Direct comparison between surface imaging and orthogonal radiographic imaging for SRS localization in phantom

Surface imaging (SI) offers a nonionizing, near real time alternative to radiographic imaging for intrafraction radiosurgery localization. In this work, we systematically compared a commercial SI system vs a commercial room mounted x-ray localization system in phantom.

An anthropomorphic head phantom with fiducial markers was imaged with linear accelerator on-board x-ray imaging, SI, and room mounted x-ray imaging (RM) at ±45° and ±90° couch angles for three different head tilts and six different isocenters (72 total positions). The shifts generated by the three systems were compared as functions of couch angle, head tilt, and isocenter position with the on-board imaging shifts used as ground truth. Two sample Kolmogorov-Smirnov tests were used to evaluate equivalence of the groups.

The magnitude of the displacement vectors for RM minus on-board imaging and SI minus on-board imaging over all 72 phantom positions were 0.7 ± 0.3 mm for both cases. The RM and SI showed no significant difference based on couch angle or isocenter position. Both systems showed decreasing accuracy with increasing couch angle, but both systems agreed with ground truth to <=1.1 mm at all couch angles. The exaggerated chin-up head orientation showed significantly different shifts for SI and RM based on increased variance in the SI measurements, although both had submillimeter accuracy on average. The standard deviation of the real time SI displacement vector was <0.06 mm over all measurements, during which the on-board imaging panels partially blocked the lateral camera pods for half the time.

RM and SI showed similar accuracy over measurements at 72 different phantom positions. SI showed minimal performance loss with camera pods blocked. SI is a feasible option for intra-fraction radiosurgery localization based on these phantom measurements.