Technical note: A universal worksheet for failure mode and effects analysis-A project of the Japanese College of Medical Physics.

BACKGROUND: Failure mode and effects analysis (FMEA), which is an effective tool for error prevention, has garnered considerable attention in radiotherapy. FMEA can be performed individually, by a group or committee, and online. PURPOSE: To meet the needs of FMEA for various purposes and improve its accessibility, we developed a simple, self-contained, and versatile web-based FMEA risk analysis worksheet. METHODS: We developed an FMEA worksheet using Google products, such as Google Sheets, Google Forms, and Google Apps Script. The main sheet was created in Google Sheets and contained elements necessary for performing FMEA by a single person. Automated tasks were implemented using Apps Script to facilitate multiperson FMEA; these functions were built into buttons located on the main sheet. RESULTS: The usability of the FMEA worksheet was tested in several situations. The worksheet was feasible for individual, multiperson, seminar, meeting, and online purposes. Simultaneous online editing, automated survey form creation, automatic analysis, and the ability to respond to the form from multiple devices, including mobile phones, were particularly useful for online and multiperson FMEA. Automation enabled through Google Apps Script reduced the FMEA workload. CONCLUSIONS: The FMEA worksheet is versatile and has a seamless workflow that promotes collaborative work for safety.

Current status of the educational environment to acquire and maintain the professional skills of radiotherapy technology and medical physics specialists in Japan: a nationwide survey.

This study aimed to investigate the educational environment of radiotherapy technology and medical physics specialists (RTMP) in Japan. We conducted a nationwide questionnaire survey in radiotherapy institutions between June and August 2022. Participants were asked questions regarding the educational system, perspectives on updating RTMP's skills and qualifications, and perspectives on higher education for RTMP at radiotherapy institutions. The results were then analyzed in detail according to three factors: whether the hospital was designed for cancer care, whether it was a Japanese Society for Radiation Oncology (JASTRO)-accredited hospital, and whether it was an intensity-modulated radiation therapy charged hospital. Responses were obtained from 579 (69%) nationwide radiation therapy institutions. For non-qualified RTMP, 10% of the institutions had their own educational systems, only 17% of institutions provided on-the-job training, and 84% of institutions encouraged participation in educational lectures and workshops in academic societies. However, for qualified RTMP, 3.0% of institutions had their own educational systems, only 8.9% of the institutions provided on-the-job training, and 83% encouraged participation in academic conferences and workshops. Less than 1% of the facilities offered salary increases for certification, whereas 8.2% offered consideration for occupational promotion. Regarding the educational environment, JASTRO-accredited hospitals were better than general hospitals. Few institutions have their own educational systems for qualified and non-qualified RTMP, but they encourage them to attend educational seminars and conferences. It is desirable to provide systematic education and training by academic and professional organizations to maintain the skills of individuals.

Development and multi-institutional evaluation of a new phantom for verifying beam-positioning errors at off-isocenter positions.

PURPOSE: Single-isocenter stereotactic radiotherapy for multiple brain metastases requires highly accurate treatment delivery at off-isocenter positions (off-iso). This study aimed to verify the beam-positioning errors at off-iso using a newly developed phantom tested at multiple institutions. METHODS: The off-iso phantom comprised five stainless-steel balls with a 3-mm diameter placed at the center and at four peripheral positions on a diagonal line. Each ball was placed 3.5 cm apart along each of the three axes. Two patterns of the phantom setup were defined as 0° and 90° phantom rotations to evaluate the beam-positioning error, which is the distance between the center of the ball and the irradiated field on the electronic portal imaging device. Furthermore, the reproducibility of the beam-positioning errors was verified by evaluating their standard deviation (SD) at a single institution, which included five measurements for two treatment machines. The errors were evaluated at multiple institutions using eight treatment machines. RESULTS: The measurement time from setup to image acquisition was approximately 20 min for two patterns. The SD of the beam-positioning errors in the reproducibility tests was 0.41 mm. In the multi-institutional evaluation, the beam-positioning error at the isocenter position was within 1.00 mm of the AAPM-RSS tolerance, with the exception of two linacs. The largest beam-positioning error (1.36 mm) was observed 7.5 cm away from the isocenter in three directions at a gantry angle of 180°. CONCLUSIONS: The developed phantom can be applied as a new tool for establishing beam-positioning errors in single-isocenter stereotactic radiotherapy at off-isocenter positions.

