Shinji Kawamura

Measurement of absorbed dose-to-water for an HDR 192Ir source with ionization chambers in a sandwich setup

PURPOSE In this study, a dedicated device for ion chamber measurements of absorbed dose-to-water for a Nucletron microSelectron-v2 HDR (192)Ir brachytherapy source is presented. The device uses two ionization chambers in a so-called sandwich assembly. Using this setup and by taking the average reading of the two chambers, any dose error due to difficulties in absolute positioning (centering) of the source in between the chambers is cancelled to first order. The methods accuracy was examined by comparing measurements with absorbed dose-to-water determination based on the AAPM TG-43 protocol. METHODS The optimal source-to-chamber distance (SCD) for (192)Ir dosimetry was determined from ion chamber measurements in a water phantom. The (192)Ir source was sandwiched between two Exradin A1SL chambers (0.057 cm(3)) at the optimal SCD separation. The measured ionization was converted to the absorbed dose-to-water using a (60)Co calibration factor and a Monte Carlo-calculated beam quality conversion factor, kQ, for (60)Co to (192)Ir. An uncertainty estimate of the proposed method was determined based on reproducibility of measurements at different institutions for the same type of source. RESULTS The optimal distance for the A1SL chamber measurements was determined to be 5 cm from the (192)Ir source center, considering the depth dependency of kQ for (60)Co to (192)Ir and the chamber positioning. The absorbed dose to water measured at (5 cm, 90°) on the transverse axis was 1.3% lower than TG-43 values and its reproducibility and overall uncertainty were 0.8% and 1.7%, respectively. The measurement doses at anisotropic points agreed within 1.5% with TG-43 values. CONCLUSIONS The ion chamber measurement of absorbed dose-to-water with a sandwich method for the (192)Ir source provides a more accurate, direct, and reference dose compared to the dose-to-water determination based on air-kerma strength in the TG-43 protocol. Due to the simple but accurate assembly, the sandwich measurement method is useful for daily dose management of (192)Ir sources.

Evaluation of a combined respiratory-gating system comprising the TrueBeam linear accelerator and a new real-time tumor-tracking radiotherapy system: a preliminary study

A combined system comprising the TrueBeam linear accelerator and a new real-time, tumor-tracking radiotherapy system, SyncTraX, was installed in our institution. The goals of this study were to assess the capability of SyncTraX in measuring the position of a fiducial marker using color fluoroscopic images, and to evaluate the dosimetric and geometric accuracy of respiratory-gated radiotherapy using this combined system for the simple geometry. For the fundamental evaluation of respiratory-gated radiotherapy using SyncTraX, the following were performed: 1) determination of dosimetric and positional characteristics of sinusoidal patterns using a motor-driven base for several gating windows; 2) measurement of time delay using an oscilloscope; 3) positional verification of sinusoidal patterns and the pattern in the case of a lung cancer patient; 4) measurement of the half-value layer (HVL in mm AL), effective kVp, and air kerma, using a solid-state detector for each fluoroscopic condition, to determine the patient dose. The dose profile in a moving phantom with gated radiotherapy having a gating window ≤4 mm was in good agreement with that under static conditions for each photon beam. The total time delay between TrueBeam and SyncTraX was <227 ms for each photon beam. The mean of the positional tracking error was <0.4 mm for sinusoidal patterns and for the pattern in the case of a lung cancer patient. The air-kerma rates from one fluoroscopy direction were 1.93±0.01, 2.86±0.01, 3.92±0.04, 5.28±0.03, and 6.60±0.05 mGy/min for 70, 80, 90, 100, and 110 kV X-ray beams at 80 mA, respectively. The combined system comprising TrueBeam and SyncTraX could track the motion of the fiducial marker and control radiation delivery with reasonable accuracy; therefore, this system provides significant dosimetric improvement. However, patient exposure dose from fluoroscopy was not clinically negligible. PACS number(s): 87.53.Bn, 87.55.km, 87.55.Qr.A combined system comprising the TrueBeam linear accelerator and a new real‐time, tumor‐tracking radiotherapy system, SyncTraX, was installed in our institution. The goals of this study were to assess the capability of SyncTraX in measuring the position of a fiducial marker using color fluoroscopic images, and to evaluate the dosimetric and geometric accuracy of respiratory‐gated radiotherapy using this combined system for the simple geometry. For the fundamental evaluation of respiratory‐gated radiotherapy using SyncTraX, the following were performed: 1) determination of dosimetric and positional characteristics of sinusoidal patterns using a motor‐driven base for several gating windows; 2) measurement of time delay using an oscilloscope; 3) positional verification of sinusoidal patterns and the pattern in the case of a lung cancer patient; 4) measurement of the half‐value layer (HVL in mm AL), effective kVp, and air kerma, using a solid‐state detector for each fluoroscopic condition, to determine the patient dose. The dose profile in a moving phantom with gated radiotherapy having a gating window ≤4 mm was in good agreement with that under static conditions for each photon beam. The total time delay between TrueBeam and SyncTraX was <227 ms for each photon beam. The mean of the positional tracking error was <0.4 mm for sinusoidal patterns and for the pattern in the case of a lung cancer patient. The air‐kerma rates from one fluoroscopy direction were 1.93±0.01, 2.86±0.01, 3.92±0.04, 5.28±0.03, and 6.60±0.05 mGy/min for 70, 80, 90, 100, and 110 kV X‐ray beams at 80 mA, respectively. The combined system comprising TrueBeam and SyncTraX could track the motion of the fiducial marker and control radiation delivery with reasonable accuracy; therefore, this system provides significant dosimetric improvement. However, patient exposure dose from fluoroscopy was not clinically negligible. PACS number(s): 87.53.Bn, 87.55.km, 87.55.Qr

