All surface tracking systems are based on the assumption that there is a perfect correlation between surface and internal organ or tumor motion, since precise, real-time information about internal anatomy is generally not available during treatment. Correlations between surface and internal structures were investigated in several studies and found to be highly dependent on treatment site.²

In particular for some regions of interest used for respiratory gating, this correlation can be inadequate for further margin reduction and normal tissue sparing. Alderliesten et al. investigated the heart position variability in deep inspiration breath hold (DIBH) radiation therapy.³ Only a modest correlation between surface and heart position was found, demonstrating that surface guidance isn’t a suitable technique to confidently monitor the dose to the heart.

During DIBH treatments for pancreatic cancer, internal organs move about 5 mm on average and in some cases movement even exceeds 1 cm.⁴ Poor correlation of surface to internal motion may be a limiting factor if the treatment beam is planned to be gated by the surface system, as it could trigger inaccurate beam starts or stops.

This may require IGRT verification and cause subsequent delays in treatment or potential inaccurate delivery. Furthermore, the calculation of couch shifts can be compromised by a lack of internal information. Comparison between surface and volumetric registration of patients showed a mean translational discrepancy of 2.7 mm.⁵ Error prone regions, such as facial soft tissues, are generally easily deformed. The option to use a surface registration technique in conjunction with internal imaging is therefore crucial for verifying the accuracy of the surface system for a particular patient and fraction.

Today, images of internal anatomy are most commonly acquired using devices mounted to the linac. However, the use of these devices throughout treatment is very limited.⁶ Cone beam computed tomography (CBCT) or electronic portal imaging devices (EPID) at non-coplanar couch positions are either confined to inconvenient gantry angles or not possible at all due to the risk of linac collisions. Gantry rotations or extended imagers can occlude the view of lateral surface cameras, compromising the accuracy achieved by the surface tracking system. Simultaneous and flexible image acquisition of patient surface and internal anatomy is therefore the gold-standard for meaningful patient monitoring.

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