Dose Calculation in the Presence of a Metallic Object using Megavoltage Cone-Beam CT Michèle Aubin, Olivier Morin, Jean-François Aubry, Josephine Chen, Martina Descovich, Lynn Verhey and Jean Pouliot University of California San Francisco, UCSF Comprehensive Cancer Center, San Francisco, CA Purpose Methods & Materials CT image artifacts caused by the presence of metallic objects hinder The treatment plan for external beam radiation Experiments were performed with a therapy is based on the delineation of the 16cm diameter cylindrical water- 16cm diameter cylindrical organ delineation and preclude water-equivalent anatomy in the planning CT scan. When precise dose calculation. In contrast, equivalent phantom. MVCBCT and phantom used for dose implanted objects of high atomic number (Z) measurements. An ion the presence of high atomic number kVCT images of the phantom were material are present, however, severe image chamber is placed in the material has relatively little impact on acquired with and without a 1.5 cm center of the phantom, artifacts are generated in conventional CT, the image quality of Megavoltage diameter steel rod (3 mm wall while mosfet detectors strongly hindering the ability to delineate some are distributed at dif- Cone-beam CT (MVCBCT). organs. This is particularly the case for the thickness) sitting on the phantom. ferent positions. Using a treatment planning system, Objectives: planning of prostate patients with hip prostheses. We have recently shown that Mega- dose distributions were calculated for ! Complement the planning CT for a 6 MV 10 cm square field AP beam Voltage Cone-Beam CT can be used to organ segmentation with MVCBCT. for these configurations. Dose Calibration curves for MVCBCT (red) and complement the regular CT for target definition Illustration of the of prostate cancer treatment of patients with hip measurements were performed using conventional CT (blue). In the MV energy range, ! Determine if MVCBCT could be dominated by Compton effect, the attenuation is 10 cm x 10 cm replacements. The purpose of the present work an ion chamber placed at isocenter mainly dependent on the electron density, with 6 MV AP beam used for accurate dose calculation in and mosfet detectors placed at 35 incident on the is to determine if MV CBCT can be calibrated little Z dependency. This results on a linear the presence of metallic objects. and provide electron density to produce accurate locations in the phantom. Measured relationship, event for bony or metallic objects. phantom. dose calculation even in presence of metallic and calculated dose values were ! Provide precise daily alignment with objects such as hip prostheses. compared. gold markers and MVCBCT. Results Conclusion TOP: The calculated dose distributions (colorwash) were • MV CBCT provides anatomical performed using a kVCT image Without metal, dose calculated in the (left) and a MVCBCT image information even in presence of phantom using either the kVCT or the (right) of the phantom for an AP ‘ Non-Compatible CT ’ objects. MVCBCT image corrected for 6MV beam. cupping artifact were all within 2% of BOTTOM: The presence of a metallic hollow rod strongly • MV CBCT allows to complement each other and of the measured values, showing the ability to use the affects the Hounsfield number Planning CT for organ segmentation accuracy of the kVCT image for patients with a hip prosthesis. MVCBCT for dose calculation. In the (left), making dose calculation presence of the metallic object, unreliable. In fact, the rod on the erroneous density values, around the kvCT image appears filled, while • MV CBCT allows daily positioning University of California San Francisco it is in fact hollow. rod, made dose calculation on the for patient with hip prosthesis. Department of Radiation Oncology However, using the MVCBCT kVCT unreliable. MVCBCT provided 1600 Divisadero Street, Suite H1031 image (right), accurate density San Francisco, CA 94143-1708 correct density values (within 5%) and • MV CBCT provides accurate values can be obtained and used differences between measured and Email: firstname.lastname@example.org calculated dose values were on average for dose calculation. electron density for dose Web: http//:www.ucsf.edu/jpouliot The numbers in white represent 3.2% (SD 3.4%). The two largest the difference (%) between the calculation, even in presence of differences (10.6% and 8.9%) were calculated dose and the high-Z material. (Useful for EBRT found in a high dose gradient region measured dose using the ion and PT) below the rod. chamber (at isocenter) and mosfet detectors. This work is supported by Siemens Oncology Care System Inc.
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