Identification of reproducible radiomic features from on-board volumetric images: A multi-institutional phantom study.

BACKGROUND: Radiomics analysis using on-board volumetric images has attracted research attention as a method for predicting prognosis during treatment; however, the lack of standardization is still one of the main concerns. PURPOSE: This study investigated the factors that influence the reproducibility of radiomic features extracted from on-board volumetric images using an anthropomorphic radiomics phantom. Furthermore, a phantom experiment was conducted with different treatment machines from multiple institutions as external validation to identify reproducible radiomic features. METHODS: The phantom was designed to be 35 × 20 × 20 cm with eight types of heterogeneous spheres (⌀ = 1, 2, and 3 cm). On-board volumetric images were acquired using 15 treatment machines from eight institutions. Of these, kilovoltage cone-beam computed tomography (kV-CBCT) image data acquired from four treatment machines at one institution were used as an internal evaluation dataset to explore the reproducibility of radiomic features. The remaining image data, including kV-CBCT, megavoltage-CBCT (MV-CBCT), and megavoltage computed tomography (MV-CT) provided by seven different institutions (11 treatment machines), were used as an external validation dataset. A total of 1,302 radiomic features, including 18 first-order, 75 texture, 465 (i.e., 93 × 5) Laplacian of Gaussian (LoG) filter-based, and 744 (i.e., 93 × 8) wavelet filter-based features, were extracted within the spheres. The intraclass correlation coefficient (ICC) was calculated to explore feature repeatability and reproducibility using an internal evaluation dataset. Subsequently, the coefficient of variation (COV) was calculated to validate the feature variability of external institutions. An absolute ICC exceeding 0.85 or COV under 5% was considered indicative of a highly reproducible feature. RESULTS: For internal evaluation, ICC analysis showed that the median percentage of radiomic features with high repeatability was 95.2%. The ICC analysis indicated that the median percentages of highly reproducible features for inter-tube current, reconstruction algorithm, and treatment machine were decreased by 20.8%, 29.2%, and 33.3%, respectively. For external validation, the COV analysis showed that the median percentage of reproducible features was 31.5%. A total of 16 features, including nine LoG filter-based and seven wavelet filter-based features, were indicated as highly reproducible features. The gray-level run-length matrix (GLRLM) was classified as containing the most frequent features (N = 8), followed by the gray-level dependence matrix (N = 7) and gray-level co-occurrence matrix (N = 1) features. CONCLUSIONS: We developed the standard phantom for radiomics analysis of kV-CBCT, MV-CBCT, and MV-CT images. With this phantom, we revealed that the differences in the treatment machine and image reconstruction algorithm reduce the reproducibility of radiomic features from on-board volumetric images. Specifically, the most reproducible features for external validation were LoG or wavelet filter-based GLRLM features. However, the acceptability of the identified features should be examined in advance at each institution before applying the findings to prognosis prediction.

Monte Carlo simulation of 6-MV dynamic wave VMAT deliveries by Vero4DRT linear accelerator using EGSnrc moving sources.