SU‐E‐J‐182: Reproducibility of Tumor Motion Probability Distribution Function in Stereotactic Body Radiation Therapy of Lung Using Real‐Time Tumor‐Tracking Radiotherapy System

Purpose: We aim to achieve new four-dimensional radiotherapy (4DRT) using the next generation real-time tumor-tracking (RTRT) system and flattening-filter-free techniques. To achieve new 4DRT, it is necessary to understand the respiratory motion of tumor. The purposes of this study were: 1.To develop the respiratory motion analysis tool using log files. 2.To evaluate the reproducibility of tumor motion probability distribution function (PDF) during stereotactic body RT (SBRT) of lung tumor. Methods: Seven patients having fiducial markers closely implanted to the lung tumor were enrolled in this study. The positions of fiducial markers were measured using the RTRT system (Mitsubishi Electronics Co., JP) and recorded as two types of log files during the course of SBRT. For each patients, tumor motion range and tumor motion PDFs in left-right (LR), anterior-posterior (AP) and superior-inferior (SI) directions were calculated using log files of all beams per fraction (PDFn). Fractional PDF reproducibility (Rn) was calculated as Kullback-Leibler (KL) divergence between PDF1 and PDFn of tumor motion. The mean of Rn (Rm) was calculated for each patient and correlated to the patient’s mean tumor motion range (Am). The change of Rm during the course of SBRT was also evluated. These analyses were performed using in-house developed software. Results: The Rm were 0.19 (0.07–0.30), 0.14 (0.07–0.32) and 0.16 (0.09–0.28) in LR, AP and SI directions, respectively. The Am were 5.11 mm (2.58–9.99 mm), 7.81 mm (2.87–15.57 mm) and 11.26 mm (3.80–21.27 mm) in LR, AP and SI directions, respectively. The PDF reproducibility decreased as the tumor motion range increased in AP and SI direction. That decreased slightly through the course of RT in SI direction. Conclusion: We developed the respiratory motion analysis tool for 4DRT using log files and quantified the range and reproducibility of respiratory motion for lung tumors.

