O Calvo

On the quantification of the dosimetric accuracy of collapsed cone convolution superposition (CCCS) algorithm for small lung volumes using IMRT

Specialized techniques that make use of small field dosimetry are common practice in todays clinics. These new techniques represent a big challenge to the treatment planning systems due to the lack of lateral electronic equilibrium. Because of this, the necessity of planning systems to overcome such difficulties and provide an accurate representation of the true value is of significant importance. Pinnacle3 is one such planning system. During the IMRT optimization process, Pinnacle3 treatment planning system allows the user to specify a minimum segment size which results in multiple beams composed of several subsets of different widths. In this study, the accuracy of the engine dose calculation, collapsed cone convolution superposition algorithm (CCCS) used by Pinnacle3, was quantified by Monte Carlo simulations, ionization chamber, and Kodak extended dose range film (EDR2) measurements for 11 SBRT lung patients. Lesions were < 3.0 cm in maximal diameter and <27.0 cm3 in volume. The Monte Carlo EGSnrc\BEAMnrc and EGS4\MCSIM were used in the comparison. The minimum segment size allowable during optimization had a direct impact on the number of monitor units calculated for each beam. Plans with the smallest minimum segment size (0.1 cm2 to 2.0 cm2) had the largest number of MUs. Although PTV coverage remained unaffected, the segment size did have an effect on the dose to the organs at risk. Pinnacle3‐calculated PTV mean doses were in agreement with Monte Carlo‐calculated mean doses to within 5.6% for all plans. On average, the mean dose difference between Monte Carlo and Pinnacle3 for all 88 plans was 1.38%. The largest discrepancy in maximum dose was 5.8%, and was noted for one of the plans using a minimum segment size of 0.1 cm2. For minimum dose to the PTV, a maximum discrepancy between Monte Carlo and Pinnacle3 was noted of 12.5% for a plan using a 6.0 cm2 minimum segment size. Agreement between point dose measurements and Pinnacle3‐calculated doses were on average within 0.7% in both phantoms. The profiles show a good agreement between Pinnacle3, Monte Carlo, and EDR2 film. The gamma index and the isodose lines support the result. PACS number: 87.56.bd

SU‐FF‐T‐386: Validation of the Delta4 Dosimetry Phantom Against Ionometric Measurements

Purpose: To validate a 3D dose calculation methodology used by the 3D Delta4™ phantom. Method and materials: Measurements were performed using a TomoTherapy™ HiArt™ and a Varian 2300C/D™. A pinpoint PTW N31006 chamber with a sensitive volume of 0.016cc was use for point dose measurements. The Delta4™ phantom (ScandiDos AB, Uppsala, Sweden) was modified to accept the ion chamber. Eight (n=8) quality assurance plans were created in both planning systems. Plans covered a range of doses from 20% to 160% (20% steps) of the prescribed dose using Pinnacle3 TPS. Tomotherapy plans covered doses from 20% to 100% (20% steps) and a DQA plan with 200% of the prescribed dose. Treatment plan, DQA plan, DQA dose and structures were exported via DICOM RT to the Delta4™ software. At each delivery, a point dose measurement was recorded. Point dose measurements and calculated doses by the Delta4™ were compared against the calculated dose of the treatment planning systems. Furthermore, point dose measurements were compared against the Delta4™ calculated dose. Results: The preliminary results showed that chamber measurements agree with Pinnacle3 within 0.3% to 2%. The Delta4™ system have an excellent response at higher doses while at lower doses the percent differences are compared to the ones obtained by the pinpoint chamber. Tomotherapy delivery results showed good agreement between the pinpoint chamber and Delta4™ with percent differences ranging from 0.03% to 5% with better response at high doses Conclusion: Delta4™ showed good agreement with the pinpoint chamber in the dose calculation, which shows that the system 3D dose calculation methodology is able to predict the same or better doses compared to the pinpoint chamber measurements.

