Thermal Ablation for Small Renal Masses

Nearly 80,000 patients in the United States will be diagnosed with kidney cancer in 2022.¹ Of these patients, an estimated 13,920 will experience kidney cancer-specific mortality.¹ The most common subtype of cortical kidney cancer is renal cell carcinoma, of which many subtypes exist.

Historically, patients present with symptoms such as hematuria, lethargy, weight loss, or a palpable abdominal mass. However, with the advent and increased utilization of cross-sectional imaging, increasing numbers of patients have received incidental diagnoses of localized kidney cancer. Moreover, patients with presumed earlier diagnoses due to incidental imaging findings tend to experience greater disease-specific survival.²

As larger volumes of renal masses are found incidentally, often falling into the category of small renal masses, the need for minimally-invasive treatment options has boomed. The latest American Urological Association (AUA) guidelines advocate for the use of partial nephrectomy as the primary management modality for all clinically staged T1a masses.³ Radical nephrectomy is recommended to be reserved for patients with high tumor complexity or those without evidence of underlying chronic kidney disease.³

Thermal Ablation Procedures

Thermal ablation (TA) techniques have been developed over the last century and provide a reasonable alternative modality for treatment of patients at high risk from surgical intervention owing to older age, multiple comorbidities, or other contraindications to receiving general anesthesia.

As the mean age of the US adult population continues to increase, and localized small renal masses continue to be discovered incidentally on imaging, thermal ablation procedures may play an important role in managing renal malignancy.

Radiofrequency Ablation

Radiofrequency ablation (RFA), a type of thermal ablation procedure, involves the use of thermal energy to induce cell death via cell membrane disintegration and protein denaturation. The first RFA probe design to be applied via a percutaneous approach was proposed by McGahan and van Raalte in 1990,⁴ which subsequently has been significantly improved.

Clinicians must take tumor characteristics into account when considering application of RFA. For example, highly vascular tumors are susceptible to a heat sink effect, where energy is preferentially transferred to flowing blood, resulting in tumor cell sparing at the site of ablation.⁵ Precise probe placement to achieve successful margins is essential because the interventional radiologist monitors RFA results by tracking impedance and temperature.

Cryoablation

Cryoablation, another type of TA technique, was described as early as the 1960s, but only came into mainstream use in the 1990s, when argon gas probes were created.⁴ Cryoablation works by inducing rapid freezing of tissue, resulting in mechanical trauma to cell membranes via formation of intracellular “ice crystals.”

The rapid freezing phase is followed by a “cooling” phase in which the extracellular fluid compartment is depleted, resulting in membrane rupture with eventual scar formation via fibrosis at the site of ablation.⁶

Cryoablation is similar to RFA in that it is also subject to the heat sink effect of highly vascular tumors. For cryoablation, side effects, such as ablation site/tumor “fracture” with subsequent bleeding or paresthesias after ablation, have been reported.⁷

Microwave Ablation

Microwave ablation is the third type of TA technology available for treating renal masses.⁴ It functions by inducing water molecule oscillation, which subsequently releases thermal energy; however, its efficacy is limited by the water content of a given tissue.

Compared with RFA and cryoablation, the benefits of microwave technology include the ability to induce ablation past the desiccation interface (a limitation specific to RFA) and its lower susceptibility to the heat sink effect.⁸

Irreversible Electroporation

Irreversible electroporation is the newest entrant into the ablation space.⁹ Unlike traditional ablation techniques, irreversible electroporation functions via induction of a direct electric current into the target tissue, inducing “nanopores” within cell membranes and resulting in apoptosis.⁹

This technique is especially useful for renal masses near sensitive segments of the kidney, such as the renal hilum or other critical structures that should not be harmed.

Guidelines for the Use of Thermal Ablation

The National Comprehensive Cancer Network® (NCCN®),¹⁰ the European Association of Urology (EAU),¹¹ and the AUA³ have issued guideline recommendations for use of thermal ablation techniques. The NCCN considers TA a viable treatment option for T1 tumors, whereas the AUA considers ablation an “alternative approach” for all clinically staged T1a masses <3 cm.¹⁰ The EAU guidelines are the most conservative, recommending ablation only for patients with multiple comorbidities and advanced age.¹¹

Patient selection is critical in the decision to use TA. Generally, TA should be considered in patients with tumors ≤T1a, with sizes >3 cm being a relative contraindication. Both tumor complexity and location on the kidney should be considered when evaluating a patient for ablation.

As with any oncologic therapeutic technique, appropriate and timely follow-up is critical to ensure optimal outcomes. The general recommendation is that all patients should undergo cross-sectional imaging via computed tomography or magnetic resonance imaging 3 to 6 months after the initial ablation,³ with appropriate mass ablation demonstrated through loss of contrast enhancement. In patients undergoing RFA, a ring of fibrosis will appear around the mass, but its size will be stable; in those undergoing cryoablation or microwave ablation, there will be a reduction in mass size.

Persistent enhancement demonstrated via repeat imaging after 12 months from the initial ablative episode is considered highly indicative of incomplete ablation.³ In such cases, patients should undergo repeat ablation or surgical excision.

Thermal Ablation Outcomes for Small Renal Masses

Outcomes of these ablative techniques vary depending on the type of ablation used. Although posttreatment renal function and cancer-specific survival rates are equivalent, these data generally indicate a slightly higher rate of local recurrence and a slightly lower rate of overall survival with ablation versus partial nephrectomy.¹²

When comparing cryoablation and radiofrequency ablation directly, no substantial differences in local recurrence, disease progression, overall survival, or cancer-specific survival have been reported.¹³ Data for newer techniques, such as microwave ablation and irreversible electroporation, are immature and cannot be reliably compared.

Although ablative techniques are often less invasive than surgical techniques, they are not without complications. When comparing percutaneous with laparoscopic ablation, the latter has a higher rate of complications,³ with intraoperative or postoperative bleeding being the most common complication of both techniques.

Other significant complications that may require urologic intervention include collecting system injury with or without development of subsequent urinary leak. Additional potential issues include injury to the pleura, resulting in pneumothorax or hemothorax. Colonic injury is another devastating complication that, in the worst circumstances, may necessitate exploratory laparotomy and diversion.

In conclusion, thermal ablation is an effective tool for treatment of small cortical renal masses, particularly in patients of advanced age or with multiple comorbidities who may not be able to withstand the demands of general anesthesia for partial nephrectomy. Learn about more therapeutic modalities in the treatment of renal cell carcinoma.

Akhil Abraham Saji, MD is a urology resident at New York Medical College / Westchester Medical Center. His interests include urology education and machine learning applications in urologic care. He is a founding and current member of the EMPIRE Urology New York AUA section team.

 

References

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