Method comparison of ultrasound and kilovoltage x-ray fiducial marker imaging for prostate radiotherapy targeting

Abstract

Several measurement techniques have been developed to address the capability for target volume reduction via target localization in image-guided radiotherapy; among these have been ultrasound (US) and fiducial marker (FM) software-assisted localization. In order to assess interchangeability between methods, US and FM localization were compared using established techniques for determination of agreement between measurement methods when a 'gold-standard' comparator does not exist, after performing both techniques daily on a sequential series of patients. At least 3 days prior to CT simulation, four gold seeds were placed within the prostate. FM software-assisted localization utilized the ExacTrac X-Ray 6D (BrainLab AG, Germany) kVp x-ray image acquisition system to determine prostate position; US prostate targeting was performed on each patient using the SonArray (Varian, Palo Alto, CA). Patients were aligned daily using laser alignment of skin marks. Directional shifts were then calculated by each respective system in the X, Y and Z dimensions before each daily treatment fraction, previous to any treatment or couch adjustment, as well as a composite vector of displacement. Directional shift agreement in each axis was compared using Altman-Bland limits of agreement, Lin's concordance coefficient with Partik's grading schema, and Deming orthogonal bias-weighted correlation methodology. 1,019 software-assisted shifts were suggested by US and FM in 39 patients. The 95% limits of agreement in X, Y and Z axes were +/-9.4 mm, +/-11.3 mm and +/-13.4, respectively. Three-dimensionally, measurements agreed within 13.4 mm in 95% of all paired measures. In all axes, concordance was graded as 'poor' or 'unacceptable'. Deming regression detected proportional bias in both directional axes and three-dimensional vectors. Our data suggest substantial differences between US and FM image-guided measures and subsequent suggested directional shifts. Analysis reveals that the vast majority of all individual US and FM directional measures may be expected to agree with each other within a range of 1-1.5 cm. Since neither system represents a gold standard, clinical judgment must dictate whether such a difference is of import. As IMRT protocols seek dose escalation and PTV reduction predicated on US- and FM-guided imaging, future studies are needed to address these potential clinically relevant issues regarding the interchangeability and accuracy of novel positional verification techniques. Comparison series with multiple image-guidance systems are needed to refine comparisons between targeting methods. However, we do not advocate interchangeability of US and FM localization methods.

Fiducial marker guided prostate radiotherapy: a review

“…They concluded that concordance of resulting TAUS prostate and SC FM measurements was "poor" or "unacceptable" with 95% of measures within 15 mm. 61 Numerous other groups have found significant differences in treatment setup data derived from TAUS soft-tissue assessment compared with FM data from other modalities, greater variation within ultrasound data and ultrasoundderived PTV margins may be up to 3-4 times smaller than those reported in the literature. 43,62,63 Fuller et al 61 also provide a useful narrative on the inherent difficulties of comparing IGRT modalities which briefly include the lack of an established ground truth and the statistical inaccuracies of comparing data generated by incomparable methods or devices.…”

“…Using the ExacTrac® 6D IGRT system (Brainlab AG, Feldkirchen, Germany), Shi et al 60 In a series of 40 patients, Fuller et al 61 compared treatment setup errors derived from in-room TAUS imaging of the prostate with those from kV SC images of prostate FMs for radiotherapy verification. As in Barney et al, the purpose of this comparison study was not to establish which modality was superior, although treatment was corrected and treated according to SC FM position.…”

Historical Development of Stereotactic Ablative Radiotherapy

“…They reported that the technique was useful in 95.8% of setups, a significant improvement from traditional ultrasound methodologies. Despite these advances, ultrasound imaging remains challenging in the vast majority of tumor sites, and the use of stereotactic ultrasound is now largely restricted to applications in prostate cancer (Chinnaiyan et al 2003;Fuller et al 2006;Peignaux et al 2006). …”

Combination of Image Registration Algorithms for Patient Alignement in Proton Beam Therapy

“…It is used in patient respiration phase monitoring to command a 4 dof robot, and is tested in proton therapy for brain tumour at Massachusetts General Hospital. For prostate radiotherapy targeting, a comparative study between ultrasound and x-ray imaging of markers is proposed in [7].…”