CT renal arteriography as a novel imaging guidance for the percutaneous ablation of small renal tumors

Abstract Purpose To report procedural data and outcomes of a novel image guidance technique, CT renal arteriography (CTRA), performed to target and ablate small intraparenchymal renal tumors. Materials and methods We retrospectively analyzed data of 2 patients undergoing CTRA-guided ablation for 3 renal intraparenchymal tumors, from February to March 2023. We previously evaluated tumor visibility with US/CEUS, and in all cases conspicuity was poor, whereas contrast-enhanced CT (CECT) clearly depicted all hypervascular nodules. Our primary endpoint was CTRA-guidance feasibility for renal ablation, defined as the precise probe deployment inside the target tumor. The secondary endpoint was CTRA-guided ablation technical success, intended as the inclusion of the whole tumor inside the necrotic volume, with 5 mm safety margins. RENAL scores, complications, procedural time, dose length product (DLP), serum creatinine variation and hospital stay length were also recorded. Results A confident deployment of the probe tip inside the nodule was accomplished in all 3 cases, with a 100% of correct targeting. We observed immediate technical success after all 3 ablations. The 3 nodules had a RENAL score <7 points, and we encountered no complications due to line placement or ablation. The average time from preablative to postablative CTRA was 54 min (50–58min), with a DLP of 3632mGy*cm (2807–4458mGy*cm). Serum creatinine didn’t show a significant variation after the procedures; both patients were hospitalized for 2 days. Conclusion Preliminary data showed that CTRA-guidance might provide unique advantages over conventional CECT-guidance to assist the ablation of small renal intraparenchymal tumor not visualized on US/CEUS.


Introduction
In the last decade, percutaneous image-guided thermal ablation (TA) has been increasingly adopted for the treatment of small renal cell cancer (RCC) [1].Compared to tumor enucleation, TA is associated with lower complications, shorter recovery time, and good preservability of renal function [2,3]; moreover, providing similar oncologic outcomes of surgery, TA has the potential of being an alternative to partial nephrectomy [4].
Intraprocedural imaging guidance to kidney thermal ablation can be performed with plain US, contrast-enhanced US (CEUS) and contrast-enhanced CT (CECT).US and in particular CEUS guidance have the advantage to provide real-time control of nodule position and enhancement, other than effectively showing small areas of reperfusion requiring further ablation [5]; however, difficulties in tumor targeting can be encountered in isoechoic or intraparenchymal nodules.For this reason, CT might be the preferred imaging modality for performing RCC TA, because both the renal mass and probe location are depicted with increased reliability [6].This notwithstanding, intraparenchymal nodules are hard to delineate on unenhanced CT, and multiple intravenous iodinated contrast-media (ICM) administrations might be needed to guide difficult tumor targeting, with an increased risk of ICMinduced acute kidney injury (AKI) [7].
To address these limitations, we shifted to kidney our experience with CT hepatic arteriography (CTHA) guidance for liver tumors.Indeed, CTHA is an established technique that improves tumor conspicuity, needle targeting, and ablation zone visualization by the selective intrahepatic administration of small boluses of intra-arterial contrast during liver TA procedures (8).As such, we started to use CT renal arteriography (CTRA) as image-guidance for the ablation of small renal masses not visualized by US.To our knowledge, CTRA has been never reported albeit it presents remarkable advantages over the standard image-guidance techniques.This is a proof of concept study to report procedural data and technical outcomes of CTRA guidance performed to target and ablate small intraparenchymal RCCs in the first two patients.

