Thermal dosimetry gains strength as ablative therapy standard

When it comes to radiofrequency ablation, one size does not fit all. Tumors and surrounding tissues react so differently to the thermal injury produced by the ablation probes that it is necessary to develop tissue-specific temperature and time of ablation protocols, according to researchers from Harvard University.

RFA has proven its value in the treatment of small solid focal tumors in many organs. Similar RFA protocols in terms of procedural time and temperature have proven inconsistent, however, across organ systems and particularly in lesions larger than 3 cm.

Physicians need to know in advance how tumors and their host tissues will react to RFA in order to balance complete tumor eradication with minimal collateral damage, researchers wrote in the study led by Dr. Pawel Mertyna from the Beth Israel Deaconess Medical Center's Laboratory for Minimally Invasive Tumor Therapies.

Mertyna and colleagues performed RFA with internally cooled and monopolar probes in 110 ex vivo and in vivo animal specimens, including some with malignant tumors. The investigators calculated thermal dosimetry using two methods to quantify the RFA-delivered heat and the procedural time at multiple tissue depths. They found a wide range of thermal sensitivity to RFA among tissues and tumor models (J Vasc Interv Radiol 2007;18:647-654).

The researchers recorded temperature differences of up to 14°C at the ablation margin for each tissue and/or tumor type (p<.01). Temperature depended on the procedure duration. For 10-minute treatments, temperatures were significantly higher in the kidney (72°C), similar for liver and muscle (51.6°C and 54.1°C, respectively), and lower compared with mammary adenocarcinomas (41.6°C).

The commonly accepted standard to induce thermal tissue coagulation is 50°C for about four to six minutes, which seems an overly unrefined threshold. Understanding the thermal dosimetry of differing tissues, particularly at the ablation margin where temperature is lowest, could be clinically beneficial, the researchers said.

"The better we understand the unique thermal characteristics of different tissue and tumor types, the more accurate our radiofrequency ablation treatment planning and control," said Dr. Debra A. Gervais, director of interventional radiology at Massachusetts General Hospital.

The study holds important implications for practice. In the future, interventional radiologists may achieve more refined and accurate tumor-specific or tissue-specific RFA protocols. Patients will have more precise treatments for their tumors and potentially less thermal injury to normal tissues, Gervais said.

More studies are needed to define the thermal dosimetry of tissue-specific ablation end points, particularly in view of the booming popularity of ablation, according to the researchers.

"Understanding and utilizing more complex thermal dosimetry will likely enable better pretreatment planning and improve real-time assessment," they said.

For more information from the Diagnostic Imaging archives:

Size and trajectory matter most in lung RFA

Cool trend in IR practice looks for warm reception

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RFA 'biopsy' shows prognostic potential