Yoshinobu Shimohigashi

Validation of fluence-based 3D IMRT dose reconstruction on a heterogeneous anthropomorphic phantom using Monte Carlo simulation

In this study, we evaluated the performance of a three‐dimensional (3D) dose verification system, COMPASS version 3, which has a dedicated beam models and dose calculation engine. It was possible to reconstruct the 3D dose distributions in patient anatomy based on the measured fluence using the MatriXX 2D array. The COMPASS system was compared with Monte Carlo simulation (MC), glass rod dosimeter (GRD), and 3DVH, using an anthropomorphic phantom for intensity‐modulated radiation therapy (IMRT) dose verification in clinical neck cases. The GRD measurements agreed with the MC within 5% at most measurement points. In addition, most points for COMPASS and 3DVH also agreed with the MC within 5%. The COMPASS system showed better results than 3DVH for dose profiles due to individual adjustments, such as beam modeling for each linac. Regarding the dose‐volume histograms, there were no large differences between MC, analytical anisotropic algorithm (AAA) in Eclipse treatment planning system (TPS), 3DVH, and the COMPASS system. However, AAA underestimated the dose to the clinical target volume and Rt‐Parotid slightly. This is because AAA has some problems with dose calculation accuracy. Our results indicated that the COMPASS system offers highly accurate 3D dose calculation for clinical IMRT quality assurance. Also, the COMPASS system will be useful as a commissioning tool in routine clinical practice for TPS. PACS number: 87.55.Qr, 87.56.Fc, 87.61.Bj

Accuracy of positioning and irradiation targeting for multiple targets in intracranial image-guided radiation therapy: a phantom study

This study investigated the accuracy of positioning and irradiation targeting for multiple off-isocenter targets in intracranial image-guided radiation therapy (IGRT). A phantom with nine circular targets was created to evaluate both accuracies. First, the central point of the isocenter target was positioned with a combination of an ExacTrac x-ray (ETX) and a 6D couch. The positioning accuracy was determined from the deviations of coordinates of the central point in each target obtained from the kV-cone beam computed tomography (kV-CBCT) for IGRT and the planning CT. Similarly, the irradiation targeting accuracy was evaluated from the deviations of the coordinates between the central point of each target and the central point of each multi-leaf collimator (MLC) field for multiple targets. Secondly, the 6D couch was intentionally rotated together with both roll and pitch angles of 0.5° and 1° at the isocenter and similarly the deviations were evaluated. The positioning accuracy for all targets was less than 1 mm after 6D positioning corrections. The irradiation targeting accuracy was up to 1.3 mm in the anteroposterior (AP) direction for a target 87 mm away from isocenter. For the 6D couch rotations with both roll and pitch angles of 0.5° and 1°, the positioning accuracy was up to 1.0 mm and 2.3 mm in the AP direction for the target 87 mm away from the isocenter, respectively. The irradiation targeting accuracy was up to 2.1 mm and 2.6 mm in the AP direction for the target 87 mm away from the isocenter, respectively. The off-isocenter irradiation targeting accuracy became worse than the positioning accuracy. Both off-isocenter accuracies worsened in proportion to rotation angles and the distance from the isocenter to the targets. It is necessary to examine the set-up margin for off-isocenter multiple targets at each institution because irradiation targeting accuracy is peculiar to the linac machine.

