Patient Selection and Data Collection
Patients who underwent percutaneous RFA of unilateral adrenal cystic masses at Asan Medical Center in Seoul, Korea, from January 2019 to December 2021 were retrospectively identified. Indications of RFA for adrenal cyst were the presence of symptoms at the time of diagnosis, a huge cyst >10 cm in diameter, or increasing size during follow-up evaluation. Patients with adrenal cysts were excluded (1) if the adrenal cyst was bilateral or functional, (2) if the cyst had a solid component, (3) if more than two septae were observed on computed tomography (CT), (4) if the cyst was suspected of being malignant, (5) if the RFA procedure was impossible because the adrenal cyst was very close to the surrounding organs (pancreas, liver, spleen), (6) if coagulopathy was present (platelet count < 50 X 103/µL or International Normalized Ratio > 1.5), (7) if the patient refused RFA for an adrenal cyst, or (8) if the patient was lost to follow-up (Figure 3).
Patient information was collected from their electronic medical records and charts, including their demographic and disease characteristics, procedure details, and outcomes. Demographic characteristics included patient sex, ages at diagnosis and at the procedure, body mass index (BMI), duration of disease from the time of diagnosis, and comorbidities. Disease characteristics included cyst location, functional status of the adrenal disorder, cyst diameter, cyst volume, and symptoms of the disease. Details of the procedure, obtained from procedure reports, included characteristics of the aspirated cyst, aspirated volume, power of the generator, duration of ablation, number of ablation sessions, and temperature of the electrode. Procedure outcomes included volume reduction rate (VRR) on follow-up CT, immediate complications, adrenal insufficiency, readmission, recurrence, re-procedure, length of hospital stay, pain scale, and use of analgesics.
Percutaneous Radiofrequency Ablation (RFA)
All procedures were performed under intravenous conscious sedation using 50 μg of Fentanyl citrate (Fentanyl®, BC World Pharm co., Ltd, Seoul, Korea) and local anesthesia. During RFA, the remaining 50 μg of Fentanyl citrate was intravenously injected if a patient complained of abdominal pain.
The insertion point on the skin was marked when a safe needle tract was secured on ultrasound. Approximately 10 mL of 2% lidocaine (Jeil Pharmaceutical; Daegu; Korea) was injected as local anesthesia, from the hepatic capsule to the insertion point along the needle tract. A single, cooled electrode (Cool-tipTM radiofrequency ablation system; Valleylab; Burlington, VT, USA) was inserted into the center of a cyst, followed by the insertion of a 21-gauge Chiba needle (COOK; Bloomington; IN, USA). The cystic content was aspirated through a Chiba needle until the cyst became less than 3.0 cm in diameter, in order to reduce the ablation time. 10 ml of 2% lidocaine was then injected into the cyst through the Chiba needle five minutes prior to RFA for the purpose of inner epithelial anesthesia.
RFA was performed under ultrasonographic guidance using a 200-W generator (Mygen M-3004; RF Medical; Seoul, Korea) and a single, cooled electrode with a 3-cm active tip by a radiologist (P.N.K.) with 20 years of clinical experience in the field of RFA. For the ablation, the current was maximally elevated from the beginning. Because a large cyst changed in shape after aspiration and presented clover-leaf like configuration, we repeatedly inserted the electrode at unablated portion of a cyst after roll-off of generator power for complete ablation over the entire cyst.
When ablation was ceased, the electrode was removed, and the remaining cystic fluid was aspirated. For monitoring, an iU22 ultrasound system (Philips Healthcare, Bothell, WA, USA) with a 2-5-MHz convex transducer was used.
Post-treatment Assessment
Although ultrasonography (US) was obtained in all patients both before and after RFA, we only used CT scans for measuring the cyst volumes. All patients underwent contrast-enhanced, abdominal CT both before and after RFA, and using the usually available scanners (LightSpeed Plus, LightSpeed Ultra16; GE Healthcare, Milwaukee, WI, USA and Somatom Sensation 16; Siemens Medical Solutions, Forchheim, Germany) with a 5-mm slice thickness. The intervals of the follow-up CTs obtained after RFA varied because hepatic cysts are benign.
The two radiologists reached a consensus by assessing the images. The cyst length and width were determined from the axial image showing the largest cyst, and the height was calculated as the total slice numbers showing a cyst. The cyst volume was calculated using the following formula: length x width x height x 4 x 3.14 / 24 = L x W x H / 0.52, and the volume reduction rate was also calculated.
After aspiration of the cystic content, we recorded the aspirated volume and the configuration of each cyst (single, septated), and measured its longest and short diameters on USG in order to calculate the volume of the residual fluid. The residual volume was calculated using the following formula: (long diameter + short diameter)3 x 4 x 3.14 / 24.
Complications occurring immediately after RFA were evaluated on contrast-enhanced abdominal-pelvic CT. Complications included inadvertent injury during the RFA procedure, bleeding, infection requiring antibiotics or drainage, and readmission. Complications were classified as major or minor, according to the guidelines of the Standards of Practice Committee of the Society of Interventional Radiology[31].
The primary outcome was response to treatment, defined as a reduction in volume of the adrenal cyst on radiologic imaging. Adrenal cyst volume was measured manually by measuring the area of each slice on axial CT axial scans, summing these areas, and multiplying the area by slice thickness. The VRR was calculated based on pre- and post-treatment lesion volumes. Responses to treatment on 3 months follow-up adrenal CT scans were defined as good (VRR ≥ 80%), intermediate (40% ≤ VRR < 80%), or poor (VRR ≤ 40%) (Table 2).
Patients were followed up 3 months after surgery and every year thereafter by adrenal CT scans (Figure 4). Cyst recurrence was determined, with recurrence defined as a cyst volume >30% higher than the volume on the 3 months follow-up CT scan.