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Dose Calculation in the Presence of a Metallic Object by ltq40826

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									                                          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: aubin@radonc17.ucsf.edu
                                                                                        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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