Treatment of localised prostate cancer with transrectal high intensity focused ultrasound.

With the advent of PSA dosing, an increasing number of prostate cancers are being detected at a local stage. Since 1989, our group has been developing a research project with the aim of establishing treatment of localised prostate cancer by means of HIFU. The treatment is performed transrectally, using ultrasound imaging guidance only. The quality of HIFU treatment depends on four factors: the intensity of the transmitted pulse, the exposure time, the signal frequency, and the time between two firing bursts. The lesions are created by a thermal effect. Their slightly conical form is due to the absorption of ultrasound by tissue, enhanced by cavitation bubbles. Results obtained since 1993 demonstrate that transrectally administered HIFU treatment achieves local control of localised prostate cancer in 80% of cases, with 70% complete success and 30% partial response. The use of an annular array probe with variable focus and frequency should significantly improve results in the future. Finally, real time visual display of the damaged tissue via differential imaging of the attenuation coefficient should give the surgeon an instant appreciation of the result of the sequence. It would thus be possible to repeat treatment of insufficiently covered zones in the same session.

Differential attenuation imaging for the characterization of high intensity focused ultrasound lesions.

High intensity focused ultrasound (HIFU) is an effective technique for creating coagulative necrotic lesions in biological tissue, with a view to treating localized tumors. Although good results have already been obtained, notably in urology, current systems lack a real time monitoring system to check the efficacy of the treatment procedures. This study describes the development and assessment of a noninvasive system for making local measurements of attentuation variations during HIFU treatment procedures. An apparatus (Ablatherm, Edap-Technomed, France), combining a 2.5 MHz therapeutic transducer and a 5.5 MHz twin plane imaging probe (connected to an ultrasound scanner), was used to produce lesions. The rf signals needed to calculate the attenuation were recorded as outputs from the ultrasound scanner, before and after the high intensity firing sequences, which were performed on ten pieces of porcine liver. Each firing sequence involved producing a lesion volume comprising 42 individual lesions. A number of recordings were also made without producing lesions, in order to test the reproducibility of the measurements. The attenuation function was evaluated locally using the centroid and the multinarrowband methods. Initially, changes in the integrated attenuation alpha (mean attenuation in the 4-7 MHz range) and the attenuation slope beta were examined for the lesion volume. beta values did not vary significantly within this range, whereas alpha values varied significantly (in the region of 86% of the initial level) in comparison to measurements performed without forming lesions. The differential attenuation delta alpha (representing local variations in alpha) was subsequently used to generate images revealing the lesion areas. There was a strong similarity between these 'delta alpha images' and the lesion volumes defined by the operator. 'delta alpha images' offer several advantages over existing attenuation imaging techniques. Any problems related to the heterogeneity of the medium are eliminated, since only the change in attenuation is taken into account. Furthermore, there is no need to compensate for diffraction when estimating delta alpha, as the rf signals are captured in exactly the same positions before and after treatment. This technique can be used during in vivo treatment procedures. It can be implemented in real time, since the computational algorithms (based primarily on FFT calculations) are very fast. The technique should provide clinical practitioners with valuable qualitative and quantitative information for use in HIFU ultrasound surgery.