An assessment of the ExacTrac intrafraction imaging capabilities for flattening filter free prostate stereotactic body radiotherapy.

The uncertainties associated with image matching using the ExacTrac® system (BrainLab, Munich, Germany) have been the subject of investigation in the literature for extra-cranial sites. However, the uncertainties involved in the use of ExacTrac in the presence of higher scatter conditions like that for intrafraction imaging of prostate stereotactic radiotherapy utilising unflattened beams is yet to be determined. A prostate phantom was created with 3 implanted gold fiducial markers. This phantom was shifted by 1 mm and 2 mm amounts in the translational planes and by 1° and 2° amounts in the rotational planes and subsequently imaged by ExacTrac during delivery of a clinical SBRT plan. ExacTrac auto-match results were compared to the known offsets with uncertainties calculated. Calculated shifts were shown to be accurate within one standard deviation of the known offsets. Uncertainties were found to vary considerably among the 6 dimensions with matching in the vertical and angle vertical directions having standard deviations of 0.7 mm and 1.3°, respectively. These results agreed with the literature cases for pre-treatment setup and lower scatter condition IMRT intrafraction delivery. Based on these values, probabilities of intrafraction inhibits were calculated based on patient movement and possible fusion tolerances. While the measured uncertainties are adequately defined in order to calculate appropriate target margins, their relatively large magnitudes made choice of intrafraction fusion tolerances problematic. A degree of compromise between the rate of false positives and false negatives is required when implementing ExacTrac into a SBRT prostate protocol.