Radiofrequency ablation of renal cell carcinoma
W. Scott McDougal, Department of Urology, Massachusetts General Hospital, 55 Fruit Street, GRB 1102, Boston, Massachusetts 02114, USA.
The treatment of RCC of <4 cm when in appropriate areas of the kidney has changed dramatically over the past several decades. With the realisation that preserving nephron mass is important, particularly in an ageing population, there has been a move to use partial nephrectomy (PN) in properly selected patients. As cancer control has been shown to be equivalent in patients who have PN and those who have total nephrectomy in the appropriately selected patient [1,2], less-invasive ablative techniques have been developed in an attempt to reduce the morbidity of treating these tumours, while maintaining the benefit of preserving a maximal amount of normal renal tissue.
Because of the widespread use of abdominal imaging, there are more incidentally discovered renal tumours and more RCCs detected before death. The incidentally discovered tumour tends to be smaller and of lower grade than tumours that are discovered when they become symptomatic [3,4]. Although tumours which are <3 cm in diameter have a low metastatic potential (≈ 2%) that patients have longer life-expectancies and that the growth of these tumours is 1–2 cm/year suggests that at least some of them will grow sufficiently to become symptomatic and metastatic. The increased life-expectancy of patients, the increased incidence of small renal tumours, and the desire to preserve as much renal parenchyma as possible has propelled minimally invasive technologies into the forefront in dealing with these problems. One of these technologies is radiofrequency ablation (RFA), the subject of this review.
RF is an energy source that has been used to destroy tissue and by the present author to treat renal tumours in vivo since 1999 . The electrical current is applied in the RF range, which results in ion agitation of the surrounding tissue, with frictional heating. The probe, which delivers the RF current, can generate temperatures in the surrounding tissue of >100 °C. At 45 °C cellular functions are disabled; at 60 °C proteins are denatured and at 100 °C cells are vaporized. Temperatures of 100 °C, which vaporize tissue, result in charring with impairment of heat conduction away from the probe, thus limiting the amount of tissue that can be destroyed. By pulsing the current and keeping the temperature below levels that result in charring (monitored by impedance) the cells can be effectively killed with a sufficient radial extent of injury from the probe. As the impedance increases with tissue charring, keeping the impedance at predetermined levels and pulsing the current, in addition to saline cooling of the probe, enhances the amount of heat that can be delivered at distances from the probe.
The protocol we use in patients who are candidates for RFA has involved biopsy of the renal mass to be certain that it is RCC. When RCC is confirmed pathologically, under CT guidance, the probe is introduced percutaneously using local anaesthesia with sedation, and the RF current applied. When we began, only patients with a life-expectancy of >1 year and <10-year, and those with significant comorbid diseases that precluded surgical extirpation, were chosen for the procedure. The follow-up is by CT with contrast medium in those who can tolerate it, or by MRI with gadolinium in those allergic to contrast media. The follow-up is at 1 month after treatment, and if there is enhancement on CT the RFA is repeated until the follow-up CT shows no enhancement. Patients are then followed at 3-month intervals for 1 year, 6-month intervals for 1 year, and yearly thereafter. The area treated should show no enhancement on follow-up CT, and decrease in size [7,8]. In some cases, over time, the area disappears all together . It is clear from our initial data that the results depend on the size and location of the primary tumour. For this reason, a classification system was developed  by which to categorize the tumours, to be able to compare the results in comparable groups of patients. Renal tumours can be classified as exophytic, central, cortical/parenchymal or mixed. Exophytic tumours are those that arise from the renal cortex, protrude into the perinephric space, and are surrounded by fat. Central tumours are those in the medulla that protrude into the hilum of the kidney. Cortical/parenchymal tumours are confined to the cortex but do not protrude into the perinephric fat or hilum, and mixed tumours are those that involve both the central and peripheral portions of the kidney.
In our initial group of patients who were selected because of significant comorbidities or limited life-expectancy, we showed that at the 5-year follow-up the cancer eradication was comparable to that in patients who had a PN . Over the past 3–4 years several other centres reported successful outcomes in patients who have had RFA, but to date there are relatively few patients from several institutions who have a long-term follow-up, to formally establish the place of RFA in the treatment of RCC [10–12]. However, there are sufficient data to establish the method as preferential in certain subpopulations of patients, and to suggest that it will play a major role in the treatment of patients, as more data are accumulated.
Tumours of 1.1–5.5 cm in diameter were treated successfully using RFA; in our series, exophytic tumours of <5.1 cm are those most successfully treated, with a success rate of almost 100%. Several treatments might be required to treat some of these tumours, particularly if they are >3 cm. However, we have not been successful in eradicating tumours of >5.5 cm. For tumours located more centrally, our success rate is 44%. About 40% of patients will require at least one additional treatment due to follow-up CT enhancement, and these tend to be those who have larger tumours .
This technology has been used in children but there are several constraints that must be considered when treating younger patients. Children have minimal retroperitoneal fat to insulate vital structures and adjacent organs, increasing the risk of injury to these structures. Therefore, injection with saline to separate vital structures from the area treated, and lower currents, are appropriate in such cases. As the grounding pads are smaller, the power must be reduced to prevent ground-pad burns. This technology has been used successfully to manage a child with Wilms’ tumour .
Although complications are uncommon after RFA of renal cancers, they can occur, and some can be quite disabling. They can be divided into five major categories: haemorrhage (collecting system, perinephric space, urinoma), collecting system injury (ureteric stricture), adjacent organ injury (colonic), neuromuscular injuries (neuropraxia), and grounding pad injuries (cutaneous thermal injury, burns) [14–16]. The most common injury is haemorrhage, and is a significant clinical problem in ≈ 6% of the patients. It can occur in the perinephric space and/or the collecting system. Occasionally a ureteric stent might be required for renal colic in those who bleed into the collecting system. We have had to transfuse two of 85 patients; five of 85 have had a significant decrease in their haematocrit. We have not needed to use an open surgical procedure to control haemorrhage. Several patients have had a urinoma develop; all were managed successfully with percutaneous drainage. Two of 85 patients had a ureteric stricture, in retrospect due to ablating a tumour adjacent to the ureter without protecting the ureter. Currently we feel it is contraindicated to treat such patients without insulating the ureter. We have had no colonic injury but such has been reported. Several patients had lumbar neuroplaxia, which was quite difficult to manage, but resolved spontaneously usually over several months. We had several patients with second-degree burns and one had a third-degree burn at the grounding pad site; this was primarily excised and closed.
In reviewing our data and comparing these patients to those who have had PN we concluded that there are subpopulations of patients who are better treated with RFA, and others who are better treated by surgical excision. Patients who appear to be at particular advantage when RFA is used, as opposed to surgical resection, are those with three or more comorbidities and those aged >60 years. Patients who are better served by surgical resection are those with tumours of >5 cm, central tumours and those adjacent to the ureter .
RFA is highly successful in eradicating small exophytic lesions surrounded by perinephric fat. Although enough patients with sufficient follow-up have not been accrued to date, the technology is promising and will have a significant place in the treatment of localized RCC. To date, exophytic lesions of <5 cm have a high likelihood of successful treatment. Moreover, elderly patients and those with significant comorbid disease have fewer complications when RFA is used than with surgical resection.
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