“Ablation” may be generally described as a therapeutic destruction and/or sealing of tissue, whether to destroy diseased tissue, remove necrotic tissue, create a lesion to produce a therapeutic effect (as in treatment of atrial fibrillation) or to otherwise dissect tissue for therapeutic benefit. As a fundamental tissue effect, ablation can in principle be accomplished by a large range of alternative modalities or energy types, but the practical application of ablation to different clinical practices has emerged from the constraints that specific target tissue types put forth — minimizing collateral tissue damage, creating ideal lesion types, limitations of the surgical approach that lend greater or lesser advantage to one modality compared to others, etc. The technologies representing the range of alternative ablation types are grouped into nine sectors:
- Electrical
- Radiation
- Light
- Radiofrequency
- Ultrasound
- Cryotherapy
- Thermal (other than cryotherapy)
- Microwave
- Hydromechanical
In 2010, given its long history in medicine, radiation represented the largest share of global revenues of energy-based ablation devices, followed by light (essentially laser) with 19% and ultrasound with 15%. The total market is forecast to grow at a compound annual growth rate (2010-2019) of 11.2%. Despite the economic slowdown of 2008-2009, the energy-based ablation devices market continued growing vigorously and is expected to continue to grow at a strong rate over the next five+ years. The total CAGR of 11.2% is deceptively modest, because these figures reflect the combined market sizes and growth rates of nine sectors. Those nine sectors, or modalities, vary widely in size and growth rates: from thermal, with an estimated CAGR of under 3%, to cryotherapy with a CAGR approaching 19.5%. Four of the modalities are forecast to experience compound annual growth rates equal to or exceeding 11%. Electrical and electrocautery devices have long been a mainstay of the surgeon’s toolbox, and they will continue to be used for the foreseeable future. Some estimates say that as much as 80% of all surgical procedures make use of one of these devices. Key among the advantages offered by these products is the ability, depending upon the procedure, to assist the surgeon to conduct a procedure rapidly—often more quickly than with a cold scalpel. Electrical ablation is used in a wide array of surgical procedures, including colon resection, hysterectomy and gastric bypass, to name a few. Radiation devices cause destruction of target tissues by disruption of cellular mechanisms, often with surgical precision, without ever cutting the skin. These systems have advanced to a high-tech level unforeseen even ten years ago. Radiation ablating equipment includes traditional radiotherapy machines, image-guided radiotherapy (IGRT) and intensity-modulated radiotherapy (IMRT). Over the last ten years or so, radiologists have been moving towards more advanced treatment techniques, such as those utilizing multiple or non-coplanar beams, 3-dimensional conformal radiotherapy (3DRT) and IMRT, to treat tumors. Physicians view the accuracy of computed tomography-based 3-dimensional target delineation, which provides more detailed targeting than does 2-dimensional design, as another very attractive treatment option. Light-based or laser devices use high-intensity light to shrink or destroy tumors. Various lasers have different effects on different tissues, depending on the laser’s wavelength. Lasers commonly used for medical and/or aesthetic purposes include Erbium:YAG, ruby, CO2, and neodymium:YAG-laser (Nd:YAG). Also in this category are femtosecond and excimer lasers. Femtosecond lasers allow extreme precision in surgery. The possibilities for its use now include but are not limited to femtosecond keratoplasty, astigmatic keratoplasty, and keratoconus. Excimer lasers typically produce ultraviolet light, and are used in LASIK eye surgery. Radiofrequency energy is characterized by a specific frequency measurable in Hz. Medical devices that emit RF energy produce a change in the electrical charges of the treated tissue, creating an electron movement. Electrosurgical cutting uses sharply focused, intense heat at the surgical site to cut the tissue. By holding the electrode a small distance away from the tissue, the surgeon can produce the most intense heat over a very short amount of time. This results in vaporization of the tissue and the desired cutting effect. Vessel sealing and ligating devices usually utilize electrical energy combined with pressure to seal vessels and to cut off small bits of tissue. Ultrasound energy relies on the fact that as an acoustic wave propagates through tissue, part of it is absorbed and converted to heat. Focusing sound waves allows concentrated energy deposition to occur deep in tissue, allowing precisely localized heating and thermal coagulation while sparing intervening tissue. High intensity focused ultrasound, or HIFU, treats a precisely defined portion of the targeted tissue. Because this technology can achieve precise ablation of diseased tissue, it is often referred to as ‘HIFU surgery’, or ‘non-invasive HIFU surgery.’ Cryotherapy uses extreme cold to freeze and destroy the target tissue, such as a cancerous tumor. It is applied in a freeze-thaw process. The cryotherapy probes, needles or catheters are carefully positioned in place using ultrasound guidance, then the freezing agent, argon gas, is allowed to circulate through the cryotherapy probes, causing an ice ball to form in the tissue at the tip of the probes. The tissue is frozen rapidly, then thawed slowly and completely, and then is put through a second freeze-thaw cycle. It is the intensity of the freezing that determines the ultimate response of the targeted tissue, which may range from chilled to inflammation to cell death. Different cell types show different sensitivities to freezing, a fact which can be used for therapeutic purposes. For example, prostate cancer cells demonstrate different susceptibilities to freezing than do other tissues, a difference that has been linked to the presence of the androgen receptor. Thermal ablation devices may be engineered to produce a variety of temperatures in tissues, depending upon the intended usage. These temperatures may range from 39 – 40 °C up to as high as 80 – 90 °C, under well-controlled conditions. When hyperthermia is used, there is evidence of a number of processes taking place, which can include enhancement of the anti-tumor effects of radiation and of various drugs; induction of immunological processes; induction of gene expression and protein synthesis; and general changes to the tumor’s environment which make the tumor more accessible to some therapies. Above 43°C, the heat itself has a cytotoxic effect on the cells. Microwave hyperthermia is a non-ionizing form of radiation therapy. Low levels of microwave energy are used to vigorously vibrate water molecules in tissue to quickly and effectively heat the tissue to a physical penetration depth defined by the microwave frequency. Microwave has also been shown to improve the results of radiation therapy for the treatment of some recurrent and progressive tumors. The resulting hyperthermia destroys cancer cells by raising the tumor temperature to a ‘high fever’ range. Recent research appears to show that cancer cells may be particularly vulnerable to microwave-induced hyperthermia due to their high acidity. Microwave energy disrupts the stability of the cellular proteins and kills the cells. Hydromechanical ablation is energy-based tissue destruction accomplished via mechanical means, such as extracorporeal shock wave lithotripsy devices, or jets of water or saline. In extracorporeal shock wave lithotripsy, the lithotriptor uses an external hydromechanical energy source to break up the stone with minimal collateral damage. The successive shock wave pressure pulses result in direct shearing forces which fragment the stones. Water jet surgery, a form of dissection which has been used successfully for several years, employs the kinetic energy of the water jet to separate different tissue types by their varying elasticity and firmness. In hepatic surgery, for example, the device can selectively differentiate between liver parenchyma, blood vessels and bile ducts. This modality does not cause thermal damage to tissue and can sculpt, ablate and cauterize bleeders.