Yoshitomo Ishihara

Evaluation of dynamic tumour tracking radiotherapy with real-time monitoring for lung tumours using a gimbal mounted linac

PURPOSE To evaluate feasibility and acute toxicities after dynamic tumour tracking (DTT) irradiation with real-time monitoring for lung tumours using a gimbal mounted linac. MATERIALS AND METHODS Spherical gold markers were placed around the tumour using a bronchoscope prior to treatment planning. Prescription dose at the isocentre was 56 Gy in 4 fractions for T2a lung cancer and metastatic tumour, and 48 Gy in 4 fractions for the others. Dose-volume metrics were compared between DTT and conventional static irradiation using in-house developed software. RESULTS Of twenty-two patients enrolled, DTT radiotherapy was successfully performed for 16 patients, except 4 patients who coughed out the gold markers, one who showed spontaneous tumour regression, and one where the abdominal wall motion did not correlate with the tumour motion. Dose covering 95% volume of GTV was not different between the two techniques, while normal lung volume receiving 20 Gy or more was reduced by 20%. A mean treatment time per fraction was 36 min using DTT. With a median follow-up period of 13.2 months, no severe toxicity grade 3 or worse was observed. CONCLUSIONS DTT radiotherapy using a gimbal mounted linac was clinically feasible for lung treatment without any severe acute toxicity.

Dosimetric characterization of a multileaf collimator for a new four‐dimensional image‐guided radiotherapy system with a gimbaled x‐ray head, MHI‐TM2000a)

PURPOSE To present the dosimetric characterization of a multileaf collimator (MLC) for a new four-dimensional image-guided radiotherapy system with a gimbaled x-ray head, MHI-TM2000. METHODS MHI-TM2000 has an x-ray head composed of an ultrasmall linear accelerator guide and a system-specific MLC. The x-ray head can rotate along the two orthogonal gimbals (pan and tilt rotations) up to ±2.5°, which swings the beam up to ±41.9 mm in each direction from the isocenter on the isocenter plane perpendicular to the beam. The MLC design is a single-focus type, has 30 pairs of 5 mm thick leaves at the isocenter, and produces a maximum field size of150×150mm2. Leaf height and length are 110 and 260 mm, respectively. Each leaf end is circular, with a radius of curvature of 370 mm. The distance that each leaf passes over the isocenter is 77.5 mm. Radiation leakage between adjacent leaves is minimized by an interlocking tongue-and-groove (T&G) arrangement with the height of the groove part 55 mm. The dosimetric characterizations including field characteristics, leaf position accuracy, leakage, and T&G effect were evaluated using a well-commissioned 6 MV photon beam, EDR2 films (Kodak, Rochester, NY), and water-equivalent phantoms. Furthermore, the field characteristics and leaf position accuracy were evaluated under conditions of pan or tilt rotation. RESULTS The differences between nominal and measured field sizes were within ±0.5 mm. Although the penumbra widths were greater with wider field size, the maximum width was<5.5mm even for the fully opened field. Compared to the results of field characteristics without pan or tilt rotation, the variation in field size, penumbra width, flatness, and symmetry was within ±1 mm/1% at the maximum pan or tilt rotational angle. The leaf position accuracy was 0.0±0.1mm, ranging from -0.3 to 0.2 mm at four gantry angles of 0°, 90°, 180°, and 270° with and without pan or tilt rotation. The interleaf leakage was up to 0.21%, whereas the intraleaf leakage was <0.12%. T&G decreased the doses by 10.7%, on average. CONCLUSIONS This study demonstrated that MHI-TM2000 has the capability for high leaf position accuracy and low leakage, leading to highly accurate intensity-modulated radiotherapy delivery. Furthermore, substantial changes in the dosimetric data on field characteristics and leaf position accuracy were not observed even at the maximum pan or tilt rotation.

Commissioning and quality assurance of Dynamic WaveArc irradiation.

