Thyroid Tumor Ablation 101: Radiofrequency Ablation of Thyroid cysts and Benign Thyroid Nodules

Thyroid Tumor Ablation 101: Radiofrequency Ablation of Thyroid cysts and Benign Thyroid Nodules

Thyroid Tumor Ablation 101:  Radiofrequency Ablation of Thyroid cysts and Benign Thyroid Nodules

Thyroid Tumor Ablation 101: Radiofrequency Ablation RFA of Thyroid cysts and Benign Thyroid Nodules

Jung Hwan Baek, MD et al
Korean J Radiol. 2011 Sep-Oct; 12(5): 525–540.

1. Surgery is curative, surgery has several drawbacks  and the efficacy of thyroid hormone-suppressive therapy has not yet been determined.

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2.Therefore, non-surgical, minimally invasive modalities, including ethanol ablation (PEI) and percutaneous laser ablation (PLA), have been used to treat thyroid nodules and both methods yield good results.

3.Although PEI is very effective when used to treat cystic thyroid nodules, it is less effective in solid nodules.

4. Radiofrequency (RF) ablation is a minimally invasive technique that has been used to treat various malignant tumors, especially those of the liver.

5. In pigs, RF ablation of normal thyroid tissue causes necrosis.

6. More recently, RF ablation has been reported to be both effective and safe when used to treat either benign thyroid nodules or recurrent thyroid cancer.

7.In Korea, RF ablation has been used to treat benign and autonomously functional thyroid nodules (AFTNs) 8. The results have been promising, and no significant complications were evident.

Radiofrequency Ablation Principles of Radiofrequency Ablation

  1. RF refers to an alternating electric current oscillating between 200 and 1200 kHz.
  2. Application of RF power to tissue agitates tissue ions as they attempt to follow the changes in direction of the alternating current.
  3. Such agitation creates frictional heat around the electrode.
  4. Although this heat creates immediate damage to tumor tissue, the damage is significant only in regions very close to (thus, within a few mm of) the electrode.
  5. Simultaneously, tumor tissue more remote from the electrode is heated slowly, via thermal conduction from the hot region adjacent to the electrode.
Heat propagation through target tumor. Immediate tissue coagulation necrosis is achieved by frictional heat generated in vicinity of electrode, but electrode-remote tumor tissue is ablated more slowly, via conductive heat.
elec-effect
  1. Several factors may reduce the efficacy of RF ablation.
  2. The first parameter is temperature. Vaporization and carbonization retard optimal ablation.
  3. When tissue vaporization occurs, a large amount of gas is formed. Gas formed around the electrode acts as an insulator, preventing the production and spread of heat.
  4. A second factor is the heterogeneous nature of target tissue, such as fibrosis or calcification.
  5. Tissue variation can alter electrical and thermal conductance.
  6. A third factor is blood flow; this results in perfusion-mediated tissue cooling and reduces the extent of thermal ablation..

Devices and Procedures Devices for thyroid radiofrequency ablation

1.The thyroid gland is relatively small, whereas thyroid nodules are relatively large and spherical in shape with almost no safety margins..Therefore, new devices and techniques have been developed for thyroid RF ablation.

2. A modified, straight internally cooled electrode was developed in Korea. This modified electrode is short (7 cm) to permit easy control; thin, (18-gauge) to minimize injury to the normal thyroid gland; and can be used with active tips of various sizes (0.5, 0.7, 1.0, or 1.5 cm).

probes

Procedures including the “trans-isthmic approach method” and the “moving shot technique”
  1. Before each procedure, a patient is placed in the supine position with mild neck extension.
  2. Two grounding pads are firmly attached to both thighs.
  3. Most operators use a local anesthetic during a procedure; although one group routinely infuses Ketorolac to prevent pain.
  4. The approaches utilized to date include the cranio-caudal approach along the greatest axis and the trans-isthmic approach along the short axis of the nodule.

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Positions of operator and patient. Patient is placed in supine position with mild neck extension, and operator stands close to patient’s head. Left hand of operator holds US probe and right hand of electrode.
shots
moving-shot
Three-method approach method employing electrode. Trans-isthmic approach via short axis of thyroid nodule (from medial to lateral aspect) is used in Korea. When vessels appear to be prominent on trans-isthmic approach route, lateral approach may prevent
1.For the cranio-caudal approach, the operator inserts the electrode tip along the long axis of the nodule.
2. To avoid overheating of surrounding critical structures, the operator maintains the prong tips at least 10 mm from the thyroid capsule and at least 5-6 mm from the margin of the nodule.
3. Another approach, the trans-isthmic approach, has been used for both RF ablation and PEI.
4. With this method, the electrode is inserted from the medial (isthmus) to the lateral aspect of a targeted nodule, and RF ablation proceeds via a transverse US view.
5. This method has several advantages compared to the cranio-caudal approach. First, the entire length of the electrode can be visualized on the transverse US view.
6.The second advantage is minimal exposure to the heat of the danger triangle, which includes the recurrent laryngeal nerve and/or esophagus.
7.During follow-up, an ablated thyroid nodule is found to decrease in size and to gradually move farther away from critical structures, allowing any undertreated nodule area to receive additional RF ablation.
8. The third advantage of this approach is that the electrode passes through a sufficient amount of thyroid parenchyma.
9. It prevents a change in the position of the electrode tip during swallowing or talking, as well as preventing leakage of hot ablated fluid outside the thyroid gland.
10.If large numbers of vessels are located in the isthmus, the lateral approach can be used to prevent hemorrhage.
Fig. 5

