Many studies have shown that the computed tomography dose index (CTDI100) which is considered as a main dose descriptor for CT dosimetry fails to provide a realistic reflection of the dose involved ...in cone beam computed tomography (CBCT) scans. Several practical approaches have been proposed to overcome drawbacks of the CTDI100. One of these is the cumulative dose concept. The purpose of this study was to investigate four different approaches based on the cumulative dose concept: the cumulative dose (1) f(0,150) and (2) f(0,∞) with a small ionization chamber 20 mm long, and the cumulative dose (3) f100(150) and (4) f100(∞) with a standard 100 mm pencil ionization chamber. The study also aimed to investigate the influence of using the 20 and 100 mm chambers and the standard and the infinitely long phantoms on cumulative dose measurements. Monte Carlo EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc codes were used to simulate a kV imaging system integrated with a TrueBeam linear accelerator and to calculate doses within cylindrical head and body PMMA phantoms with diameters of 16 cm and 32 cm, respectively, and lengths of 150, 600, 900 mm. f(0,150) and f100(150) approaches were studied within the standard PMMA phantoms (150 mm), while the other approaches f(0,∞) and f100(∞) were within infinitely long head (600 mm) and body (900 mm) phantoms. CTDI∞ values were used as a standard to compare the dose values for the approaches studied at the centre and periphery of the phantoms and for the weighted values. Four scanning protocols and beams of width 20-300 mm were used. It has been shown that the f(0,∞) approach gave the highest dose values which were comparable to CTDI∞ values for wide beams. The differences between the weighted dose values obtained with the 20 and 100 mm chambers were significant for the beam widths <120 mm, but these differences declined with increasing beam widths to be within 4%. The weighted dose values calculated within the infinitely long phantoms with both the chambers for the beam widths ≤140 were within 3% of those within the standard phantoms, but the differences rose to be within 15% at wider beams. By comparing the approaches studied in this investigation with other methodologies taking into account the efficiency of the approach as a dose descriptor and the simplicity of the implementation in the clinical environment, the f(0,150) method may be the best for CBCT dosimetry combined with the use of correction factors.
The IEC has introduced a practical approach to overcome shortcomings of the CTDI100 for measurements on wide beams employed for cone beam (CBCT) scans. This study evaluated the efficiency of this ...approach (CTDIIEC) for different arrangements using Monte Carlo simulation techniques, and compared CTDIIEC to the efficiency of CTDI100 for CBCT. Monte Carlo EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc codes were used to simulate the kV imaging system mounted on a Varian TrueBeam linear accelerator. The Monte Carlo model was benchmarked against experimental measurements and good agreement shown. Standard PMMA head and body phantoms with lengths 150, 600, and 900 mm were simulated. Beam widths studied ranged from 20-300 mm, and four scanning protocols using two acquisition modes were utilized. The efficiency values were calculated at the centre (εc) and periphery (εp) of the phantoms and for the weighted CTDI (εw). The efficiency values for CTDI100 were approximately constant for beam widths 20-40 mm, where εc(CTDI100), εp(CTDI100), and εw(CTDI100) were 74.7 ± 0.6%, 84.6 ± 0.3%, and 80.9 ± 0.4%, for the head phantom and 59.7 ± 0.3%, 82.1 ± 0.3%, and 74.9 ± 0.3%, for the body phantom, respectively. When beam width increased beyond 40 mm, ε(CTDI100) values fell steadily reaching ~30% at a beam width of 300 mm. In contrast, the efficiency of the CTDIIEC was approximately constant over all beam widths, demonstrating its suitability for assessment of CBCT. εc(CTDIIEC), εp(CTDIIEC), and εw(CTDIIEC) were 76.1 ± 0.9%, 85.9 ± 1.0%, and 82.2 ± 0.9% for the head phantom and 60.6 ± 0.7%, 82.8 ± 0.8%, and 75.8 ± 0.7%, for the body phantom, respectively, within 2% of ε(CTDI100) values for narrower beam widths. CTDI100,w and CTDIIEC,w underestimate CTDI∞,w by ~55% and ~18% for the head phantom and by ~56% and ~24% for the body phantom, respectively, using a clinical beam width 198 mm. The CTDIIEC approach addresses the dependency of efficiency on beam width successfully and correction factors have been derived to allow calculation of CTDI∞.
