Soo Il Kwon

Guiding curve based on the normal breathing as monitored by thermocouple for regular breathing

Adapting radiation fields to a moving target requires information continuously on the location of internal target by detecting it directly or indirectly. The aim of this study is to make the breathing regular effectively with minimizing stress to the patient. A system for regulating patients breath consists of a respiratory monitoring mask (ReMM), a thermocouple module, a screen, inner earphones, and a personal computer. A ReMM with thermocouple was developed previously to measure the patients respiration. A software was written in LabView 7.0 (National Instruments, TX), which acquires respiration signal and displays its pattern. Two curves are displayed on the screen: One is a curve indicating the patients current breathing pattern; the other is a guiding curve, which is iterated with one period of the patients normal breathing curve. The guiding curves were acquired for each volunteer before they breathed with guidance. Ten volunteers participated in this study to evaluate this system. A cycle of the representative guiding curve was acquired by monitoring each volunteers free breathing with ReMM and was then generated iteratively. The regularity was compared between a free breath curve and a guided breath curve by measuring standard deviations of amplitudes and periods of two groups of breathing. When the breathing was guided, the standard deviation of amplitudes and periods on average were reduced from 0.0029 to 0.00139 (arbitrary units) and from 0.359 s to 0.202 s, respectively. And the correlation coefficients between breathing curves and guiding curves were greater than 0.99 for all volunteers. The regularity was improved statistically when the guiding curve was used.

Evaluation of the Accuracy of the CyberKnife

The use of stereotactic radiosurgical systems to treat intracranial and extracranial tumors and other lesions requires a high degree of accuracy in target identification and localization. CyberKnife can deliver, with a high degree of precision, a single or several fractions of radiation dose to a well-defined small intracranial or extracranial target. The accuracy of the output factor directly affects the accuracy of dose delivery in CyberKnife system. The purpose of this study was to evaluation the total system accuracy of the CyberKnife and also to estimate an output factor for CyberKnife using the several detectors. Accuracy of target localization was measured in anthropomorphic head phantom containing a spherical target, fiducial markers, and two pieces of film. The accuracy measured is the displacement of the dose contours from the treatment plan to that measured in the exposed phantom. All measurements of the output factors for collimators were performed by six different detectors: diode detector, X-Omat V film, Gafchromic EBT film, 0.015, 0.125 and 0.6 cc ionization chamber. Each collimator normalized with respect to the output factor of the largest collimator. We performed the E2E test and the general film dosimetry for estimation target localization in CyberKnife. The targeting error of the skull tracking mode and fiducial tracking mode were 0.956 mm and 0.923 mm. We could confirm the accuracy of total system is less than the 1 mm. For larger collimators, the output factors from six detectors showed a good agreement. For the collimators less than 15 mm, there were substantial differences in the output factors among different detectors. That is, the value of output factor for the 5 mm collimator of a diode and Gafchromic film was each 0.656 and 0.777. The Gafchromic EBT film was considered more accurate than the others detectors.

The Effect of the CT Number for Each CT on Photon Dose Calculation

The CT numbers obtained from computed tomography (CT) are potentially useful for inhomogeneity correction in CT based treatment planning. The purpose of this study is to evaluate the variation of CT number from each scanner and the effect of this variation on photon dose calculations. Five kinds of CT scanners were used to obtain images of electron density calibration phantom (Gammex RMI 467). The images were transferred to treatment planning system (Pinnacle, USA) and drawing contour of each materials for obtaining the CT number. We obtained relationship between the CT numbers and electron densities from each CT scanner. In order to investigate the influence of CT number to dose calculation, patients’ thoracic CT images were used. Difference of dose was evaluated with organ (Heart, Esophagus, Tumor, Lung, Liver, Vertebra body). The differences between CT numbers for each scanner were ±2% in homogeneous medium and 9.5% in high density medium. The maximum dose difference was 0.48% for each organ. It acquired the phantom images inserted high density material in the water phantom. Comparing the doses calculated with CT images from each CT scanner, the maximum dose difference was 2.1% in 20 cm. There was not significant difference of CT number and dose in general condition but it was different in high density medium. The exact density to CT number conversion according to CT scanner is required to minimize the uncertainty of dose depends on CT number. Especially the each hospital with various CT scanners has to discriminate CT numbers for each CT scanner. Moreover a periodic quality assurance is required for reproducibility of CT number.

In Vivo Dosimetry with MOSFET Detector during Radiotherapy

In Vivo dosimetry is a method to evaluate the radiotherapy; it is used to find the dosimetric and mechanical errors of radiotherapy unit. In this study, on-line In Vivo dosimetry was enabled by measuring the skin dose with MOSFET detectors attached to patient’s skin during treatment. MOSFET dosimeters were found to be reproducible and independent on beam directions. MOSFET detectors were positioned on patient’s skin underneath of the dose buil-up material which was used to minimize dosimetric error. Delivered dose calculated by the plan verification function embedded in the radiotherapy treatment planning system (RTPs), was compared with measured data point by point. The dependency of MOSFET detector used in this study for energy and dose rate agrees with the specification provided by manufacturer within 2% error. Comparing the measured and the calculated point doses of each patient, discrepancy was within 5%. It was enabled to verify the IMRT by using MOSFET detector. However, skin dosimetry using conventional ion chamber and diode detector is limited to the simple radiotherapy.

