Abstract
High intensity focused ultrasound (HIFU) provides a non-invasive salvage treatment option for patients with recurrence after external beam radiation therapy (EBRT). As part of EBRT the prostate is frequently implanted with permanent fiducial markers. To date, the impact of these markers on subsequent HIFU treatment is unknown. The objective of this work was to systematically investigate, using computational simulations, how these fiducial markers affect the delivery of HIFU treatment. A series of simulations was performed modelling the propagation of ultrasound pressure waves in the prostate with a single spherical or cylindrical gold marker at different positions and orientations. For each marker configuration, a set of metrics (spatial-peak temporal-average intensity, focus shift, focal volume) was evaluated to quantify the distortion introduced at the focus. An analytical model was also developed describing the marker effect on the intensity at the focus. The model was used to examine the marker's impact in a clinical setting through case studies. The simulations show that the presence of the marker in the pre-focal region causes reflections which induce a decrease in the focal intensity and focal volume, and a shift of the maximum pressure point away from the transducer's focus. These effects depend on the shape and orientation of the marker and become more pronounced as its distance from the transducer's focus decreases, with the distortion introduced by the marker greatly increasing when placed within 5mm of the focus. The analytical model approximates the marker's effect and can be used as an alternative method to the computationally intensive and time consuming simulations for quickly estimating the intensity at the focus. A retrospective review of a small patient cohort selected for focal HIFU after failed EBRT indicates that the presence of the marker may affect HIFU treatment delivery. The distortion introduced by the marker to the HIFU beam when positioned close to the focus may result in an undertreated region beyond the marker due to less energy arriving at the focus, and an overtreated region due to reflections. Further work is necessary to investigate whether the results presented here justify the revision of the patient selection criteria or the markers' placement protocol.
Highlights
Prostate cancer is the most commonly occurring male cancer and the second leading cause of cancer-related death in men in the European Union (EU) and the United States of America (USA).[1,2] in 2012 of all reported cancer cases in men, prostate cancer accounted for 24.1% in the EU, with a 10% mortality rate, and 28.3% in the USA, with aMed
The purpose of this work was to perform an extensive quantitative investigation of the effect of fiducial markers on the propagation and focusing of the ultrasound (US) waves when the beam path is obstructed by an external beam radiation therapy (EBRT) fiducial marker
The results presented in the previous sections suggest that the marker distorts the high intensity focused ultrasound (HIFU) beam with its effect increasing the closer it is positioned to the focus
Summary
Prostate cancer is the most commonly occurring male cancer and the second leading cause of cancer-related death in men in the European Union (EU) and the United States of America (USA).[1,2] in 2012 of all reported cancer cases in men, prostate cancer accounted for 24.1% in the EU, with a 10% mortality rate, and 28.3% in the USA, with aMed. Selecting an appropriate therapy depends on several factors, such as the stage of the tumor, biochemical indicators (e.g. prostate specific antigen value), Gleason score, other associated diseases, the patient’s age and life expectancy, as well as the patient’s personal preference.[5] For localized or locally advanced prostate cancer, recommended treatments for primary therapy include active surveillance, radical prostatectomy (RP), external beam radiation therapy (EBRT) and temporary (high-dose rate) or permanent (lowdose rate) brachytherapy (BT) with or without additional EBRT. These treatments may be offered independently or in combination with hormonal therapy (androgen deprivation therapy). New minimally invasive modalities have emerged and provide alternative treatment options with the most notable being high intensity focused ultrasound (HIFU) and cryosurgery.[5,6,7]
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