US image showing trans-isthmic approach. This transverse US image shows electrode in its entirety, thyroid nodule, and possible location of recurrent laryngeal nerve (small yellow circle). Electrode passes through volume of thyroid parenchyma (white circle)
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US images showing danger triangle.

10. Baek et al.baek

 have proposed a moving shot technique for thyroid nodules, in lieu of the fixed electrode technique, which has been used to treat liver tumors.

11. Baek et al. have suggested to divide thyroid nodules into multiple small conceptual ablation units, and perform RF ablation unit-by-unit, by moving the electrode.

12.The conceptual units are smaller at the periphery of the nodule and large in the center of the nodule or in regions remote from critical structures.

13. Initially, the electrode tip is positioned in the deepest, most remote, portion of the nodule to enable the tip to be easily monitored in the absence of any disturbance caused by microbubbles.

14. Ablation is commenced with 30 W (1-cm active tip) or 50 W (1.5-cm active tip) of RF power.

15. When a transient echogenic area appears at the targeted unit, RF power is decreased, and the electrode tip is moved to an untreated area.

16. The electrode is continuously moved both backward and in the superficial direction within the thyroid nodule.

17.The extent of the ablated area is determined by echogenic changes around the electrode.

18. If a transient hyperechoic zone does not form at the electrode tip within 5-10 seconds, RF power is increased in 10 W increments to a maximum of 100-110 W.

19.If a patient cannot tolerate the pain associated with ablation, the power is reduced or turned off for several seconds.

20.To treat predominantly cystic thyroid nodules, we usually first aspirate all cystic fluid and next perform an RF ablation.

21. The RF ablation is terminated when all conceptual units of the targeted nodule have become transient hyperechoic zones.

22.However, some regions of each thyroid nodule, including those close to critical structures such as the recurrent laryngeal nerve and esophagus, may remain undertreated.

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Sequential US images of moving shot technique showing complete ablation of nodule margins.

23. Operators must monitor any adverse events that occur during and immediately after a procedure.

24.The patient is observed for 1-2 hours with mild compression of the neck.

25. Patients who complain of severe neck pain or discomfort usually receive an oral or IV painkiller.

Follow-Up Evaluation

  1. Follow-up US examinations at 1, 3, 6, and 12 months, and every 6-12 months were routinely performed .
  2. Changes in size, echogenicity, and intranodular vascularity are all evaluated by US.
  3. Reduction in volume is determined by US imaging.
  4. The echogenicity of a nodule would be lower than noted before ablation, and intra-nodular vascularity should disappear in regions receiving complete ablation.
  5. Additional ablation should be performed if a viable portion of the nodule (i.e., showing the same echogenicity as the index nodule and the presence of intra-nodular vascularity) remains on follow-up.
  6. If a patient complains of incompletely resolved clinical problems, including cosmetic and symptomatic issues. An external file that holds a picture, illustration, etc.
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Transverse US images of well-ablated thyroid nodule, before (A) and 12 months after (B) radiofrequency ablation, showing size reduction, decreased echogenicity, and lack of internal vascularity.

Clinical Results

1.The results of RF ablation are evaluated by change in nodule volume and improvement in clinical problems, including pressure symptoms and cosmetic issues.

2.The reduction in nodule volume after RF ablation has been found to range from 33-58% at one month and from 51-85% at six months.

3. The greatest volume reduction is usually observed within the first month after RF ablation, followed by a more gradual decrease.

4.The efficacy of RF ablation in treatment of benign, predominantly solid, thyroid nodules has been proven in prospective randomized trials that compared the use of RF ablation with conservative treatment.

5.The first report on the use of RF ablation demonstrated a reduction of nodule volume and improvement of nodule-related symptoms (29). The results of this study was confirmed by Jeong et al. (2) in a large series volume reduction at six months after ablation, which found that about 85% and 28% of index nodules disappeared on follow-up US.

6. A single session of RF ablation also showed efficacy in both cold and AFTNs.

7. However, volume reductions after RF ablation were less than noted in other reports, which may be attributable to the large nodule volume and the use of only a single session of RF ablation.

8. RF ablation is also safe and effective in elderly patients who are poor candidates for surgery.

9. RF ablation has been reported to be effective in patients with AFTN, thereby reducing the volume of treated nodules, improving nodule-related symptoms and cosmetic problems, and improving abnormal thyroid function .