The commissioning and benchmark of a Monte Carlo (MC) model of the 6-MV Brainlab-Mitsubishi Vero4DRT linear accelerator for the purpose of quality assurance of clinical dynamic wave arc (DWA) treatment plans is reported. Open-source MC applications based on EGSnrc particle transport codes are used to simulate the medical linear accelerator head components. Complex radiotherapy irradiations can be simulated in a single MC run using a shared library format combined with BEAMnrc "source20." Electron energy tuning is achieved by comparing measured vs simulated percentage depth doses (PDDs) for MLC-defined field sizes in a water phantom. Electron spot size tuning is achieved by comparing measured and simulated inplane and crossplane beam profiles. DWA treatment plans generated from RayStation (RaySearch) treatment planning system (TPS) are simulated on voxelized (2.5 mm3 ) patient CT datasets. Planning target volume (PTV) and organs at risk (OAR) dose-volume histograms (DVHs) are compared to TPS-calculated doses for clinically deliverable dynamic volumetric modulated arc therapy (VMAT) trajectories. MC simulations with an electron beam energy of 5.9 MeV and spot size FWHM of 1.9 mm had the closest agreement with measurement. DWA beam deliveries simulated on patient CT datasets results in DVH agreement with TPS-calculated doses. PTV coverage agreed within 0.1% and OAR max doses (to 0.035 cc volume) agreed within 1 Gy. This MC model can be used as an independent dose calculation from the TPS and as a quality assurance tool for complex, dynamic radiotherapy treatment deliveries. Full patient CT treatment simulations are performed in a single Monte Carlo run in 23 min. Simulations are run in parallel using the Condor High-Throughput Computing software1 on a cluster of eight servers. Each server has two physical processors (Intel Xeon CPU E5-2650 0 @2.00 GHz), with 8 cores per CPU and two threads per core for 256 calculation nodes.

Radiotherapy planning techniques to reduce lung irradiation in head and neck cancer patients with mediastinal involvement.

The number of patients with head and neck squamous cell carcinoma (HNC) with mediastinal involvement is small, and appropriate treatment techniques have not been widely discussed. This study aimed to compare the efficacy of radiotherapy planning techniques in reducing lung irradiation while retaining target coverage. Among all HNC patients with mediastinal involvement who underwent radiotherapy between 2007 and 2014 at our hospital, seven consecutive patients were included in this study. Four different treatment plans were generated for each patient as follows: seven-field intensity-modulated radiation therapy (IMRT), modified IMRT in which the lateral beams avoided the lungs, three-full-arc volumetric-modulated arc therapy (VMAT), and VMAT with lung avoidance. We compared the outcomes of IMRT and VMAT plans using the paired t-test. After modifications were made to avoid lung irradiation, IMRT values for V5Gy and V20Gy decreased from 713.2 to 503.6 cm3 (p = 0.011) and from 338.8 cm3 to 267.0 cm3 (p = 0.058), respectively. In the case of VMAT, lung V5Gy and V20Gy values decreased from 754.8 to 601.0 cm3 (p = 0.004) and from 328.5 to 255.7 cm3 (p = 0.020), respectively. Other factors did not significantly differ between the plans. In both IMRT and VMAT planning, lung doses were significantly reduced following the modification of the beams that cross the lungs with target coverage maintenance.

Variation in accumulated dose of volumetric-modulated arc therapy for pancreatic cancer due to different beam starting phases.

PURPOSE: To assess the effects of different beam starting phases on dosimetric variations in the clinical target volume (CTV) and organs at risk (OARs), and to identify the relationship between plan complexity and the dosimetric impact of interplay effects in volumetric-modulated arc therapy (VMAT) plans for pancreatic cancer. METHODS: Single and double full-arc VMAT plans were generated for 11 patients. A dose of 50.4 Gy in 28 fractions was prescribed to cover 50% of the planning target volume. Patient-specific Digital Imaging and Communications in Medicine-Radiation Therapy plan files were divided into 10 files based on the respiratory phases in four-dimensional computed tomography (4DCT) simulations. The phase-divided VMAT plans were calculated in consideration of the beam starting phase for each arc and were then combined in the mid-ventilation phase of 4DCT (4D plans). The dose-volumetric parameters were compared with the calculated dose distributions without consideration of the interplay effects (3D plans). Additionally, relationships among plan parameters such as modulation complexity scores, monitor units (MUs), and dose-volumetric parameters were evaluated. RESULTS: Dosimetric differences in the median values associated with different beam starting phases were within ± 1.0% and ± 0.2% for the CTV and ± 0.5% and ± 0.9% for the OARs during single and double full-arc VMAT, respectively. Significant differences caused by variations in the beam starting phases were observed only for the dose-volumetric parameters of the CTV during single full-arc VMAT (P < 0.05), associated with moderate or strong correlations between the MUs and the dosimetric differences between the 4D and 3D plans. CONCLUSIONS: The beam starting phase affected CTV dosimetric variations of single full-arc VMAT. The use of double full-arc VMAT mitigated this problem. However, variation in the dose delivered to OARs was not dependent on the beam starting phase, even for single full-arc VMAT.

