Purpose: 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.
Methods: 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.
Results: 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 5 mm 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.
Conclusions: 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.