3D Dose Maps Research Articles

Patient specific plan verification of a VMAT plan using 3D polymer Gel dosimeter in a phantom reproducing patient anatomy

Introduction Patient specific plan verification becomes necessary in the advent of complex treatment delivery options with current linear accelerator technology. Purpose To use a patient-specific end-to-end quality assurance approach for plan verification and overall accuracy evaluation of a Volumetric Modulated Arc Therapy (VMAT) irradiation. Materials and methods Patient CT-scans were used to construct a phantom reproducing the patient anatomy in terms of external surface and bone structures using 3D-printing technology. The phantom was filled with a polymer-gel dosimeter and utilized to accurately reproduce every link in the treatment chain and irradiated using a 6 MV flattening filter free (fff) Elekta Versa linear accelerator. Upon irradiation, the phantom was MRI-scanned using a specially designed T2 pulse sequence and T2-maps were converted to 3D relative dose measurements. MR-images were imported to Monaco-TPS and co-registered to patient CT-images. TPS dose calculations were exported and used for dose comparison with measurements in an independent software. Results Radiation-induced polymerization area was clearly evident in the T2-images and found to coincide to the high-dose target area while organs at risk (OARs) were adequately spared in agreement with the TPS dose distribution. In addition, a quantitative evaluation was performed by comparing measured and TPS-calculated 3D dose-maps in terms of dose distributions, gamma index maps and Dose Volume Histogram (DVH) indices clinically used for plan evaluation and acceptance. Conclusion A patient-specific plan verification method offering the unique characteristic of providing 3D-dose distribution measurements in patient anatomy, including DVHs was implemented and revealed accurate delivery of the VMAT plan.

SU‐F‐T‐513: Dosimetric Validation of Spatially Fractionated Radiotherapy Using Gel Dosimetry

Purpose:Spatially fractionated radiation therapy, also known as GRID therapy, is used to treat large solid tumors by irradiating the target to a single dose of 10–20Gy through spatially distributed beamlets. We have investigated the use of a 3D gel for dosimetric characterization of GRID therapy.Methods:GRID therapy is an external beam analog of volumetric brachytherapy, whereby we produce a distribution of hot and cold dose columns inside the tumor volume. Such distribution can be produced with a block or by using a checker‐like pattern with MLC. We have studied both types of GRID delivery. A cube shaped acrylic phantom was filled with polymer gel and served as a 3D dosimeter. The phantom was scanned and the CT images were used to produce two plans in Pinnacle, one with the grid block and one with the MLC defined grid. A 6MV beam was used for the plan with a prescription of 1500cGy at dmax. The irradiated phantom was scanned in a 3T MRI scanner.Results:3D dose maps were derived from the MR scans of the gel dosimeter and were found to be in good agreement with the predicted dose distribution from the RTP system. Gamma analysis showed a passing rate of 93% for 5% dose and 2mm DTA scoring criteria. Both relative and absolute dose profiles are in good agreement, except in the peripheral beamlets where the gel measured slightly higher dose, possibly because of the changing head scatter conditions that the RTP is not fully accounting for. Our results have also been benchmarked against ionization chamber measurements.Conclusion:We have investigated the use of a polymer gel for the 3D dosimetric characterization and evaluation of GRID therapy. Our results demonstrated that the planning system can predict fairly accurately the dose distribution for GRID type therapy.

SU‐F‐T‐179: Fast and Accurate Profile Acquisition for Proton Beam Using Multi‐Ion Chamber Arrays

Purpose:Proton beam profile measurement is more time‐consuming than photon beam. Due to the energy modulation during proton delivery, chambers have to move step‐by‐step instead of continuously. Multi‐ion chamber arrays are appealing to this task since multiple measurements can be performed at once. However, their utilization suffers from sparse spatial resolution and potential intrinsic volume‐averaging effect of the disk‐shaped ion chambers. We proposed an approach to measure proton beam profiles accurately and efficiently.Methods:Mevion S250 proton system and IBA Matrixx ion chamber arrays were used in this study. Matrixx has interchamber distance of 7.62 mm, and chamber diameter of 4.5 mm. We measured the same beam profile by moving the Matrixx seven times with 1 mm each time along y axis. All 7 measurements were superimposed to get a “finer” profile with 1 mm spatial resolution. Coarser resolution profiles of 2 mm and 3 mm were also generated by using subsets of measurements. Those profiles were compared to the TPS calculated beam profile. Gamma analysis was performed for 2D dose maps to evaluate the difference to TPS dose plane.Results:Preliminary results showed a large discrepancy between the TPS calculated profile and the single measurement profile with 7.6 mm resolution. A good match could be achieved when the resolution reduced to 3 mm by adding one extra measurement. Gamma analysis for 2D dose map of a 10×10 field showed a passing rate (γ ≤ 1) of 90.6% using a 3% and 3mm criterion for single measurement, which increased to 92.3% for 2‐measurement superimposition, and slightly further increased to 92.9% for 7‐measurement superimposition.Conclusion:The results indicated that 2 measurements shifted by 3mm using Matrixx generated a smooth proton beam profile with good matching to Eclipse beam profile. We suggest using this 2‐measurement approach in clinic for double scattering proton beam profile measurement.

