Precision of “target of the day” radiotherapy

We have previously reported that computer-controlled table repositioning can increase the reproducibility of patient setup in 2D using a megavoltage imager and bony anatomy as reference of position. This investigation was extended to daily 3D localization of soft tissue structures (“target of the day” treatment). Research to date has involved liver (under ventilatory immobilization) and prostate patients. Both protocols involve aligning the specified target to the reference position (determined via CT) using a diagnostic imager and alignment software. The liver position is determined by the position of the diaphragm and the spine, the prostate target is automatically localized and positioned through computer-based extraction of 3 implanted gold markers. Adjustments to correct target position are executed using a computer-controlled table. Measuring the residual offset of the target after the corrective action determines the accuracy of measurement and table adjustment, however this uncertainty is also influenced by organ motion between images. For the prostate protocol, the accuracy of the computer-controlled target repositioning is dx:0.56 mm (σ=3.69 mm), dy:2.74 mm (σ=3.87 mm), dz:2.38 mm (σ=4.5 mm) after 24 treatment fractions. For the liver protocol, the accuracy is dx:-0.7 mm (σ=1.67 mm), dy:-0.9 mm (σ=1.89 mm), dz:-1.0 mm (σ=3.18 mm) after 23 treatment fractions. The extra time necessary to image, align, and adjust the target has also been calculated. The average time for the localization and subsequent verification is 12.2 minutes (σ=5.7 min)