Implementation of Back-projection Filtering Algorithm for Image

I. J. Radiation Oncology d Biology d Physics

S752

3222

Volume 78, Number 3, Supplement, 2010

Implementation of Back-projection Filtering Algorithm for Image Reconstruction of On-board Cone-Beam CT

J. C. Park1, W. T. Watkins1, S. H. Park2, J. S. Kim3, Z. Liu1, W. Y. Song1 University of California San Diego, La Jolla, CA, 2Asan Medical Center, Seoul, Republic of Korea, 3Sam Sung Medical Center, Seoul, Republic of Korea

1

Purpose/Objective(s): The success of on-line image guided radiation therapy (IGRT) depends critically on the quality of images obtained from on-board CBCT systems. CBCT images are especially poor in terms of low-contrast visualization (e.g., in the case of mobile liver metastases). In order to enhance CBCT images, and specifically to enhance the low contrast visualization of these images, a novel back-projection filtering (BPF) image reconstruction algorithm was implemented and compared to the conventional and clinically implemented FDK approach. Materials/Methods: Instead of using the well-known FDK image reconstruction algorithm which filters projection data prior to back-projection, we have implemented the back-projection process prior to filtration. The chosen filter was subsequently applied to the reconstructed volume. The direction of the filter was set orthogonal to the back projection angle and a ramp filter was used during the filtering process. For comparison, projections data of the CatPhan phantom was acquired using the Varian On-Board Imager (OBI, Varian Medical Systems, Palo Alto, CA). Images were acquired using the standard full-fan mode with the aluminum full-bow-tie filter. In this setting, 379 2D cone-beam projections were acquired over 200 degrees using 100 kVp, 20 mA, and 20 ms/ frame. The projections data were then reconstructed with both the FDK and BPF methods. The contrast layer of the CatPhan phantom was evaluated and the contrast-to-noise ratio (CNR) was calculated on all inserts available. Results: Comparing the CNR on both of the reconstructed images using the CatPhan phantom, the BPF method resulted in significant increase in CNR for all inserts evaluated. Upon measuring Air and Teflon inserts (CT numbers of -1000 and 990, respectively), the measured CNR improved by .2.5 times. This increase was due both to increased contrast and decreased noise throughout the reconstructed volume. Moreover, it was observed that the BPF images showed visually clean images with noticeably better edge preservation than that of the FDK-reconstructed images. Conclusions: In this study, a novel BPF image reconstruction algorithm was developed and compared with the standard FDK algorithm. It was shown that the image quality reconstructed with the BPF algorithm is superior to the standard FDK reconstruction algorithm in terms of CNR and edge preservation. However, further studies are required to determine its clinical impact. Author Disclosure: J.C. Park, None; W.T. Watkins, None; S.H. Park, None; J.S. Kim, None; Z. Liu, None; W.Y. Song, None.

3223

Intrafraction Organ Motion during Prostate Radiotherapy: Quantitative Correlation of Treatment Time and Margin Size

D. Schmitt1, S. Nill1, K. Herfarth2, M. Mu¨nter2, J. Pfitzenmaier2, A. Zabel-du Bois1, F. Ro¨der1, P. Huber1, U. Oelfke1 1

German Cancer Research Center, Heidelberg, Germany, 2University of Heidelberg, Heidelberg, Germany

Purpose/Objective(s): Data of intrafraction prostate motion from 10 IMRT patients obtained over a course of 35 fractions with the CalypsoÒ System were analyzed to evaluate the impact of daily treatment delivery time on population-based CTV to PTV margins. Materials/Methods: Intrafraction prostate motion was collected for 10 IMRT patients during at least 9 minutes of 340 daily fractions. Data were acquired with the electromagnetic tumor tracking system of Calypso Medical. Population-based CTV to PTV margins for intrafraction motion were determined from the observed variances of systematic and random prostate displacements for daily treatment times ranging between 2 and 9 minutes. Perfect patient positioning prior to treatment was assumed. For each fraction of all patients the data obtained within the first minutes of the treatment were employed for the investigation. These motion patterns are assumed to reflect the clinically relevant prostate motion without any bias related to patient relaxation or discomfort which might occur for extended treatment times. Isotropic 3-dimensional CTV to PTV margins were calculated according to the formula of van Herk et al. Additional geometrical uncertainties, e.g. related to the target delineation, were not accounted for. In addition we determined the maximum displacements for each treatment time interval. Results: An almost linear increase of the CTV to PTV margin with increasing treatment time per fraction was observed, e.g. the intrafraction prostate motion observed within the first 2 minutes of the treatment could be accounted for by a margin of 1.5 mm while a treatment time of 9 minutes required a margin of 3.5 mm. For all treatment times the variance of the random error was much larger than the variance of the systematic error, but because of the different time dependences of these variances, the margin component related to systematic displacements dominated the margin increase after 3.5 minutes. The maximum displacements from the isocenter ranged from 14.7 mm for 2 minutes to 41.0 mm for 9 minutes treatment time. Conclusions: According to our data, intrafraction prostate motion requires increasing population-based CTV to PTV margins with increasing treatment times ranging from 2 to 9 minutes. Starting with a margin of 1.5 mm for a treatment fraction of 2 minutes each additional minute of treatment requires an additional margin of 0.3 mm. These data may serve as a useful reference for the design of population-based CTV to PTV margins of hypofractionated prostate radiotherapy protocols. Given the large observed maximum displacements, margins for individuals may need to be larger, or the margin values can be used as intervention limits for the Calypso System as well. This work was partially supported by Calypso Medical Technologies, Inc. Author Disclosure: D. Schmitt, None; S. Nill, None; K. Herfarth, None; M. Mu¨nter, None; J. Pfitzenmaier, None; A. Zabel-du Bois, None; F. Ro¨der, None; P. Huber, None; U. Oelfke, None.

3224

Dosimetric Impact of Image Guidance on Bolus Electron Conformal Therapy

O. A. Zeidan, B. D. Chauhan, W. W. Estabrook, M. S. Curry, T. R. Willoughby, R. R. Manon, S. L. Meeks M.D. Anderson Cancer Center Orlando, Orlando, FL Purpose/Objective(s): We report on our initial experience with the use of daily image guidance for the treatment of H&N patients using bolus electron conformal therapy. We demonstrate the dosimetric effect of misalignment on PTV dose coverage by