Monte Carlo modelling the dosimetric effects of electrode material on diamond detectors.

Diamond detectors for radiation dosimetry were modelled using the EGSnrc Monte Carlo code to investigate the influence of electrode material and detector orientation on the absorbed dose. The small dimensions of the electrode/diamond/electrode detector structure required very thin voxels and the use of non-standard DOSXYZnrc Monte Carlo model parameters. The interface phenomena was investigated by simulating a 6 MV beam and detectors with different electrode materials, namely Al, Ag, Cu and Au, with thickens of 0.1 µm for the electrodes and 0.1 mm for the diamond, in both perpendicular and parallel detector orientation with regards to the incident beam. The smallest perturbations were observed for the parallel detector orientation and Al electrodes (Z = 13). In summary, EGSnrc Monte Carlo code is well suited for modelling small detector geometries. The Monte Carlo model developed is a useful tool to investigate the dosimetric effects caused by different electrode materials. To minimise perturbations cause by the detector electrodes, it is recommended that the electrodes should be made from a low-atomic number material and placed parallel to the beam direction.

Automatic tracking of gold seed markers from CBCT image projections in lung and prostate radiotherapy.

PURPOSE: To construct a method and software to track gold seed implants in prostate and lung patients undergoing radiotherapy using CBCT image projections. METHODS: A mathematical model was developed in the MatLab (Mathworks, Natick, USA) environment which uses a combination of discreet cosine transforms and filtering to enhance several edge detection methods for identifying and tracking gold seed fiducial markers in images obtained from Varian (Varian Medical Systems, Palo Alto, USA) and Elekta (Kungstensgatan, Sweden) CBCT projections. RESULTS: Organ motion was captured for 16 prostate patients and 1 lung patient. CONCLUSION: Image enhancement and edge detection is capable of automatically tracking markers for up to 98% (Varian) and 79% (Elekta) of CBCT projections for prostate and lung markers however inclusion of excessive bony anatomy (LT and RT LAT) inhibit the ability of the model to accurate determine marker location.

Estimate of organ radiation absorbed doses in clinical CT using the radiation treatment planning system.

Organ absorbed doses in computed tomography (CT) scans can be measured with anatomical phantoms but not inside the human body. In this study, a straightforward method was investigated to estimate organ doses in clinical CT using the radiation treatment planning system (RTPS) and compared them with experimental results of photoluminescence dosemeters (PLD). In a heterogeneous phantom, the average difference between PLD and RTPS values were -5.0% for the body and 7.1% for the lung. Using CT data, organ doses in 30 clinical cases were then calculated. There was a significant inverse correlation between the calculated values of organ doses and body mass index (BMI, correlation coefficients (r) = -0.69 (whole body), -0.80 (right lung), -0.81 (left lung), -0.76 (spinal cord), -0.74 (vertebra bone), -0.74 (heart), and -0.79 (oesophagus), all p < 0.01). An RTPS can be a simple and useful tool for estimating equivalent doses inside the human body, during whole-body CT scans.

Converging stereotactic radiotherapy using kilovoltage X-rays: experimental irradiation of normal rabbit lung and dose-volume analysis with Monte Carlo simulation.

PURPOSE: To validate the feasibility of developing a radiotherapy unit with kilovoltage X-rays through actual irradiation of live rabbit lungs, and to explore the practical issues anticipated in future clinical application to humans through Monte Carlo dose simulation. METHODS AND MATERIALS: A converging stereotactic irradiation unit was developed, consisting of a modified diagnostic computed tomography (CT) scanner. A tiny cylindrical volume in 13 normal rabbit lungs was individually irradiated with single fractional absorbed doses of 15, 30, 45, and 60 Gy. Observational CT scanning of the whole lung was performed every 2 weeks for 30 weeks after irradiation. After 30 weeks, histopathologic specimens of the lungs were examined. Dose distribution was simulated using the Monte Carlo method, and dose-volume histograms were calculated according to the data. A trial estimation of the effect of respiratory movement on dose distribution was made. RESULTS: A localized hypodense change and subsequent reticular opacity around the planning target volume (PTV) were observed in CT images of rabbit lungs. Dose-volume histograms of the PTVs and organs at risk showed a focused dose distribution to the target and sufficient dose lowering in the organs at risk. Our estimate of the dose distribution, taking respiratory movement into account, revealed dose reduction in the PTV. CONCLUSIONS: A converging stereotactic irradiation unit using kilovoltage X-rays was able to generate a focused radiobiologic reaction in rabbit lungs. Dose-volume histogram analysis and estimated sagittal dose distribution, considering respiratory movement, clarified the characteristics of the irradiation received from this type of unit.