3D Dose Reconstruction of Pretreatment Verification Plans Using Multiple 2D Planes from the OCTAVIUS/Seven29 Phantom Array

The purpose of this study is to evaluate 3D dose reconstruction of pretreatment verification plans using multiple 2D planes acquired from the OCTAVIUS phantom and the Seven29 detector array. Eight VMAT patient treatment plans of different sites were delivered onto the OCTAVIUS phantom. The plans span a variety of tumor site locations from low to high plan complexity. A patient specific quality assurance (QA) plan was created and delivered for each of the 8 patients using the OCTAVIUS phantom in which the Seven29 detector array was placed. Each plan was delivered four times by rotating the phantom in 45° increments along its longitudinal axis. The treatment plans were delivered using a Novalis Tx with the HD120 MLC. Each of the four corresponding planar doses was exported as a text file for further analysis. An in-house MATLAB code was used to process the planar dose information. A cylindrical geometry-based, linear interpolation method was utilized to generate the measured 3D dose reconstruction. The TPS calculated volumetric dose was exported and compared against the measured reconstructed volumetric dose. Dose difference, dose area histograms (DAH), isodose lines, profiles, 2D and 3D gamma were used for evaluation. The interpolation method shows good agreement (<2%) between the planned dose distributions in the high dose region but shows discrepancies in the low dose region. Horizontal profiles, dose area histograms and isodose lines show good agreement for the sagittal and coronal planes but demonstrate slight discrepancies in the transverse plane. The 3D gamma index average was 92.4% for all patients when a 5%/5 mm gamma passing rate criteria was employed but dropped to <80.1% on average when parameters were reduced to 2%/2 mm. A simple cylindrical geometry-based, linear interpolation method is able to predict good agreement in the high dose region between the reconstructed volumetric dose and the planned volumetric dose. It is important to mention that the interpolation algorithm introduces dose discrepancies in small regions within the high dose gradients due to the interpolation itself. However, the work presented serves as a good starting point to establish a benchmark for the level of manipulation necessary to obtain 3D dose delivery quality assurance using current technology.

SU‐FF‐T‐358: Evaluation of the Delta4 ™ 3D Cylindrical Phantom for Delivery Quality Assurance of Stereotactic Body Radiotherapy (SBRT) Treatments Using Helical Tomotherapy

Purpose: To evaluate the use of the ScandiDos® Delta4 ™ cylindrical 3D phantom for delivery quality assurance (DQA) of stereotactic body radiotherapy(SBRT) treatments using helical tomotherapy. Method and Materials: Five (n=5) patients with single or multiple lesions in either the lung or liver were treated with SBRT techniques using the Hi‐Art II™ unit in our Institution. SBRT plans delivered dose per fraction doses in the range of 10.0–20.0Gy. For DQA plan creation, the Delta4 ™ was scanned using the MVCT of tomotherapy and imported into the tomotherapy database as a phantom. A “fine” dose grid resolution was used for the dose calculation. Patient plan, structures, and dose were exported via DICOM RT protocol and uploaded into the ScandiDos® software platform. Profiles and gamma index analysis were used for plan validation. Results: Good agreement between calculated and measured doses was observed using the Delta4 ™. Results were consistent among the ten treatment plans evaluated over a period of 6 months. No diode calibration drift was noticed. For all the cases, good agreement in the high and low dose regions were recorded. All of the DQA measurements passed with at least 95% of the diode measurements within the gamma analysis criteria of 3% or 3mm. Additionally, a 20–30 minute time saving per DQA was noted using the Delta4 ™ when compared to film and ion chamber techniques. Time reduction primarily occurs during post‐delivery processing since analysis is performed instantly after measurement. Conclusion: Our results indicate that the ScandiDos® Delta4 ™ cylindrical phantom is an effective and efficient method for the delivery quality assurance of SBRT treatments using helical tomotherapy. The capability of measuring large doses per fraction eliminates the necessity of scaling the DQA procedure thus minimizing any possible scaling errors such as those associated with MLC latency.

SU‐DD‐A1‐03: On the Quantification of the Dosimetric Accuracy of Collapsed Cone Convolution Superposition Algorithm for Small Lung Volumes Using IMRT

