Patient position verification in ion-beam therapy using ion-beam radiography and fiducial markers

The basic rationale for radiation therapy using ion-beams is its high local precision of dose deposition. Therefore accurate patient positioning prior to and during beam application is a crucial part of the therapy. The current standard position verification procedure uses X-ray based imaging before each beam application. The patient is assumed to remain in his position throughout irradiation. Currently there is no monitoring of the patient position or organ movement during treatment. The aim of this study is to investigate the possibility of verifying the position of a fiducial marker during therapy using ion radiography. Some modern ion therapy facilities like the Heidelberg Ion-Beam Therapy Center (HIT), where our measurements were carried out, use scanning pencil beams to apply dose. Exploiting them for imaging allows to solely irradiate regions of interest in the patient's body, e.g. tissue containing medical markers. The advantage of this technique is that it can be performed quickly in turn with therapeutic beam application and irradiates only very little tissue. For our measurements we used conventional medical metal markers embedded in phantom material mimicking body tissue. To image the residual beam we use a Perkin Elmer RID256-L flat panel detector. In an idealized setup the marker contrast was measured to be as high as 60%, which was reduced by a factor of 2–2.5 when the marker was placed at distances to the detector in the phantom material larger than 10 cm. It was shown that applying 2⋅10⁵ carbon ions suffices to make the markers' position visible in a setup of realistic material thickness and marker depth. While the dose is comparable to X-ray imaging, the irradiated volume and, consequently, also the integral dose is considerably reduced. However, in realistic geometries there are large particle range differences in lateral direction yielding steep signal gradients in the radiography. Thus, the useful image area with unambiguous signal information is largely reduced.