The ability of radiation therapy to deliver radiation doses accurately to tumour targets, while sparing normal tissues, has advanced remarkably over recent decades. A development that has been key to ...achieving this improvement has been the application of image-guidance that facilitates more precise delivery of the prescribed dose to the treatment site. A recent international survey showed that kV cone beam CT (CBCT) is the imaging modality used most widely for image guided radiation therapy (IGRT) procedures. However, IGRT will deliver additional radiation doses to patients that should be considered. The aims of this study were to (1) estimate organ and size-specific effective doses resulting from kV-CBCT chest and pelvis scans based on body mass index (BMI) of the patient, and (2) investigate the influence of patient size on imaging dose. A validated Monte Carlo model developed with BEAMnrc/EGSnrc was employed to simulate kV spectra generated by Varian on-board imaging (OBI) system. Organ and size-specific effective doses were assessed for adult phantoms in the National Cancer Institute (NCI) phantom library using DOSXYZnrc/EGSnrc code. The library has been reconstructed from images of CT patients, and contains 100 male and 93 female phantoms of a wide range of sizes. The phantoms were grouped into six categories based on body mass index (BMI) as classified by the world health organization. For all sizes, average doses to organs that lay fully or partially inside the field of the chest and pelvic scans were in the range of (0.20–3.06 mGy/100 mAs) and (0.17–4.47 mGy/100 mAs), and size-specific effective doses were (0.63 – 1.78 mSv/100 mAs) and (0.30 – 1.17 mSv/100 mAs), respectively. Patient size played a significant role in determining the imaging doses resulting from both scans. In general, organ and effective doses of chest and pelvic scans of thin patients were about 2–3 times larger than those of obese patients, with the impact of patient size on pelvis doses being slightly higher than on chest ones. Results of this study suggests that patient size should be taken into consideration when estimation and optimization of the imaging doses resulting from CBCT. Although development of patient size-specific CBCT protocols is a practical approach that can account for patient size, it has not been implemented worldwide. Results from this investigation might be used as a simple approach to obtain an estimation of the imaging doses for patients of various sizes based on BMI.
•Organ dose and size-specific effective doses were affected significantly by patient size.•Differences in organ doses of 2–3 times were found between patients with high and low body mass indices (BMIs).•The BMI could be used to estimate imaging doses for individuals from CBCT scans based on the standard dose parameter.•Use of patient size-specific CBCT protocols would enable better optimization of radiological protection for IGRT procedures.
Cone beam CT (CBCT) is used in improving delivery of image guided radiation therapy (IGRT). However, less attention is paid to optimization of imaging for individual patients than in diagnostic ...applications. Methods are required to evaluate patient doses resulting from imaging procedures in order to facilitate optimization.
A modified version of the CT dose index (CTDI) has been proposed by the International Electrotechnical Commission (CTDIIEC) based on measurements with longer CTDI chambers. A dose–width product can be calculated by multiplying the CTDIIEC by the width of the CBCT beam and this is proposed as a quantity that can be used for dose assessment and linked to effective dose for optimization. Monte Carlo simulations based on a Varian On Board Imaging (OBI) system have been used to derive conversion coefficients that enable organ doses and effective dose for ICRP reference phantoms to be determined from the CTDIIEC for different scan protocols.
Coefficients to allow estimation of effective doses for reference phantoms from the dose-width product have been derived for head (0.0034 mSv(mGy cm)−1), thorax (0.0252 mSv(mGy cm)−1) abdomen, (0.0216 mSv(mGy cm)−1) and pelvis (0.0150 mSv(mGy cm)−1) protocols. These can be helpful in assessing overall optimization of the imaging process. However, organ doses are required for optimization of procedures for individual patients. Since standard exposure factors and field sizes are used frequently for CBCT, more organs are irradiated and dose levels are proportionately higher for smaller patients, than they are in diagnostic procedures.
Data on effective doses from CBCT imaging can be used in making judgements on the contributions to patient dose from imaging, and thereby assist in optimization of IGRT regimes. More attention is required in selection of exposure factors and field sizes for CBCT used with IGRT.