Purpose: To quantify the accuracy of a collapsed cone convolution superposition (CCCS) algorithm against Monte Carlo(MC) simulations for small lung lesions subject to electronic disequilibrium when very small segments are used during the IMRT optimization process. Method and Materials: IMRT plans for eleven (n=11) lung patients were created using Pinnacle3 7.6 planning system (TPS). Lung lesions measuring <3 cm in max. diameter and <27 cm3 were previously treated in our institution with SBRT techniques. The optimized intensity maps of each plan were then used to calculate the dose distributions using the CCCS algorithm. For each patient, seven optimized plans were created with varying MLC minimum segment sizes—0.25cm2 to 6cm2. The linear accelerator was modeled using MC code EGSnrc\BEAMnrc and verified against commissioned measured data. Intensity maps for each plan and the patient CT dataset from the TPS were exported to our MCsoftware. All patients were planned using a 5‐field IMRT plan (Millennium 120‐leaf MLC and Varian 2100C 6MV beam). Dose distributions were calculated and normalized so that the isocenter receives 45.0Gy. Isodose distributions, DVH, and ROI statistics were used for comparison between the two calculation methods. Results: Comparison of the DVHs from Pinnacle3 and MC show similar target coverage between CCCS algorithm and MC. Differences in the minimum, maximum and mean dose of the PTV were less than ±5% while doses to critical structures agreed within ±6% for all cases investigated. Discrepancies in the dose to very small structures have been observed and due to volume effects between the two systems. Conclusion: Good agreement exists in the dose distributions predicted by the CCCS algorithm and MC method. The CCCS algorithm can accurately predict doses to small lungtumors.

Clinical experience with a novel reference chamber “stealth chamber” by IBA

The purpose of this research is to clinically evaluate the performance of a novel reference chamber (Stealth Chamber by IBA). Experimental data were acquired in water with IBA three-dimensional (3D) blue phantom2. Percent depth dose (PDD) comparisons for fields ranging from 1 × 1 to 25 × 25 cm2 were performed for photon energies of 6 and 15 MV. Profile comparison for fields ranging from 1 × 1 to 25 × 25 cm2 were executed for the depths of dmax, 5, 10 and 20 cm. PDD curves and dose profiles obtained from measurements with the Stealth Chamber and the CC13 as the reference chamber were compared to each other for field sizes ranging from 1 × 1 to 25 × 25 cm2 for 6 and 15 MV. Experimental results from this investigation indicate the benefits associated with chamber positioning and time expended during the acquisition of the relative measurements of PDDs and profiles for the beam commissioning of photon beams when the Stealth Chamber is used as a reference chamber to perform these tasks.

SU-E-T-242: Monte Carlo Simulations Used to Test the Perturbation of a Reference Ion Chamber Prototype Used for Small Fields

PURPOSE To study the perturbation due to the use of a novel Reference Ion Chamber designed to measure small field dosimetry (KermaX Plus C by IBA). METHODS Using the Phase-space files for TrueBeam photon beams available by Varian in IAEA-compliant format for 6 and 15 MV. Monte Carlo simulations were performed using BEAMnrc and DOSXYZnrc to investigate the perturbation introduced by a reference chamber into the PDDs and profiles measured in water tank. Field sizes ranging from 1×1, 2×2,3×3, 5×5 cm2 were simulated for both energies with and without a 0.5 mm foil of Aluminum which is equivalent to the attenuation equivalent of the reference chamber specifications in a water phantom of 30×30×30 cm3 and a pixel resolution of 2 mm. The PDDs, profiles, and gamma analysis of the simulations were performed as well as a energy spectrum analysis of the phase-space files generated during the simulation. RESULTS Examination of the energy spectrum analysis performed shown a very small increment of the energy spectrum at the build-up region but no difference is appreciated after dmax. The PDD, profiles and gamma analysis had shown a very good agreement among the simulations with and without the Al foil, with a gamma analysis with a criterion of 2% and 2mm resulting in 99.9% of the points passing this criterion. CONCLUSION This work indicates the potential benefits of using the KermaX Plus C as reference chamber in the measurement of PDD and Profiles for small fields since the perturbation due to in the presence of the chamber the perturbation is minimal and the chamber can be considered transparent to the photon beam.

SU‐E‐T‐213: Preliminary Testing of a 2‐D Fluence Measurement Prototype Device (Delta4‐AT) for In‐Vivo Patient Verification Dosimetry

