In case of radiation treatment using small field high-energy photon beams, an accurate dosimetry is a challenging task because of dosimetrically unfavorable phenomena such as dramatic changes of the dose at the field boundaries, dis-equilibrium of the electrons, and non-uniformity between the detector and the phantom materials. In this study, the absorbed dose in the phantom was measured by using an ion chamber and a diode detector widely used in clinics. $GAFCHROMIC^{(R)}$ EBT films composed of water equivalent materials was also evaluated as a small field detector and compared with ionchamber and diode detectors. The output factors at 10 cm depth of a solid phantom located 100 cm from the 6 MV linear accelerator (Varian, 6 EX) source were measured for 6 field sizes ($5{\times}5\;cm^2$, $2{\times}2\;cm^2$, $1.5{\times}1.5\;cm^2$, $1{\times}1\;cm^2$, $0.7{\times}0.7\;cm^2$ and $0.5{\times}0.5\;cm^2$). As a result, from $5{\times}5\;cm^2$ to $1.5{\times}1.5\;cm^2$ field sizes, absorbed doses from three detectors were accurately identified within 1%. Wheres, the ion chamber underestimated dose compared to other detectors in the field sizes less than $1{\times}1\;cm^2$. In order to correct the observed underestimation, a convolution method was employed to eliminate the volume averaging effect of an ion chamber. Finally, in $1{\times}1\;cm^2$ field the absorbed dose with a diode detector was about 3% higher than that with the EBT film while the dose with the ion chamber after volume correction was 1% lower. For $0.5{\times}0.5\;cm^2$ field, the dose with the diode detector was 1% larger than that with the EBT film while dose with volume corrected ionization chamber was 7% lower. In conclusion, the possibility of $GAFCHROMIC^{(R)}$ EBT film as an small field dosimeter was tested and further investigation will be proceed using Monte Calro simulation.
This study examined the dosimetric influence of implanted gold markers in proton therapy and the effects of their positions in the spread-out Bragg peak (SOBP) proton beam. The implanted cylindrical gold markers were 3 mm long and 1.2 mm in diameter. The dosimetric influence of the gold markers was determined with markers at various locations in a proton-beam field. Spatial dose distributions were measured using a three-dimensional moving water phantom and a stereotactic diode detector with an effective diameter of 0.5 mm. Also, a film dosimetry was performed using Gafchromic External Beam Treatment (EBT) film. The GEANT4 simulation toolkit was used for Monte-Carlo simulations to confirm the measurements and to construct the dose-volume histogram with implanting markers. Motion data were obtained from the portal images of 10 patients to investigate the effect of organ motions on the dosimetric influence of markers in the presence of a rectal balloon. The underdosed volume due to a single gold marker, in which the dose was less than 95% of a prescribed amount, was 0.15 cc. The underdosed volume due to the presence of a gold marker is much smaller than the target volume. However, the underdosed volume is inside the gross tumor volume and is not smeared out due to translational prostate motions. The positions of gold markers and the conditions of the proton-beam field give different impacts on the dose distribution of a target with implanted gold markers, and should be considered in all clinical proton-based therapies.
Kim, Tae-Ho;Yoon, Jai-Woong;Kang, Seong-Hee;Suh, Tae-Suk
Progress in Medical Physics
/
v.23
no.3
/
pp.145-153
/
2012
In this study, we quantify the residual motion artifact in 4D-CT scan using the dynamic lung phantom which could simulate respiratory target motion and suggest a simple one-dimension theoretical model to explain and characterize the source of motion artifacts in 4DCT scanning. We set-up regular 1D sine motion and adjusted three level of amplitude (10, 20, 30 mm) with fixed period (4s). The 4DCT scans are acquired in helical mode and phase information provided by the belt type respiratory monitoring system. The images were sorted into ten phase bins ranging from 0% to 90%. The reconstructed images were subsequently imported into the Treatment Planning System (CorePLAN, SC&J) for target delineation using a fixed contour window and dimensions of the three targets are measured along the direction of motion. Target dimension of each phase image have same changing trend. The error is minimum at 50% phase in all case (10, 20, 30 mm) and we found that ${\Delta}S$ (target dimension change) of 10, 20 and 30 mm amplitude were 0 (0%), 0.1 (5%), 0.1 (5%) cm respectively compare to the static image of target diameter (2 cm). while the error is maximum at 30% and 80% phase ${\Delta}S$ of 10, 20 and 30 mm amplitude were 0.2 (10%), 0.7 (35%), 0.9 (45%) cm respectively. Based on these result, we try to analysis the residual motion artifact in 4D-CT scan using a simple one-dimension theoretical model and also we developed a simulation program. Our results explain the effect of residual motion on each phase target displacement and also shown that residual motion artifact was affected that the target velocity at each phase. In this study, we focus on provides a more intuitive understanding about the residual motion artifact and try to explain the relationship motion parameters of the scanner, treatment couch and tumor. In conclusion, our results could help to decide the appropriate reconstruction phase and CT parameters which reduce the residual motion artifact in 4DCT.
