Evaluation of coefficients to derive organ and effective doses from cone-beam CT (CBCT) scans: a Monte Carlo study

Estimating organ dose with optimized peak dose index in cone-beam CT scans

PURPOSE

Organ dose evaluation is important for optimizing cone beam computed tomography (CBCT) scan protocols. However, an evaluation method for various CBCT scanners is yet to be established. In this study, we developed scanner-independent conversion coefficients to estimate organ doses using appropriate peak dose (f(0)) indices.

METHODS

This study included various scanners (angiography scanners and linear accelerators) and protocols for the head and body (thorax, abdomen, and pelvis) scan regions. f(0) was measured at five conventional positions (center position (f(0)c) and four peripheral positions (f(0)p) at 90° intervals) in the CT dose index (CTDI) phantom. To identify appropriate measurement positions for organ dose estimation, various f(0) indices were considered. Organ doses were measured by using optically stimulated luminescence dosimeters positioned in an anthropomorphic phantom. Thereafter, the conversion coefficients were calculated from each obtained f(0) value and organ or tissue dose using a linear fit for all scanners, and the coefficient of variation (CV) of the conversion coefficients was calculated for each organ or tissue. The f(0) index with the minimum CV value was proposed as the appropriate index.

RESULTS

The appropriate f(0) index was determined as f(0)c for the body region and a maximum of four f(0)p values for the head region. Using the proposed conversion coefficients based on the appropriate f(0) index, the organ/tissue doses were well estimated with a mean error of 14.2% across all scanners and scan regions.

CONCLUSIONS

The proposed scanner-independent coefficients are useful for organ dose evaluation using CBCT scanners.

Variations in size-specific effective dose with patient stature and beam width for kV cone beam CT imaging in radiotherapy

The facilities now available on linear accelerators for external beam radiotherapy enable radiation fields to be conformed to the shapes of tumours with a high level of precision. However, in order for the treatment delivered to take advantage of this, the patient must be positioned on the couch with the same degree of accuracy. Kilovoltage cone beam computed tomography systems are now incorporated into radiotherapy linear accelerators to allow imaging to be performed at the time of treatment, and image-guided radiation therapy is now standard in most radiotherapy departments throughout the world. However, because doses from imaging are much lower than therapy doses, less effort has been put into optimising radiological protection of imaging protocols. Standard imaging protocols supplied by the equipment vendor are often used with little adaptation to the stature of individual patients, and exposure factors and field sizes are frequently larger than necessary. In this study, the impact of using standard protocols for imaging anatomical phantoms of varying size from a library of 193 adult phantoms has been evaluated. Monte Carlo simulations were used to calculate doses for organs and tissues for each phantom, and results combined in terms of size-specific effective dose (SED). Values of SED from pelvic scans ranged from 11 mSv to 22 mSv for male phantoms and 8 mSv to 18 mSv for female phantoms, and for chest scans from 3.8 mSv to 7.6 mSv for male phantoms and 4.6 mSv to 9.5 mSv for female phantoms. Analysis of the results showed that if the same exposure parameters and field sizes are used, a person who is 5 cm shorter will receive a size SED that is 3%-10% greater, while a person who is 10 kg lighter will receive a dose that is 10%-14% greater compared with the average size.

STUDY ON A SPECIFIC TEST METHOD FOR DOSIMETRIC CHARACTERIZATION OF ULTRA-WIDE DETECTOR COMPUTED TOMOGRAPHY

The purpose of this study was to establish a specific test method for dosimetric characterization of wide-beam computed tomography (CT). For a wide beam, the dose distribution curve and the area of the curve were obtained by using pencil-like ionization chamber, a long CT dose profiler probe, a head phantom and a body phantom. The absolute dose conversion coefficient was multiplied to obtain the total integration integral of the absolute dose distribution, and then the computed tomography dose index (CTDI) value under any wide beam condition was obtained by dividing the collimation width. It was calculated that the absolute dose conversion coefficient was 1.135 under the narrow beam of 8 mm. To a 160 mm-wide beam, the value of CTDI was 7.57 mGy/100mAs after normalized in the head 80 kV CT scanning, and it was 9.80 mGy/100mAs after normalized in the body 120 kV CT scanning. The specific test method solves the problem that the previous measurement method underestimates the CTDI value.

A Monte Carlo investigation of dose length product of cone beam computed tomography scans

The dose length product (DLP) provides a measurement related to energy imparted from a computed tomography (CT) scan. The DLP is based on the volume-averaged CT dose index (CTDI _vol), which is designed for fan beams. The aims of this study were to investigate the use of DLP for scans with wide beams used in cone beam CT (DLP _CBCT) in radiotherapy that would be analogous to the DLP of fan beam scans (DLP _CT), and to compare the efficiencies of DLP _CT and DLP _CBCT in reporting the total energy imparted in patients. A validated Monte Carlo model of a kV imaging system integrated into a Varian TrueBeam linac was employed. The DLP _CT was assessed by multiplying the CTDI _vol for a 20 mm fan beam by scan length, and the DLP _CBCT determined through multiplying the CTDI _vol, estimated for wide beams using a correction factor based on free-in-air measurements, by the beam width. Two scan protocols for head and body were investigated for tube potentials between 80 and 140 kV and a range of scan lengths/widths. Efficiency values were estimated by normalising the DLP _CT and DLP _CBCT with respect to the corresponding dose profile integrals (DPIs), which were evaluated within 900 mm long phantoms. The results show that the DLP _CBCT values were within 1% of those for DLP _CT of similar length performed on the same system, and the efficiencies decrease with tube potential. However, whereas DLP values for fan beams are approximately proportional to scan length, those for wide beams decrease by ∼2% between beam widths of 20 and 320 mm. As a result, while the DLP _CT efficiency is similar over all scan lengths, that for DLP _CBCT increases slightly with beam width. The DLP _CT and DLP _CBCT underestimated the total energy imparted by comparable amounts with efficiencies within the range of 80-81% and 80-83% for the head scans, and 71-76% and 70-77% for the body scans, respectively. The results indicate that the DLP _CBCT can be considered as an analogous dose index to the DLP _CT. It could, therefore, be used for quantification of doses from imaging in radiotherapy and provide a valuable tool to aid optimisation.

Estimation of size-specific dose estimates (SSDE) for paediatric and adults patients based on a single slice

Volume averaged CT dose index (CTDI_vol) is an important dose index utilized for CT dosimetry. Measurements of CTDI_vol 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 (D_w) to allow calculation of a dose value appropriate for the patient. Estimation of D_w, however, may be challenging and time consuming as it requires assessment of D_w for each slice within the scanned region. A study has been carried out to investigate the suitability of using D_w,mid for a single slice at the middle of the scanned region to estimate a value of D_w,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 D_w,mid can lead to over or underestimation of D_w,mean by up to 13% for paediatric and adult patients. SSDE values based on D_w,mid and D_w,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 D_w,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.