Influence of cone beam CT (CBCT) scan parameters on size specific dose estimate (SSDE): a Monte Carlo study

The CT dose index (CTDI_vol) is the dosimetric quantity used for multi-slice CT (MSCT) with beams  ⩽4 cm. Conversion factors (f _size) based on patient size are applied to CTDI_vol to adjust for differences in patient size and derive size-specific dose estimates (SSDE) relating to patient dose. The aim of this study is to: (1) investigate use of a similar technique to provide SSDE values for cone beam CT (CBCT) scans, (2) determine whether factors derived for narrow beam MSCT are suitable for CBCT, and (3) investigate the influence of CBCT parameters on f _size values. Monte Carlo simulations were used to model an on-board imager system integrated into a Varian Truebeam linear accelerator, and to assess doses for imaging. The CTDI_vol is unsuitable for CBCT dosimetry, thus the assessments were applied to the modification CTDI_IEC, recommended by the international electrotechnical commission (IEC). Conversion factors (f _size,CBCT) were derived for CBCT to allow adjustment of values for the absorbed dose in water (AD) averaged over the beam width in the middle of the phantoms. Values of AD were evaluated as for CTDI₃₀₀ measured with a 30 cm long chamber in centres and peripheries of long water phantoms with diameters of 10-40 cm for head and body protocols. Three beam widths 8, 16, and 24 cm were used with tube potentials ranging from 80-140 kV, for full and partial rotation modes. In order to derive f _size,CBCT values, calculated values for AD were normalized with respect to the weighted CTDI_IEC assessed in standard CTDI phantoms and free-in-air. Variations in f _size,CBCT with beam width were minimal, 1%-5%, but those with tube potential were greater for 80 kV at small diameters reaching 11%. Acquisition mode affected f _size,CBCT values by up to 7%. Best-fit curves were derived from the f _size,CBCT values and compared to those reported by AAPM TG-204 for MSCT. Conversion factors estimated from these curves for 120 kV were within  ±8% and  ±13% of the MSCT values over head and body diameters, respectively, representing the majority of the adult population. Therefore, the use of MSCT factors to convert CTDI_IEC for CBCT scans may lead to under/overestimation of doses to patients by 5% and 6%, on average, for the head and body protocols, respectively. Best-fit curves of the results from this study provide values that could be used to convert CTDI_IEC for specific water-equivalent diameter (D _w) to a SSDE.