Optical-CT 3D Dosimetry Using Fresnel Lenses with Minimal Refractive-Index Matching Fluid.

Telecentric optical computed tomography (optical-CT) is a state-of-the-art method for visualizing and quantifying 3-dimensional dose distributions in radiochromic dosimeters. In this work a prototype telecentric system (DFOS—Duke Fresnel Optical-CT Scanner) is evaluated which incorporates two substantial design changes: the use of Fresnel lenses (reducing lens costs from $10-30K t0 $1-3K) and the use of a ‘solid tank’ (which reduces noise, and the volume of refractively matched fluid from 1ltr to 10cc). The efficacy of DFOS was evaluated by direct comparison against commissioned scanners in our lab. Measured dose distributions from all systems were compared against the predicted dose distributions from a commissioned treatment planning system (TPS). Three treatment plans were investigated including a simple four-field box treatment, a multiple small field delivery, and a complex IMRT treatment. Dosimeters were imaged within 2h post irradiation, using consistent scanning techniques (360 projections acquired at 1 degree intervals, reconstruction at 2mm). DFOS efficacy was evaluated through inspection of dose line-profiles, and 2D and 3D dose and gamma maps. DFOS/TPS gamma pass rates with 3%/3mm dose difference/distance-to-agreement criteria ranged from 89.3% to 92.2%, compared to from 95.6% to 99.0% obtained with the commissioned system. The 3D gamma pass rate between the commissioned system and DFOS was 98.2%. The typical noise rates in DFOS reconstructions were up to 3%, compared to under 2% for the commissioned system. In conclusion, while the introduction of a solid tank proved advantageous with regards to cost and convenience, further work is required to improve the image quality and dose reconstruction accuracy of the new DFOS optical-CT system.

2D mapping of the MV photon fluence and 3D dose reconstruction in real time for quality assurance during radiotherapy treatment

Summary: the photon irradiation response of a 2D solid state transmission detector array mounted in a linac block tray is used to reconstruct the projected 2D dose map in a homogenous phantom along rays that diverge from the X-ray source and pass through each of the 121 detector elements. A unique diode response-to-dose scaling factor, applied to all detectors, is utilised in the reconstruction to demonstrate that real time QA during radiotherapy treatment is feasible. Purpose: to quantitatively demonstrate reconstruction of the real time radiation dose from the irradiation response of the 11×11 silicon Magic Plate (MP) detector array operated in Transmission Mode (MPTM). Methods and Materials: in transmission mode the MP is positioned in the block tray of a linac so that the central detector of the array lies on the central axis of the radiation beam. This central detector is used to determine the conversion factor from measured irradiation response to reconstructed dose at any point on the central axis within a homogenous solid water phantom. The same unique conversion factor is used for all MP detector elements lying within the irradiation field. Using the two sets of data, the 2D or 3D dose map is able to be reconstructed in the homogenous phantom. The technique we have developed is illustrated here for different depths and irradiation field sizes, (5 × 5 cm2 to 40 × 40 cm2) as well as a highly non uniform irradiation field.Results: we find that the MPTM response is proportional to the projected 2D dose map measured at a specific phantom depth, the sweet depth. A single factor, for several irradiation field sizes and depths, is derived to reconstruct the dose in the phantom along rays projected from the photon source through each MPTM detector element. We demonstrate that for all field sizes using the above method, the 2D reconstructed and measured doses agree to within ± 2.48% (2 standard deviation) for all in-field MP detector elements. Conclusions: a 2D detector system and method to reconstruct the dose in a homogeneous phantom and in real time has been demonstrated. The success of this work is an exciting development toward real time QA during radiotherapy treatment.

