Prostate Planning Treatment Volume Margin Calculation Based on the ExacTrac X-Ray 6D Image-Guided System: Margins for Various Clinical Implementations

Purpose: To assess the prostate motion from day-to-day setup, as well as during irradiation time, to calculate planning target volume (PTV) margins. PTV margins differ depending on the clinical implementation of an image-guided system. Three cases were considered in this study: daily bony anatomy match, center of gravity of the implanted marker seeds calculated with a limited number of imaged days, and daily online correction based on implanted marker seeds. Methods and Materials: A cohort of 30 nonrandomized patients and 1,330 pairs of stereoscopic kV images have been used to determine the prostate movement. The commercial image guided positioning tool employed was ExacTrac X-Ray 6D (BrainLAB AG, Feldkirchen, Germany). Results: Planning target volume margins such that a minimum of 95% of the prescribed dose covers the clinical target volume for 90% of the population are presented. PTV margins based on daily bony anatomy match, including intrafraction correction, would be 11.5, 13.5, and 4.5 mm in the anterior-posterior, superior-inferior, and right-left directions, respectively. This margin can be further reduced to 8.1, 8.6, and 4.8 mm (including intrafraction motion) if implanted marker seeds are used. Finally, daily on line correction based on marker seeds would result in the smallest of the studied margins: 4.7, 6.2, and 1.9 mm. Conclusion: Planning target volume margins are dependent on the local clinical use of the image-guided RT system available in any radiotherapy department