers or follicular neoplasm, the results of thermal ablation are limited and further research is required to provide the efficacy and effectiveness over surgical intervention. However, considering the increase in conservative management in small thyroid cancers, nonsurgical treatment may be the alternative to surgery in the future.
It seems that there is more than enough evidence on thyroid ablation. Is it too early for interventional radiologists to bring this rich opportunity to their patients?
For more information, visit the website of “The Study Group of Thyroid Ablation in North America,” www.thyroidablation.net.