Evaluation of a single-scan protocol for radiochromic film dosimetry

The purpose of this study was to evaluate a single‐scan protocol using Gafchromic EBT3 film (EBT3) by comparing it with the commonly used 24‐hr measurement protocol for radiochromic film dosimetry. Radiochromic film is generally scanned 24 hr after film exposure (24‐hr protocol). The single‐scan protocol enables measurement results within a short time using only the verification film, one calibration film, and unirradiated film. The single‐scan protocol was scanned 30 min after film irradiation. The EBT3 calibration curves were obtained with the multichannel film dosimetry method. The dose verifications for each protocol were performed with the step pattern, pyramid pattern, and clinical treatment plans for intensity‐modulated radiation therapy (IMRT). The absolute dose distributions for each protocol were compared with those calculated by the treatment planning system (TPS) using gamma evaluation at 3% and 3 mm. The dose distribution for the single‐scan protocol was within 2% of the 24‐hr protocol dose distribution. For the step pattern, the absolute dose discrepancies between the TPS for the single‐scan and 24‐hr protocols were 2.0±1.8 cGy and 1.4±1.2 cGy at the dose plateau, respectively. The pass rates were 96.0% for the single‐scan protocol and 95.9% for the 24‐hr protocol. Similarly, the dose discrepancies for the pyramid pattern were 3.6±3.5 cGy and 2.9±3.3 cGy, respectively, while the pass rates for the pyramid pattern were 95.3% and 96.4%, respectively. The average pass rates for the four IMRT plans were 96.7%±1.8% for the single‐scan protocol and 97.3%±1.4% for the 24‐hr protocol. Thus, the single‐scan protocol measurement is useful for dose verification of IMRT, based on its accuracy and efficiency. PACS number: 87.55.Qr

Validation of a method for in vivo 3D dose reconstruction in SBRT using a new transmission detector

Abstract Stereotactic body radiation therapy (SBRT) involves the delivery of substantially larger doses over fewer fractions than conventional therapy. Therefore, SBRT treatments will strongly benefit patients using vivo patient dose verification, because the impact of the fraction is large. For in vivo measurements, a commercially available quality assurance (QA) system is the COMPASS system (IBA Dosimetry, Germany). For measurements, the system uses a new transmission detector (Dolphin, IBA Dosimetry). In this study, we evaluated the method for in vivo 3D dose reconstruction for SBRT using this new transmission detector. We confirmed the accuracy of COMPASS with Dolphin for SBRT using multi leaf collimator (MLC) test patterns and clinical SBRT cases. We compared the results between the COMPASS, the treatment planning system, the Kodak EDR2 film, and the Monte Carlo (MC) calculations. MLC test patterns were set up to investigate various aspects of dose reconstruction for SBRT: (a) simple open fields (2 × 2–10 × 10 cm2), (b) a square wave chart pattern, and (c) the MLC position detectability test in which the MLCs were changed slightly. In clinical cases, we carried out 6 and 8 static IMRT beams for SBRT in the lung and liver. For MLC test patterns, the differences between COMPASS and MC were around 3%. The COMPASS with the dolphin system showed sufficient resolution in SBRT. For clinical cases, COMPASS can detect small changes for the dose profile and dose–volume histogram. COMPASS also showed good agreement with MC. We can confirm the feasibility of SBRT QA using the COMPASS system with Dolphin. This method was successfully operated using the new transmission detector and verified by measurements and MC.

Plan quality and delivery time comparisons between volumetric modulated arc therapy and intensity modulated radiation therapy for scalp angiosarcoma: A planning study

Due to its spherical surface, scalp angiosarcoma requires careful consideration for radiation therapy planning and dose delivery. Herein, we investigated whether volumetric modulated arc therapy (VMAT) is superior to intensity modulated radiation therapy (IMRT) in terms of the plan quality and delivery time.

SU-F-T-541: Impact of VMAT Dose Calculations with Respiratory Movements in Lung

PURPOSE To evaluate dose impact due to respiratory movements for volumetric modulated arc therapy (VMAT) in lung, using deformation technique. METHODS The dose impact due to respiratory movements was evaluated for VMAT in lung. VMAT dose distributions were calculated with 4D-CT images for a 3-D dynamic lung phantom and seven patients. First, one-arc VMAT dose distributions were calculated at the reference phase (the maximum of expiratory phase) using the Varian Eclipse (ver.10) treatment planning system. All VMAT were planned with 48Gy for 4 fractions. The VMAT dose distributions were divided into 88 control points (CPs) and were assigned to each one of ten phases divided through one respiratory cycle time. Next, the dose distributions were recalculated at each phase and were deformed at the reference phase using DIRART ver.1.0a and Velocity AI ver.3.1 software. The deformed dose distributions were summed up. Finally, VMAT dose distributions with respiratory movements were evaluated from dose volume histograms (DVHs). RESULTS VMAT dose distributions deformed by DIRART and the Velocity AI in the phantom study agreed within 2% with that of the reference phase for 95% of GTV (D95 ). In the patient study, deformed VMAT dose distributions decreased up to 4.15% and 3.46% for DIRART and Velocity AI, respectively. The maximum dose of GTV (Dmax) tended to be higher with increasing tumor movements. In constant, D95 for DVHs decreased with increasing tumor movements. The discrepancy in the VMAT dose distributions deformed by two software depended on specific properties of respective algorithms and parameters. CONCLUSION Respiratory movements impact on the dose distributions for GTV. The Dmax may relate with amplitude of tumor movements.

