Modeling of a digital couch for a proton treatment planning system

To evaluate the modeling of a proton treatment couch for a commercial proton treatment planning system, a real couch was scanned using a CT scanner and was inserted into the commercial proton treatment planning system (TPS, Eclipse, v8.12) by replacing the CT couch of all transverse CT slices. The couch inserted in the proton TPS was validated by dosimetric measurements of percentage depth dose (PDD), transverse profile, and output. When the beam direction was perpendicular to the proton couch (0° in our configuration), insertion of the proton couch shortened the range of the proton beam by 0.99 ± 0.01 cm and caused the spread-out Bragg peak (SOBP) to vary by 0.10 ± 0.09 cm. Increasing the angle of the beam from 0° to 55° increased the average range shift of the proton beam from 0.99 cm to 1.70 cm, but had little effect on the SOBP, which varied by less than 0.1 cm. Insertion of the proton couch had no effect on the transverse dose profiles at various depths. When we measured the doses at points set at various depths, located in the middle of the SOBP for various beam energies, with/without proton couch, we found no distinct changes in output with insertion of the couch, resulting in a variation of less than 1%. These results suggest that dosimetric factors, such as range, profile, and output factors, were little changed by insertion of a proton couch whereas the beam range depended greatly on both couch insertion and beam angle. Using the inserted CT scanned proton couch in the TPS, we found that the range of the proton beam was shortened by ∼0.7 cm in the TPS, a ∼0.3 cm difference from the actual measurement. The difference between the computed and the measured ranges was compensated by assigning appropriate densities to the inserted proton couch, called a digital couch. Measured and computed percent depth dose (PDD) with the digital couch showed that the beam range and the SOBP were well matched for various beam angles, with differences of 0.01 ± 0.02 cm and 0.12 ± 0.03 cm, respectively. Our results suggest that a digital couch with relevant density assignment may be a good solution for proton couch modeling, which can be used for a commercial proton TPSs.