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Science to Practice |
1 West Reading Radiology Associates, PO Box 16052, Reading, PA 19612-6052 rduszak@att.net
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As an alternative to the elusive "magic bullet," modern cancer therapy often combines several treatment options to maximize tumor destruction and minimize host toxicity. With increasing frequency, minimally invasive treatment options offered by interventional radiologists play a role in such multimodality cancer care. In the current issue of Radiology, Chang et al (1) combine two distinct procedures—radiofrequency (RF) ablation and arterial occlusion—to potentially expand the size of renal tumors that could otherwise be treated with RF ablation alone.
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A relatively new method of image-guided cancer therapy, RF ablation is limited in its applicability in part because of the relatively small size (usually <4 cm) of tumors that can be ablated with a single probe position. Recently, several investigators have shown that treatable tumor size might be expanded by selectively occluding either the arterial (2,3) or the venous (4) blood flow to the liver or by nonselectively occluding arterial flow to the kidney (5).
By using an animal model, Chang et al (1) performed RF ablation in the normal kidneys of swine both without and with vascular occlusion. They measured the size of the resulting thermal lesions (ie, zones of cellular nonviability) and compared the two lesion groups (ie, lesions created with and those created without occlusion). Vascular occlusion was performed by either surgically ligating the renal artery or selectively embolizing it with Embosphere particles (Biosphere Medical, Rockland, Mass).
The mean diameter of thermal injuries was 0.86 cm ± 0.07 when RF ablation was performed alone (ie, without occlusion) and 1.38 cm ± 0.05 when it was performed with vascular occlusion. This amounts to a 60% increase in lesion diameter, or a threefold increase in ablation lesion volume. There was no substantial difference in ablation zone size with surgical ligation (ie, clamping) versus with transcatheter embolization (1.40 cm ± 0.06 vs 1.33 cm ± 0.07, respectively).
To understand why the ablation zone is enlarged in the setting of vascular occlusion, one needs a basic understanding of how heat is transferred out of soft tissues (see diagram below). The bioheat transfer equation referenced by Chang et al is complicated, but the concept is simple: Heat loss occurs by means of either conduction or convection. With conduction, energy is dissipated directly into contiguous tissues. This occurs with or without perfusion and is a relatively inefficient method of cooling. With convection, heat is transferred into nearby flowing blood, analogous to the transfer of heat from a car’s engine to the circulating coolant in its radiator. The body’s bloodstream thus acts as a "heat sink," allowing rapid dissipation of thermal energy. By occluding blood flow to an organ, one effectively "breaks the radiator." This slows cooling, expands the zone of thermal injury, and results in more cell death.
The Practice
Clinical use.—Most commercially available RF ablation systems create a sphere of thermal injury measuring approximately 3 cm in diameter. Many solid organ tumors, however, are larger. Their treatment usually necessitates overlapping ablations, which are both time consuming and difficult to plan (6). Unless precisely created, the resulting ablation zones are heterogeneous, nonspherical, and sometimes unpredictable. Vascular occlusion at the time of RF ablation may allow larger tumors to be more reliably ablated in their entirety.
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Future opportunities and challenges.—Contrast material–enhanced computed tomography and magnetic resonance imaging are typically used in follow-up to assess for residual or recurrent viable tumor. Imaging at the time of RF ablation currently facilitates very precise probe placement. Ideally, this imaging would also allow accurate contemporaneous monitoring of the zone of thermal injury so that problems with asymmetric ablation or incomplete margin control could be identified and addressed at the time of the initial procedure.
While adjunctive vascular occlusion during RF ablation appears promising, numerous technical details are still unresolved. For example, with balloon occlusion, will anticoagulation help (by minimizing nontarget thrombosis and ischemia) or hurt (by causing excessive bleeding, even with access tract cautery)? Is selective embolization (more time consuming) preferable to temporary balloon occlusion (more expedient)? Only actual clinical experience will answer these and other practical questions.
Finally, despite the attractiveness of performing percutaneous RF ablation on increasingly large tumors, its effects on quality of life and survival are yet unknown. As with any emerging cancer therapy, well-designed clinical trials will ultimately be necessary to rigorously evaluate outcomes.
Summary
By using vascular occlusion to augment the thermal effect of RF ablation on renal tumors, Chang et al have added to the small but growing literature experience demonstrating the promise of these complementary techniques in treating large solid organ tumors. Further investigation will undoubtedly cast more light on the clinical indications and technical details that may eventually bring this science into widespread practice.
FOOTNOTES
See also the article by Chang et al in this issue.
REFERENCES
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