SU-F-T-549: Validation of a Method for in Vivo 3D Dose Reconstruction for SBRT Using a New Transmission Detector

PURPOSE Recently, there has been increased clinical use of stereotactic body radiation therapy (SBRT). SBRT treatments will strongly benefit from in vivo patient dose verification, as any errors in delivery can be more detrimental to the radiobiology of the patient as compared to conventional therapy. In vivo dose measurements, a commercially available quality assurance platform which is able to correlate the delivered dose to the patients anatomy and take into account tissue inhomogeneity, is the COMPASS system (IBA Dosimetry, Germany) using a new transmission detector (Dolphin, IBA Dosimetry). In this work, we evaluate a method for in vivo 3D dose reconstruction for SBRT using a new transmission detector, which was developed for in vivo dose verification for intensity-modulated radiation therapy (IMRT). METHODS We evaluated the accuracy of measurement for SBRT using simple small fields (2×2-10×10 cm2), a multileaf collimator (MLC) test pattern, and clinical cases. The dose distributions from the COMPASS were compared with those of EDR2 films (Kodak, USA) and the Monte Carlo simulations (MC). For clinical cases, we compared MC using dose-volume-histograms (DVHs) and dose profiles. RESULTS The dose profiles from the COMPASS for small fields and the complicated MLC test pattern agreed with those of EDR2 films, and MC within 3%. This showed the COMPASS with Dolphin system showed good spatial resolution and can measure small fields which are required for SBRT. Those results also suggest that COMPASS with Dolphin is able to detect MLC leaf position errors for SBRT. In clinical cases, the COMPASS with Dolphin agreed well with MC. The Dolphin detector, which consists of ionization chambers, provided stable measurement. CONCLUSION COMPASS with Dolphin detector showed a useful in vivo 3D dose reconstruction for SBRT. The accuracy of the results indicates that this approach is suitable for clinical implementation.

Comparison of 3-dimensional dose reconstruction system between fluence-based system and dose measurement-guided system.

COMPASS system (IBA Dosimetry, Schwarzenbruck, Germany) and ArcCHECK with 3DVH software (Sun Nuclear Corp., Melbourne, FL) are commercial quasi-3-dimensional (3D) dosimetry arrays. Cross-validation to compare them under the same conditions, such as a treatment plan, allows for clear evaluation of such measurement devices. In this study, we evaluated the accuracy of reconstructed dose distributions from the COMPASS system and ArcCHECK with 3DVH software using Monte Carlo simulation (MC) for multi-leaf collimator (MLC) test patterns and clinical VMAT plans. In a phantom study, ArcCHECK 3DVH showed clear differences from COMPASS, measurement and MC due to the detector resolution and the dose reconstruction method. Especially, ArcCHECK 3DVH showed 7% difference from MC for the heterogeneous phantom. ArcCHECK 3DVH only corrects the 3D dose distribution of treatment planning system (TPS) using ArcCHECK measurement, and therefore the accuracy of ArcCHECK 3DVH depends on TPS. In contrast, COMPASS showed good agreement with MC for all cases. However, the COMPASS system requires many complicated installation procedures such as beam modeling, and appropriate commissioning is needed. In terms of clinical cases, there were no large differences for each QA device. The accuracy of the compass and ArcCHECK 3DVH systems for phantoms and clinical cases was compared. Both systems have advantages and disadvantages for clinical use, and consideration of the operating environment is important. The QA system selection is depending on the purpose and workflow in each hospital.

