Tumor Ablation: Treatment and Palliation Using Image-Guided Therapy

Imagine destroying a tumor in 6 minutes with a small needle electrode placed through the skin into a tumor deep in the human body-the patient is cured after spending only a few hours in the hospital and leaves with just a small bandage. This may sound like science fiction, but it is reality in many medical centers around the world. It seems like yesterday that I was a resident rotating through Massachusetts General Hospital, helping Dan Rosenthal treat an osteoid osteoma by radiofrequency (RF) ablation. That defining event, which took place 14 years ago, was my first exposure to image-guided tumor ablation; that seminal work[1] encouraged me and others to become involved in this burgeoning field of medicine. As guest editor of this supplement to ONCOLOGY devoted to image-guided ablation, I would like not only to share my thoughts on the past and present, but also to shed some light on patient management and follow-up. I thank my fellow contributors to this supplement for their concise and clinically relevant overviews of the most important areas of image-guided tumor ablation. I found them balanced and extremely well written. Over the past decade we have seen a continued expansion of clinical applications for ablative techniques through single-center trials and several multicenter trials designed to rigorously document safety and efficacy. The proliferation of ablation technology and its dissemination into the treatment of various solid tumors[2] is the reason for this update, which will be invaluable for those interested in introducing this new technology into their radiology practices as well as those who are already using it. The short reviews highlight the current knowledge of image-guided tumor ablation in liver, kidney, bone, and lung and illustrate its use with pertinent clinical examples. We understand that some areas of utilization are further along in widespread implementation than others. For those areas that are less well researched, we provide provocative viewpoints for future expansion. History and Development
The concept of killing a tumor in situ without ionizing radiation or surgery is not new, as the quotation from Hippocrates attests. Previously, direct visualization was necessary, as in the pouring of liquid nitrogen into the cavity created by a giant cell tumor of bone.[2] The advent of cross-sectional imaging with ultrasound or CT enabled physicians to precisely and directly place needles percutaneously into solid tumors in almost all regions of the human body. The procedure was initially performed for diagnosis by needle biopsy, but direct therapeutic intervention closely followed. In 1986, Livraghi and colleagues described the direct injection of chemotherapy into tumors of the liver, pancreas, pelvis, and lung under crosssectional imaging guidance.[3] This work was shortly followed by the direct injection of absolute ethanol.[4] Heat-based ablative technologies had been available for decades and were initially applied to the trigeminal ganglion for treating patients with trigeminal neuralgia.[5] This same technology was then applied to bone[6] and eventually led to the treatment of osteoid osteomas.[1] As the RF technology improved, newer devices[ 7,8] allowed the destruction of larger volumes of tissue, opening up new applications that were quickly applied to liver tumors. With the creation of cryoprobe technology in the early 1960s[9] and refinements in the late 1970s and 1980s, surgeons and radiologists could place probes directly into liver tumors in the operative setting, using ultrasound guidance. Newer percutaneous cryoprobes developed over the past decade have enabled additional clinical applications, including treatment of prostate cancer.[10] Other heat-based ablative technologies also being used to treat tumors under image guidance, such as laser and microwave, show considerable promise. The ultimate goal of these techniques is to provide excellent local control rates with fast treatment times at reasonable cost. Head-to-head comparisons in the literature are thus far lacking, and this presents a fertile area for image-guided interventional research. Patient Selection
Appropriate patient selection is essential to the success of these new technologies. Similar oncologic principles apply to ablation as to radiation therapy and surgery. Removal or destruction of a local tumor makes sense when tumor staging suggests localized disease or when symptom control or palliation is the goal. Choosing the appropriate patient also means choosing the appropriate tumor and taking the tumor biology into consideration. Rapidly growing tumors with rapid dissemination characteristics are poor candidates for local therapy. Many patients have comorbid medical conditions that make traditional oncologic treatments such as surgery, radiation, and chemotherapy inappropriate for them. Treating these patients makes sense if tumor control is likely to improve their quality or quantity of life. Implementing local ablative therapy does not make sense if the patient is likely to succumb to an underlying medical condition. As cancer treatment is a relatively new area for most radiologists, conferring with a team of subspecialists at the tumor board level is an excellent means of acquiring the knowledge and credibility of the other treating physicians. The team approach becomes increasingly important for patients who are candidates for multimodal therapy, given the systemic nature of many cancers and the known synergy of ablative techniques with chemotherapy and radiation. Imaging Evaluation
Since ablative therapy is not extirpative, identifying residual or recurrent disease against the background of treatment effects can be difficult in the early posttreatment period. Positron emission tomography and contrast CT/MRI may identify areas of persistent metabolically active tissue or tumor neovascularity. The precise timing and sensitivity of these techniques has not been defined, but suffice to say that growth in size beyond the postablation baseline examination is more than likely tumor growth, and new metabolically active areas with qualitative and quantitative assessment in the neoplastic range are also likely evidence of progression. Current scientific study on the most appropriate modality and timing of utilization is not yet conclusive. Ongoing multicenter trials have incorporated imaging evaluation as part of the scientific design in the hopes of answering these important questions. As with many patient examinations, definitive answers in equivocal cases can be obtained by biopsy evaluation or additional follow- up over time. Conclusion
Because the field of image-guided tumor ablation is young, many questions remain unanswered. Early results are very encouraging, however, and the future scientific progress in technology and clinical trials will surely lead to its continued success and implementation.