[Comparison of Monitor Unit between Electron Monte Carlo Algorithm by Different Dose Normalization Methods and Conventional Manual Calculation].

The purpose of this study was to evaluate the discrepancy between the monitor unit (MU) calculated by different dose normalization methods in the electron Monte Carlo (eMC) algorithm and the conventional manual MU. In the water phantom condition, the manual MU obtained from the measured output factor was compared with the calculated MU by the eMC algorithm, using 24 different irradiation field shapes and several different energies of electron beam. In the breast boost condition, calculated MUs by both calculation methods were evaluated for 45 cases. As a result, the MUs computed by the eMC algorithm in the water phantom varied according to the dose normalization methods, and the mean±standard deviation of the difference between the manual and calculated MU were 1.1±1.4%, 0.0±1.0% and 0.4±1.2% in peak depth normalization (PN), no plan normalization (NPN) and 100% at body maximum (100%BM), respectively. In breast-boost cases, the MU difference between the manual and the calculated MU were 6.1±3.7%, 3.4±2.8% and 1.1±2.9% in PN, NPN and 100%BM, respectively. We revealed that the resultant MU calculated by eMC algorithm was dependent on the dose normalization method and the averaged differences exceeded 6% in PN, especially in breast boost condition. When using the eMC in the breast boost condition, it is desirable to select an appropriate dose normalization method according to dose prescription policies at each facility.

Evaluation of Dynamic Tumor-tracking Intensity-modulated Radiotherapy for Locally Advanced Pancreatic Cancer.

Intensity-modulated radiotherapy (IMRT) is now regarded as an important treatment option for patients with locally advanced pancreatic cancer (LAPC). To reduce the underlying tumor motions and dosimetric errors during IMRT as well as the burden of respiratory management for patients, we started to apply a new treatment platform of the dynamic tumor dynamic tumor-tracking intensity-modulated radiotherapy (DTT-IMRT) using the gimbaled linac, which can swing IMRT toward the real-time tumor position under patients' voluntary breathing. Between June 2013 and March 2015, ten patients were treated, and the tumor-tracking accuracy and the practical benefits were evaluated. The mean PTV size in DTT-IMRT was 18% smaller than a conventional ITV-based PTV. The root-mean-squared errors between the predicted and the detected tumor positions were 1.3, 1.2, and 1.5 mm in left-right, anterior-posterior, and cranio-caudal directions, respectively. The mean in-room time was 24.5 min. This high-accuracy of tumor-tracking with reasonable treatment time are promising and beneficial to patients with LAPC.

Development of a portable quality control application using a tablet-type electronic device.

PURPOSE: Our aim was to develop a portable quality control (QC) application using a thermometer, a barometer, an angle gauge, and a range finder implemented in a tablet-type consumer electronic device (CED) and to assess the accuracies of the measurements made. METHODS: The QC application was programmed using Java and OpenCV libraries. First, temperature and atmospheric pressure were measured over 30 days using the temperature and pressure sensors of the CED and compared with those measured by a double-tube thermometer and a digital barometer. Second, the angle gauge was developed using the accelerometer of the CED. The roll and pitch angles of the CED were measured from 0 to 90° at intervals of 10° in the clockwise (CW) and counterclockwise (CCW) directions. The values were compared with those measured by a digital angle gauge. Third, a range finder was developed using the tablet's built-in camera and image-processing capacities. Surrogate markers were detected by the camera and their positions converted to actual positions using a homographic transformation method. Fiducial markers were placed on a treatment couch and moved 100 mm in 10-mm steps in both the lateral and longitudinal directions. The values were compared with those measured by the digital output of the treatment couch. The differences between CED values and those of other devices were compared by calculating means ± standard deviations (SDs). RESULTS: The means ± SDs of differences in temperature and atmospheric pressure were -0.07 ± 0.25°C and 0.05 ± 0.10 hPa, respectively. The means ± SDs of the difference in angle was -0.17 ± 0.87° (0.15 ± 0.23° degrees excluding the 90° angle). The means ± SDs of distances were 0.01 ± 0.07 mm in both the lateral and longitudinal directions. CONCLUSIONS: Our portable QC application was accurate and may be used instead of standard measuring devices. Our portable CED is efficient and simple when used in the field of medical physics.

