By Nishita Kothary
Over the years, we have seen a rapid development of tools, tricks and techniques that enable ablation of tumours located in regions that are in close proximity to thermally sensitive organs or structures. Despite this, what remains of paramount importance and central to ablating tumours, especially those that are deemed high-risk, is a complete understanding of the type of thermal injury applied, tissue properties of the target organ, electrode design, configuration of the isotherm created when applying one or several of these electrodes and, most importantly, a thorough understanding of anatomical structures that may inadvertently be in the ablative zone.
Percutaneous image-guided thermal ablation of benign and malignant tumours is usually considered to be safe, with a low complication rate. According to literature, most reported complications result from unintended thermal damage to the organ or an adjacent structure. This is especially true for tumours located in regions that are in close proximity to thermally sensitive organs, such as a sub-diaphragmatic hepatic tumour, wherein the diaphragm is injured; renal tumours abutting the ureter; or an ischial metastasis in close proximity to the sciatic nerve.
Over the years, we have seen a rapid development of tools, tricks and techniques that enable ablation of tumours in these “high-risk” zones. Despite this, what remains of paramount importance and central to ablating tumours, especially those that are deemed high-risk, is a complete understanding of the type of thermal injury applied, tissue properties of the target organ, electrode design, configuration of the isotherm created when applying one or several of these electrodes and, most importantly, a thorough understanding of anatomical structures that may inadvertently be in the ablative zone. While necessary to minimise the procedural risk, a detailed discussion on equipment and tissue sensitivity are beyond the scope of this article. What follows is a comprehensive, but by no means all-inclusive, list of “pearls” for optimising the safety profile of ablative technologies in these high-risk zones.
All the bells and whistles on an ablative device cannot replace a well-thought out and thorough plan. Although most thermal ablative technologies have been used interchangeably, a particular situation may call for a specific technology. For example, cryotherapy is especially suited for chest wall lesions and lung tumours close to the brachial plexus. However, the same technology has a reported higher incidence of haemorrhage in patients with hepatic tumours in a cirrhotic liver, when compared to radiofrequency ablation. Understanding the advantages and limitations of each technology, as well as the properties of the tissue, allows for maximisation of results while minimising the risk of injury. Another consideration during the pre-planning phase is the modality for image guidance. Although most ablations are carried out under CT or ultrasound guidance, occasionally MR-guided ablations may provide anatomical information or thermal mapping, which may be critical for the safe ablation of a tumour. For example, visualisation of the course of an adjacent nerve can often be better appreciated on an MR. Finally a pre-procedure planning exercise allows one to determine the best approach for needle placement and patient positioning.
A variety of techniques have been described to displace and hence protect an organ(s) that may be vulnerable to thermal injury. A simple, but not always effective, approach used for relatively mobile organs, such as the bowel, is to reposition the patient, such that the bowel falls away from the target, hence increasing the distance between the thermal ablation zone and the tissue at risk.
Hydrodissection involves injecting fluid between the target and the thermal sensitive organ, mechanically displacing, and consequently insulating, the organ at risk. This technique is often used to displace the colon during liver, renal or adrenal ablation. Similarly, fluid injected in the epidural space protects the spinal cord during thermal ablation of spinal tumours. This technique involves image-guided placement of a 19–21 gauge needle, interposed between the target and the organ at risk. Sterile saline or 5% dextrose in water (D5W) is commonly used. A small amount (5–7mL) of iodinated contrast can be mixed with the fluid to improve visualisation when using CT guidance. The fluid is injected under intermittent image guidance until adequate separation of the two structures is achieved, following which ablation can commence. The use of saline in radiofrequency ablation has been discouraged due to the high electrical conductivity of saline. Similarly caution should be used when injecting large amounts of dextrose solution in diabetic patients.
As an alternative to fluid dissection and displacement, some operators prefer the use of gas, specifically the use of CO2. The high solubility of CO2 and the fact that it is not toxic makes it an ideal agent for dissection. However, as with any gas, it obeys the laws of gravity and may distribute in a suboptimal position. Intermittent imaging while injecting CO2 helps confirm satisfactory distribution. Finally, the use of balloons to interface between colon and an ablation target has been described, although this is often challenging in the peritoneal cavity.
Target torqueing technique
Instead of displacing the thermal sensitive organ at risk, one could potentially attempt to displace or move the target itself. This technique has been used for relatively mobile targets, such as the kidney and the lung. The technique involves placement of the probe(s) and then pulling back on the probe in order to torque the target itself. Once adequate distance is achieved, tension can be maintained manually or mechanically with a clamp through the ablation process.
Warming or cooling of at risk hollow organ
Circulating warm (for cryotherapy) or cool (for radiofrequency or microwave) fluid in the ureter, the urethra and rectum have been described to decrease the risk of injury. On a similar note, ureteral stents have been used to protect the ureter when ablating a renal tumour in close proximity to the ureter. The theoretical advantage of this manoeuvre is to provide a mechanical buttress to the ureter.
Prior to applying thermal energy for ablative purposes, it is essential that the position of the uninsulated portion of the electrode is critically examined. This is especially true for the multitined radiofrequency probe, where a single tine can deposit high amounts of energy into an adjacent heat sensitive organ. For example, radiofrequency ablation of hepatic dome tumours has resulted in permanent diaphragmatic injuries. Similarly, an electrode less than 5mm from a major bile duct can cause ductal injury. Hence three-dimensional imaging prior to ramping up the thermal energy can alert an operator to inadvertent injuries.
Intra-operative monitoring is critical in high-risk ablation and comes in various forms depending on the tissue ablated and the modality. Ablations in close proximity to the sciatic nerve would benefit from neuromonitoring. An intermittent visual check, through imaging, of the iceball during the freeze cycle of cryoablation as it increases in size can help monitor its proximity to the colonic wall. Similarly, live monitoring of the ablation zone by microwave will alert the physician of imminent risk.
Finally thermal monitoring, either directly with thermocouples or fibreoptic thermosensors, or MRI temperature mapping, is another tool that can be used to monitor the temperature reached by the ablation zone or in the organ at risk. For spinal tumours, thermal monitoring is essential, especially if the interposition of the bony lamella is scant.
In conclusion, our understanding of ablations in high-risk zones has significantly increased. Detailed knowledge of the tissue, the modality, techniques and technology has evolved over the years. Once armed with this knowledge, ablations that once carried a high risk of injury can now be done safely.
Nishita N Kothary is an associate professor of Radiology at the Stanford University Medical Center, Stanford, USA. She is also director, Clinical Operations (Interventional Radiology), Stanford University Medical Center, Division of Interventional Radiology. Kothary reports no disclosures pertaining to this article