2D mapping of the MV photon fluence and 3D dose reconstruction in real time for quality assurance during radiotherapy treatment

Summary: the photon irradiation response of a 2D solid state transmission detector array mounted in a linac block tray is used to reconstruct the projected 2D dose map in a homogenous phantom along rays that diverge from the X-ray source and pass through each of the 121 detector elements. A unique diode response-to-dose scaling factor, applied to all detectors, is utilised in the reconstruction to demonstrate that real time QA during radiotherapy treatment is feasible. Purpose: to quantitatively demonstrate reconstruction of the real time radiation dose from the irradiation response of the 11×11 silicon Magic Plate (MP) detector array operated in Transmission Mode (MPTM).Methods and Materials: in transmission mode the MP is positioned in the block tray of a linac so that the central detector of the array lies on the central axis of the radiation beam. This central detector is used to determine the conversion factor from measured irradiation response to reconstructed dose at any point on the central axis within a homogenous solid water phantom. The same unique conversion factor is used for all MP detector elements lying within the irradiation field. Using the two sets of data, the 2D or 3D dose map is able to be reconstructed in the homogenous phantom. The technique we have developed is illustrated here for different depths and irradiation field sizes, (5 × 5 cm2 to 40 × 40 cm2) as well as a highly non uniform irradiation field.Results: we find that the MPTM response is proportional to the projected 2D dose map measured at a specific phantom depth, the “sweet depth”. A single factor, for several irradiation field sizes and depths, is derived to reconstruct the dose in the phantom along rays projected from the photon source through each MPTM detector element. We demonstrate that for all field sizes using the above method, the 2D reconstructed and measured doses agree to within ± 2.48% (2 standard deviation) for all in-field MP detector elements.Conclusions: a 2D detector system and method to reconstruct the dose in a homogeneous phantom and in real time has been demonstrated. The success of this work is an exciting development toward real time QA during radiotherapy treatment.

Dosimetric accuracy of a treatment planning system for actively scanned proton beams and small target volumes: Monte Carlo and experimental validation

This study was performed to evaluate the accuracy of a commercial treatment planning system (TPS), in optimising proton pencil beam dose distributions for small targets of different sizes (5–30 mm side) located at increasing depths in water. The TPS analytical algorithm was benchmarked against experimental data and the FLUKA Monte Carlo (MC) code, previously validated for the selected beam-line. We tested the Siemens syngo® TPS plan optimisation module for water cubes fixing the configurable parameters at clinical standards, with homogeneous target coverage to a 2 Gy (RBE) dose prescription as unique goal. Plans were delivered and the dose at each volume centre was measured in water with a calibrated PTW Advanced Markus® chamber. An EBT3® film was also positioned at the phantom entrance window for the acquisition of 2D dose maps. Discrepancies between TPS calculated and MC simulated values were mainly due to the different lateral spread modeling and resulted in being related to the field-to-spot size ratio. The accuracy of the TPS was proved to be clinically acceptable in all cases but very small and shallow volumes. In this contest, the use of MC to validate TPS results proved to be a reliable procedure for pre-treatment plan verification.

TH-AB-201-01: A Real-Time Skin-Dose Mapping System for Region-Of-Interest (ROI) Fluoroscopy

Purpose: The real-time dose-tracking system (DTS) which we developed for fluoroscopically–guided procedures has been upgraded so that it can track the patient skin dose when using a region-of-interest (ROI) beam attenuator. Methods: ROI fluoroscopy is a method for greatly reducing patient dose by inserting an attenuator with an aperture into the x-ray beam. The aperture defines an ROI with normal dose and image quality, while under the attenuator the dose and quality are reduced. A moveable copper ROI attenuator has been installed in the collimator assembly of a Toshiba Infinix system. The DTS provides a 3D-dose mapping on a patient graphic and, to track dose for ROI fluoroscopy, the software now models the attenuator shape and position. A calibration file provides values for the transmission of the attenuator as a function of kVp and beam filtration and dose corrections are performed if a ray to a given point on the patient graphic is determined to intersect the attenuator. Backscatter is included in the skin-dose determination and is dependent on the ROI beam size projected onto the patient. In the current version, a spatially invariant factor corrects for backscatter within the ROI and from the ROI region into the attenuated region. The agreement of the dose distribution calculated by the DTS with the actual distribution was verified by measurement with calibrated radiochromic film and a 20-cm PMMA phantom. Results: The DTS is able to accurately model the size and position of the ROI attenuator as projected onto the patient graphic. Measurements with radiochromic film show the skin-dose estimation agreement to be within +/−12%, with most error arising from the spatial variation of backscatter. Conclusion: The dose distribution on the patient can now be tracked in real-time with acceptable accuracy when the new dose-saving feature of ROI fluoroscopy is used. This research was supported in part by Toshiba Medical Systems Corporation and NIH Grant R01EB002873.

