Interventional Imaging in the Oncologic Patient

Introduction

The use of image-guided procedures has recently experienced tremendous growth in the setting of oncologic applications. There are several reasons for this increased use. Advances in diagnosis and therapy have led to increased survival benefit in this patient population. Earlier detection translates into more and more patients now presenting with their primary or metastatic disease still confined to a single organ. Therefore, these patients have the potential to be cured through the application of regional therapies, thus reducing systemic toxicity. A solid target lesion can now be accurately defined using novel imaging modalities, subsequently followed by the use of minimally invasive techniques to confirm the diagnosis and provide local curative or palliative therapies. In addition, recent advances in catheter technology, embolic agents, chemotherapy drugs, and delivery systems have been linked to further improvement of patient outcomes, sparking interest in combination approaches with systemic therapies. In this chapter, we discuss the most commonly performed interventional procedures in oncology and the central role of diagnostic imaging in the pre- and postprocedural care of these patients.

Image-Guided Tissue Ablation

Technical Background

Image-guided tumor ablation is considered a potential first-line treatment in many patients with small tumors, and it can be accomplished using chemical agents or thermal energy. Chemical ablation can be achieved by direct intratumoral percutaneous ethanol injection or, less commonly, acetic acid or chemotherapeutic agents that induce tumor cell death. Thermal ablation modalities include high-energy radiofrequency ablation (RFA), microwave ablation (MWA), cryoablation, interstitial laser photocoagulation, and high-intensity focused ultrasound (US), which causes coagulation necrosis. Irreversible electroporation constitutes a relatively novel, predominantly nonthermal, technique that is being increasingly investigated for the treatment of lesions in difficult locations, such as in the vicinity of the main bile duct in the liver. These procedures can be performed under imaging guidance by interventional radiologists or by surgeons in the operating suite. A complete analysis of each image-guided tumor ablation is beyond the scope of this review; thus, the chapter focuses on RFA, MWA, and cryoablation, because these are the most commonly used ablative techniques in North America.

Radiofrequency and Microwave Ablation

Image-guided percutaneous thermal ablation is typically guided by computed tomography (CT), US, or a combination of both. RFA and MWA are performed by connecting a generator that provides an electric current to an electrode that deposits the thermal energy. The tissues surrounding the tip are destroyed within seconds as temperatures reach 55°C, and are destroyed immediately at temperatures greater than 60°C. During RFA, an alternating electrical current (frequency range 480–500 kHz) is deposited within the tissues via an electrode placed directly into the tumor. Ions in the tissue follow these high-frequency directional changes, inducing heat proportionate to the strength of applied energy in the vicinity of the electrode. RFA is subject to the heat sink effect, which limits its effectiveness for perivascular tumors (vessels >3 mm in diameter). MWA is based on dielectric heating, where thermal energy is created by electromagnetic microwaves operating at frequencies between 900 and 2450 MHz. These microwaves force the water molecules within the field to continuously realign with the oscillating electromagnetic field, producing friction and heat. This mechanism generates heat faster than radiofrequency ionic excitement and is more resistant to the heat sink effect. The primary endpoint of successful curative ablative therapy is obtaining a zone of complete necrosis of at least 5 to 10 mm around the external margin of the target lesion. The size and shape of the ablation zone will vary depending on the amount of energy, type of electrode, duration of ablation, and characteristics of the inherent tissue. Image-guided RFA has been used to treat tumors in a wide variety of organs such as the liver, kidneys, lung, and bone, among many others. Initial RFA indications included the treatment of small lesions in patients who were not surgical candidates or for palliation of large lesions. However, owing to the efficacy and safety profile of the technique, its use has greatly expanded in certain diseases, to the extent that it is now used even in patients who are surgical candidates, with comparable outcomes. The limitations of the techniques include incomplete ablation of large (>5 cm), complex lesions, and proximity to delicate structures, such as gastrointestinal wall, gallbladder, diaphragm, and nerves, among other effects.

Cryoablation

Cryoablation is typically guided by CT or magnetic resonance imaging (MRI). Cryoablation consists of the application of freezing temperatures to tumors to cause tissue destruction. This technique has been used to treat tumors in a variety of tissues such as the liver, kidney, prostate, lung, and cervix. To perform cryoablation, a metallic probe is directly inserted into the target lesion. Argon gas circulates through the probe, causing a rapid drop in the local temperature. The ensuing low temperatures cause disruption of the cellular membrane and local ischemia. Ice crystals form within the cells and the adjacent interstitium, causing cell dehydration and surrounding vascular thrombosis. Subsequently, when the tissues thaw, vascular occlusion leads to further ischemic injury. Consistent tumor cell death is accomplished when the tissues are exposed to temperatures of at least –20°C, corresponding to the area approximately 3 mm inside the margins of the ice ball. The temperatures along the interface between the ice ball and the adjacent tissues, as well as in the ice ball’s peripheral rim, are suboptimal for tissue necrosis and represent only a reference for the interventional oncologist. As with RFA, the main limitations of cryoablation include proximity to blood vessels, gastrointestinal organs, nerves, and skin. Treatment of large tumor volumes with cryoablation can lead to the development of important systemic complications, such as cryoshock (a cytokine-mediated inflammatory response associated with coagulopathy and multiorgan failure), myoglobinuria, and severe thrombocytopenia. Otherwise, most complications of cryotherapy—such as hemorrhage and injury to adjacent organs—are generally similar to those of RFA. Tumor antigens released after cryoablation are taken up by antigen-presenting cells and can potentially function as an in situ vaccination stimulating antitumor immune response.

Clinical Applications

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