Dosimetric properties and commissioning of cone-beam CT image beam line with a carbon target

Background and purpose

Accurate patient positioning before radiotherapy is often verified using advanced imaging techniques such as cone-beam computed tomography (CBCT). Even for dedicated imaging beam lines, the applied dose is not necessarily negligible with respect to the treatment dose and should be considered in the treatment plan.

Materials and methods

This study presents measurements of the beam properties of the Siemens kView (Siemens AG, Munich, Germany) image beam line (IBL) and the commissioning in the Philips Pinnacle³ treatment planning system (TPS; Philips, Amsterdam, Netherlands).

Results

The percent depth dose curve reaches its maximum at a depth of 10 mm, with a surface dose of 44?%. The IBL operates in flattening filter-free mode, showing the characteristic dose falloff from the central axis. Stability over several days to months is within less than 2?% dose deviation or 1 mm distance-to-agreement. Modelling of the IBL beam line was performed using the Pinnacle³ automatic modelling routine, with absolute dosimetric verification and film measurements of the fluence distribution.

Conclusion

After commissioning of the IBL beam model, the dose from the imaging IBL CBCT can be calculated. Even if the absolute dose deposited is small, repeated imaging doses may sum up to significant amounts and can shift the position of the dose maximum by several centimetres.