Attenuation and de-focusing during high-intensity focused ultrasound therapy through peri-nephric fat.
High-intensity focused ultrasound (HIFU) is an attractive therapy for kidney cancer, but its efficacy can be limited by heat deposition in the pre-focal tissues, notably in fat around the kidney (peri-nephric fat), the acoustic properties of which have not been well characterized. Measurements of attenuation were made using a modified insertion-loss technique on fresh, unfixed peri-nephric fat obtained from patients undergoing kidney surgery for cancer. The de-focusing effect of changing the position of the fat layers was also investigated using fresh subcutaneous fat from euthanized pigs. The mean attenuation of human peri-nephric fat was found to be 11.9 ± 0.9 Np/m (n = 10) at 0.8 MHz, the frequency typically used for HIFU ablation of kidney tumors, with a frequency dependence of f(1.2). A typical 2- to 4-cm thickness of peri-nephric fat would result in a de-rated intensity of 3% - 62% at 0.8 MHz compared with a hypothetical patient with no peri-nephric fat. Through the use of freshly excised porcine subcutaneous fat, the presence of fat 100 mm in front of the focus was found to have a de-focusing effect of approximately 1 mm in both transverse directions, which corresponds to a full HIFU beam width off-target. Peri-nephric fat may significantly affect both the intensity and accuracy of HIFU fields used for the ablation of kidney cancer.
Nuffield Department of Surgical Sciences, Oxford University Hospitals, Oxford, UK. robritchie1@gmail.com
Robert
R
Ritchie
Jamie
J
Collin
Constantin
C
Coussios
Tom
T
Leslie
D000273:Adipose Tissue / Q000503:physiopathology* / Q000528:radiation effects* / Q000601:surgery; D000818:Animals; D019220:High-Energy Shock Waves; D057086:High-Intensity Focused Ultrasound Ablation / Q000379:methods*; D006801:Humans; D066298:In Vitro Techniques; D007668:Kidney / Q000503:physiopathology* / Q000528:radiation effects* / Q000601:surgery; D011829:Radiation Dosage; D012542:Scattering, Radiation; D013552:Swine
Attenuation; De-focusing; High-intensity focused ultrasound; Kidney; Peri-nephric fat; Renal cancer; Thermal ablation