Clinical assessment of coiled fiducial markers as internal surrogates for hepatocellular carcinomas during gated stereotactic body radiotherapy with a real-time tumor-tracking system

BACKGROUND AND PURPOSE To report the clinical usefulness of coiled fiducial markers as an internal surrogate in gated stereotactic body radiotherapy (SBRT) for hepatocellular carcinoma (HCC) using a real-time tumor-tracking radiotherapy (RTRT) system. MATERIALS AND METHODS Seventeen HCC patients with Child-Pugh (CP) scores of A or B received gated SBRT (45-50Gy in 5-10 fractions) using an RTRT system and Visicoil markers. Local control (LC), progression-free (PFS), and overall survival (OS) rates were assessed using the Kaplan-Meier method. Toxicities were assessed using the Common Terminology Criteria for Adverse Events, Version 4.0. RESULTS Of the 17 patients, 14 had a CP score A. The mean planning target volume was 54.6cc. Only 1 patient developed pneumothorax after marker implantation. Visicoil tracking during SBRT was possible in all cases. With a median follow-up of 16months, 1-year LC, PFS, and OS rates were 100%, 53%, and 82%, respectively. Grade≥2 late toxicity was observed in 2 patients (grade 2 duodenal ulcer and grade 3 temporary transaminase elevation). CONCLUSIONS Using an RTRT system and Visicoil markers, gated SBRT was well tolerated in patients with HCC. This can be considered a safe treatment strategy with potential for delivering favorable outcomes.

SU‐G‐JeP1‐08: Dual Modality Verification for Respiratory Gating Using New Real‐ Time Tumor Tracking Radiotherapy System

PURPOSE The respirato ry gating system combined the TrueBeam and a new real-time tumor-tracking radiotherapy system (RTRT) was installed. The RTRT system consists of two x-ray tubes and color image intensifiers. Using fluoroscopic images, the fiducial marker which was implanted near the tumor was tracked and was used as the internal surrogate for respiratory gating. The purposes of this study was to develop the verification technique of the respiratory gating with the new RTRT using cine electronic portal image device images (EPIDs) of TrueBeam and log files of the RTRT. METHODS A patient who underwent respiratory gated SBRT of the lung using the RTRT were enrolled in this study. For a patient, the log files of three-dimensional coordinate of fiducial marker used as an internal surrogate were acquired using the RTRT. Simultaneously, the cine EPIDs were acquired during respiratory gated radiotherapy. The data acquisition was performed for one field at five sessions during the course of SBRT. The residual motion errors were calculated using the log files (Elog ). The fiducial marker used as an internal surrogate into the cine EPIDs was automatically extracted by in-house software based on the template-matching algorithm. The differences between the the marker positions of cine EPIDs and digitally reconstructed radiograph were calculated (EEPID ). RESULTS Marker detection on EPID using in-house software was influenced by low image contrast. For one field during the course of SBRT, the respiratory gating using the RTRT showed the mean ± S.D. of 95th percentile EEPID were 1.3 ± 0.3 mm,1.1 ± 0.5 mm,and those of Elog were 1.5 ± 0.2 mm, 1.1 ± 0.2 mm in LR and SI directions, respectively. CONCLUSION We have developed the verification method of respiratory gating combined TrueBeam and new real-time tumor-tracking radiotherapy system using EPIDs and log files.

SU-G-JeP1-11: Feasibility Study of Markerless Tracking Using Dual Energy Fluoroscopic Images for Real-Time Tumor-Tracking Radiotherapy System.

PURPOSE The new real-time tumor-tracking radiotherapy (RTRT) system was installed in our institution. This system consists of two x-ray tubes and color image intensifiers (I.I.s). The fiducial marker which was implanted near the tumor was tracked using color fluoroscopic images. However, the implantation of the fiducial marker is very invasive. Color fluoroscopic images enable to increase the recognition of the tumor. However, these images were not suitable to track the tumor without fiducial marker. The purpose of this study was to investigate the feasibility of markerless tracking using dual energy colored fluoroscopic images for real-time tumor-tracking radiotherapy system. METHODS The colored fluoroscopic images of static and moving phantom that had the simulated tumor (30 mm diameter sphere) were experimentally acquired using the RTRT system. The programmable respiratory motion phantom was driven using the sinusoidal pattern in cranio-caudal direction (Amplitude: 20 mm, Time: 4 s). The x-ray condition was set to 55 kV, 50 mA and 105 kV, 50 mA for low energy and high energy, respectively. Dual energy images were calculated based on the weighted logarithmic subtraction of high and low energy images of RGB images. The usefulness of dual energy imaging for real-time tracking with an automated template image matching algorithm was investigated. RESULTS Our proposed dual energy subtraction improve the contrast between tumor and background to suppress the bone structure. For static phantom, our results showed that high tracking accuracy using dual energy subtraction images. For moving phantom, our results showed that good tracking accuracy using dual energy subtraction images. However, tracking accuracy was dependent on tumor position, tumor size and x-ray conditions. CONCLUSION We indicated that feasibility of markerless tracking using dual energy fluoroscopic images for real-time tumor-tracking radiotherapy system. Furthermore, it is needed to investigate the tracking accuracy using proposed dual energy subtraction images for clinical cases.

