Evaluation of cumulative dose for cone‐beam computed tomography (CBCT) scans within phantoms made from different compositions using Monte Carlo simulations

摘要

Measurement of cumulative dose f ( 0 , 150 ) with a small ionization chamber within standard polymethyl methacrylate (PMMA) CT head and body phantoms, 150 mm in length, is a possible practical method for cone-beam computed tomography (CBCT) dosimetry. This differs from evaluating cumulative dose under scatter equilibrium conditions within an infinitely long phantom f ( 0 , ∞ ) , which is proposed by AAPM TG-111 for CBCT dosimetry. The aim of this study was to investigate the feasibility of using f ( 0 , 150 ) to estimate values for f ( 0 , ∞ ) in long head and body phantoms made of PMMA, polyethylene (PE), and water, using beam qualities for tube potentials of 80 ? 140 k V . The study also investigated the possibility of using 150 mm PE phantoms for assessment of f ( 0 , ∞ ) within long PE phantoms, the ICRU/AAPM phantom. The influence of scan parameters, composition, and length of the phantoms was investigated. The capability of f ( 0 , 150 ) to assess f ( 0 , ∞ ) has been defined as the efficiency and assessed in terms of the ratios ? ( f ( 0 , 150 ) / f ( 0 , ∞ ) ) . The efficiencies were calculated using Monte Carlo simulations for an On-Board Imager (OBI) system mounted on a TrueBeam linear accelerator. Head and body scanning protocols with beams of width 40 ? 500 mm were used. Efficiencies ? ( PMMA / PMMA ) and ? ( PE / PE ) as a function of beam width exhibited three separate regions. For beam widths 150 mm , ? ( PMMA / PMMA ) and ? ( PE / PE ) values were greater than 90% for the head and body phantoms. The efficiency values then fell rapidly with increasing beam width before levelling off at 74% for ? ( PMMA / PMMA ) and 69% for ? ( PE / PE ) for a 500 mm beam width. The quantities ? ( PMMA / PE ) and ? ( PMMA / Water ) varied with beam width in a different manner. Values at the centers of the phantoms for narrow beams were lower and increased to a steady state for ～ 100 ? 150 mm wide beams, before declining with increasing the beam width, whereas values at the peripheries decreased steadily with beam width. Results for ? ( PMMA / PMMA ) were virtually independent of tube potential, but there was more variation for ? ( PMMA / PE ) and ? ( PMMA / Water ) . f ( 0 , 150 ) underestimated f ( 0 , ∞ ) for beam widths used for CBCT scans, thus it is necessary to use long phantoms, or apply conversion factors ( C f s ) to measurements with standard PMMA CT phantoms. The efficiency values have been used to derive ( C f s ) to allow evaluation of f ( 0 , ∞ ) from measurements of f ( 0 , 150 ) . The ( C f s ) only showed a weak dependence on scan parameters and scanner type, and so may be suitable for general application.PACS number: 87.55.K-, 87.57.Q-, 87.57.uq.