The purpose of this study is to analyze motion-induced dose error generated by each tumor motion parameters of irregular tumor motion in helical tomotherapy. To understand the effect of the irregular tumor motion, a simple analytical model was simulated. Moving cases that has tumor motion were divided into a slightly irregular tumor motion case, a large irregular tumor motion case and a patient case. The slightly irregular tumor motion case was simulated with a variability of 10% in the tumor motion parameters of amplitude (amplitude case), period (period case), and baseline (baseline case), while the large irregular tumor motion case was simulated with a variability of 40%. In the phase case, the initial phase of the tumor motion was divided into end inhale, mid exhale, end exhale, and mid inhale; the simulated dose profiles for each case were compared. The patient case was also investigated to verify the motion-induced dose error in 'clinical-like' conditions. According to the simulation process, the dose profile was calculated. The moving case was compared with the static case that has no tumor motion. In the amplitude, period, baseline cases, the results show that the motion-induced dose error in the large irregular tumor motion case was larger than that in the slightly irregular tumor motion case or regular tumor motion case. Because the offset effect was inversely proportion to irregularity of tumor motion, offset effect was smaller in the large irregular tumor motion case than the slightly irregular tumor motion case or regular tumor motion case. In the phase case, the larger dose discrepancy was observed in the irregular tumor motion case than regular tumor motion case. A larger motion-induced dose error was also observed in the patient case than in the regular tumor motion case. This study analyzed motion-induced dose error as a function of each tumor motion parameters of irregular tumor motion during helical tomotherapy. The analysis showed that variability control of irregular tumor motion is important. We believe that the variability of irregular tumor motion can be reduced by using abdominal compression and respiratory training.
In this study, we aim to design the architecture of the kV imaging system for tumor tracking in the dual-head gantry system and analyze its accuracy by simulations. We established mathematical formulas and algorithms to track the tumor position with the two-pair kV imaging systems when they are in the non-orthogonal positions. The algorithms have been designed in the homogeneous coordinate framework and the position of the source and the detector coordinates are used to estimate the tumor position. 4D XCAT (4D extended cardiac-torso) software was used in the simulation to identify the influence of the angle between the two-pair kV imaging systems and the resolution of the detectors to the accuracy in the position estimation. A metal marker fiducial has been inserted in a numerical human phantom of XCAT and the kV projections were acquired at various angles and resolutions using CT projection software of the XCAT. As a result, a positional accuracy of less than about 1mm was achieved when the resolution of the detector is higher than 1.5 mm/pixel and the angle between the kV imaging systems is approximately between $90^{\circ}$ and $50^{\circ}$. When the resolution is lower than 1.5 mm/pixel, the positional errors were higher than 1mm and the error fluctuation by the angles was greater. The resolution of the detector was critical in the positional accuracy for the tumor tracking and determines the range for the acceptable angle range between the kV imaging systems. Also, we found that the positional accuracy analysis method using XCAT developed in this study is highly useful and will be a invaluable tool for further refined design of the kV imaging systems for tumor tracking systems.
