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

Measurement of cumulative dose [Formula: see text] 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 [Formula: see text] , which is proposed by AAPM TG‐111 for CBCT dosimetry. The aim of this study was to investigate the feasibility of using [Formula: see text] to estimate values for [Formula: see text] in long head and body phantoms made of PMMA, polyethylene (PE), and water, using beam qualities for tube potentials of [Formula: see text]. The study also investigated the possibility of using 150 mm PE phantoms for assessment of [Formula: see text] within long PE phantoms, the ICRU/AAPM phantom. The influence of scan parameters, composition, and length of the phantoms was investigated. The capability of [Formula: see text] to assess [Formula: see text] has been defined as the efficiency and assessed in terms of the ratios [Formula: see text]. 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 [Formula: see text] were used. Efficiencies [Formula: see text] and [Formula: see text] as a function of beam width exhibited three separate regions. For beam widths [Formula: see text] , [Formula: see text] and [Formula: see text] 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 [Formula: see text] and 69% for [Formula: see text] for a 500 mm beam width. The quantities [Formula: see text] and [Formula: see text] 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 [Formula: see text] wide beams, before declining with increasing the beam width, whereas values at the peripheries decreased steadily with beam width. Results for [Formula: see text] were virtually independent of tube potential, but there was more variation for [Formula: see text] and [Formula: see text]. [Formula: see text] underestimated [Formula: see text] for beam widths used for CBCT scans, thus it is necessary to use long phantoms, or apply conversion factors ([Formula: see text]) to measurements with standard PMMA CT phantoms. The efficiency values have been used to derive ([Formula: see text]) to allow evaluation of [Formula: see text] from measurements of [Formula: see text]. The ([Formula: see text]) 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.