Experimental stereotactic irradiation of normal rabbit lung: computed tomographic analysis of radiation injury and the histopathological features.

PURPOSE: The aim of this study was to establish an animal experimental model of pulmonary stereotactic irradiation and clarify the morphological patterns of pulmonary radiation injury with computed tomography and the histopathological features. MATERIALS AND METHODS: Tiny spherical regions in the lungs of seven anesthetized rabbits were irradiated stereotactically with a single fractional dose of 21-60 Gy. Subsequently, the irradiated lungs were observed biweekly with computed tomography (CT) for 24 weeks. Radiation injury of the lung was examined histopathologically in one specimen. RESULTS: Localized hypodense changes were observed 7-15 weeks after irradiation in three rabbits irradiated with 60 Gy, and the findings persisted beyond that time. The electron density ratios in the lung fields obtained from the CT images were shown to be decreasing, corresponding to the hypodensity changes. No clear increased density opacity was observed in any rabbit in the 60-Gy irradiated group. Severe localized fibrotic change was observed in the histopathological specimens. CONCLUSION: Specific localized hypodensity changes were found in only three rabbits irradiated with 60 Gy, the highest dose we employed.

Differences in the definition of internal target volumes using slow CT alone or in combination with thin-slice CT under breath-holding conditions during the planning of stereotactic radiotherapy for lung cancer.

PURPOSE: To investigate how the delineations of the internal target volume (ITV) made from 'slow' CT alter with reference to 'thin-slice' CT. MATERIALS AND METHODS: Thin-slice CT images taken under breath-holding conditions and slow CT images taken under shallow-breathing conditions (8s/image) of 11 lung cancers were used for this study. Five radiation oncologists delineated ITV of the 11 lesions using slow CT images (ITV1), and then redefined them with reference to thin-slice CT images (ITV2). SD-images (standard deviation image) were created for all patients from ITV images in order to visualize the regional variation of the ITVs. RESULTS: The mean value of ITV2 was smaller than that initially defined by ITV1. There was no significant change in ITV1 and ITV2 between operators with regard to standard deviation in volume. There was a significant difference in the distribution of the ratio of ITV1 to ITV2 obtained on thin-slice CTs between cases with and without ground glass opacity. In cases without ground glass opacity there was a tendency for ITV2 to have a smaller volume than ITV1. CONCLUSIONS: Combined use of slow CT and thin-slice CT in delineation of ITV contours appeared to be useful in making adjustments for obscured tumor images caused by respiratory movement.

Interface software for DOSXYZnrc Monte Carlo dose evaluation on a commercial radiation treatment planning system.

PURPOSE: As the conventional graphical user interface (GUI) associated with DOSXYZnrc or BEAMnrc is unable to define specific structures such as gross tumor volume (GTV) on computed tomography (CT) data, the quantitative analysis of doses in the form of dose-volume histograms (DVHs) is difficult. The purpose of this study was to develop an interface that enables us to analyze the results of DOSXYZnrc output with a commercial radiation treatment planning (RTP) system and to investigate the validity of the system. MATERIALS AND METHODS: Interface software to visualize three-dimensional radiotherapy Monte Carlo (MC) dose data from DOSXYZnrc on the XiO RTP system was developed. To evaluate the interface, MC doses for a variety of photon energies were calculated using the CT data of a thorax phantom and a uniform phantom as well as data from patients with lung tumors. RESULTS: The dose files were analyzed on the XiO RTP system in the form of isodose distributions and DVHs. In all cases, the XiO RTP system perfectly displayed the MC doses for quantitative evaluation in the form of differential and integral DVHs. CONCLUSION: Three-dimensional display of DOSXYZnrc doses on a dedicated RTP system could provide all the existing facilities of the system for quantitative dose analysis.

Rotational and translational reproducibility of newly developed Leksell frame-based relocatable fixation system.

