Anthropomorphic Heterogeneous Phantom Research Articles

Clinical application of a ct dose reduction simulator

Purpose To validate an exposure reduction algorithm for head CT examinations in clinical practice. Material and methods CT images from Head Adult protocol in a Siemens CT Somatom Definition AS+ was used for this study. Dose reduction was simulated through the addition of noise to each pixel value of the original image. A homogeneous phantom was used to measure the autocorrelation function, which is convolved with a white noise matrix to get the spatially correlated statistical noise to be added. A Matlab code was written to perform this task. Firstly, the algorithm was validated using two phantoms, i.e. a homogeneous and an anthropomorphic heterogeneous phantom, CT-scanned with different exposure reductions, up to 75%, from the original reference mAs. Noise values comparison between CT-simulated and CT-real images was done. Secondly, from our PACS 10 patients previously CT-scanned, with subtle pathologies and normal brain, were selected. CT-simulated images, up to an exposure reduction of 50%, were reviewed by two experienced radiologists, not aware of the applied exposure reduction. Results The algorithm reproduces well the image noise variation with the reduction of the mAs. Differences were less than 2% in both phantoms in the exposure range analysed. CT-simulated images with exposure reductions equal or larger than 30% were clearly identified by both radiologists. Mostly, a 20% reduction was deemed appropriate for clinical diagnosis. Conclusion Dose reduction simulation software might be a powerful tool for patient dose optimization.

Monte Carlo evaluation of a treatment planning system for helical tomotherapy in an anthropomorphic heterogeneous phantom and for clinical treatment plans

Helical tomotherapy is an increasingly common form of intensity modulated radiation therapy that allows for image guided adaptive radiotherapy. Its treatment planning system (TPS) uses a convolution superposition algorithm for dose distribution calculations. The accuracy of this algorithm in the presence of heterogeneities was evaluated against Monte Carlo (MC) calculations and measurements. This work performed BEAMnrc-and DOSXYZnrc-based MC dose calculations of tomotherapy deliveries to a CIRS anthropomorphic heterogeneous phantom with typical clinical inverse planning and delivery settings. Point measurements with A1SL ion chambers and relative measurements with Kodak EDR2 film were carried out in the phantom. The experimental results were used to evaluate both the TPS and MC dose calculations. Furthermore, the dose distribution for a clinical head-and-neck cancer plan was calculated on the TPS and MC systems. The results support this MC system as a viable option for the accurate simulation of the tomotherapy process in the presence of heterogeneities where direct measurement may not be practical. Ion chamber measurements in the CIRS phantom suggested the TPS has an average relative difference of 2.3%, with the largest difference being -4.1% in one of the organs at risk. The MC system accurately predicted the dose to these measurement points within statistical uncertainty. The film measurements in the CIRS phantom demonstrated 90.7% (of pixels) agreed with the MC system using a +/-3%/3 mm acceptance criteria, where only 50.3% agreed with the TPS. In the clinical head-and-neck cancer plan evaluation where MC served as a reference against which to compare the TPS result, an average of 92.7% of the voxels within volumes of interest passed a 3%/3 mm criteria. The PTV54 showed the worst agreement with 85.4% of the volume passing the 3% /3 mm criteria. In general, the +/-3%/3 mm criterion was found to be a challenge for the TPS in the presence of lung inhomogeneity.

TH‐E‐224A‐04: IMRT Film QA in a Heterogeneous Anthropomorphic Phantom

Purpose: To study the agreement of heterogeneous IMRT treatment planning dose calculations with radiochromic film (RCF) and radiographic film (RGF) measured in anthropomorphic heterogeneous phantoms. We intend to use heterogeneous‐phantom film dosimetry to improve IMRT quality assurance (QA). Method and Materials: A commercial heterogeneous anthropomorphic head‐and‐neck phantom, with equivalent tissue bone, soft tissue and air regions was employed for dosimetry verification (CIRS, inc.). High sensitivity EBT radiochromic film (ISP, Inc.) and EDR2 radiographic film (Kodak) were employed simultaneously for film dosimetry. The phantom had 12 axial sections, each 25mm thick, with alignment pins and a compression plate. We selected a plane containing bone, soft tissue and an air gap for IMRT verification. The RCF and RGF were cut to match the selected plane, inserted between the sections and light sealed. A 6MV IMRT head & neck treatment plan with 7 beams was delivered using a commercial linac (Varian Clinac 2100/CD). The RCF was scanned avoiding artifact described in a companion work using a CCD film scanner (Epson 1680 scanner). The dose distributions were compared with the treatment plan. Results: Both RCF and RGF disagree with the treatment plan dose near the air cavities by 15%. Away the air cavities 5% agreement was obtained with RCF. Pronounced artifacts were observed in the RGF that were not observed in the RCF. Conclusion: IMRT dosimetry and quality assurance can be improved by using anthropomorphic phantoms that incorporate realistic heterogeneities such as bone, soft tissue and air gaps, and corrections to dose calculations can be applied. We hypothesize that RGF suffers from Cerenkov in air cavities and in less opaque phantom materials, and this will be investigated in further studies.This work was supported in part by NCI grants R01‐CA‐100636 and N43‐CM‐52214.

Reconstruction of the dose to the victim as a result of accidental irradiation in Lia (Georgia)

The spatial distribution of the irradiation dose to a victim who accidentally came into contact with a radioactive source is reconstructed. The computer program ROBOT and an ICRP standard anthropomorphic heterogeneous phantom are used to perform a mathematical simulation of the accident. The mass density distribution of the critical organs with respect to dose is calculated in order to predict the severity of the sickness.

Organ doses in mathematical phantoms with different elemental composition of organs and tissues

A Monte Carlo technique applied to the GSF anthropomorphic heterogeneous phantoms has been realized. Mass attenuation coefficients for ICRP reference man calculated from White and Fitzgerald have been introduced in our mathematical model. Linear attenuation coefficients for each organ and tissue have been utilized instead of the four groups of the GSF phantoms. This work consists of comparative evaluation of organ doses in ours and in GSF phantoms. Calculations have been carried out for external irradiation with monochromatic and distributed photon energy in the geometrical conditions of two typical radiographic techniques.