Kenneth Shung Hu MD, Abstract Co-Author: Speakers Bureau, Bristol-Myers Squibb Company Speakers Bureau, ImClone Systems Incorporated Speakers Bureau, Cytogen Corporation Research grant, MedImmune, Inc

Aparna Surapaneni MD, Presenter: Nothing to Disclose

The widespread use of intensity modulated radiation increases the risk for underdosage of tumor and overtreatment of normal tissues due to tumor shrinkage, setup error and dynamic changes in the geometry between tumor and patient anatomy. The routine use of replanning CT (rCT) is impractical and increases radiation exposure. CBCT offers a rapid, practical assessment of setup accuracy and volumetric changes. Additionally, it may provide insight into dosimetric changes which require adaptive replanning. The purpose of this study is to determine its accuracy to track cumulative dose compared to a rCT.

Eight patients treated definitively with IMRT who had rapid tumor shrinkage or weight loss underwent midtreatment rCT as well as CBCT. Volumes from the original treatment planning CT (iCT) were deformed to the rCT and CBCT. Dosimetry of the PTV and normal structures calculated from the CBCT and rCT were compared. For one patient, the iCT was deformed to match the CBCT and the corresponding dosimetry calculated using the deformed CT (dCT) as a dose surrogate.

Dose volume histogram comparisons between the CBCT and rCT were generated. The median percentage differences between CBCT and rCT were as follows:PTV-D95: 3.68% [0.18-13.0%]PTV-Dmean: 1.07% [0.17-4.39%]Brainstem-Dmax: 4.30% [0.85-83.7%]Brainstem-D5: 2.40% [.02-24.7%]Cord-Dmax: 5.67% [0.16-38.1%]Cord-D5: 1.11% [0.60-32.0%]L Parotid-D50:10.1% [0.38-51.7%]R Parotid-D50 : 2.55% [0.98-82.5%]Mandible-Dmax :1.87% [0.20-12.4%]This corresponded to absolute dose differences of:PTV-D95: 245cGy [12-819cGy]PTV-Dmean: 60cGy [13-296cGy]Brainstem-Dmax: 164cGy [40-731cGy]Brainstem-D5 112cGy[1-303cGYCord-Dmax: 136cGy [8-1274cGy]Cord-D5: 253cGy [25-987]L Parotid-D50:245cGy [16-655cGy]R Parotid-D50 :181cGy [9-712cGy]Mandible-Dmax : 129cGy [14-625cGy]Given the possibility of electron density artifacts of CBCT impacting on dosimetric accuracy, we deformed the iCT to match the anatomic information of the CBCT in one patient with large variance between dose computed from CBCT and rCT. Dosimetric calculations derived from the dCT were then compared to those from CBCT. Use of the dCT reduced the error calculated from the CBCT as follows:PTV-D95: 0.60% -> 0.40%PTV-Dmean 0.85% -> 0.47%Spinal cord dmax 0.4% -> 1.29%Brainstem Dmax 6.41% -> 0.03%R parotid 11.4% ->1.06%L parotid 26.2% -> 2.57%Mandible Dmax 0.90% ->1.84%

CBCT appears to be a reasonable first approximation to track dose with an accuracy generally <5% and absolute difference of 200cGy for both PTV and most normal structures. The use of the dCT deformed to a midtreatment CBCT appears to yield an even more consistent and accurate dosimetric calculation compared to that generated from the CBCT.

Hu, K, Surapaneni, A, Value of Kilovoltage Cone Beam CT (CBCT) to Track Dose in the Adaptive Radiation Treatment of Head and Neck Cancer.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/7002219.html