A novel three‐dimensional unicursal irradiation technique “Dynamic WaveArc” (DWA), which employs simultaneous and continuous gantry and O‐ring rotation during dose delivery, has been implemented in Vero4DRT. The purposes of this study were to develop a commissioning and quality assurance procedure for DWA irradiation, and to assess the accuracy of the mechanical motion and dosimetric control of Vero4DRT. To determine the mechanical accuracy and the dose accuracy with DWA irradiation, 21 verification test patterns with various gantry and ring rotational directions and speeds were generated. These patterns were irradiated while recording the irradiation log data. The differences in gantry position, ring position, and accumulated MU (EG,ER, and EMU, respectively) between the planned and actual values in the log at each time point were evaluated. Furthermore, the doses delivered were measured using an ionization chamber and spherical phantom. The constancy of radiation output during DWA irradiation was examined by comparison with static beam irradiation. The mean absolute error (MAE) of EG and ER were within 0.1° and the maximum error was within 0.2°. The MAE of EMU was within 0.7 MU, and maximum error was 2.7 MU. Errors of accumulated MU were observed only around control points, changing gantry, and ring velocity. The gantry rotational range, in which EMU was greater than or equal to 2.0 MU, was not greater than 3.2%. It was confirmed that the extent of the large differences in accumulated MU was negligibly small during the entire irradiation range. The variation of relative output value for DWA irradiation was within 0.2%, and this was equivalent to conventional arc irradiation with a rotating gantry. In conclusion, a verification procedure for DWA irradiation was designed and implemented. The results demonstrated that Vero4DRT has adequate mechanical accuracy and beam output constancy during gantry and ring rotation. PACS number: 87

Dynamic tumor-tracking radiotherapy with real-time monitoring for liver tumors using a gimbal mounted linac.

PURPOSE Dynamic tumor-tracking stereotactic body radiotherapy (DTT-SBRT) for liver tumors with real-time monitoring was carried out using a gimbal-mounted linear accelerator and the efficacy of the system was determined. In addition, four-dimensional (4D) dose distribution, tumor-tracking accuracy, and tumor-marker positional variations were evaluated. MATERIALS AND METHODS A fiducial marker was implanted near the tumor prior to treatment planning. The prescription dose at the isocenter was 48-60 Gy, delivered in four or eight fractions. The 4D dose distributions were calculated with a Monte Carlo method and compared to the static SBRT plan. The intrafractional errors between the predicted target positions and the actual target positions were calculated. RESULTS Eleven lesions from ten patients were treated successfully. DTT-SBRT allowed an average 16% reduction in the mean liver dose compared to static SBRT, without altering the target dose. The average 95th percentiles of the intrafractional prediction errors were 1.1, 2.3, and 1.7 mm in the left-right, cranio-caudal, and anterior-posterior directions, respectively. After a median follow-up of 11 months, the local control rate was 90%. CONCLUSIONS Our early experience demonstrated the dose reductions in normal tissues and high accuracy in tumor tracking, with good local control using DTT-SBRT with real-time monitoring in the treatment of liver tumors.

Development of a dose verification system for Vero4DRT using Monte Carlo method.

Vero4DRT is an innovative image‐guided radiotherapy system employing a C‐band X‐ray head with gimbal mechanics. The purposes of this study were to propose specific MC models of the linac head and multileaf collimator (MLC) for the Vero4DRT and to verify their accuracy. For a 6 MV photon beam delivered by the Vero4DRT, a simulation code was implemented using EGSnrc. The linac head model and the MLC model were simulated based on its specification. Next, the percent depth dose (PDD) and beam profiles at depths of 15, 100, and 200 mm were simulated under source‐to‐surface distance of 900 and 1000 mm. Field size was set to 150×150mm2 at a depth of 100 mm. Each of the simulated dosimetric metrics was then compared with the corresponding measurements by a 0.125 cc ionization chamber. After that, intra‐ and interleaf leakage, tongue‐and‐groove, and rounded‐leaf profiles were simulated for the static MLC model. Meanwhile, film measurements were performed using EDR2 films under similar conditions to simulation. The measurement for the rounded‐leaf profile was performed using the water phantom and the ionization chamber. The leaf physical density and abutting leaf gap were adjusted to obtain good agreement between the simulated intra‐ and interleaf leakage profiles and measurements. For the MLC model in step‐and‐shoot cases, a pyramid and a prostate IMRT field were simulated, while film measurements were performed using EDR2. For the linac head, exclusive of MLC, the difference in PDD was <1.0% after the buildup region. The simulated beam profiles agreed to within 1.3% at each depth. The MLC model has been shown to reproduce dose measurements within 2.5% for static tests. The MLC is made of tungsten alloy with a purity of 95%. The leaf gap of 0.015 cm and the MLC physical density of 18.0g/cm3, which provided the best agreement between the simulated and measured leaf leakage, were assigned to our MC model. As a result, the simulated step‐and‐shoot IMRT dose distributions agreed with the film measurements to within 3.3%, with exception of the penumbra region. We have developed specific MC models of the linac head and the MLC in the Vero4DRT system. The results have demonstrated that our MC models have high accuracy. PACS numbers: 87.55.K‐, 87.56.nk, 87.56.bd

Development of a four-dimensional Monte Carlo dose calculation system for real-time tumor-tracking irradiation with a gimbaled X-ray head