PURPOSE: The aim of this study was to evaluate three-dimensional movement of the cranium in a relocatable frame using positions of anatomical landmarks obtained from repeated quality-assurance (QA) computed tomography (CT) studies. MATERIALS AND METHODS: We analyzed 17 series of QA-CT data representing five patients who underwent stereotactic radiotherapy for treatment of acoustic neurinoma. Helical-CT scans with 1-mm collimation were obtained at the time of treatment planning and during the course of treatment. The right and left short processes of the incus and the top of the crista galli were used as the three anatomical reference points. RESULTS: Fluctuations in distance among the reference points were all <1 mm. The translational displacements for these points were <2 mm, with standard deviations (SD) of <2 mm. A plane that included all three reference points was defined as the reference plane. To investigate the direction of cranial rotation for each QA-CT scan, unit normal vectors of the reference plane were obtained. Three-dimensional analyses indicated that cranial rotation was greatest along the X-axis, followed by the Y-axis, with the least rotation along the Z-axis. CONCLUSION: The result suggested that movement of the craniocaudal axis in the sagittal plane was a major factor behind displacement of the cranium.

System design and development of a pinhole SPECT system for quantitative functional imaging of small animals.

Recently, small animal imaging by pinhole SPECT has been widely investigated by several researchers. We developed a pinhole SPECT system specially designed for small animal imaging. The system consists of a rotation unit for a small animal and a SPECT camera attached with a pinhole collimator. In order to acquire complete data of the projections, the system has two orbits with angles of 90 degrees and 45 degrees with respect to the object. In this system, the position of the SPECT camera is kept fixed, and the animal is rotated in order to avoid misalignment of the center of rotation (COR). We implemented a three dimensional OSEM algorithm for the reconstruction of data acquired by the system from both the orbitals. A point source experiment revealed no significant COR misalignment using the proposed system. Experiments with a line phantom clearly indicated that our system succeeded in minimizing the misalignment of the COR. We performed a study with a rat and 99mTc-HMDP, an agent for bone scan, and demonstrated a dramatic improvement in the spatial resolution and uniformity achieved by our system in comparison with the conventional Feldkamp algorithm with one set of orbital data.

Investigations of different kilovoltage X-ray energy for three-dimensional converging stereotactic radiotherapy system: Monte Carlo simulations with CT data.

We are investigating three-dimensional converging stereotactic radiotherapy (3DCSRT) with suitable medium-energy x rays as treatment for small lung tumors with better dose homogeneity at the target. A computed tomography (CT) system dedicated for non-coplanar converging radiotherapy was simulated with BEAMnrc (EGS4) Monte-Carlo code for x-ray energy of 147.5, 200, 300, and 500 kilovoltage (kVp). The system was validated by comparing calculated and measured percentage of depth dose in a water phantom for the energy of 120 and 147.5 kVp. A thorax phantom and CT data from lung tumors (<20 cm3) were used to compare dose homogeneities of kVp energies with MV energies of 4, 6, and 10 MV. Three non-coplanar arcs (0 degrees and +/-25 degrees ) around the center of the target were employed. The Monte Carlo dose data format was converted to the XiO RTP format to compare dose homogeneity, differential, and integral dose volume histograms of kVp and MV energies. In terms of dose homogeneity and DVHs, dose distributions at the target of all kVp energies with the thorax phantom were better than MV energies, with mean dose absorption at the ribs (human data) of 100%, 85%, 50%, 30% for 147.5, 200, 300, and 500 kVp, respectively. Considering dose distributions and reduction of the enhanced dose absorption at the ribs, a minimum of 500 kVp is suitable for the lung kVp 3DCSRT system.

Differences in pulmonary function before vs. 1 year after hypofractionated stereotactic radiotherapy for small peripheral lung tumors.