Quantification of the kV X-ray imaging dose during real-time tumor tracking and from three- and four-dimensional cone-beam computed tomography in lung cancer patients using a Monte Carlo simulation.

Knowledge of the imaging doses delivered to patients and accurate dosimetry of the radiation to organs from various imaging procedures is becoming increasingly important for clinicians. The purposes of this study were to calculate imaging doses delivered to the organs of lung cancer patients during real-time tumor tracking (RTTT) with three-dimensional (3D), and four-dimensional (4D) cone-beam computed tomography (CBCT), using Monte Carlo techniques to simulate kV X-ray dose distributions delivered using the Vero4DRT. Imaging doses from RTTT, 3D-CBCT and 4D-CBCT were calculated with the planning CT images for nine lung cancer patients who underwent stereotactic body radiotherapy (SBRT) with RTTT. With RTTT, imaging doses from correlation modeling and from monitoring of imaging during beam delivery were calculated. With CBCT, doses from 3D-CBCT and 4D-CBCT were also simulated. The doses covering 2-cc volumes (D2cc) in correlation modeling were up to 9.3 cGy for soft tissues and 48.4 cGy for bone. The values from correlation modeling and monitoring were up to 11.0 cGy for soft tissues and 59.8 cGy for bone. Imaging doses in correlation modeling were larger with RTTT. On a single 4D-CBCT, the skin and bone D2cc values were in the ranges of 7.4-10.5 cGy and 33.5-58.1 cGy, respectively. The D2cc from 4D-CBCT was approximately double that from 3D-CBCT. Clinicians should Figure that the imaging dose increases the cumulative doses to organs.

Geometric and dosimetric accuracy of dynamic tumor tracking during volumetric-modulated arc therapy using a gimbal mounted linac.

PURPOSE: The aim was to examine the feasibility of a dynamic tumor-tracking volumetric modulated arc therapy (DTT-VMAT) technique using a gimbal-mounted linac and assess its positional, mechanical and dosimetric accuracy. MATERIALS AND METHODS: DTT-VMAT was performed using a surrogated signal-based technique. The positional tracking accuracy was evaluated as the difference between the predicted and detected target positions for various wave patterns. Mechanical accuracy measurements included gantry, multileaf collimator (MLC) and gimbal positions. The differences between the command and the measured positions were evaluated for various wave patterns. Dosimetric verification was performed using Gafchromic EBT3 films in the benchmark phantom and two clinical cases. RESULTS: The root mean square error (RMSE) of the positional accuracy was within 0.31 mm. The RMSE of mechanical accuracy was within 0.14° for the gantry, 0.11 ±â€¯0.02 mm for the MLC and 0.13 mm for the gimbal positions. The passing rate of the 3%/3 mm gamma index was greater than 83.3% and 91.2% for the benchmark phantom and two clinical cases, respectively. CONCLUSIONS: The positional, mechanical and dosimetric accuracy of DTT-VMAT were evaluated. DTT-VMAT with a gimbal-mounted linac had sufficient accuracy and presents a new strategy for treatment of several tumors with respiratory motion.

Dosimetric feasibility of using tungsten-based functional paper for flexible chest wall protectors in intraoperative electron radiotherapy for breast cancer.