Evaluation of target localization accuracy for image-guided radiation therapy by 3D and 4D cone-beam CT in the presence of respiratory motion: a phantom study

We evaluate the target definition accuracy of four-dimensional CT (4D-CT) simulation and target localization accuracy of 3D or 4D cone-beam CT (CBCT) in the presence of respiration. The target motion is modelled by a sine curve or a cos6 curve. The target volumes, shapes, and positions obtained from the 4D-CT simulation are compared with the static CT image and theoretical values of the phantom. Reference average intensity projection (AIP) and maximum intensity projection (MIP) images for target localization are generated from the 4D-CT simulation. Localization involves aligning the AIP/MIP to 3D cone-beam CT (3D-CBCT) or 4D cone-beam CT (4D-CBCT), and localization accuracy is evaluated from the difference in target position between the reference AIP/MIP image and 3D-CBCT/4D-CBCT measurements. 4D-CBCT also allows measurement of the target motion standard deviation (SD) and excursion (EX). The SD and EX errors are calculated with respect to the theoretical value of the phantom. The target volume and position accuracies obtained via 4D-CT at each phase are within 3.0% and 2.5 mm, respectively, of the static and theoretical values for the sine and cos6 curves. The target localization errors for 3D-CBCT are within 1.0 mm regardless of the EX variation and reference image for the sine curve, whereas the errors for the cos6 curve increase from 0.1 to 5.1 mm with increasing EX variation. In contrast, the 4D-CBCT localization errors are within 1.0 mm regardless of EX variation, reference image, and motion pattern. In addition, SD and EX errors, respectively, range from −1.3 to 0.1 mm and −2.2 to 0.1 mm (AIP) and from −4.4 to −2.7 mm and −13.5 to −4.2 mm (MIP). 4D-CBCT for AIP is more accurate than that for MIP. Target localization is simple and accurate for 3D-CBCT and 4D-CBCT with the AIP. However, 3D-CBCT is more inaccurate than 4D-CBCT when considering EX variations with the cos6 curve.

SU-E-T-336: Dosimetric Properties of a New Solid Water High Equivalency Phantom for High-Energy Photon Beams

Purpose: To investigate dosimetric properties in high-energy photon beams for a Solid Water High Equivalency (SWHE, SW557) phantom (Gammex) which was newly developed as water mimicking material. Methods: The mass density of SWHE and SWHE/water electron density ratio are 1.032 g/cm3 and 1.005 according to the manufacturer information, respectively. SWHE is more water equivalent material in physical characteristics and uniformity than conventional SW457. This study calculated the relative ionization ratio of water and SWHE as a function of depth from the cavity dose in PTW30013 and Exradin A19 Farmer-type ionization chambers using Monte Caro simulations. The simulation was performed with a 10 x 10 cm2 field at SAD of 100 cm for 4, 6, 10, 15, and 18 MV photons. The ionization ratio was also measured with the PTW30013 chamber for 6 and 15 MV photons. In addition, the overall perturbation factor of both chambers was calculated for both phantoms. Results: The relative ionization ratio curves for water and SWHE was in good agreement for all photon energies. The ionization ratio of water/SWHE for both chambers was 0.999–1.002, 0.999–1.002, 1.001–1.004, 1.004–1.007, and 1.006–1.010 at depths of over the buildup region for 4, 6, 10, 15, and 18 MV photons, respectively. The ionization ratio of water/SWHE increased up to 1% with increasing the photon energy. The measured ionization ratio of water/SWHE for 6 and 15 MV photons agreed well with calculated values. The overall perturbation factor for both chambers was 0.983–0.988 and 0.978–0.983 for water and SWHE, respectively, in a range from 4 MV to 18 MV. Conclusion: The depth scaling factor of water/SWHE was equal to unity for all photon energies. The ionization ratio of water/SWHE at a reference depth was equal to unity for 4 and 6 MV and larger up to 0.7% than unity for 18 MV.