Tips and tricks for a safe and effective image-guided percutaneous renal tumour ablation
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Image-guide thermal ablations are nowadays increasingly used to provide a minimally invasive treatment to patients with renal tumours, with reported good clinical results and low complications rate. Different ablative techniques can be applied, each with some advantages and disadvantages according to the clinical situation. Moreover, percutaneous ablation of renal tumours might be complex in cases where there is limited access for image guidance or a close proximity to critical structures, which can be unintentionally injured during treatment. In the present paper we offer an overview of the most commonly used ablative techniques and of the most important manoeuvres that can be applied to enhance the safety and effectiveness of percutaneous image-guided renal ablation. Emphasis is given to the different technical aspects of cryoablation, radiofrequency ablation, and microwave ablation, on the ideal operating room setting, optimal image guidance, application of fusion imaging and virtual navigation, and contrast enhanced ultrasound in the guidance and monitoring of the procedure. Moreover, a series of protective manoeuvre that can be used to avoid damage to surrounding sensitive structures is presented. A selection of cases of image-guided thermal ablation of renal tumours in which the discussed technique were used is presented and illustrated.
• Cryoablation, radiofrequency and microwave ablation have different advantages and disadvantages.
• US, CT, fusion imaging, and CEUS increase an effective image-guidance.
• Different patient positioning and external compression may increase procedure feasibility.
• Hydrodissection and gas insufflation are useful to displace surrounding critical structures.
• Cold pyeloperfusion can reduce the thermal damage to the collecting system.
KeywordsRenal tumours Renal ablation Protective manoeuvres Image-guided tumour ablation Image guidance
Kidney cancer are among the most prevalent tumours in western countries, with an incidence of 2,9–15 per 100,000 in Europe and 50,000 new cases in the United States each year [1, 2, 3]. Conventional treatment has been historically represented by radical nephrectomy, while, more recently, surgical nephron sparing techniques have been developed in order to reduce the invasiveness of the treatment [4, 5, 6]. For the same reason, image-guided thermal ablations have been successfully applied in several different organs, including liver, lung, bone, and even neck tumours [5, 7, 8, 9, 10, 11, 12, 13, 14]. The necessity to offer a treatment to patients not suitable for surgery, and to spare the highest amount of normal renal parenchyma has pushed for the application of image-guided ablative techniques to renal cancer [15, 16] with reported good clinical results [17, 18, 19]. However, some factors could limit the feasibility and/or safety of these techniques. In particular, the proximity to the tumour of sensitive structures that might be damaged by thermal injury can limit the application of image-guided ablations .
The availability of the most advanced imaging techniques, including a dedicated room with computed tomography (CT) and a last generation ultrasound (US) machine, possibly equipped with fusion imaging, may enhance the correct targeting of the tumour and maximize the technical result [21, 22, 23, 24, 25]. Furthermore, different ablative techniques might be used, each one with its own specific characteristics that should be taken into account in order to minimize the risk of complications. Moreover, the application of some protective manoeuvres, such as hydrodissection, gas insufflation, external compression, device bending and retrograde pyeloperfusion are useful to overcome these limitations. A good knowledge of differences among ablative techniques, optimal room setting and image guidance, and of possible protective manoeuvre is of paramount importance for a safe and effective application of image guided ablation to renal tumours.
Thus, in the present paper we offer an overview of the most frequently used ablative techniques, the best image guidance setting and of the most important protective manoeuvres that can be applied to image-guided thermal ablation of renal tumours, also presenting some didascalic cases.
The most widely used ablative techniques used to treat renal tumours are cryoablation, radiofrequency ablation, and microwave ablation [9, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41] .