Intraoperative electron radiotherapy (IOERT), which is an accelerated partial breast irradiation method, has been used for early-stage breast cancer treatment. In IOERT, a protective disk is inserted behind the target volume to minimize the dose received by normal tissues. However, to use such a disk, the surgical incision must be larger than the field size because the disk is manufactured from stiff and unyielding materials. In this study, the applicability of newly developed tungsten-based functional paper (TFP) was assessed as an alternative to the existing protective disk. The radiation-shielding performance of the TFP was verified through experimental measurements and Monte Carlo simulations. Percentage depth dose curves and lateral dose profiles with and without TFPs were measured and simulated on a dedicated IOERT accelerator. The number of piled-up TFPs was changed from 1 to 40. In the experimental measurements, the relative doses at the exit plane of the TFPs for 9 MeV were 42.7%, 9.2%, 0.2%, and 0.1% with 10, 20, 30, and 40 TFPs, respectively, whereas those for 12 MeV were 63.6%, 27.1%, 8.6%, and 0.2% with 10, 20, 30, and 40 TFPs, respectively. Slight dose enhancements caused by backscatter radiation from the TFPs were observed at the entrance plane of the TFPs at both beam energies. The results of the Monte Carlo simulation indicated the same tendency as the experimental measurements. Based on the experimental and simulated results, the radiation-shielding performances of 30 TFPs for 9 MeV and 40 TFPs for 12 MeV were confirmed to be acceptable and close to those of the existing protective disk. The findings of this study suggest the feasibility of using TFPs as flexible chest wall protectors in IOERT for breast cancer treatment.

Use of a second-dose calculation algorithm to check dosimetric parameters for the dose distribution of a first-dose calculation algorithm for lung SBRT plans.

PURPOSE: To verify lung stereotactic body radiotherapy (SBRT) plans using a secondary treatment planning system (TPS) as an independent method of verification and to define tolerance levels (TLs) in lung SBRT between the primary and secondary TPSs. METHODS: A total of 147 lung SBRT plans calculated using X-ray voxel Monte Carlo (XVMC) were exported from iPlan to Eclipse in DICOM format. Dose distributions were recalculated using the Acuros XB (AXB) and the anisotropic analytical algorithm (AAA), while maintaining monitor units (MUs) and the beam arrangement. Dose to isocenter and dose-volumetric parameters, such as D2, D50, D95 and D98, were evaluated for each patient. The TLs of all parameters between XVMC and AXB (TLAXB) and between XVMC and AAA (TLAAA) were calculated as the mean±1.96 standard deviations. RESULTS: AXB values agreed with XVMC values within 3.5% for all dosimetric parameters in all patients. By contrast, AAA sometimes calculated a 10% higher dose in PTV D95 and D98 than XVMC. The TLAXB and TLAAA of the dose to isocenter were -0.3±1.4% and 0.6±2.9%, respectively. Those of D95 were 1.3±1.8% and 1.7±3.6%, respectively. CONCLUSIONS: This study quantitatively demonstrated that the dosimetric performance of AXB is almost equal to that of XVMC, compared with that of AAA. Therefore, AXB is a more appropriate algorithm for an independent verification method for XVMC.

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 Verification around High-density Materials for External Beam Radiotherapy.

It is generally known that the dose distribution around the high-density materials is not accurate with commercially available radiation treatment planning systems (RTPS). Recently, Acuros XB (AXB) has been clinically available for dose calculation algorithm. The AXB is based on the linear Boltzmann transport equation - the governing equation - that describes the distribution of radiation particles resulting from their interactions with matter. The purpose of this study was to evaluate the dose calculation accuracy around high-density materials for AXB under three X-rays energy on the basis of measured values with EBT3 and compare AXB with various dose calculation algorithms (AAA, XVMC) in RTPS and Monte Carlo. First, two different metals, including titanium and stainless steel, were inserted at the center of a water-equivalent phantom, and the depth dose was measured with EBT3. Next, after a phantom which reproduced the geometry of measurement was virtually created in RTPS, dose distributions were calculated with three commercially available algorithms (AXB, AAA, and XVMC) and MC. The calculated doses were then compared with the measured ones. As a result, compared to other algorithms, it was found that the dose calculation accuracy of AXB at the exit side of high-density materials was comparable to that of MC and measured value with EBT3. However, note that AXB underestimated the dose up to approximately 30% at the plane of incidence because it cannot exactly estimate the impact of the backscatter.

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.

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.

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 × 150 mm2 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 18.0 g/ 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.