Principles and types of ablation energies
The main goal of image-guided ablation is tumor destruction while preserving surrounding healthy tissue and minimizing trauma compared to surgery. Most methods are based on thermal effects causing coagulation necrosis. To reduce the risk of local recurrence, it is necessary to destroy not only the tumor itself but also the adjacent margin of normal parenchyma where microsatellites may be present.
Main types of thermal ablation:
- Radiofrequency ablation (RFA): uses high-frequency alternating current (460–500 kHz). The current causes water molecule movement, friction, and heating. Temperatures of 100–110°C are reached around the probe, and heat is further transferred via conductive heating. A disadvantage is its dependence on tissue characteristics, which can lead to marginal recurrences.
- Microwave ablation (MWA): electromagnetic radiation (900–2400 MHz) causes water molecule oscillation. Heating is more active and rapid (radius of about 10 mm from the tip), less dependent on tissue limitations and perfusion-mediated cooling.
- Cryoablation (CRA): uses freezing. A major advantage is the formation of an «iceball» (iceball), which is clearly visible on imaging, facilitating monitoring.
Tissue protection and image guidance
A critical task is to avoid damaging adjacent organs. To achieve this, the following are used:
- Hydrodissection: injection of fluid (5% dextrose or saline, sometimes with iodinated contrast) to physically displace the bowel.
- CO2 insufflation: also used to displace bowel loops.
- Patient positioning: for example, lateral decubitus positioning for adrenal tumors.
For targeting (targeting), ultrasound (provides real-time imaging), CT (or a combination), and MRI (good soft tissue contrast, but hindered by the narrow bore of the scanner) are used.
During radiofrequency and microwave ablation, the issue of gas formation (outgassing) arises — tissue vaporization creates artifacts that obscure the target and hinder evaluation. Cryoablation does not have this issue, as the formed «iceball» is clearly visible, e.g., on MRI as a signal void zone (signal void).
Outcome evaluation and clinical application
Treatment success is assessed by achieving a 360-degree resection margin. The main sign of complete necrosis is the absence of contrast enhancement on follow-up CT or MRI performed at 2–4 weeks.
Specific markers:
- In the kidneys: the «halo» artefact (halo artefact) in the perirenal fat.
- In the lungs: the «ground-glass opacity» zone (ground-glass opacification).
Application in various locations:
- Renal cell carcinoma: image-guided ablation is the priority nephron-sparing method for small tumors (< 35 mm). Cryoablation shows results comparable to resection, but with less invasiveness.
- Hepatocellular carcinoma (HCC): image-guided ablation is the method of choice for nodules < 3 cm if resection or transplantation is not feasible. For larger nodules, it is combined with chemoembolization.
- Colorectal liver metastases: ablation is used for lesions < 30 mm, often as an adjunct to chemotherapy. They are more difficult to treat than HCC.
- Lung tumors: ablation is indicated for unresectable patients. Metastasis selection: < 3 per hemithorax, size < 35 mm, distant from the pulmonary hilum (> 2–3 cm).
Briefly: answers to topical questions
What types of electrodes (probes) are used for radiofrequency ablation?
Needle applicators, as well as straight or expandable electrodes, are used for radiofrequency ablation. To achieve larger volumes of tissue destruction, the following types of devices are used:
- Needle applicator — used to deliver high-frequency alternating current to the target tissue.
- Clusters (clustering) — a combination of multiple needles on a single handle.
- Expandable multitined devices (expandable multitined devices) — allow for reproducible spheres of tissue destruction.
- Multipolar arrays — used for multipolar ablation.
- Straight or expandable electrodes (applicators) — inserted into the center of the tumor under image guidance.
What is the cellular mechanism of tissue damage in cryoablation?
The cellular mechanism of tissue damage in cryoablation is based on the development of necrosis induced by low temperatures. Freezing gradually spreads around the probe, forming an ovoid iceball. Cellular necrosis occurs at temperatures below −20°C. Main damaging factors:
- Ice formation and osmosis — the combination of these physical processes triggers cell death mechanisms.
- Protein denaturation — occurs due to extreme tissue cooling.
- Cell membrane rupture — is the final stage of cell destruction, leading to cell death.
Class and exam questions
What is the main advantage of microwave ablation over radiofrequency ablation?
Microwave ablation provides more active local heating and is less dependent on tissue conductivity limitations and convective cooling (heat-sink effect).
Why is real-time monitoring difficult in RFA and MWA?
Due to the outgassing effect: tissue vaporization creates artifacts that obscure the visualization of the target tumor.
What is the heat-sink effect?
This is the cooling of the ablation zone by blood flow in adjacent vessels larger than 3 mm in diameter, which prevents reaching the lethal temperature and can lead to the persistence of viable tumor at the margins.
How is the completeness of renal ablation assessed on postoperative imaging?
The main criterion is the absence of intravenous contrast enhancement. The appearance of a halo artifact in the perirenal fat surrounding the tumor is also characteristic.
What else to study on this topic
- Combined treatment of hepatocellular carcinoma (image-guided ablation + chemoembolization)
- Barcelona Clinic Liver Cancer (BCLC) algorithm and treatment selection
- Follow-up protocols after ablation of colorectal metastases
- Differential diagnosis of recurrence and blood breakdown products on MRI
- Stereotactic ablative radiotherapy (SABR) for lung tumors
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