MagicPlate-512: A 2D silicon detector array for quality assurance of stereotactic motion adaptive radiotherapy.

Spatial and temporal resolutions are two of the most important features for quality assurance instrumentation of motion adaptive radiotherapy modalities. The goal of this work is to characterize the performance of the 2D high spatial resolution monolithic silicon diode array named "MagicPlate-512" for quality assurance of stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS) combined with a dynamic multileaf collimator (MLC) tracking technique for motion compensation. MagicPlate-512 is used in combination with the movable platform HexaMotion and a research version of radiofrequency tracking system Calypso driving MLC tracking software. The authors reconstruct 2D dose distributions of small field square beams in three modalities: in static conditions, mimicking the temporal movement pattern of a lung tumor and tracking the moving target while the MLC compensates almost instantaneously for the tumor displacement. Use of Calypso in combination with MagicPlate-512 requires a proper radiofrequency interference shielding. Impact of the shielding on dosimetry has been simulated by (GEANT)4 and verified experimentally. Temporal and spatial resolutions of the dosimetry system allow also for accurate verification of segments of complex stereotactic radiotherapy plans with identification of the instant and location where a certain dose is delivered. This feature allows for retrospective temporal reconstruction of the delivery process and easy identification of error in the tracking or the multileaf collimator driving systems. A sliding MLC wedge combined with the lung motion pattern has been measured. The ability of the MagicPlate-512 (MP512) in 2D dose mapping in all three modes of operation was benchmarked by EBT3 film. Full width at half maximum and penumbra of the moving and stationary dose profiles measured by EBT3 film and MagicPlate-512 confirm that motion has a significant impact on the dose distribution. Motion, no motion, and motion with MLC tracking profiles agreed within 1 and 0.4 mm, respectively, for all field sizes tested. Use of electromagnetic tracking system generates a fluctuation of the detector baseline up to 10% of the full scale signal requiring a proper shielding strategy. MagicPlate-512 is also able to reconstruct the dose variation pulse-by-pulse in each pixel of the detector. An analysis of the dose transients with motion and motion with tracking shows that the tracking feedback algorithm used for this experiment can compensate effectively only the effect of the slower transient components. The fast changing components of the organ motion can contribute only to discrepancy of the order of 15% in penumbral region while the slower components can change the dose profile up to 75% of the expected dose. MagicPlate-512 is shown to be, potentially, a valid alternative to film or 2D ionizing chambers for quality assurance dosimetry in SRS or SBRT. Its high spatial and temporal resolutions allow for accurate reconstruction of the profile in any conditions with motion and with tracking of the motion. It shows excellent performance to reconstruct the dose deposition in real time or retrospectively as a function of time for detailed analysis of the effect of motion in a specific pixel or area of interest.

High resolution 3D dosimetry for microbeam radiation therapy using optical CT

Optical Computed Tomography (CT) is a promising technique for dosimetry of Microbeam Radiation Therapy (MRT), providing high resolution 3D dose maps. Here different MRT irradiation geometries are visualised showing the potential of Optical CT as a tool for future MRT trials. The Peak-to-Valley dose ratio (PVDR) is calculated to be 7 at a depth of 3mm in the radiochromic dosimeter PRESAGE®. This is significantly lower than predicted values and possible reasons for this are discussed.

