LAUSR

To the Editor, Microwave ablation (MWA) is potentially more effective than radiofrequency ablation (RFA), with some studies reporting a lower rate of local tumour progression in MWA [1]. Nonetheless, coagulative necrosis may be difficult to achieve in lesions close to major ([3 mm) hepatic venous tributaries or portal venous radicals due to perfusion-mediated convective cooling. This ‘heat sink’ phenomenon might be less pronounced in MWA as compared to RFA due to the higher temperatures reached with MWA, making it relatively insensitive to perfusion-mediated cooling of small calibre vessels. Temporary venous occlusion has already been described as a way to decrease the magnitude of this effect in RFA [2, 3] and non-clinical MWA studies [4]. We present three cases of balloon-occluded MWA for lesions located close to a major vein. Patient A is a 41-year-old male patient who underwent distal pancreatectomy in 2012 for acinar cell carcinoma. Three liver metastases were noted in December 2013, for which he underwent chemotherapy followed by RFA. Three new liver metastases were noted on follow-up in March 2015. One of these was located in segment VII in direct contact with, but not invading, the main trunk of the right hepatic vein (RHV), 8 mm from the inferior vena cava. US-guided MWA was performed. Residual disease was noted in segment VII on follow-up, in the part of the metastatic deposit located adjacent to the RHV (Fig. 1). CT-guided balloon-occluded MWA was subsequently performed. Patient B is a 46-year-old lady with primary biliary cirrhosis and deranged liver function despite optimal medical management (bilirubin 66 lmol/L; ALP 844 U/L; AST 108 U/L; GGT 508 U/L; MELD 12), but no signs of portal hypertension. She had a history of sigmoid adenocarcinoma, resected in 2014. Four suspicious liver lesions were noted on follow-up MRI, and biopsy showed metastatic colonic adenocarcinoma. Given her poor liver reserve, she was referred for MWA. The metastatic deposit located in segment VIII was closely related to a major tributary of the RHV (Fig. 2). US/CT-guided balloon-occluded MWA was performed in April 2016. Patient C is a 55-year-old male diagnosed with welldifferentiated small bowel neuroendocrine tumour (NET) in September 2015, for which he underwent surgical resection. A single liver metastasis was noted in the right liver lobe (segment VI/VII) on the staging CT—this was very close to both the segment VI branch of the portal vein and to a major tributary of the RHV (Fig. 3). The patient refused surgery and was referred for ablation. Balloonoccluded MWA with temporary occlusion of both right hepatic vein and segment VI branch of the right portal vein was done in May 2016 under ultrasound guidance. In all cases, a 2–6-cm-long 10-mm-diameter balloon (Admiral Xtreme, Medtronic, Dublin, Republic of Ireland) was advanced over a standard guidewire in the RHV from a jugular approach and inflated at a safe distance from the metastatic deposit. Access to the right portal venous branch in Patient C was achieved percutaneously (Neff Percutaneous Access Set, Cook Medical, Bloomington, & Kelvin Cortis [email protected]