Targeting cancer tumours in the upper body requires precision. However, lying down to receive radiation therapy does not appear to be the most natural way to do this, especially when it comes to internal movement of our organs.
A study was conducted with 12 volunteers by the Switzerland-based Paul Scherrer Institute (PSI).
The research paper, published in 2007 was titled Systematic errors in respiratory gating due to intrafraction deformations of the liver. It found that when we are lying parts of the liver drifts for up to 35 minutes moving in some cases up to 20 millimetres. In terms of targeting, this is a considerable shift and means that without careful monitoring, a tumour could move out of the treatment field causing healthy tissue around it to be hit by the radiation beam, thus damaging it.
While respiratory gating effectively compensated for the respiratory motion within short sequences (three minutes), deformations after 20 minutes from the setup position of more than 5mm in seven of the 12 subjects in some parts of the liver were recorded. The study concluded that measurements over a few breathing cycles should not be used as proof of accurate reproducibility of motion, not even with the same fraction, if it is longer than a few minutes.
It can be argued that with treatment in an upright position the direction of gravity is not changing, the amount of liver drift in the upright position will be less than the results from this PSI study.