Lee, Chang Yeol;Kim, Woo Chul;Kim, Hun Jeong;Park, Jeong Hoon;Min, Chul Kee;Shin, Dong Oh;Choi, Sang Hyoun;Park, Seungwoo;Huh, Hyun Do
Progress in Medical Physics
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v.26
no.3
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pp.127-136
/
2015
The purpose of this study is to perform a dosimetric evaluation of amplitude-based respiratory gating for the delivery of volumetric modulated arc therapy (VMAT). We selected two types of breathing patterns, subjectively among patients with respiratory-gated treatment log files. For patients that showed consistent breathing patterns (CBP) relative to the 4D CT respiration patterns, the variability of the breath-holding position during treatment was observed within the thresholds. However, patients with inconsistent breathing patterns (IBP) show differences relative to those with CBP. The relative isodose distribution was evaluated using an EBT3 film by comparing gated delivery to static delivery, and an absolute dose measurement was performed with a $0.6cm^3$ Farmer-type ion chamber. The passing rate percentages under the 3%/3 mm gamma analysis for Patients 1, 2 and 3 were respectively 93.18%, 91.16%, and 95.46% for CBP, and 66.77%, 48.79%, and 40.36% for IBP. Under the more stringent criteria of 2%/2 mm, passing rates for Patients 1, 2 and 3 were respectively 73.05%, 67.14%, and 86.85% for CBP, and 46.53%, 32.73%, and 36.51% for IBP. The ion chamber measurements were within 3.5%, on average, of those calculated by the TPS and within 2.0%, on average, when compared to the static-point dose measurements for all cases of CBP. Inconsistent breathing patterns between 4D CT simulation and treatment may cause considerable dosimetric differences. Therefore, patient training is important to maintain consistent breathing amplitude during CT scan acquisition and treatment delivery.
The purpose of this study is to analyze dosimetric parameters of patient with large and pendulous breast receiving breast radiotherapy in the prone versus supine position. The patient underwent computed tomography simulation in both prone and supine position. The homogeneity index (HI), conformity index (CI), coverage index (CVI) to the left breast as planning target volume (PTV) and the doses to the lung, heart, and right breast as organ at risk (OAR) were compared by using dose-volume histogram. The lifetime attributable risk (LAR) according to the prone and supine position was measured for the lung and right breast. The HI, CI of the PTV decreased 21.7%, 6.49%, respectively and the CVI increased 10.8% with the prone position. The mean and maximum dose to the left lung decreased 91.6%, 87.0%, respectively and the volume parameters also decreased over 99% with the prone position. The parameters to the right lung were same regardless of the position. The mean and maximum dose to the heart decreased 51.6%, 14.2% with the prone position. But the mean and maximum dose to the right breast increased unlike the other OARs. The LARs to the lung decreased 80.3% (left), 24.2% (right) but the LAR to the right breast doubled with the prone position. The prone position is a favorable alternative for irradiation of breast in patients with large and pendulous breasts.
The accuracy and uniformity of CT numbers are the main causes of radiation dose calculation error. Especially, for the dose calculation based on kV-Cone Beam Computed Tomography (CBCT) image, the scatter affecting the CT number is known to be quite different by the object sizes, densities, exposure conditions, and so on. In this study, the scatter impact on the CBCT based dose calculation was evaluated to provide the optimal condition minimizing the error. The CBCT images was acquired under three scatter conditions ("Under-scatter", "Over-scatter", and "Full-scatter") by adjusting amount of scatter materials around a electron density phantom (CIRS062, Tissue Simulation Technology, Norfolk, VA, USA). The CT number uniformities of CBCT images for water-equivalent materials of the phantom were assessed, and the location dependency, either "inner" or "outer" parts of the phantom, was also evaluated. The electron density correction curves were derived from CBCT images of the electron density phantom in each scatter condition. The electron density correction curves were applied to calculate the CBCT based doses, which were compared with the dose based on Fan Beam Computed Tomography (FBCT). Also, 5 prostate IMRT cases were enrolled to assess the accuracy of dose based on CBCT images using gamma index analysis and relative dose differences. As the CT number histogram of phantom CBCT images for water equivalent materials was fitted with a gaussian function, the FHWM (146 HU) for "Full-scatter" condition was the smallest among the FHWM for the three conditions (685 HU for "under scatter" and 264 HU for "over scatter"). Also, the variance of CT numbers was the smallest for the same ingredients located in the center and periphery of the phantom in the "Full-scatter" condition. The dose distributions calculated with FBCT and CBCT images compared in a gamma index evaluation of 1%/3 mm criteria and in the dose difference. With the electron density correction acquired in the same scatter condition, the CBCT based dose calculations tended to be the most accurate. In 5 prostate cases in which the mean equivalent diameter was 27.2 cm, the averaged gamma pass rate was 98% and the dose difference confirmed to be less than 2% (average 0.2%, ranged from -1.3% to 1.6%) with the electron density correction of the "Full-scatter" condition. The accuracy of CBCT based dose calculation could be confirmed that closely related to the CT number uniformity and to the similarity of the scatter conditions for the electron density correction curve and CBCT image. In pelvic cases, the most accurate dose calculation was achievable in the application of the electron density curves of the "Full-scatter" condition.