Characterization of ferric ions diffusion in Fricke gel dosimeters by using inverse problem techniques

Diffusion of ferric ions in ferrous sulfate (Fricke) gels represents one of the main drawbacks of some radiation detectors, such as Fricke gel dosimeters. In practice, this disadvantage can be overcome by prompt dosimeter analysis, and constraining strongly the time between irradiation and analysis, implementing special dedicated protocols aimed at minimizing signal blurring due to diffusion effects. This work presents a novel analytic modeling and numerical calculation approach of diffusion coefficients in Fricke gel radiation sensitive materials. Samples are optically analyzed by means of visible light transmission measurements by capturing images with a charge-coupled device camera provided with a monochromatic filter corresponding to the XO-infused Fricke solution absorbance peak. Dose distributions in Fricke gels are suitably delivered by assessing specific initial conditions further studied by periodical sample image acquisitions. Diffusion coefficient calculations were performed using a set of computational algorithms based on inverse problem formulation. Although 1D approaches to the diffusion equation might provide estimations of the diffusion coefficient, it should be calculated in the 2D framework due to the intrinsic bi-dimensional characteristics of Fricke gel layers here considered as radiation dosimeters. Thus a suitable 2D diffusion model capable of determining diffusion coefficients was developed by fitting the obtained algorithm numerical solutions with the corresponding experimental data. Comparisons were performed by introducing an appropriate functional in order to analyze both experimental and numerical values. Solutions to the second-order diffusion equation are calculated in the framework of a dedicated method that incorporates finite element method. Moreover, optimized solutions can be attained by gradient-type minimization algorithms. Knowledge about diffusion coefficient for a Fricke gel radiation detector is helpful in accounting for effects regarding elapsed time between dosimeter irradiation and further analysis. Hence, corrections might be included in standard dependence of optical density differences and actual, non-diffused, absorbed dose distributions. The obtained values for ferric ion diffusion coefficient are around 0.65 mm2 h−1, being in good agreement with previous works corresponding to similar Fricke gel dosimeter compositions. Therefore, more accurate 2D and 3D dose mapping might be attained, thus constituting valuable improvements in Fricke gel dosimetry, and parallely a high precision method of diffusion modeling and calculation has been developed.

Correlation between ferrous ammonium sulfate concentration, sensitivity and stability of Fricke gel dosimeters exposed to clinical X-ray beams

This work describes the characterization of various Fricke-Agarose-Xylenol gels (FXG) dosimeters using NMR relaxometry and MRI analysis. Using X-rays from a clinical linear accelerator (LINAC), the gels were irradiated in the dose range from 0Gy to 20Gy. The photon sensitivity of the FXGs was measured in terms of NMR relaxation rates; its dependence on radiation dose was determined as a function of ferrous ammonium sulfate contents (from 0.5mM to 5mM). Furthermore, the stability of the NMR signal was monitored over several days after irradiation. These measurements were aided by Magnetic Resonance Imaging (MRI) scans which allowed three-dimensional (3D) dose mapping. In order to maximize the MRI response, a systematic study was performed to optimize acquisition sequences and parameters. In particular, we analyzed the dependence of MRI signal on the repetition time (TR) and on the inversion time (TI) using inversion recovery sequences. The results are reported and discussed from the point of view of the dosimeter use in clinical radiotherapy. This work highlights that the optimization of additive content inside gel matrix is fundamental for optimizing photon sensitivity of these detectors.

SU-E-J-236: Feasibility of Using Infrared Imaging to Verify the Accuracy of the Radiotherapy Delivery

Purpose: To assess whether infrared imaging can be used to verify the accuracy of the radiation treatment delivery. Methods: Radiation treatment induced skin reactions include acute changes (erythema and pigmentation) and chronic changes (telangiectasia and ulceration). Thermal imaging can detect these reactions before they become visible. A thermal camera with infrared spectral band 7.5 μm to 14 μm, and temperature measurement range from −20 °C to +150 °C and accuracy of ±2 °C was used. Beams-eye-view images were taken by the end of each treatment for a head and neck patient. The temperature changes of three points were monitored throughout the treatment course, including two points in the field (canthus and vessel) and one point outside of the field. The thermal images were warped and registered to the corresponding planning CT skin rendering. Dose map was exported from the treatment planning system and warped and registered to the planning CT skin rendering as well. The correlation between these two warped and registered images was calculated. The time spent to take the images was recorded. Results: The temperatures of the two points inside the field were always higher than the temperature of the point outside of the field. The temperature insidemore » the field tended to increase as more fractions delivered to the patient. A correlation of 0.90 was found between the registered thermal image and the 3D dose map. On average, half minute was needed to take the images after each treatment. Conclusion: Thermal imaging can monitor the temperature variations of the skin, and the temperatures were proportional to the dose map. It can therefore potentially be an inexpensive and non-ionization tool to verify the accuracy of the radiation treatment delivery.« less