PURPOSE To develop a four-dimensional (4D) dose calculation system for real-time tumor tracking (RTTT) irradiation by the Vero4DRT. METHODS First, a 6-MV photon beam delivered by the Vero4DRT was simulated using EGSnrc. A moving phantom position was directly measured by a laser displacement gauge. The pan and tilt angles, monitor units, and the indexing time indicating the phantom position were also extracted from a log file. Next, phase space data at any angle were created from both the log file and particle data under the dynamic multileaf collimator. Irradiation both with and without RTTT, with the phantom moving, were simulated using several treatment field sizes. Each was compared with the corresponding measurement using films. Finally, dose calculation for each computed tomography dataset of 10 respiratory phases with the X-ray head rotated was performed to simulate the RTTT irradiation (4D plan) for lung, liver, and pancreatic cancer patients. Dose-volume histograms of the 4D plan were compared with those calculated on the single reference respiratory phase without the gimbal rotation [three-dimensional (3D) plan]. RESULTS Differences between the simulated and measured doses were less than 3% for RTTT irradiation in most areas, except the high-dose gradient. For clinical cases, the target coverage in 4D plans was almost identical to that of the 3D plans. However, the doses to organs at risk in the 4D plans varied at intermediate- and low-dose levels. CONCLUSIONS Our proposed system has acceptable accuracy for RTTT irradiation in the Vero4DRT and is capable of simulating clinical RTTT plans.

Dosimetric impact of gold markers implanted closely to lung tumors: a Monte Carlo simulation.

We are developing an innovative dynamic tumor tracking irradiation technique using gold markers implanted around a tumor as a surrogate signal, a real‐time marker detection system, and a gimbaled X‐ray head in the Vero4DRT. The gold markers implanted in a normal organ will produce uncertainty in the dose calculation during treatment planning because the photon mass attenuation coefficient of a gold marker is much larger than that of normal tissue. The purpose of this study was to simulate the dose variation near the gold markers in a lung irradiated by a photon beam using the Monte Carlo method. First, the single‐beam and the opposing‐beam geometries were simulated using both water and lung phantoms. Subsequently, the relative dose profiles were calculated using a stereotactic body radiotherapy (SBRT) treatment plan for a lung cancer patient having gold markers along the anteriorposterior (AP) and right‐left (RL) directions. For the single beam, the dose at the gold marker‐phantom interface laterally along the perpendicular to the beam axis increased by a factor of 1.35 in the water phantom and 1.58 in the lung phantom, respectively. Furthermore, the entrance dose at the interface along the beam axis increased by a factor of 1.63 in the water phantom and 1.91 in the lung phantom, while the exit dose increased by a factor of 1.00 in the water phantom and 1.12 in the lung phantom, respectively. On the other hand, both dose escalations and dose de‐escalations were canceled by each beam for opposing portal beams with the same beam weight. For SBRT patient data, the dose at the gold marker edge located in the tumor increased by a factor of 1.30 in both AP and RL directions. In clinical cases, dose escalations were observed at the small area where the distance between a gold marker and the lung tumor was ≤ 5 mm, and it would be clinically negligible in multibeam treatments, although further investigation may be required. PACS number: 87.10.Rt

SU‐E‐J‐142: Gafchromic Film Dosimetry in Fluoroscopy for Dynamic Tumor Tracking Irradiation of the Lung Using XR‐SP2 Model ‐ A Phantom Study ‐

PURPOSE We have recently developed a dynamic tumor tracking irradiation system using Vero4DRT (MHI-Tm2 000). It is needed to create a 4D correlation model between a fiducial marker implanted near a tumor and an external surrogate as a function of time by continuously acquiring both fluoroscopy images and external surrogate signals. The purpose of this study was to propose a new dosimetry method using Gafchromic XR-SP2 films to measure surface dose by fluoroscopy imaging. METHODS First, half-value layers (HVLs) were measured using aluminum (Al) thicknesses (15 mm) at 40125 kVp. Subsequently, several films were irradiated using various milliampere second values on a solid water phantom. The surface air kerma were also measured using the chamber to calculate the surface doses under the same condition. Then, the calibration curve of dose vs. pixel values was calculated. Finally, surface dose by fluoroscopy imaging was measured using several pieces of film taped on the chest phantom. Orthogonal X-ray fluoroscopy imaging was simultaneously performed until completion of data acquisition for creating a 4D correlation model. Those films were scanned after irradiation using a flat-bed scanner and converted to dose by calibration curve. RESULTS The HVLs for tube voltage within 40125 kVp ranged from 2.35 to 5.98 mm Al. The calibration curve between surface dose and pixel values was reasonably smooth. The differences between the measured and the calibrated doses were less than 3%. The hot spots with the maximum dose of 37.12 mGy were observed around the area overlapped by both fluoroscopic fields. CONCLUSIONS We have proposed a new dosimetry method using Gafchromic XR-SP2 films to measure surface dose by fluoroscopy imaging. This phantom study has demonstrated that it may be feasible to assess surface dose to patients during dynamic tumor tracking irradiation in clinic with ease after further investigation. This research was supported by the Japan Society for the Promotion of Science (JSPS) through its Funding Program for World-Leading Innovation R&D on Science and Technology (FIRST Program). Research sponsored in part by Mitsubishi Heavy Industries, Ltd.