Patients
The procedures described in this study were performed in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki declaration and its later amendments.Informed consent for anonymous data review and publication was obtained from all individual participants included.
We retrieved from our institutional archive clinical data of two male patients undergoing renal ablation of small intraparenchymal RCCs, with CTRA guidance, from 1 February to 31 March 2023.
Two male patients (66 and 68 years old, respectively) with a total of 3 RCCs (mean dimension 15 mm, range 11-17 mm) were included.Patient #1 had 1 RCC (ventral, mid-third of the left kidney), while Patient #2 had 2 (both in the upper pole of the right kidney).
Both patients received an indication for percutaneous TA after a multidisciplinary discussion among urologists, oncologists and interventional radiologists.Although both microwave (MW) and cryoablation are widely used for non-surgical patients, with comparable safety and costs for single-nodule treatments, we opted for MW due to our higher expertise with this technique [8,9].All patients have already been diagnosed with RCC and underwent an earlier partial nephrectomy; tumor recurrence with evidence of time-interval growth was detected during follow-up CT scans, and biopsy was deemed not necessary.
We evaluated RCC visibility with US and CEUS prior to TA, and in all cases tumor conspicuity was poor to absent (Figure 1), whereas CECT clearly depicted all mildly hypervascular nodules (Figure 2).

CTRA-guided thermal ablation
The treatments were performed by two interventional radiologists with 5 and 2 years of experience, respectively.
After the induction of general anesthesia, we selectively catheterized the mid-third of the renal artery ipsilateral to the tumors (5 Fr Cobra C2 catheter, Cordis, USA) through a 5 Fr femoral access, under fluoroscopic guidance.Ten mL of contrast agent (Imeron 300, Bracco, Italy) were injected to check the catheter position and, when at target, we fixed the catheter to the vascular access sheath and to the patient skin.This maneuver is deemed fundamental to avoid catheter luxation, if any change of patient position is needed to gain a safe probe route from skin to tumor.
For the preoperative planning, a CTRA was performed acquiring a volume including the pathologic kidney with the following injection protocol: intra-arterial bolus of 15 ml of 1:1 mixed 7.5 ml contrast medium and 7.5 ml saline, with a flow-rate of 3.5 ml/s.To obtain the arterial and venous phases, CT scans were acquired at 6 and 22 s, respectively.The mean waiting time from acquisition to image visualization was about 10 s.
After entering the MW probe (Emprint, Medtronic, USA) through the skin, the path to the nodule was guided by the administration of small boluses of 5 ml of pure contrast agent inside the catheter and the probe was advanced under fluoro-CT guidance, with almost real-time enhancement of the RCCs.After deploying the probe tip inside the nodules, MW TA was performed for 6 min at 75 W in Patient #1, and for 7 min and 5 min at 75 W for the nodules of Patient #2.
After all treatments, we performed track ablation (100 W) to reduce hemorrhagic risks.Then, a final CTRA with the same preoperative injection protocol was acquired to depict the necrotic volumes, to evaluate the immediate technical success and to exclude early complications (Figure 3).A total amount of 30 ml of contrast was injected in Patient #1 and 45 ml in Patient #2.
Finally, we removed the vascular access and the Cobra catheter and performed manual femoral compression.

Variables
Our primary endpoint was the feasibility of CTRA-guidance for renal TA, defined as the possibility to obtain precise probe deployment inside the target tumor.
The secondary endpoint was the technical success of CTRA-guided TA, intended as the inclusion of the whole tumor inside the necrotic volume, with safety margins of at least 5 mm in all three-dimensions.This was evaluated with the visualization of the 'ghost tumor' inside the ablated volume [10] at the CTRA final control, in line with the SIO-DATECAN guidelines [11].

Results
A confident deployment of the probe tip inside the nodule was accomplished in all cases.No manual corrections nor needle repositioning maneuvers were needed, with a 100% of correct targeting.
We observed immediate technical success after all ablations, and no additional probe insertion was required.
All nodules consisted of intraparenchymal tumors with a RENAL score <7 points.
We encountered no complications related to both line placement and ablation; hydrodissection or pyeloperfusion were not required in both patients.
Post-operative serum creatinine levels showed a less than 10% variation to the admission values.Both patients were hospitalized for 2 days.

