Meredith Sadinski PhD, Presenter: Nothing to Disclose

Erik A. Pearson BS, BEng, Abstract Co-Author: Nothing to Disclose

Charles A. Pelizzari PhD, Abstract Co-Author: Research Grant, Varian Medical Systems, Inc Scientific Advisory Board, RefleXion Medical Inc

To evaluate dose reduction with intensity weighted (partially transmitting collimation) and true (opaque collimation) region of interest imaging (IWROI and ROI) for CBCT in radiation therapy using Monte Carlo simulations of the kV imager and film measurements during CBCT of an anthropomorphic phantom.

The bowtie filter of an onboard imager was replaced by 3 mm thick Copper or Lead filter blades for IWROI and true ROI imaging respectively, set to illuminate a 5 cm diameter ROI. CBCT scans were performed with filtered and open beams on a RANDO head phantom containing radiochromic film. The films were scanned, converted to dose using locally developed calibration software, and compared. The BEAMnrc Monte Carlo system was also used to simulate the on board imager with the three different filters (Cu, Pb, none) in place and phase space files were generated for each condition. These were propagated through the voxelized RANDO phantom at 360 projection angles mimicking the experimental setup, using the DOSXYZnrc code, generating simulated dose distributions for true ROI, IWROI and standard CBCT scans. Using Monte Carlo in addition to film enabled us to quantitatively evaluate doses in 3D for the three imaging methods.

IWROI and ROI scans delivered similar doses, both significantly lower than the unfiltered standard scan. Dose-volume histogram (DVH) analysis on CT-defined regions representing target (i.e., the CBCT ROI) and normal structures yielded median doses (per 1019 particle histories) to the (target, eyes, all other tissue) in a 5cm thick slab of (9.307, 14.84, 10.54) cGy for unfiltered; (6.556, 4.586 ,4.296) cGy for true ROI; (6.766, 5.146, 4.801) cGy for IWROI scans. Monte Carlo doses on the film plane were consistent with the film results. Doses in practice are of course dependent on tube current settings, but for any mAs setting, the relation among the three techniques demonstrated here will persist.

ROI and IWROI CBCT allow significant and similar dose sparing. Since IWROI CBCT also allows reconstruction free from truncation artifacts using standard algorithms, it is potentially a valuable clinical capability.

CBCT is widely used for image guidance in radiation therapy but dose sparing methods are needed. IWROI CBCT allows significant dose sparing and artifact free reconstruction with standard algorithms.

Sadinski, M, Pearson, E, Pelizzari, C, Dose Reduction Intensity Weighted Region of Interest and True Region of Interest Conebeam CT Imaging: A Monte Carlo and Phantom Study.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12036957.html