With cryoablation, repeated freeze-thaw cycles determine extracellular and intracellular ice formation with subsequent injury to cell membranes [26, 27, 28]. Cryoablation provide good visualization of the ice ball with both US and CT, which facilitates precise control of the procedure. Furthermore, as ice forming during the ablation firmly sticks to the surrounding tissues, it is possible to perform a gentle torqueing in order to displace the tumour from sensitive structures. Finally, as with cryoablation, the collagenous architecture is preserved, this technique appears to be ideal in tumours close to the calyx, as it might reduce the risk of fistula . On the contrary, as cryoablation does not provide the “coagulative effect”, this technique is burdened by an higher bleeding risk than other techniques [30, 31, 32].
With radiofrequency an alternating current determines ionic friction that leads to slow heat generation, with subsequent protein denaturation, blood coagulation, and coagulative necrosis [33, 34]. RFA determines also blood coagulation, thus reducing the bleeding risk in comparison with cryoablation . Moreover, with a dedicated umbrella-shaped device it is possible to stick the electrode to the tissue. One of the most important limitations of RFA is represented by the loss of heat occurring close to vessels due to the blood flow (heat sink effect). However, heat propagation from the tip of the electrode might determine injury to close sensitive structures [37, 38].
With the microwave technique the ablative antenna generates an electromagnetic radiation that produces oscillation of water molecules with formation of frictional heating, achieving very high temperature in very short time, with larger and faster ablations . This technique seems to be less affected by the heat sink effect that limits radiofrequency. However, as temperatures higher than with radiofrequency can be rapidly obtained, particular caution should be used for tumours abutting the renal sinus, as urine boiling can be fast obtained with subsequent damage on the collecting system [40, 41].
Operating room setting, optimal image guidance, and ideal patient management
Particularly helpful is to perform renal ablations under general anesthesia. General anesthesia allows for avoiding patients’ movements during the procedure and even achieves a controlled breath-hold that enhances the targeting of the tumour and decrease the risk of unintended puncture of undesired structures.
After the procedure it is always recommended to perform a contrast enhanced CT to control for immediate result and eventual presence of complications.
Patient positioning and external compression
Patients’ position change might be sometimes enough to obtain a sufficient distance between the tumour and the surrounding structure to perform a safe ablation. For this purpose, careful planning, and application of dedicated devices such as vacuum mattress are of paramount importance. A simulation can be made performing CT in different positions before the procedure. This can be done the same day of the procedure or some days in advance, changing the position of the patient up until the identification of the best and easiest trajectory for the ablative device.
Some authors applied injection of normal air , while others reported the use of carbon dioxide (CO2) for organ displacement [45, 46]. CO2 seems to be particularly useful, as it is quickly reabsorbed by the body and eliminated by respiration. CO2 has a low thermal conductivity, and large volumes can be safely injected.
This technique can be used mainly with umbrella-shaped radiofrequency devices and with cryoablation. The tip of the ablation device is anchored into the tumour, and small movements of the organ can be achieved by gentle traction. It can be used in adjunct to other methods, such as hydrodissection or gas insufflation, to increase the distance between the tumour and surrounding structures. Great caution has to be used, as forceful torqueing can potentially cause vascular or parenchymal injuries of the organ [42, 48]. An example is shown in Fig. 4.
This is usually a second-line technique: after abdominal wall puncture with an 18 or 19G coaxial needle, a 0.035-in. wire is placed trough the needle. A sheath is then advanced over the wire, and a balloon is advanced over the wire and trough the sheath. Once the position is adjusted under image guidance, the balloon is expanded completely [49, 50].
During the last few years, percutaneous thermal ablation has become a widespread alternative to nephrectomy and nephron sparing surgery thanks to the good clinical results, low invasiveness, low morbidity and rapid patient recovery.
Although percutaneous thermal ablation is considered safe, it can be complicated with unintended thermal injury to the surrounding structures, such as bowel loops, nerves, or the urinary collecting system.
Interventional radiologists who perform renal thermal ablation should have a fully equipped operating theater, possibly including CT, US, and a system for fusion imaging, and they should be familiar with all the ablative techniques and protective manoeuvres that can be applied to maximize the result and reduce complications.
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