SU‐E‐CAMPUS‐T‐05: Preliminary Results On a 2D Dosimetry System Based On the Optically Stimulated Luminescence of Al2O3

Purpose:To develop a precise 2D dose mapping technique based on the optically stimulated luminescence (OSL) from Al2O3 films for medical applications.Methods:A 2D laser scanning reader was developed using fast F+‐center (lifetime of <7 ns) and slow F‐center (lifetime of 35 ms) OSL emission from newly developed Al2O3 films (Landauer Inc.). An algorithm was developed to correct images for both material and system properties. Since greater contribution of the F??‐center emission in the recorded signal increases the readout efficiency and robustness of image corrections, Al2O3:C,Mg film samples are being investigated in addition to Al2O3:C samples. Preliminary investigations include exposure of the films to a 6 MV photon beam at 10 cm depth in solid water phantom with an SSD of 100 cm, using a 10 cm × 10 cm flat field or a 4 cm × 4 cm field with a 60° wedge filter. Kodak EDR2 radiographic film and EBT2 Gafchromic film were also exposed for comparison.Results:The results indicate that the algorithm is able to correct images and calculate 2D dose. For the wedge field irradiation, the calculated dose at the center of the field was 0.9 Gy for Al2O3:C and 0.87 Gy for Al2O3:C,Mg, whereas, the delivered dose was 0.95 Gy. A good qualitative agreement of the dose profiles was obtained between the OSL films and EDR2 and EBT2 films. Laboratory tests using a beta source suggest that a large dynamic range (10−2−102 Gy) can be achieved using this technique.Conclusion:A 2D dosimetry system and an in‐house image correction algorithm were developed for 2D film dosimetry in medical applications. The system is in the preliminary stage of development, but the data demonstrates the feasibility of this approach. This work was supported by Landauer, Inc.

SU-E-T-154: Establishment and Implement of 3D Image Guided Brachytherapy Planning System

Purpose: Cannot observe the dose intuitionally is a limitation of the existing 2D pre-implantation dose planning. Meanwhile, a navigation module is essential to improve the accuracy and efficiency of the implantation. Hence a 3D Image Guided Brachytherapy Planning System conducting dose planning and intra-operative navigation based on 3D multi-organs reconstruction is developed. Methods: Multi-organs including the tumor are reconstructed in one sweep of all the segmented images using the multiorgans reconstruction method. The reconstructed organs group establishs a three-dimensional visualized operative environment. The 3D dose maps of the three-dimentional conformal localized dose planning are calculated with Monte Carlo method while the corresponding isodose lines and isodose surfaces are displayed in a stereo view. The real-time intra-operative navigation is based on an electromagnetic tracking system (ETS) and the fusion between MRI and ultrasound images. Applying Least Square Method, the coordinate registration between 3D models and patient is realized by the ETS which is calibrated by a laser tracker. The system is validated by working on eight patients with prostate cancer. The navigation has passed the precision measurement in the laboratory. Results: The traditional marching cubes (MC) method reconstructs one organ at one time and assembles them together. Compared to MC, presentedmore » multi-organs reconstruction method has superiorities in reserving the integrality and connectivity of reconstructed organs. The 3D conformal localized dose planning, realizing the 'exfoliation display' of different isodose surfaces, helps make sure the dose distribution has encompassed the nidus and avoid the injury of healthy tissues. During the navigation, surgeons could observe the coordinate of instruments real-timely employing the ETS. After the calibration, accuracy error of the needle position is less than 2.5mm according to the experiments. Conclusion: The speed and quality of 3D reconstruction, the efficiency in dose planning and accuracy in navigation all can be improved simultaneously.« less

Dependence of Coronary 3-Dimensional Dose Maps on Coronary Topologies and Beam Set in Breast Radiation Therapy: A Study Based on CT Angiographies