Comparison of linac-based fractionated stereotactic radiotherapy and tomotherapy treatment plans for intra-cranial tumors

This study compares and analyzes stereotactic radiotherapy using tomotherapy and linac-based fractionated stereotactic radiotherapy in the treatment of intra-cranial tumors, according to some cases. In this study, linac-based fractionated stereotactic radiotherapy and tomotherapy treatment were administered to five patients diagnosed with intra-cranial cancer in which the dose of 18–20 Gy was applied on 3–5 separate occasions. The tumor dosing was decided by evaluating the inhomogeneous index (II) and conformity index (CI). Also, the radiation-sensitive tissue was evaluated using low dose factors V1, V2, V3, V4, V5, and V10, as well as the non-irradiation ratio volume (NIV). The values of the II for each prescription dose in the linac-based non-coplanar radiotherapy plan and tomotherapy treatment plan were (0.125±0.113) and (0.090±0.180), respectively, and the values of the CI were (0.899±0.149) and (0.917±0.114), respectively. The low dose areas, V1, V2, V3, V4, V5, and V10, in radiation-sensitive tissues in the linac-based non-coplanar radiotherapy plan fell into the ranges 0.3%−95.6%, 0.1%−87.6%, 0.1%−78.8%, 38.8%-69.9%, 26.6%-65.2%, and 4.2%−39.7%, respectively, and the tomotherapy treatment plan had ranges of 13.6%−100%, 3.5%−100%, 0.4%−94.9%, 0.2%−82.2%, 0.1%−78.5%, and 0.3%−46.3%, respectively. Regarding the NIV for each organ, it is possible to obtain similar values except for the irradiation area of the brain stem. The percentages of NIV 10%, NIV20%, and NIV30%for the brain stem in each patient were 15%−99.8%, 33.4%−100%, and 39.8%−100%, respectively, in the fractionated stereotactic treatment plan and 44.2%-96.5%, 77.7%-99.8%, and 87.8%−100%, respectively, in the tomotherapy treatment plan. In order to achieve higher-quality treatment of intra-cranial tumors, treatment plans should be tailored according to the isodose target volume, inhomogeneous index, conformity index, position of the tumor upon fractionated stereotactic radiosurgery, and radiation dosage for radiation-sensitive tissues.

A study on the characteristic of normoxic polymer gel dosimeter according to its composition

In this study, to find the optimal composition of the gel as therapeutic radiation, the amounts of methacrylic acid and gelatin were varied. The polymer gel with various compositions were evaluated for its sensitivity, reproducibility, and accuracy. As the concentration of the gelatine is high, the threshold R2 value increases and the dose response was decreases. As the concentration of the methacrylic acid is high, both the threshold R2 value and the dose response were decrease. As both concentrations of the gelatine and the methacrylic acid is high, the sensitivity to the dose was increases within some range. It was found that the polymer gel composed in this study can be optimized for measuring the therapeutic radiation.

Quantitative Evaluation of the Guided Respiration for Motion Adaptive Radiotherapy

Targets inside lung or liver usually move significantly due to respiratory motion. A generous margin must be allowed and large volume of normal tissue is irradiated with intensive radiation. Several techniques have been suggested to minimize the PTV relating to respiratory motion. One of methods, adapting radiation fields continuously to a moving target needs information on the location of internal target by detecting it directly or indirectly. In our previous study, each volunteer’s representative respiration signal was generated iteratively for guiding signal, and the audio guiding and visual guiding was compared. The purpose of this study is to evaluate the guided breath quantitatively. A system regulates patient’s respiration, consists of a ReMM (Respiratory monitoring mask), a thermocouple module, a screen, a inner earphones, and a laptop. ReMM with thermocouple was developed to measure the patient’s respiration. A software was written in LabView 7.0 (National Instruments, USA), which acquires respiration signal and display its pattern. Two curves are displayed on the screen: one is a curve indicating patient’s current breathing pattern, the other is guiding signal, which is iterated as a real respiration signal. A cycle of representative guiding curves are acquired by monitoring each volunteer’s free respiration, then it generated iteratively. Five healthy volunteers are enrolled to regulate their breaths by this system. The time domain of the guided respiratory curves was converted to frequency domain by fourier-transform. The full width half maximum (FWHM) of each respiratory curve in the frequency domain was measured and compared. The smaller FWHM is, the more regulated breath is. The FWHM of breathing curves followed by audio and video guidance were estimated as less than 0.2 /sec, and 0.3 /sec for audio guidance. The discrepancies between guiding curves and breathing curves agreed with 26% and 24% of standard deviation for audio guidance only and for audio and visual guidance, respectively.

Investigation of high-radiation-sensitive normoxic polymer gel dosimeter

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One-stop daily beam verification with electronic portal imaging device

Dosimetric properties of an amorphous silicon electronic portal imaging device (EPID) for the computerbased routine quality assurance (QA) of the linear accelerator (linac) were developed in this study. The template phantom consists of a 25×25 cm2 size and 2 mm thick acrylic slab with right angled lead wires. The lead wires were inset in acrylic template phantom with exact 2 cm-off from the cross hair of the phantom. The difference between the center of radiation field and light field would be the light/radiation filed congruence. It enables the software to determine the edge of the radiation field and the center of the field light field easily at various SSDs and field sizes. The light-radiation field congruence was determined by measuring the distance between the edges of the radiation field and that of the light field determined by above method. It was designed that each light fields could be aligned with the scale on top of the phantom. The software to verify the images from the EPID was developed in this study using the IDL (version 6.0, Research Systems, USA). After the irradiation, the images from the EPID were transferred to the local computer as a DICOM format. The software designed to analyze the filed size, the field symmetry, and the constancy of beam energy at one time. To assure the QA system, the ion-chamber and the films (X-Omat V2, Kodak, USA) were used. The symmetry, the energy and the field size of the beam from the linac could be verified at once. The verification performed everyday. The discrepancy between the measurements of the EPID agrees well with the measurements of the film. It was found that this QA tool using the EPID could be substituted the film test which is time-consuming for the daily routine QA.