TU‐C‐BRB‐02: Identification of a Targeting Error in a Small Field Biological Irradiator Using 3D Dosimetry Techniques

Purpose: To characterize the targeting accuracy of a small animal irradiator, (XRad225Cx, Precision X‐Ray Inc.), producing 225kVp circular or square fields ranging from 1mm–40mm dimensions. Targeting accuracy is critical for accurate interpretation of pre‐clinical dose‐volume response, but is extremely challenging, due to the extremely small field sizes. Characterization was performed with a uniquely suited novel high‐resolution 3D dosimetry system. Methods: Targeting accuracy was evaluated on cylindrical 3D PRESAGE™dosimeters (5cm diameter, 7cm long) containing a range of physical targets. Each target corresponded to the tip‐point of a 1.3mm channel drilled into the dosimeter at various locations and depths. Conebeam‐CT image guided targeting was applied for treatment positioning, according to normal preclinical procedures. Each target was treated with 7 beam, varying angle, co‐planar plan using the 2.5mm circular cone, including a delivery to an undrilled position, for undisturbed optical‐CT data. Post‐treatment, the dosimeter was imaged with the Duke Midsized Optical‐CT Scanner (DMOS), producing 0.5mm isotropic 3D dose measurements. Targeting accuracy was evaluated by comparing the central position of the dose distribution to the targeted position (visible on optical‐CT). Results: High‐quality optical‐CT scans were obtained, revealing both the physical target and the delivered dose distribution. High‐resolution 3D analysis revealed a consistent targeting error of ∼1.5mm in all targets (along the long axis of the channel). The full‐width half maximum of the beam intersection region was 2.2mm. Intersection of the beam central axes showed an error of 1.1mm in the direction perpendicular to the long axis. Conclusions: The value of high‐resolution 3D dosimetry data was critical for this challenging problem of precisely evaluating the targeting accuracy of a very small field irradiator. The study enabled precise quantification of 3D targeting accuracy. The reason for the observed targeting errors is under investigation, and the accuracy of other cones will be explored. This work was supported by NIH R01CA100835, S103034539, NCI S10RR027610, NIAID K02AI093866 and NASA NSCOR NNX11AC60G. One potential conflict of interest is the dosimeter used in this work, PRESAGE™ is manufactured by Heuris Pharma, LLC, a company owned by John Adamovics.