In left-side breast radiation therapy (RT), doses to the left main (LM) and left anterior descending (LAD) coronary arteries are usually assessed after delineation by prior anatomic knowledge on the treatment planning computed tomography (CT) scan. In this study, dose sensitivity due to interindividual coronary topology variation was assessed, and hot spots were located. Twenty-two detailed heart models, created from heart computed tomography angiographies, were fitted into a single representative female thorax. Two breast RT protocols were then simulated into a treatment planning system: the first protocol comprised tangential and tumoral bed beams (TGs_TB) at 50 + 16 Gy, the second protocol added internal mammary chain beams at 50 Gy to TGs_TB (TGs_TB_IMC). For the heart, the LAD, and the LM, several dose indicators were calculated: dose-volume histograms, mean dose (Dmean), minimal dose received by the most irradiated 2% of the volume (D2%), and 3-dimensional (3D) dose maps. Variations of these indicators with anatomies were studied. For the LM, the intermodel dispersion of Dmean and D2% was 10% and 11%, respectively, with TGs_TB and 40% and 80%, respectively, with TGs_TB_IMC. For the LAD, these dispersions were 19% (Dmean) and 49% (D2%) with TGs_TB and 35% (Dmean) and 76% (D2%) with TGs_TB_IMC. The 3D dose maps revealed that the internal mammary chain beams induced hot spots between 20 and 30 Gy on the LM and the proximal LAD for some coronary topologies. Without IMC beams, hot spots between 5 and 26 Gy are located on the middle and distal LAD. Coronary dose distributions with hot spot location and dose level can change significantly depending on coronary topology, as highlighted by 3D coronary dose maps. In clinical practice, coronary imaging may be required for a relevant coronary dose assessment, especially in cases of internal mammary chain irradiation.

Phantom based KV-CBCT dose plan calculations for re-planning decision: Comparison with CT-based dose plans

Objective Kilovoltage cone-beam computed tomography scans (kVCBCT) are helpful for patients positioning and viewing anatomical changes. This study investigates, on an anthropomorphic phantom, with no movement and anatomical changes, the possibility and accuracy of using CBCT-based dosimetry for re-planning decision. Materials and method Rando phantom's brain, Head and Neck (H&N), thorax and pelvis were scanned. Targets and main organ at risks (OAR) were contoured. IMRT was used for H&N and VMAT for brain, lung and pelvis. Dose distributions were calculated using Eclipse 10.0 AAA Algorithm (Varian Medical System). CBCT scans were acquired per region on a Novalis Tx. After registration with the reference CT, structures were copied and dosimetric plans were recalculated on the CBCT. The dose volume histograms (HVH) for targets and OAR and 2D dose maps were compared. Furthermore the Hounsfield units to electron density (HU to ED) curves for the different CBCT protocols were obtained using a Catphan® 504 phantom. To end examples of patients with anatomical changes are presented. Results PTV and OAR average mean dose differences for the brain, H&N, lungs and pelvic region were −1.1 ± 0.7%, −0.6 ± 0.4%, −0.9 ± 1.0% and 0.1 ± 0.5% respectively. Passing rate superior than 98% was achieved (gamma index criteria of 2%/1 mm) for absolute 2D CBCT and CT-based dose distributions. CBCT HU to ED curves were within ±30 HU compared with the reference HU of the materials contained inside phantom, amply inside requested CT tolerance values. Conclusion Decision to re-plan VMAT or IMRT treatments seems feasible using CBCT data. Future investigations on the HU to ED curve and curve stability will be done with the help of a CBCT electron density phantom (CIRS).

SU‐C‐500‐01: Quantifying the Impact of Intra‐Prostatic Calcifications and Seed Orientation in Low‐Dose Rate Prostate Brachytherapy Using Monte‐Carlo Dose Calculations

Purpose: To quantify the impact intra‐prostatic calcifications and seed orientation in prostate brachytherapy using Monte‐Carlo (MC) dose calculations. Methods: MC simulations were made with ALGEBRA (based on Geant4 C/C++ toolkit v4.9.6.p01) to simulate dose deposition in 1mm3 voxelized geometries using post‐planning exported DICOMRT files. Materials are assigned from the physicians contours, based on TG186 recommendations, and electron density from CT number. SelectSeed and OncoSeed6711 seeds are modeled and place in the CT geometries. 3D dose maps, isodoses, DVHs and dosimetric indices for each structure are calculated. Orientation study: Orientations are obtained using automated seed detection from five fluoroscopic projections. Five dose algorithms are compared: TG43 1D(1), TG43 2D (parallel to the CT scans (2)/ with orientations (3)) and MC (parallel (4) /with orientations (5)). Calcifications study: Breast calcification definition is used to define the prostate calcification. 6 different scenarios were tested: water‐based MC with (1) and without (2) seeds fully modeled, 2) water‐based prostate MC with densities from the CT and calcification areas with (3) and without seeds (4) and full heterogeneities with (5) and without seeds (6). Results: Orientation study: Calculations on 296 clinical geometries indicate no significant difference between parallel and oriented seeds for prostate and urethra but an average difference of 3% (min‐max = X‐Y%) for rectum and bladder between TG43 and MC. Calcification study: 42 patients (out of 296) with observable calcifications were analyzed. We measure, large differences relative to TG43, up to −27(−28)% for D90 (V100) when all materials are defined and seeds modeled, and up to −25(−22)% when only the calcified areas are considered. Conclusion: Large patient cohorts have been analyzed to extract the (previously neglected) effect of seed orientations and calcification. Significant discrepancies relative to TG‐43 have been outline. Correlation to toxicities and survival will be studied in a forthcoming study. NSERC ‐ Natural Sciences and Engineering Research Council of Canada

