It is a non-enhancing ring-like area of soft tissue that appears five to 13 months after ablation.


WASHINGTON, D.C.—A “halo sign” is a common delayed finding following radiofrequency ablation (RFA) of kidney tumors, according to researchers at the University of Toronto.

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“The halo sign is an interesting phenomenon,” said principal investigator John Kachura, MD, assistant professor in the department of medical imaging.


“We notice it takes a while for it to show up. By halo sign, we mean a ring-like area of soft tissue that does not enhance, which is surrounded by fat on either side. It usually surrounds an ablation zone and it usually shows up five to 13 months after the procedure.”


Dr. Kachura and a colleague, Philippe Sarliève, MD, studied 33 patients (mean age, 61 years) with kidney tumors treated with RFA at Toronto General Hospital and Mount Sinai Hospital in Toronto. Of the 33 patients, 29 had a solitary tumor and four had two tumors, for a total of 37 treated tumors.


The mean maximum tumor diameter was 2.3 cm (range, 1.1-4.2 cm). The patients had imaging follow-up with triphasic CT. Typical CT follow-up was performed four weeks after ablation and then every four to six months thereafter. The mean CT imaging follow-up was 13 months (range, 0.3-43 months).


The ablation zone diameter was 10% larger, on average, than the tumor diameter on the first follow-up CT; this decreased to 7% smaller than the original tumor size at 10 months. It then decreased to 19% smaller than the original tumor size at 27 months post-ablation. The mean ablation zone attenuation was 40 Hounsfield units (HU) at initial follow-up and 27 HU at 33 months, excluding three tumors that contained calcification. Findings were reported here at the Society for Interventional Radiology’s 33rd Annual Scientific Meeting.


The halo signs, consisting of curvilinear soft tissue attenuation in perinephric fat, were found around 16 ablation zones (43%).They were first identified at a mean of 11 months post ablation (range, 5-13 months). 


In an interview with Renal & Urology News, Dr. Kachura said he believes the halo sign is benign, and noted that the study is important “because we need to know what to expect when we follow these patients. Any patient who has any kind of thermal therapy or ablative therapy for cancer needs to be followed life long.”