Organ doses for patients having CT scans can be assessed using Monte Carlo simulations, but this would require significant computing power if carried out for every patient. An alternative method is ...to use scanner-independent dose conversion coefficients (DCCs) to estimate organ doses. DCCs, calculated by normalizing organ doses with respect to the corresponding CTDIvol, have been developed based on patient size, in terms of patient perimeter or effective diameter (ED) in the region of interest (ROI). However, patient dimensions do not take account of x-ray attenuation within the patient, which will affect the accuracy of dose assessment, as the attenuations of tissues are different for each patient. The aim of this study was to develop size-specific and scanner-independent DCCs by quantifying patient size in terms of water-equivalent diameter (Dw). A library of 370 anatomical paediatric and adult phantoms was used. An algorithm was developed on MATLAB version 2015b to calculate Dw values for each phantom over six scan regions in the trunk. Organ doses and CTDIvol resulting from body scans acquired with 120 kVp were assessed using NCICT software for the phantom library to obtain DCCs. Good correlations were obtained between Dw as a measure for patient size and the DCCs, for which R2 values of correlations were above 0.92 for the majority of organs. These could provide better assessments of organ doses than ones based on patient dimensions. Comparisons of lung doses from scans of different regions assessed using NCICT software and DCCs showed that values based on Dw values gave lower variations in dose by up to 12% compared to those based on ED. The correlations could potentially be utilized to assess organ doses for patients undergoing scans with any CT scanner. This would simplify organ dose estimation for patients and overcome the limitations associated with Monte Carlo simulations.
•Dose conversion coefficients (DCCs) are used to estimate organ doses for CT patients.•DCCs are established in terms of patient size, which is assessed from the patient dimensions.•DCCs based on patient dimensions do not consider spectral attenuation of the scan region.•Water-equivalent diameter (Dw) used for patient size can be applied for scanner–independent DCCs.•Excellent correlations between Dw and DCC values for a range of organs were obtained.
The CT dose index (CTDI
) is the dosimetric quantity used for multi-slice CT (MSCT) with beams ⩽4 cm. Conversion factors (f
) based on patient size are applied to CTDI
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
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
is unsuitable for CBCT dosimetry, thus the assessments were applied to the modification CTDI
, recommended by the international electrotechnical commission (IEC). Conversion factors (f
) 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 CTDI
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
values, calculated values for AD were normalized with respect to the weighted CTDI
assessed in standard CTDI phantoms and free-in-air. Variations in f
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
values by up to 7%. Best-fit curves were derived from the f
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
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
for specific water-equivalent diameter (D
) to a SSDE.
•The Deffρe method was proposed to overcome the challenges of using the water equivalent diameter (Dw) method used for SSDE.•The Deffρe method is based on correcting effective diameter (Deff) with ...electron densities of tissues of a scan area.•Results of the Deffρe method showed to be promise for the chest protocols, but was less accurate for other body protocols.•The Deff method was poorer for the chest protocols, but gave good assessments for other body protocols.•Correction of both dimensions, lateral (LAT) and anterior-posterior (AP), did not improve the accuracy of the Deffρe method.
An assessment of the effective diameter of a patient’s body using electron densities of tissues inside the scan area (Deffρe) was proposed to overcome challenges associated with the estimation of water-equivalent diameter (Dw), which is used for size-specific dose estimate (SSDE). The aims of this study were to (1) investigate the Deffρe method in two different forms using a wide range of patient sizes and scanning protocols, and (2) compare between four methods used to estimate the patient size for SSDE.
Under IRB approval, a total of 350 patients of varying sizes have been collected retrospectively from the Hospital. The Dw values were assessed over six different CT body protocols: (1) chest with contrast media, (2) chest High-Resolution Computed Tomography (HRCT) without contrast media, (3) abdomen-pelvis with contrast media, (4) abdomen-pelvis without contrast media, (5) chest-abdomen-pelvis with contrast media, and (6) pelvis without contrast media. A MATLAB-based code was developed in-house to assess the size of each patient using the conventional effective diameter method (Deff), Deffρe by correcting either both the lateral (LAT) and anterior-posterior (AP) dimensions (Deff,LAT+APρe) or LAT only (Deff,LATρe), and Dw at the mid-CT slice of the patient images.
The results of Deff,LAT+APρe and Deff,LATρe provided a better estimation for the chest protocols with the averages of absolute percentage difference (PD) values in the range of 3 – 7 % for all patient sizes as compared to the Dw method, whereas the averages of PD values for the Deff method were 9 – 15 %. However, Deff gave a better estimation for Dw values for the other body protocols, with differences of 2 – 4 %, which were lower than those obtained with the Deff,LAT+APρe and Deff,LATρe methods. For the chest protocols, statistically significant differences were found between Deff and the other methods, but there were no significant differences between all the methods for the other scanning protocols. The results show that the correction of both dimensions, LAT and AP, did not improve the accuracy of the Deffρe method, and, for most protocols, Deff,LAT+APρe gave larger range differences compared to those based on correction of the LAT dimension only.
