AJR: US guidance may suffice for renal RFA guidance

A venous phase CT scan shows mass in mid to lower pole of left kidney of a 53-year-old woman. Source: American Journal of Roentgenology

While CT remains the preferred modality to guide renal radiofrequency ablation (RFA) procedures, ultrasound provides a safe and effective method for preselected cases, according to a retrospective study published in the November edition of American Journal of Roentgenology. The authors suggested the protocol could decrease CT utilization and cut radiation exposure when applied in an appropriate patient population.

CT, MRI and ultrasound have been used to guide percutaneous renal RFA. John P. McGahan, MD, of the department of radiology at University of California, Davis Medical Center in Sacramento, and colleagues sought to determine whether preablation imaging could be used to triage patients to RFA with ultrasound alone (group 1) or to CT and ultrasound combined (group 2). The researchers examined ablation effectiveness and complication rates in each group of patients.

In the retrospective study, which covered procedures that took place from 2002 until 2009, pretreatment imaging was used to determine whether the mass was well visualized by ultrasound and if the mass was in close proximity to structures that might be injured by ablation. If those two criteria were met, RFA was performed with ultrasound alone (27 patients). In the remaining 29 patients, RFA was performed using CT and ultrasound guidance.

In the ultrasound cohort, 93 percent of masses were initially successfully ablated without recurrence, compared with 84 percent in the combined imaging group. One patient in each group had one major complication, for an overall major complication rate of 3.3 percent. Univariate and multivariate analysis showed no statistically significant difference in complication rates.

Ultrasound guidance was technically unsuccessful for one patient: a 50-year old, 141-kg man with a mass that could not be reliably visualized after the patient was anesthetized. A second patient in this group required a repeat ablation after the six-month follow-up exam.

Five masses in the CT group were not completely ablated on the first attempt: four tumors had nodular enhancement on CT imaging and one revealed abnormal enhancement on CT 26 months after treatment.

“Our results show better technical effectiveness and nearly equal major complication rates for RFA guided by ultrasound alone compared with RFA guided by combined CT and ultrasound, a finding possibly attributable to selection bias,” wrote McGahan et al.

The researchers outline multiple advantages to the ultrasound alone method. These include:

• Eliminating ionizing radiation;
• Reducing CT scheduling pressures; and
• Ability to transport ultrasound throughout the hospital.

In addition, “the use of color-flow ultrasound with ultrasound may help avoid vessels in the path of the needle, within the lesion or note postprocedural bleeding,” wrote the researchers. Furthermore, contrast-enhanced ultrasound may be employed to assess the success of ablation.

However, unlike CT, ultrasound cannot verify needle position in relation to other structures, and ultrasound guidance may be difficult in large patients.

Although McGahan and colleagues concluded that ultrasound guidance alone can be used for guidance of RFA, the researchers issued a caveat, noting that prescreening is important to select patients who may require CT guidance to avoid risk of injury to adjacent structures.