Thermal Ablation Dosimetry for Radiofrequency and Microwave Ablation

How thermal ablation works

In RFA, a radiofrequency alternating current (typically 200 to 500 kHz) is delivered through a needle applicator placed inside the tumor. Resistive heating raises tissue temperature above 60°C, causing coagulative necrosis in a zone around the electrode tip. In MWA, microwave energy at 915 MHz or 2.45 GHz heats tissue through dielectric heating. MWA achieves higher temperatures more rapidly and produces larger ablation zones than RFA in most tissues.

Both modalities are used for liver (HCC and colorectal metastases), lung, kidney, adrenal, and bone tumors. Liver cancer is the largest indication by procedure volume. For HCC in particular, percutaneous thermal ablation is a first-line curative option for tumors up to 3 cm and an important bridge or alternative to surgical resection for larger lesions.

The ablation margin problem

A clear safety margin of at least 5 mm around the tumor boundary is the accepted standard for local tumor control. A 2026 international multisociety Delphi consensus on liver tumor thermal ablation margin assessment — published in Lancet Oncology and endorsed by ESSE, CIRSE, and SIO, with 72 expert participants across North America, South America, Europe, and Asia — confirmed margin assessment as a central determinant of ablation success.^{[4, 5]}

Achieving a reliable 5 mm margin is harder than it appears. The obstacles are physical, not just technical:

• The heat sink effect: blood vessels larger than 3 mm actively cool adjacent tissue, significantly reducing the ablation zone on the vascular side. A tumor sitting next to a hepatic vein will have a smaller zone in that direction regardless of energy delivered.
• Tissue heterogeneity: the thermal conductivity and perfusion of liver parenchyma, tumor, fibrotic tissue, and adjacent fat differ substantially. Generic manufacturer charts cannot capture this.
• Intra-operative deformation: the liver shifts and deforms between the pre-operative scan and the actual procedure due to patient positioning, respiratory motion, and gas from the procedure itself. A plan based on pre-op imaging can be off by centimeters by the time needles are in place.
• Device variability: manufacturer ablation zone charts are based on ex vivo or in vivo animal models and represent averages, not patient-specific predictions.

Why elastic image registration matters

Most existing planning tools use rigid image registration — they assume the organ stays in the same position and shape between the planning scan and the procedure. For the liver, this assumption is routinely violated. The liver can shift several centimeters between a pre-operative MRI (acquired under breath-hold) and a cone-beam CT taken intra-operatively.

hapchot uses elastic (deformable) image registration, which estimates the three-dimensional deformation field mapping the pre-op anatomy onto the intra-operative anatomy. The tumor, vessels, and surrounding structures are remapped accordingly, giving the simulation an accurate geometric foundation at the time of treatment — not at the time of the planning scan.