Endocrine (2017) 56:681–682 DOI 10.1007/s12020-017-1302-9
LETTER TO THE EDITOR
Values and limitations of the comparing thyroid radiofrequency and microwave ablation using propensity score Hye Sun Park1 Jung Hwan Baek1 Auh Whan Park2 ●
●
Received: 8 March 2017 / Accepted: 5 April 2017 / Published online: 19 April 2017 © Springer Science+Business Media New York 2017
Dear Editor, We read the article written by Yue et al. entitled “Radiofrequency ablation vs. microwave ablation for patients with benign thyroid nodules: a propensity score matching study” in Endocrine with great interest [1]. It is the first study comparing two thermal ablation modalities—radiofrequency ablation (RFA) and microwave ablation (MWA) —for thyroid. This study [1] indicated that there were no significant differences in any of the nodule volume-related endpoints at 6 and 12 months after ablation without major complications. RFA and MWA were considered to be effective and safe for benign thyroid nodules [1]. In 2015, Ha et al. performed a Bayesian network meta-analysis to compare RFA with laser ablation (LA), and concluded that RFA is better than LA in terms of volume reduction at the 6–12-month follow-up [2]. Based on this meta-analysis, RFA has been considered as the best thermal ablation tool for treating benign solid thyroid nodules. We appreciate that authors reported valuable results. They attempted to maintain the balance while comparing the two modalities. They used the propensity score method to minimize selection bias of this observational study. For successful propensity score analysis, proper propensity score adjustment is essential. There are several key factors
* Jung Hwan Baek [email protected] 1
Department of Radiology and Research Institute of Radiology, University of Ulsan College of Medicine, Asan Medical Center, Seoul, Korea
2
Vascular & Interventional Radiology Section, Department of Radiology, University of Virginia Health System, 1215 Lee StreetCharlottesville, VA 22908-0170, USA
that readers should consider when understanding a propensity score study. First, to generate a reasonable propensity score model, patient groups need to have sufficient patient overlaps. Second, a list of appropriate clinical variables should be used. Third, proper methods should be used to balance the groups. Finally, the results of the propensity score study should be clinically realistic. Yue et al. [1] used reasonable inclusion criteria based on the 2012 Korean Society of Thyroid Radiology Guidelines [3]. They enrolled only mainly solid nodule (solid portion >50%). They also used standard techniques (i.e., movingshot technique), standard outcome measurement methods (i.e., 10-cm visual analog scale and volume reduction), and standard follow-up protocols [3]. The experiences of the operators were also well-balanced (6 years with thyroid RFA and 5 years with thyroid MWA). With regard to the experience of the operator, experience with approximately 50–100 cases is essential for an effective thyroid RFA procedure [4]. Hence, the operators participating in the comparison study should have sufficient experience under the supervision of an expert in the ablation field. Moreover, they used reasonable propensity score variables such as index nodule volume, proportion of solid component, and follow-up period [2]. Unfortunately however, they performed multiple treatment sessions, and the number of treatment sessions was unclear in both groups. The number of treatment sessions is an important factor to determine volume reduction; therefore, all randomized controlled trials performed single-session treatments [2]. Hence, we believe that the number of treatment sessions should be included among the propensity score variables. Yue et al. have noted several limitations of their study [1], which should be taken into account for a proper interpretation and contextualization of the study results. We
682
agree and readers should consider these limitations when reviewing this article. First, as mentioned above, the number of treatment sessions in both groups was unclear. Second, the authors used a bipolar electrode, even though this is not a popular device for thyroid RFA. Most thyroid centers use the thyroid-dedicated monopolar electrode (i.e., 18-gauge straight-type internally cooled electrode). Hence, thyroid-dedicated electrodes may be used in future comparison studies. Third, although the mean volume of the two groups was not different (P = 0.527), the Yue study only included small-sized thyroid nodules (mean volume, 5.7 vs. 5.5 mL). Due to the small index nodule volume, the volume reductions of the two groups can be similar. Hence, medium-sized nodules may represent appropriate candidates for such comparative studies. The first randomized controlled international trial also used medium-sized (10–20 mL) nodules to verify the efficacy of singlesession ablation. Fourth, although they enrolled only mainly solid nodule, the ultrasound feature is also important factor. Ahn et al. [5] emphasized that the margin of a nodule correlated with therapeutic success. Most of the successfully ablated nodules showed well-defined margins rather than ill-defined margins. Finally, a longer follow-up result (mean, 10.7 and 10.6 months in the Yue study) is necessary. Over a longer follow-up period, marginal recurrence is a major factor for insufficient volume reduction and recurrence. Previous studies have reported that marginal recurrence is apparent in 2 years after ablation. In conclusion, Yue et al. [1] have obtained valuable findings when comparing two ablation tools—RFA and MWA. Although we agree with the opinion of the authors,
Endocrine (2017) 56:681–682
we believe that readers should consider the limitations mentioned above when interpreting these results, and future additional trials should be addressed. Compliance with ethical standards Conflict of interest JHB is a consultant of STARmed and RF Medical. The other authors declare that they have no conflict of interest.