Purpose: To perform preliminary sensitivity testing on the performance of a prototype device (Delta4‐AT) that measures the 2D linear acceleratorphoton fluence during IMRT treatments. The measured fluence is then projected onto the Delta4 phantom to assess the dosimetric impact. Methods: Two Smart‐Arc patient treatment plans were delivered on the Delta4 for pretreatment dose (PT) verification with the At‐treatment (AT) Delta4 prototype. The measurement process of Delta4‐AT is the same as the Delta4‐PT. Delta4‐AT acquires the fluence data and reconstructs a semi‐measured dose in the Delta4 geometry based on daily changes such as MLC position and output. To assess the sensitivity, MLC patterns for the two patients were modified to introduce two sets of known errors: 1) Systematic error—all leaves moved relative to isocenter, and 2) Random error— selected leaves shifted. Data was collected and an inter‐comparison of the TPS calculated dose, AT semi‐measured, and PT semi‐measured dose was performed. All data was analyzed using the Delta4 software. Results: Measurements showed a good gamma index (GI) agreement of 90.0% (3%/3mm) between the Delta4‐AT and Delta4‐PT when small discrepancies were introduced. When larger errors were present, a larger discrepancy existed between both measurement devices partly due to the decreased resolution of the prototype Delta4‐AT. Relative to the TPS dose, both showed good agreement within the GI criteria. Conclusions: The Delta4‐AT prototype together with the Delta4‐PT is a promising solution for the quantification of the dosimetric impact of daily variations in linear accelerators during IMRT delivery. The Delta4‐PT can serve as a vital, addon tool for patient dose verification.

WE‐E‐BRB‐10: Can 2D and 3D Detector Arrays Identify Multileaf Collimator (MLC) Positional Errors during Smart‐Arc Pre‐Treatment Verification Plan Delivery?

Purpose: Evaluate the sensitivity of 2D and 3D dose arrays in detecting multileaf collimator(MLC) positional errors during Smart‐Arc pre‐treatment verification delivery. Materials and Methods: Four Smart‐Arc patient treatment plans of different tumor sites were delivered on three different phantoms: 1) Delta4, 2) Seven29 OCTAVIUS and 3) MatriXX. MLC files for each plan were exported to an in‐house MATLAB program and two set of errors were introduced: 1) Systematic error: MLC bank shifted 1.0, 3.0 and 5.0mm and 2) Random error: Selected MLCs (ten leaf pairs) shifted 1.0, 3.0 and 5.0mm. The new sets of MLCs were used to create six additional plans. A total of seven plans were delivered for each of the patients. The original delivered plans were compared to the TPS plans and statistics acquired. After which, each of the error MLC plans were compared against the original TPS plan and statistics acquired. Finally, each of the error plans were compared against each other and statistics acquired. Results: When the original plan was compared against the error plans, the 1.0mm and 3mm systematic error plans were not detected by any of the arrays: gamma analysis >95% (3%/3mm), 3.0mm gamma analysis >90% (3%,3mm) but bigger errors than 3mm were detected: 5mm 97% (3%/3mm), but significant differences were observed between errors plans when random error plans were compared showing gamma analysis 3.0mm) systematic type of error introduced in the MLC.

SU‐GG‐T‐250: 3D Dose Reconstruction for Delivery Quality Assurance (DQA) from Multiple 2D Planes Using the OCTAVIOUS Phantom

Purpose: To perform a 3D dose reconstruction for delivery quality assurance (DQA) from multiple 2D planes using the OCTAVIOUS phantom. Method and Materials: Ten RapidArc patient treatment plans of different sites were delivered on two phantoms. Two different DQA plans were delivered for each of the 10 patients: 1) OCTAVIOUS phantom and 2) 30×30×30 cube solid water phantom in which the detector array (Seven29) was placed. The corresponding 3D dose of each of the DQA plans was exported. All DQA plans were delivered by means of a NovalisTX with the HD120 MLC. For the solid water phantom, the same plan was delivered six times with the array varying in the coronal plane in increments of 0.5cm. For the OCTAVIOUS, the same plan was delivered four times by rotating the phantom in 45° increments along its longitudinal axis. An in‐house MATLAB code was used to read the planar dose information. A linear reconstruction was performed for the cube phantom while a circular reconstruction was used for the OCTAVIOUS. Dose statistics per plane were obtained to validate the reconstruction method. Results: Both interpolation methods showed good agreement to the planned dose distribution in the high dose region (<1.0%) but showed discrepancies in the low dose region. A DAH comparison shows good agreement for the sagittal and coronal planes but demonstrates some discrepancies in the transversal plane. Conclusion: A simple linear interpolation method is able to predict good matching in the high dose region between the reconstructed dose and the planned dose. This technique is a good starting point to establish a benchmark in the level of manipulation necessary to obtain good 3D dose delivery quality assurance using current technology. Conflict of Interest: Research Sponsored by PTW Company