SU-E-T-673: Development of Monte Carlo Dose Calculation System for Tumor-Tracking Irradiation with a Gimbaled X-Ray Head

Purpose: To propose dose calculation model of tumor‐tracking irradiation by gimbals mechanism for MHI‐TM2000 (Vero). Methods: Our MC simulation for a 6 MV photon beam delivered by the MHI‐TM2000 system was performed using EGSnrc. First, the lateral doses (5 × 5 cm2 at 100 mm depth) with pan/tilt rotation were simulated under the SSD of 900 mm. Rotation angles along pan/tilt directions were set in the range of −2.5 to +2.5° at intervals of 0.5°. The corresponding measurements were performed using EDR2 films with water‐equivalent phantoms. Differences between simulated and measured doses were calculated. Furthermore, difference of output characteristic without setup error was estimated using the simulated lateral doses under the reference condition at the maximum and minimum rotation angles. Next, the moving phantom was driven at frequency of 0.25 Hz with amplitude of 10 mm along tilt direction. From the infrared marker positions, the corresponding pan/tilt angles were calculated. Time, phantom position by laser displacement gauge, current angle of pan/tilt, and MU were recorded in the log file. A phase space data at any angles was created from the log file and particle data under the MLC. Finally, both stationary X‐ray head and tumor‐tracking irradiation were simulated under the SSD of 950 mm (4 × 4 cm2 at 50 mm depth). Each of them was compared with the corresponding measurement using EDR2 film. Results: The lateral doses within flat region showed agreement of within 1.1% in any angles. The differences between doses at the minimum angle and at the maximum were within 0.5% and 4.5% in flat and penumbra region, respectively. The averaged differences between the simulated and the measured doses with stationary and tumor‐tracking irradiation were 1.9%. Conclusions: We have developed dose calculation model of tumor‐tracking irradiation for MHI‐TM2000. This study demonstrated our proposed model has acceptable accuracy. Research sponsored in part by Mitsubishi Heavy Industries, Ltd

SU-GG-T-427: Development of Monte Carlo Dose Verification System for MHI-TM2000

Purpose: MHI‐TM2000 is an innovative image‐guidedradiotherapy system with a C‐band X‐ray head on an O‐ring shaped gantry. We are developing an integrated Monte Carlo (MC)dose calculation system for four‐dimensional radiotherapy using MHI‐TM2000. The purposes of this study were to propose specific MC models of the X‐ray head and the multi‐leaf collimator(MLC) for MHI‐TM2000 and to validate their accuracy. Methods and Materials: 6 MV photon beam delivered by the MHI‐TM2000 unit was implemented by EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc. Subsequently, the X‐ray head composed of a target, a primary collimator, a flattening filter, a monitor chamber, a fixed secondary collimator, and a MLC was simulated based on the specification. Next, the central axis depth doses and the lateral doses at 15, 100, and 200 mm depth were simulated under the source to surface distance (SSD) of 900 mm. Then, Each of them was compared with the corresponding measurements using a CC06 ionization chamber and a water phantom. For the MLC model, Tongue‐and‐Groove (100 mm depth, SSD 900 mm), leaf leakage (50 mm depth, SSD 950 mm), and round leaf effects (100 mm depth, SSD 900 mm) were simulated. Meanwhile film measurements were performed using EDR2 film and a solid water phantom under similar conditions. These doses were normalized to the corresponding doses for the open field at the isocenter. The differences between simulated and measured doses were calculated. Results: For the X‐ray head, depth doses beyond the buildup region and lateral doses within the region of flatness showed agreement of within 1.3%. For each MLC test, the simulated and measured doses agreed less than 3.0%, respectively. Conclusions: We have demonstrated that the proposed MC models of the X‐ray head and the MLC for MHI‐TM2000 have reasonable accuracy. Conflicts of Interest: Research sponsored in part by Mitsubishi Heavy Industries, Ltd.