Intraprostatic fiducials for image guidance: Workflow implications in a single linac department

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Purpose To assess the accuracy and implications on workflow of an online correction electronic portal imaging (EPI) protocol utilising bony anatomy in the online environment and an assessment of three implanted gold seed fiducial markers in the offline environment. This paper summarises an initial trial to establish the range of systematic and random errors present in patient set-up for both bony anatomy and fiducial markers, and to calculate optimal clinical target volume (CTV) to planning target volume (PTV) margins. The impact of the introduction of such a technique was also assessed in terms of impact on workflow and resource management in a single machine unit (SMU). Methods and materials Pre treatment electronic portal images (EPIs) were acquired and bony anatomy was matched with CT derived digitally reconstructed radiographs (DRRs). Intervention in field placement was made if field placement fell outside the range of 4 mm on any of the orthogonal axes. In the offline environment the position of the implanted gold seed fiducials was aligned with that of the DRRs. An analysis of set-up error, total error and internal organ motion was then undertaken, with full statistical analysis of systematic and random errors. Results Eleven patients completed treatment as specified, with 1006 EPIs available for analysis. Treatment times were in the order of 10.4 min. Set-up errors were in the order of 2.7 mm right–left, 2.4 mm sup–inf and 1.6 mm ant–pst. These were reduced to 1.2 mm, 0.7 mm and 0.9 mm respectively utilising an online correction protocol. However there was minimal impact on total error and internal organ motion. Using the data obtained in both the online and offline environments optimal CTV–PTV margins were calculated for correcting to bone, correcting to gold seed fiducials and also the possibility of EPI malfunction. Conclusions Daily targeting of the prostate is both technically feasible and can be carried out in an efficient and accurate manner. An online correction protocol using gold seeds as the matching mechanism provides the ability to significantly reduce treatment margins for most prostate patients, and importantly does not impact on a busy clinical workflow.