PURPOSE: To evaluate long-term pulmonary toxicity of stereotactic radiotherapy (SRT) by pulmonary function tests (PFTs) performed before and after SRT for small peripheral lung tumors. METHODS AND MATERIALS: A total of 17 lesions in 15 patients with small peripheral lung tumors, who underwent SRT between February 2000 and April 2003, were included in this study. Twelve patients had primary lung cancer, and 3 patients had metastatic lung cancer. Primary lung cancer was T1-2N0M0 in all cases. Smoking history was assessed by the Brinkman index (number of cigarettes smoked per day multiplied by number of years of smoking). Prescribed radiation doses at the 80% isodose line were 40-60 Gy in 5-8 fractions. PFTs were performed immediately before SRT and 1 year after SRT. Test parameters included total lung capacity (TLC), vital capacity (VC), forced expiratory volume in 1 s (FEV1.0), and diffusing capacity of lung for carbon monoxide (DLCO). PFT changes were evaluated in relation to patient- and treatment-related factors, including age, the Brinkman index, internal target volume, the percentages of lung volume irradiated with >15, 20, 25, and 30 Gy (V15, V20, V25, and V30, respectively), and mean lung dose. RESULTS: There were no significant changes in TLC, VC, or FEV1.0 before vs. after SRT. The mean percent change from baseline in DLCO was significantly increased by 128.2%. Univariate and multivariate analyses revealed a correlation between DLCO and the Brinkman index. CONCLUSIONS: One year after SRT as compared with before SRT, there were no declines in TLC, VC, and FEV1.0. DLCO improved in patients who had been heavy smokers before SRT, suggesting a correlation between DLCO and smoking cessation. SRT seems to be tolerable in view of long-term lung function.

Measurement of beam-axis displacement from the isocenter during three-dimensional conformal radiosurgery with a micro-multileaf collimator.

We describe the displacement of the beam-axis from the planning isocenter in clinical situations during three-dimensional conformal radiosurgery using an Acculeaf bi-directional micro-multileaf collimator. The displacements were recorded for 64 ports using a video imaging system and a stereotactic arc. The mean displacement was 0.41+/-0.25 mm.

Dependency of energy and spatial distributions of photons on edge of object in brain SPECT.

OBJECTIVES: Accurate mu maps are important for quantitative image reconstruction in SPECT. The Compton scatter energy window (CSW) technique has been proposed to define the outline of objects. In this technique, a lower energy window image is acquired in addition to the main photo-peak energy window. The image of the lower energy window is used to estimate the edge of the scanned object to produce a constant attenuation map. The aim of this study was to investigate the dependency of CSW on the spatial and energy distribution of radioisotope to predict the edges of objects. METHODS: Two particular cases of brain study were considered, namely uniform distribution and non-uniform distribution using Monte Carlo simulation and experiments with uniform cylindrical phantom and hotspot phantom. The phantoms were filled with water and a radioactive solution of 99mTc. For each phantom, 20%, 30%, 40% and 50% thresholds of the mean profile were applied to estimate Ewt, the energy window for minimum difference between the estimated and true edge of objects. RESULTS: The Ewt's were 100-120 keV with a 40% threshold and 92-114 keV with a 30% threshold for uniform and hotspot phantoms, respectively. CONCLUSIONS: Edge of the objects with CSW technique varies with energy window and thresholds. Careful setting of the energy window is required to use the CSW technique.

Evaluation of penetration and scattering components in conventional pinhole SPECT: phantom studies using Monte Carlo simulation.

In quantitative pinhole SPECT, photon penetration through the collimator edges (penetration), and photon scattering by the object (object scatter) and collimator (collimator scatter) have not been investigated rigorously. Monte Carlo simulation was used to evaluate these three physical processes for different tungsten knife-edge pinhole collimators using uniform, hotspot and donut phantoms filled with 201Tl, 99mTc, 123I and 131I solutions. For the hotspot phantom, the penetration levels with respect to total counts for a 1 mm pinhole aperture were 78%, 28% and 23% for 131I, 123I and 99mTc, respectively. For a 2 mm aperture, these values were 65% for 131I, 16% for 123I and 12% for 99mTc. For all pinholes, 201Tl penetration was less than 4%. The evaluated scatter (from object and collimator) with a hotspot phantom for the 1 mm pinhole was 24%, 16%, 18% and 13% for 201Tl, 99mTc, 123I and 131I, respectively. Summation of the object and collimator scatter for the uniform phantom was approximately 20% higher than that for the hotspot phantom. Significant counts due to penetration and object and collimator scatter in the reconstructed image were observed inside the core of the donut phantom. The collimator scatter can be neglected for all isotopes used in this study except for 131I. Object scatter correction for all radionuclides used in this study is necessary and correction for the penetration contribution is necessary for all radionuclides but 201Tl.