For estimation of regional myocardial blood flow with O-15 water PET, a few modifications considering partial volume effect based on single compartment model have been proposed. In this study, we attempted to quantify the degree of heterogeneity and to show the effect of tissue flow heterogeneity on partition coefficient(${\lambda}$) and to find the relation between perfusable tissue index(PTI) and ${\lambda}$ by computer simulation using two modified models. We simulated tissue curves for the regions with homogeneous and heterogeneous blood flow over a various flow range(0.2-4.0ml/g/min). Simulated heterogeneous tissue composed of 4 subregions of the same or different size of block which have different homogeneous flow and different degree of slope of distribution of blood flow. We measured the index representing heterogeneity of distribution of blood flow for each heterogeneous tissue by the constitution heterogeneity(CH). For model I, we assumed that tissue recovery coefficient ($F_{MME}$) was the product of partial volume effect($F_{MMF}$) and PTI. Using model I, PTI, flow, and $F_{MM}$ were estimated. For model II, we assumed that partition coefficient was another variable which could represent tissue characteristics of heterogeneity of flow distribution. Using model II, PTI, flow and ${\lambda}$ were estimated. For the simulated tissue with homogeneous flow, both models gave exactly the same estimates, of three parameters. For the simulated tissue with heterogeneous flow distribution, in model I, flow and $F_{MM}$ were correctly estimated as CH was increased moderately. In model II, flow and ${\lambda}$ were decreased curvi-linearly as CH was increased. The degree of underestimation of ${\lambda}$ obtained using model II, was correlated with CH. The degree of underestimation of flow was dependent on the degree of underestimation of ${\lambda}$. PTI was somewhat overestimated and did not change according to CH. We conclude that estimated ${\lambda}$ reflect the degree of tissue heterogeneity of flow distribution. We could use the degree of underestimation of ${\lambda}$ to find the characteristic heterogeneity of tissue flow and use ${\lambda}$ to recover the underestimated flow.
For overall system test, hidden-target test have been used using film which leads to inherent analysis error. The purpose of our study is to quantify this error and to propose gel dosimeter based verification technique for 3-dimensional target point error. The phantom was made for simulation of human head and this has ability to equip 10 gel-dosimeter. $BANGkit^{TM}$ which we are able to manufacture whenever it is needed as well as to easily change the container with different shapes was used as a gel dosimeter. The 10 targets were divided into two groups based on shapes of areas with a planned 50% isodose line. All treatment and analysis was performed three times using Novalis and $BrainSCAN^{TM}$. The target point error is $0.77{\pm}0.15mm$ for 10 targets and directional target point error in each direction is $0.54{\pm}0.23mm$, $0.37{\pm}0.08mm$, $0.33{\pm}0.10mm$ in AP (anterior-posterior), LAT (lateral), and VERT (vertical) direction, respectively. The result of less than 1 mm shows that the treatment was performed through each precise step in treatment procedure. In conclusion, the 3-dimensional target point verification technique can be one of the techniques for overall system test.
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