Parham Alaei PhD, Presenter: Equipment support, Koninklijke Philips Electronics NV

George Ding, Abstract Co-Author: Research grant, Varian, Inc

Huaiqun Guan, Abstract Co-Author: Nothing to Disclose

The results obtained from modeling the CBCT beam and utilizing it in calculating dose from this imaging modality in the treatment planning provides for an easy estimation and accounting of the dose from imaging for all patients as part of their treatment plans.

 Cone beam CT is increasingly being used for daily patient positioning verification during radiation therapy treatments. The daily use of CBCT would lead to accumulated patient doses higher than the older technique of weekly portal imaging. There have been several studies focusing on measurement or calculation of the patient dose from CBCT recently. This study focuses on the inclusion of and accounting for the CBCT dose in the patient treatment plans.

We have modeled the CBCT beam from a Varian OBI system on a Trilogy linear accelerator in the Philips Pinnacle treatment planning system. The beam produced by the system has a range of 40-125 kVp and the images can be acquired in a full- or half-fan mode. An additional bowtie filter is inserted in the beam to improve image quality. This study concentrates on the 125 kVp beam and a half-beam acquisition with a half bowtie, but can be extended to include other beam qualities and acquisition modes. The Pinnacle system's capabilities have been extended by addition of low energy deposition kernels in the range of 20-110 keV, details of which have been explained in the past (Med. Phys. 26: 1687 and 27: 2821). In order to model the CBCT beam with a bow-tie filter, a wedge mimicking the bowtie has been added to the beam. The resultant modeled depth dose and cross profiles have a very good match with the measured and Monte Carlo-calculated profiles reported previously (Med. Phys. 35:1135). The CBCT beam was then used to compute the dose from the imaging procedure on a Rando phantom and the results compared to measurements performed previously (Phys. Med. Biol. 52: 2267).  

Preliminary results indicate a difference in the range of 2% to 30% between planned and measured doses. The large discrepancies are observed on the left side and may be due to variations in the gantry speed at the start and stopping points which have not yet been accounted for in Pinnacle.

Alaei, P, Ding, G, Guan, H, Inclusion of the Dose from Cone Beam CT in the Radiation Therapy Treatment Plans.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6008920.html