Absorbed dose-to-water measurement of an HDR 192Ir source with Farmer ionization chambers in a sandwich setup

This study extends a sandwich method for ion chamber measurements of absorbed dose to water of a Nucletron microSelectron-v2 high dose rate (HDR) 192Ir brachytherapy source presented in the previous paper (Araki et al 2013 Med. Phys. 40 092101) to Farmer-type ion chambers. The goal in this study is to verify the calculation dose based on the AAPM TG-43 protocol by direct absorbed dose-to-water measurement with the Farmer chamber. The measurement device uses two Farmer chambers in a so-called sandwich assembly. A microSelectron-v2 HDR 192Ir source and a PTW30013 Farmer chamber were modeled with the EGSnrc/egs_chamber code and its beam quality conversion factor, kIr, for 60Co to 192Ir was calculated as a function of a distance from the 192Ir source. From calculation results, the optimal distance for the Farmer chamber measurements was determined to be 8 cm from the 192Ir source center, considering the depth dependency of kIr and the chamber positioning, and the required measurement time. The developed device consists of 1 mm-thick poly-methyl methacrylate (PMMA) waterproof sleeves located at both sides of the 192Ir source with a separation of 16 cm. The chamber readings measured with the Farmer chambers inserted into the PMMA sleeves were converted to the absorbed dose to water using a 60Co calibration factor and kIr. The average dose of two Farmer chamber measurements was compared with values based on TG-43 for eight different 192Ir sources at four institutes. kIr for the PTW30013 Farmer chamber ranged from 1.174 to 0.985 at distances of 1.5–10 cm from the source and it was 0.992 at a distance of 8 cm. The measured dose to water was 0.73% lower than TG-43 values and its reproducibility, the chamber positioning accuracy, and the combined standard uncertainty were 0.28%, 0.25%, and 0.68%, respectively. The sandwich measurement with the more popular Farmer chamber is useful and convenient for daily dose management of 192Ir sources in clinics.

SU‐E‐T‐102: Measurement of Absorbed Dose‐To‐Water for An HDR Ir‐192 Source with a Farmer Ionization Chamber in a Sandwich Setup

PURPOSE This study developed a dedicated device for ion chamber measurements of absorbed dose-to-water for an HDR Ir-192 brachytherapy source. The device uses two Farmer chambers in a so-called sandwich assembly. The accuracy of the sundwich method was examined by comparing chamber measurements with absorbed dose-to-water determination based on the AAPM TG-43 protocol. METHODS A microSelectron-v2 HDR Ir-192 source was modeled with the EGSnrc/egs_chamber code. The accuracy of modeling was confirmed by comparing calculated results for gL(r) and F(r, θ) with those of TG-43. The PTW30013 Framer chamber was also modeled with the egs_chamber code and its beam quality conversion factors, kQ, of Ir-192 to Co-60 were calculated as a function of distance from the Ir-192 source. From calculation results, we developed a dedicated device for the sandwich setup with 1 mm PMMA waterproof sleeves located at both side of 8 cm from the Ir-192 source. The chamber readings measured with the Farmer chambers inserted into the 1 mm PMMA sleeves were converted to the absorbed dose to water using kQ. The average dose of two Farmer chamber measurements with the sandwich setup were compared with the TPS values based on TG-43 for eight Ir-192 sources at four institutes. RESULTS Calculated gL(r) and F(r, θ) values agreed well with those of TG-43. The calculated kQ values for the PTW30013 Framer chamber ranged from 1.103 to 0.986 at distance of 2∼10 cm from the source and it was 0.992 at distance of 8 cm. The doses to water measured with the sandwich setup were -0.73 +/- 0.28% (n=10) compared to TG-43 values. CONCLUSION The dose to water measurement by the Farmer chamber with the sandwich setup is useful for daily dose management for Ir-192 sources.