References 1. W.W. Yue, S.R. Wang, F. Lu, L.P. Sun, L.H. Guo, Y.L. Zhang, X. L. Li, H.X. Xu, Radiofrequency ablation vs. microwave ablation for patients with benign thyroid nodules: a propensity score matching study. Endocrine 55, 485–495 (2017) 2. E.J. Ha, J.H. Baek, K.W. Kim, J. Pyo, J.H. Lee, S.H. Baek, H. Dossing, L. Hegedus, Comparative efficacy of radiofrequency and laser ablation for the treatment of Benign thyroid nodules: systematic review including traditional pooling and bayesian network meta-analysis. J. Clin. Endocrinol Metab. 100, 1903–1911 (2015) 3. D.G. Na, J.H. Lee, S.L. Jung, J.H. Kim, J.Y. Sung, J.H. Shin, E.K. Kim, J.H. Lee, D.W. Kim, J.S. Park, K.S. Kim, S.M. Baek, Y.H. Lee, S. Chong, J.S. Sim, J.Y. Huh, J.I. Bae, K.T. Kim, S.Y. Han, M.Y. Bae, Y.S. Kim, J.H. Baek, Radiofrequency ablation of benign thyroid nodules and recurrent thyroid cancers: consensus statement and recommendations. Korean J. Radiol 13, 117–125 (2012) 4. J.Y. Sung, J.H. Baek, S.L. Jung, J.H. Kim, K.S. Kim, D. Lee, W.B. Kim, D.G. Na, Radiofrequency ablation for autonomously functioning thyroid nodules: a multicenter study. Thyroid 25, 112–117 (2015) 5. H.S. Ahn, S.J. Kim, S.H. Park, M. Seo, Radiofrequency ablation of benign thyroid nodules: evaluation of the treatment efficacy using ultrasonography. Ultrasonography 35, 244–252 (2016)
LETTER TO THE EDITOR
Values and limitations of the comparing thyroid radiofrequency and microwave ablation using propensity score Hye Sun Park1 Jung Hwan Baek1 Auh Whan Park2 ●
●
Received: 8 March 2017 / Accepted: 5 April 2017 / Published online: 19 April 2017 © Springer Science+Business Media New York 2017
Dear Editor, We read the article written by Yue et al. entitled “Radiofrequency ablation vs. microwave ablation for patients with benign thyroid nodules: a propensity score matching study” in Endocrine with great interest [1]. It is the first study comparing two thermal ablation modalities—radiofrequency ablation (RFA) and microwave ablation (MWA) —for thyroid. This study [1] indicated that there were no significant differences in any of the nodule volume-related endpoints at 6 and 12 months after ablation without major complications. RFA and MWA were considered to be effective and safe for benign thyroid nodules [1]. In 2015, Ha et al. performed a Bayesian network meta-analysis to compare RFA with laser ablation (LA), and concluded that RFA is better than LA in terms of volume reduction at the 6–12-month follow-up [2]. Based on this meta-analysis, RFA has been considered as the best thermal ablation tool for treating benign solid thyroid nodules. We appreciate that authors reported valuable results. They attempted to maintain the balance while comparing the two modalities. They used the propensity score method to minimize selection bias of this observational study. For successful propensity score analysis, proper propensity score adjustment is essential. There are several key factors
* Jung Hwan Baek [email protected] 1
Department of Radiology and Research Institute of Radiology, University of Ulsan College of Medicine, Asan Medical Center, Seoul, Korea
2
Vascular & Interventional Radiology Section, Department of Radiology, University of Virginia Health System, 1215 Lee StreetCharlottesville, VA 22908-0170, USA
that readers should consider when understanding a propensity score study. First, to generate a reasonable propensity score model, patient groups need to have sufficient patient overlaps. Second, a list of appropriate clinical variables should be used. Third, proper methods should be used to balance the groups. Finally, the results of the propensity score study should be clinically realistic. Yue et al. [1] used reasonable inclusion criteria based on the 2012 Korean Society of Thyroid Radiology Guidelines [3]. They enrolled only mainly solid nodule (solid portion >50%). They also used standard techniques (i.e., movingshot technique), standard outcome measurement methods (i.e., 10-cm visual analog scale and volume reduction), and standard follow-up protocols [3]. The experiences of the operators were also well-balanced (6 years with thyroid RFA and 5 years with thyroid MWA). With regard to the experience of the operator, experience with approximately 50–100 cases is essential for an effective thyroid RFA procedure [4]. Hence, the operators participating in the comparison study should have sufficient experience under the supervision of an expert in the ablation field. Moreover, they used reasonable propensity score variables such as index nodule volume, proportion of solid component, and follow-up period [2]. Unfortunately however, they performed multiple treatment sessions, and the number of treatment sessions was unclear in both groups. The number of treatment sessions is an important factor to determine volume reduction; therefore, all randomized controlled trials performed single-session treatments [2]. Hence, we believe that the number of treatment sessions should be included among the propensity score variables. Yue et al. have noted several limitations of their study [1], which should be taken into account for a proper interpretation and contextualization of the study results. We
682
agree and readers should consider these limitations when reviewing this article. First, as mentioned above, the number of treatment sessions in both groups was unclear. Second, the authors used a bipolar electrode, even though this is not a popular device for thyroid RFA. Most thyroid centers use the thyroid-dedicated monopolar electrode (i.e., 18-gauge straight-type internally cooled electrode). Hence, thyroid-dedicated electrodes may be used in future comparison studies. Third, although the mean volume of the two groups was not different (P = 0.527), the Yue study only included small-sized thyroid nodules (mean volume, 5.7 vs. 5.5 mL). Due to the small index nodule volume, the volume reductions of the two groups can be similar. Hence, medium-sized nodules may represent appropriate candidates for such comparative studies. The first randomized controlled international trial also used medium-sized (10–20 mL) nodules to verify the efficacy of singlesession ablation. Fourth, although they enrolled only mainly solid nodule, the ultrasound feature is also important factor. Ahn et al. [5] emphasized that the margin of a nodule correlated with therapeutic success. Most of the successfully ablated nodules showed well-defined margins rather than ill-defined margins. Finally, a longer follow-up result (mean, 10.7 and 10.6 months in the Yue study) is necessary. Over a longer follow-up period, marginal recurrence is a major factor for insufficient volume reduction and recurrence. Previous studies have reported that marginal recurrence is apparent in 2 years after ablation. In conclusion, Yue et al. [1] have obtained valuable findings when comparing two ablation tools—RFA and MWA. Although we agree with the opinion of the authors,
Endocrine (2017) 56:681–682
we believe that readers should consider the limitations mentioned above when interpreting these results, and future additional trials should be addressed. Compliance with ethical standards Conflict of interest JHB is a consultant of STARmed and RF Medical. The other authors declare that they have no conflict of interest.
References 1. W.W. Yue, S.R. Wang, F. Lu, L.P. Sun, L.H. Guo, Y.L. Zhang, X. L. Li, H.X. Xu, Radiofrequency ablation vs. microwave ablation for patients with benign thyroid nodules: a propensity score matching study. Endocrine 55, 485–495 (2017) 2. E.J. Ha, J.H. Baek, K.W. Kim, J. Pyo, J.H. Lee, S.H. Baek, H. Dossing, L. Hegedus, Comparative efficacy of radiofrequency and laser ablation for the treatment of Benign thyroid nodules: systematic review including traditional pooling and bayesian network meta-analysis. J. Clin. Endocrinol Metab. 100, 1903–1911 (2015) 3. D.G. Na, J.H. Lee, S.L. Jung, J.H. Kim, J.Y. Sung, J.H. Shin, E.K. Kim, J.H. Lee, D.W. Kim, J.S. Park, K.S. Kim, S.M. Baek, Y.H. Lee, S. Chong, J.S. Sim, J.Y. Huh, J.I. Bae, K.T. Kim, S.Y. Han, M.Y. Bae, Y.S. Kim, J.H. Baek, Radiofrequency ablation of benign thyroid nodules and recurrent thyroid cancers: consensus statement and recommendations. Korean J. Radiol 13, 117–125 (2012) 4. J.Y. Sung, J.H. Baek, S.L. Jung, J.H. Kim, K.S. Kim, D. Lee, W.B. Kim, D.G. Na, Radiofrequency ablation for autonomously functioning thyroid nodules: a multicenter study. Thyroid 25, 112–117 (2015) 5. H.S. Ahn, S.J. Kim, S.H. Park, M. Seo, Radiofrequency ablation of benign thyroid nodules: evaluation of the treatment efficacy using ultrasonography. Ultrasonography 35, 244–252 (2016)
