The evolution of microballoons have enabled various procedures such as balloon-occluded transarterial chemoembolization for hepatocellular carcinoma (Lucatelli et al., 2019; Matsumoto et al., 2015; Matsumoto et al., 2016; Matsumoto et al., 2017), balloon-occluded retrograde transvenous obliteration for gastric varices (Mine et al., 2017), and coaxial microballoon-occluded coil embolization for vascular disorders (Yasumoto et al., 2015). The present study shows, for the first time, the effectiveness and safety of TAE using a 1.8-Fr tip microballoon catheter with a mixture of ethanol and Lipiodol in the treatment of unruptured renal AML.
In this study, technical success was achieved in all cases. We were able to advance the 1.8-Fr tip microballoon catheter selectively into all target arteries without using a guiding catheter, while 2.2-Fr tip or bigger tip microballoon catheters used with a 5-Fr guiding catheter sometimes cannot be inserted into target arteries (Baba et al., 2014; Sawada et al., 2017). It suggests that the 1.8-Fr tip microballoon catheter have excellent selectability and can be advanced into target vessels less invasively.
Clinical success was also achieved in all cases. The median AML diameter reduction was 34% at a median follow-up of 13 months. Murray et al. reported a mean AML diameter reduction of 39% at a mean follow-up of 39 months. Moreover, it was reported that the tumor shrinkage continues for more than a year (Planche et al., 2011). Thus, the effectiveness of this technique for tumor shrinkage is comparable to the previous reports, although it is affected by the initial volume and tissue composition of the tumor (Hocquelet et al., 2014; Planche et al., 2011).
In our study, ethanol reflux did not occur, and there was no significant decrease in renal function in any patient. In previous reports, renal infarction due to reflux of ethanol occurred in selective TAE using microcatheters in up to 22.5% of cases (Sawada et al., 2017). Thus, it is reasonable to assume that using a microballoon catheter can reduce the risk of renal dysfunction due to the reflux of ethanol. Reducing the risk of renal dysfunction is a significant advantage in the safety of TAE procedure for renal AML because patients with AML often have multiple and bilateral lesions, and sometimes require repeated treatments. Moreover, patients with tuberous sclerosis, which sometimes occurs as a comorbidity with AML, are more likely to present at a younger age, have bilateral lesions, and develop recurrence more often than those without tuberous sclerosis (Kothary et al., 2005; Steiner et al., 1993).
It is known that an ethanol concentration of 70% or more is necessary to denature proteins irreversibly. Therefore, Although there is no consensus on the optimal mixing ratio of ethanol and Lipiodol, 70 to 75% proved sufficient (Hiraki et al., 2009; Kothary et al., 2005; Park et al., 1986; Sawada et al., 2017). The microballoon prevents dilution of ethanol by blood flow, keeping the concentration of ethanol at the desired level in a target vessel. Thus, a microballoon can contribute to increasing the embolization effect of the ethanol mixture.
In this study, pulmonary vasospasm, which is generally a rare complication due to the intravascular administration of ethanol, possibly leading to cardiopulmonary collapse, did not occur. Respiratory complications have been reported to occur in 2% of patients after TAE for AML (Murray et al., 2015). Hiraki et al. reported that pulmonary edema, which is presumed to result from pulmonary vasospasm (Hammer et al., 2001; Mitchell et al., 2006), occurred after TAE for renal AML with lymphangioleiomyomatosis, which sometimes is a comorbid condition with tuberous sclerosis, despite a 4.1 ml (0.07 ml/Kg) ethanol injection (Hiraki et al., 2009). Thus, prevention of pulmonary vasospasm might be required in TAE for renal AML. Most reports of pulmonary vasospasm are cases that occurred after TAE for vascular malformations (Mitchell et al., 2006). Ko et al. reported that the vascular occlusion significantly reduced the elevation of pulmonary artery ethanol concentration and pulmonary artery pressure by reducing the massive washout of ethanol into the systemic artery in TAE for vascular malformations (Ko et al., 2009). Therefore, applying a microballoon catheter in TAE for renal AML can reduce the risk of pulmonary vasospasm by preventing the elevation of pulmonary artery ethanol concentration.
There are some limitations to the present study. First, this study was a retrospective study. Second, the sample size was small. Third, this study included only patients with sporadic renal AML. Renal AMLs in patients with tuberous sclerosis are more likely to recur than those without tuberous sclerosis. Thus, a further study of the effectiveness of this treatment strategy in patients with tuberous sclerosis may be needed.