If the Dw cannot be assessed, the Deff,LATρe method may only be considered for the chest protocols as an alternative approach. The Deff method may also be used for all regions taking into account the application of a correction factor for the chest protocols to avoid a significant under or overestimation of the patient dose.
•Assessment of SSDE for an individual patient may be challenging and time consuming.•An advanced software is needed to calculate size of patients in terms of water equivalent diameter accurately.•Use ...of a single slice at middle of a region of interest leads to an over/underestimation.•Correlation between SSDE based on a single slice and a region size appears to be a practical approach.•Separate correlations should be considered for paediatric and adult patients.
Volume averaged CT dose index (CTDIvol) is an important dose index utilized for CT dosimetry. Measurements of CTDIvol are performed in reference cylindrical phantoms of specified diameters. A size-specific dose estimate (SSDE) has been recommended for assessment of doses delivered to individual patients. Evaluation of the SSDE requires the size of the scanned region of the patient to be estimated in terms of water-equivalent diameter (Dw) to allow calculation of a dose value appropriate for the patient. Estimation of Dw, however, may be challenging and time consuming as it requires assessment of Dw for each slice within the scanned region. A study has been carried out to investigate the suitability of using Dw,mid for a single slice at the middle of the scanned region to estimate a value of Dw,mean to apply to all slices. 351 phantoms (158 paediatric and 193 adult) developed from reconstructed CT images of patients were employed. Six scan regions were studied: chest, abdomen, pelvis, chest and abdomen, abdomen and pelvis, and the whole trunk. Results show that the use of Dw,mid can lead to over or underestimation of Dw,mean by up to 13% for paediatric and adult patients. SSDE values based on Dw,mid and Dw,mean were assessed for each phantom, and a linear regression analysis was performed. Use of the analysis could provide a simple and practical approach to assessing SSDE for a given scan based on Dw,mid with the root-mean-square errors estimated to be in the range of 1.2%–4.0% for paediatric and 1.2%–5.9% for adults.
The size-specific dose estimate (SSDE) concept is based on converting volume averaged CT dose index (CTDIvol) measured in cylindrical PMMA head and body phantoms of reference sizes to values for ...doses absorbed in individual patients. The original conversion factors used for SSDE is related to the effective diameter (ED) of the region of interest for the patient. ED is simple to measure, but does not include attenuation, which affects estimation of the dose absorbed. Water-equivalent diameter (Dw) which takes account of attenuation through the CT number, based on the linear attenuation coefficients, has been recommended to replace ED. The main scan parameter that affects Dw values is the x-ray spectrum. In this study, a library of 363 phantoms (168 paediatric and 195 adult) of a wide range of sizes from newborn to large adult were used to investigate the influence of the x-ray spectrum on Dw and the correlation between ED and Dw. Values were assessed for each phantom over seven scanned regions: head, chest, abdomen, pelvis, chest-abdomen, abdomen-pelvis, and the whole trunk using x-ray spectra of 80–140 kVp. The results show the effect of x-ray spectrum on the relationship between Dw and ED for the trunk regions was minimal, and almost all Dw/ED ratios were within a factor of ±1.01. However, Dw/ED values for the head region varied by a factor of up to 1.10 with the x-ray spectra. Energy-independent correlations between ED and Dw values for the trunk regions have been established. The correlations were compared with published results calculated by AAPM using another method and gave an average difference of 2.9%. Use of the correlations should simplify determination of the SSDE taking account of attenuation based on ED that is more readily measurable than Dw, which requires advanced software.
•The influence of CT x-ray spectrum on patient size for SSDE has been investigated.•Water-equivalent diameter (Dw) is recommended for evaluating patient size for SSDE.•The effect of spectrum was minimal for the trunk, but was significant for the head.•Energy-independent correlations have been established to simply assessment of Dw.
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![[P135] Should there be more...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/1120-1797_1724-191X/small "[P135] Should there be more optimization of cone beam ct imaging used with image guided radiotherapy")