WE‐G‐108‐05: A Technique to Detect Dosimetric Errors as Low as 0.5% Arising From MLC Leaf Shifts in Rapid Arc

Purpose: RapidArc is a more complicated technique than fixed field IMRT both in terms of treatment planning and delivery. Due to its speedy delivery, RapidArc (RA) is becoming popular in the clinics. Previous work has shown that the detectors used to perform patient specific IMRT QA cannot detect MLC leaf position error induced dosimetric differences even large as 5.6%. We present a patient specific QA technique with the ability to detect MLC leaf error induced dosimetric errors as small as 0.5%. Methods: Dual arc RA plans were made for treatment of head and neck, gastrointestinal and spine cancers of 10 patients on a Trilogy unit. For each patient, four plans were created: i) clinically acceptable RA plan ii) RA plan with a single MLC leaf shifted for only three consecutive control points iii) delivered original plan recreated using linac log file measurements iv) delivered shifted plan recreated using linac log file measurements. Delivered plans were recreated using only MLC positional errors from linac log file measurements. The results were analyzed by comparing voxel by voxel 3D dose differences. To quantify the actual dose difference errors due to intentional MLC shifts, a comparison was performed between the shifted dose map and the original dose map. Results: Simulated MLC shifts gave rise to a trajectory of dose differences with maximum percentage dose differences mean (range) of 0.52 (0.31 – 0.80)%, while measurements from linac log files show a maximum percentage dose differences mean (range) of 0.56 (0.31 – 0.81)%. Maximum background percentage dose differences mean (range) were 0.19 (0.05 – 0.35)%. Measurement related 3D dose map had an average signal to noise ratio of 3.89 for the set of patients analyzed. Conclusion: Linac log file measurements provide the means to detect MLC shift induced dosimetric deviations as small as 0.5% from the prescription dose. UVA Radiation Oncology George Amarino Grants ‐ 2011

SU-E-T-279: Towards a Personalized Cardiovascular Dosimetry in Radiation Therapy

Purpose: To improve cardiovascular dosimetry in radiation therapy (RT), as recommended in the NCRP report N°170, using hybrid computational phantoms (HCPs). Methods: For the Hodgkin lymphoma, HCPs were proposed as patient models to improve dose reconstructions for a multifactorial study on the coronary diseases following radiation therapy. HCPs were built using RT CT images and heart CT angiograms, merging the detailed heart morphology and the thoracic morphology at the treatment time. DICOM images generated from the HCPs were inserted into the treatment planning system. A personalized dose reconstruction is performed using the ballistics from the medical record. For the left‐side breast cancer, different coronary topologies based on 22 heart CT angiograms were fitted into a representative thorax in terms of breast and heart volumes. Two treatments were simulated: BeamSet01 with tangential, tumoral bed and internal mammary chain (IMC) beams; BeamSet02 without IMC beams.For both studies, coronaries dose‐volume histograms and dose statistics were collected. A 3D dose mapping displayed the coronary and aorta doses. Results: For the Hodgkin study, a dose was assessed for each coronary stenosis location.For the breast cancer study, the interventricular artery (IVA) topology differences induced a standard deviation for the IVA mean dose and the minimal dose received by 2% of the IVA volume of 35% and 76%, respectively, with BeamSet01 and 19% and 50%, respectively, with BeamSet02. CMI beams are responsible for the IVA highest doses. Conclusion: This work shows the potential of HCPs for research on radiation therapy and the necessity to know the precise topology of coronaries in retrospective studies and in clinical practice. For the Hodgkin study, a comparison with doses for patients without stenoses will be performed. For the left‐side breast cancer study, other morphological parameters will be changed and errors in dose reconstructions with non‐personalized anatomy will be assessed.