10. Hyperthyroid patients with AFTN required reduced or no methimazole therapy at the end of follow-up.

11.RF ablation has also been useful to treat patients who showed incompletely resolved clinical symptoms after PEI.

12.These findings suggest that PEI is effective in treating 80% of thyroid nodules (with cystic portions > 50%).

12. 20% of patients who showed incompletely resolved symptoms because of residual solid nodule regions, were effectively treated by RF ablation.

13. These results showed that RF ablation is effective modality, regardless of the proportion of solid component.

14.The moving shot technique is less effective when used to treat thyroid nodules > 4 cm in diameter.

15. Thus, larger nodules may require several treatment sessions to achieve complete ablation which induce the treated nodule as a small scar-like lesion.

Fig. 9

Longitudinal US images showing marginal re-growth of ablated nodule.
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Transverse and longitudinal US images showing marked shrinkage of ablated nodule.

Complications

complications

1. Various complications of thyroid RF ablation have been reported, including pain, voice change, hematoma, skin burn at the puncture site, thyrotoxicosis, hypothyroidism, edema, and fever.

2.However, most patients recovered without sequela.

3. Pain is the most common symptom experienced during the procedure. Most of the patients complain of various degrees of pain in the neck and/or radiating to the head, ear, shoulders, chest, back, or teeth.

4. However pain decreases rapidly when the generator output is reduced or turned off .

5.Voice change has been reported in four patients, with three showing complete recovery within three months and the fourth patient being lost to follow-up.

6.One patient with AFTN developed subclinical hypothyroidism at 12 months. However, any association of hypothyroidism with RF ablation was unclear because the patient showed persistent elevation of serum anti-thyroid peroxidase antibody both before and after RF ablation.

7. Thyrotoxicosis, as diagnosed by elevated free T4 and decreased TSH concentrations, has been reported in six patients on the day after RF ablation. Thyrotoxicosis may be caused by the release of thyroid hormone from thyroid follicles, because of thermal and/or mechanical injury to the normal thyroid gland. None of the six patients showed any thyrotoxic symptoms, and the levels of free T4 and TSH became normal at one month.

8. Skin burns may occur at the sites of electrode insertion and ground attachment.

9.To date, only first-degree burns have been reported at the electrode insertion site; such burns, however, were self-limiting, without sequela.

10.The ground pads act as dispersion electrodes in the RF circuit, and skin burns at pad attachment sites have been reported in patients treated with liver tumors. However, the risk of ground pad burn during thyroid RF ablation is not reported yet, because the RF power is much lower than what is employed in liver tumors.

11.Although hematoma, edema, and fever are usually self-limiting, medications may reduce patient symptoms.

12. Hematoma is usually caused by mechanical injury to the vessels around the thyroid gland or in the thyroid nodule.

13. When a large hematoma develops, RF procedure should be delayed for 1-2 weeks.

14. Although large vessels such as the carotid artery and jugular vein can be mechanically damaged by the electrode, the vessels are resistant to thermal injury because of perfusion-mediated tissue cooling.

15. As thyroid nodules located in the isthmus are very close to the trachea, heat propagation to the trachea is common. This may induce coughing, and ablation should be stopped. To date, permanent tracheal injury has not been reported.

16. Although fatal complications have not been observed, procedure-related deaths, esophageal perforation, tracheal injury, infection, abscess formation, and permanent voice change are possible complications.

17.These complications could be minimized by improvements in knowledge and by the study of complication-prevention techniques.

18. Several technical tips have been suggested in a recent consensus meeting of the Korean Society of Thyroid Radiology.

19.For example, use of the trans-isthmic approach method and the moving shot technique may reduce complications.

20. Injuries to the recurrent laryngeal nerve and esophagus may be minimized by undertreating the danger triangle.

21.In addition, esophageal injury may be prevented by asking the patient to swallow cold water during ablation.

22.The swallowing of cold water during ablation induces esophageal peristalsis, while cooling of the esophagus may reduce thermal damage to the esophagus.

23.During the treatment of thyroid nodules lying just beneath the skin, frequent application of ice bags can prevent skin burn.

24.Unsuccessful attempts have been made to inject saline between critical structures and nodules in order to avoid thermal damage. Because of the longitudinal arrangement of the neck muscles and fascias, injected saline spread rapidly along the muscle plan.

25. Hyrodissection with dextrose was able to work as it does not conduct electricity. We call it the Dextrose avoidance method.

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The image shows the space created by infusion of Dextrose to move the target nodule away from the danger structure.

Advance training for prospective RFA ablation endocrinologists in preparation for the FDA release of thyroid RFA in the USA is available to qualified ECNU or those doing their own USG FNA The fee for the classes will be reduced for those that qualify. A RF generator and electrode system with models to practice RFA ablation on “beef nodules” inserted in superconcentrated gelatin or bovine liver. All RF techniques will be practiced using simulated models.

We need several to do the first clinical trials on RFA treatment of benign thyroid nodules.

Call Matt for details. 310-393-8860 or thyroid.manager@protonmail.com

Richard Guttler MD,
FACE,ECNU

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