Introduction

Advances in medical imaging have created the opportunity for minimally invasive, image-guided oncologic care by allowing: (1) procedure planning; (2) device delivery; (3) intraprocedure monitoring; and (4) therapy assessment. Although most current image-guided therapy still utilizes standard diagnostic imaging equipment, interventional use of imaging equipment has in fact different priorities compared with diagnostic uses of such equipment. Therefore, interventional procedures prioritize imaging equipment that: (1) provides real-time imaging; (2) lowers radiation dose; and (3) provides greater physician access to the patient. In contrast to diagnostic imaging, lower image quality is an acceptable compromise for real-time imaging for interventional procedures. Patients have already undergone high-quality diagnostic imaging when they are referred to interventional therapies. Moreover, high-quality diagnostic imaging may require more time and more radiation dose than fast imaging of a restricted region of interest as performed for image guidance of interventions.

Although current imaging systems provide some of the required features for interventional procedures, none provides all of them. Ultrasound (US) is a real-time, multiplanar technique, but is limited in terms of detection or tumor visualization. Computed tomography (CT) provides partial access and can be used to guide procedures intermittently, but exposes patients and staff to ionizing radiation. Moreover, CT is primarily a two-dimensional (2D) planar tool; real-time, three-dimensional (3D) imaging is not yet fully integrated into interventional CT applications. Magnetic resonance imaging (MRI) seems to be the most reliable technique, allowing interventions to be performed for tumors that are visible only with MRI, such as in case of soft-tissue tumors, and provides thermal monitoring of ablations, but access to MRI systems is limited and MR tool compatibility is lacking. However, using these techniques, recent intervention-focused improvements have helped broaden the applications of image-guided therapy.

Imaging for procedure planning

The first critical application of imaging in any image-guided procedure or, for that matter, any surgical procedure is in the planning phase. In this application, the most relevant, high-quality diagnostic imaging study available must be evaluated. In many cases, the evaluation requires an assortment of imaging studies. Some may be anatomic studies such as contrast CT or MR, and other studies may be physiologic studies such as positron emission tomography (PET) or single-photon emission computed tomography (SPECT).

Introduction

In the last decades, the incidence of prostate cancer tripled to 152 new cases of prostate cancer per 100,000 men in 2013. Among cancer in men, it is the third most common cause of death in men, with 23 per 100,000 men/year. A better understanding of prostate cancer biology and earlier detection with prostate-specific antigen (PSA) screening and imaging have contributed to interest in less invasive alternatives to surgical resection.

Focal tumor ablation has been increasingly used for local control or with curative intent in solid-organ tumors such as kidney, thyroid, breast, liver, and lung. While stimulating thought about ablation application in prostate cancer, clinical application has been limited despite encouraging functional and short-term oncological outcomes. Furthermore, despite evidence that men with low-risk localized prostate cancer may not benefit from treatment in terms of prostate cancer-specific mortality, many men still elect to undergo radical treatment. For these patients, minimally invasive options that could provide oncologic efficacy with little impact on quality of life may be attractive. More importantly, focal ablation of intermediate-risk tumors may enable more men to remain on active surveillance, thereby sparing them the consequent harms associated with radical treatment, including sexual, urinary, and bowel complications.

Prostate ablation is an emerging treatment modality offering promise for local cancer control with reduced morbidity relative to alternatives. The image-guided nature of focal ablation techniques is particularly appealing as preservation of erectile, urinary, and rectal function can be achieved by minimizing damage to the neurovascular bundles, external sphincter, bladder neck, and rectum. The reality, however, is that ablation strategies have not been standardized in the prostate. A trend toward targeted treatments in men with smaller tumor volume and away from whole- or half-gland therapy has been observed. One explanation is that the natural history of the disease is driven by the largest lesion with the highest grade, the so-called “index lesion,” and not by the presence of multiple foci of disease observed in surgical series. Improvements in imaging techniques, particularly magnetic resonance imaging (MRI), now enable visualization of small foci of prostate cancer. Whether focal ablation should play an increasing role in prostate cancer management is a matter of ongoing debate, though clinical outcomes after treatment of smaller-volume disease in other organs have been encouraging.