Discussion
Our pilot study intended to propose a new practical method to guide the probe deployment for RCC TA, overcoming some specific US and CECT limitations; to our knowledge, this is the first descriptive paper of CTRA-guidance for RCC TA.
Conceptually, enhanced tumor conspicuity provided by real-time intra-arterial contrast injection may mirror nodule opacification obtained with preoperative embolization: in this view, CTRA may be considered an hybrid technique between CECT-guidance and TA combined with trans-arterial embolization [14].
Driven by the experience gained on liver tumors with CTHA [15], we assumed CTRA could enhance the detection of poorly visible intraparenchymal RCCs.This can be particularly useful to treat small recurrent tumors, detected during CECT follow-up after a previous kidney resection.As previously described, CECT provides excellent visualization of renal tumors and is considered the principal imaging modality for guiding renal ablation [16].However, patients undergoing renal TA might have already undergone previous kidney enucleation or total nephrectomy, or are unfit to surgery due to clinical conditions; for these reasons, we believe it is important to minimize the administration of ICM in these vulnerable patients.
In this clinical setting, CTRA guidance guarantees the optimal nodule visibility of CECT, while reducing to 7.5 ml the amount of contrast medium needed per volume acquisition.Moreover, administering little boluses through the renal artery, the MW probe can be followed along its path through the skin to inside the enhancing nodule with almost realtime visualization under fluoro-CT, overcoming the possible mismatch between different imaging techniques burdening US/CT fusion guidance [17].
The minimization of repositioning maneuvers, thanks to constant RCC enhancement provided by repeated intra-arterial contrast medium injection, is the key advantage of CTRA that could help in reducing the risks of hemorrhage or collecting system damages [18,19], which are known complications encumbering the conventional CT-guided kidney TA.
We acknowledge the main technical limits of CTRA stands in the risks related to arterial catheterization and the accessibility of the CT room in high workload centers; moreover, the additional time and costs of a line placement can worsen the respective total amounts for the whole procedure.However, we judged these drawbacks negligible if compared to the advantages described.Furthermore, we have to consider radiation dose and contrast administration as limitations if comparing CTRA guidance to US guidance, albeit both values seems reasonable.
Despite the promising premise, larger studies are needed to implement such imaging modality in routinary activity and determine whether CTRA may provide additional benefits, also in terms of ionizing radiation exposure.Furthermore, the CTRA-guided renal TA requires a learning process and it is an undeniably time-consuming procedure for the inexperienced operator.
In conclusion, preliminary data showed that CTRA-guidance might provide unique advantages over conventional CECT-guidance to assist the ablation of small intraparenchymal RCCs not visualized on US or CEUS.

Figure 1 .
Figure 1.(A) Patient #1 preoperative US, completed by contrast administration.The renal tumor of the left kidney is hard to depict.(B) Unenhanced, left kidney CT: the borders of the intraparenchymal nodule are impossible to identify, as well as the full dimensions.Notes: US: ultrasound; CT: computed tomography.

Figure 2 .
Figure 2. (A) Patient #1 CECT showing a small, intraparenchymal renal cell carcinoma at the ventral mid-third of the left kidney (red arrow).(B) Arterial phase CTRA of the left kidney; in this image the renal nodule shows an increased conspicuity if compared to the CECT enhancement.C) Microwave probe with its tip precisely deployed inside the nodule.D) Venous phase CTRA showing the necrotic area, with the ghost tumor fully included (blue arrow).Notes: CECT: contrastenhanced CT; CTRA: CT renal arteriography.

Figure 3 .
Figure 3. Patient #2 arterial phase CTRA in axial (A) and coronal (B) views, showing two adjacent hyperenhancing intraparenchymal nodules (red arrows) at the upper pole of the right kidney.After the ablation, two merging areas of necrosis (blue arrows) covering the whole nodules, in axial (C) and coronal (D) views.Notes: CTRA: CT renal arteriography.