IBA Dosimetry Research Articles

P19. Intra-institutional end-to-end accuracy testing of modern technologies

Introduction To perform end-to-end testing of conformal radiotherapy (3D-CRT), and various Intensity modulated radiotherapy (IMRT) techniques for Addington, Greys and Inkosi Albert Luthuli central hospital (IALCH) using a Varian Eclipse treatment planning system (TPS) and Elekta Mosaic TPS with Varian and Elekta linear accelerator delivery systems. Materials and methods A MatrixX with MULTICube was CT scanned, transferred to TPS, and treatment plans were created for each treatment technique. A virtual water phantom was also created in the TPS and the same plans were generated. The plans were transferred, verified and delivered on the Linacs. The planned dose was then compared with the measured dose in both phantoms. Results The maximum dose (D2) difference was found to be −2.27 ± 0.60%, −1.35 ± 0.51%, −3.68 ± 0.31% and −4.67 ± 0.56% for 3D-CRT, step-and-shoot IMRT (ssIMRT), dynamic IMRT (dIMRT) and intensity modulated arc therapy (IMAT) respectively with the MatrixX. The dose difference in water measured at an isocenter with a Farmer ionization chamber (FC65-G, IBA Dosimetry Group, Germany) was found to be −0.19 ± 0.41%, 0.02 ± 0.47%, 0.08 ± 0.34% and −0.27 ± 0.44% for 3D-CRT, ssIMRT, dIMRT and IMAT respectively. The MatrixX detectors sampling time and 7.6 mm resolution contributed to high dose differences in dIMRT and IMAT techniques. Conclusion End-to-end testing at our institutions was carried out successfully for a range of techniques. The measured and planned doses for both MatrixX and water measurements were comparable and acceptable.

Assessment of portal dosimetry accuracy as a QA tool for VMAT clinical treatment plans using dolphin/compass tools

Introduction and purpose Thanks to intensity modulated radiotherapy and dynamic arctherapy techniques, the degree of complexity of modern radiotherapy treatments has reached an important level, especially when attempting to better spare organs at risk, while trying to escalate the dose to target volumes. In order to achieve this, quality has to be ensured through the whole RT quality assurance chain. At CHU de Charleroi, patients with head and neck and pelvic cancers are treated on a Novalis powered by Truebeam STx (VARIAN) linac using VMAT technique, and thus patient specific QA is performed for each single clinical treatment plan. Materials and methods VMAT patient QA is known to be very time and energy consuming; our current routine methodology is based on a 2D coronal plane comparison containing measured and calculated absorbed fluences in the detector plane in a phantom. Portal dosimetry of each clinical treatment arc is also performed and looks attractive thanks to its rapid measurement and potential efficiency but is not a dose measurement. As measurement with flat panel for portal dosimetry shows a totally different geometry, in terms of measurement conditions, both measurements cannot be correlated. Therefore, for localizations requiring high intensity modulation such as head and neck and some challenging pelvic cases, a special methodology has been developed in order to be able to compare such fluences. This is made feasible thanks to a new dosimetry equipment acquiring radiation at the exit of the collimator, called DOLPHIN using COMPASS software in the framework of a scientific collaboration with IBA Dosimetry. Basically, the aim of this detector system is to reconstruct the dose into the patient CT, injecting delivered fluence instead of the calculated one from the TPS model software. Results Until now, more than 30 clinical cases have been investigated but the feasibility is demonstrated. So far, good agreement between predictions and measurements is obtained. Actually, for 14 clinical arcs evaluated (for all localizations), in average 97.4% of the pixels passed, and 0.5% of the points are between 1 and 1.05 for a gamma criterion of 3%–3 mm. Results for more than 30 patients will be presented sorting out dosimetry information from different localizations. Conclusion For all cases – including head and neck and pelvic localizations- analyzed and showed in this study, measurements performed by linac flat panel and considered detector showed excellent agreement.

Dosimetric characterization of a commercial 2-D scintillation detector for quality assurance tests in scanned proton and carbon ion beams

Introduction Pencil beam scanning technique used at CNAO requires beam characteristics to be carefully and periodically checked to guarantee patient safety. Radiochromic films are used within daily QA checks. Purpose This study aimed at characterizing the Lynx PT system (IBA Dosimetry) and assessing its suitability for QA tests, for proton and carbon ion beams. Materials and methods The Lynx PT is a 2D high-resolution dosimetry system consisting of a scintillating screen coupled with a CCD camera, in a compact light-tight box. A dedicated software is used for image acquisition, raw data correction and fast data analysis. Lynx was characterized in terms of short-term stability, image geometrical distortion, uniformity, signal proportionality, response dependence on IRIS and on particle intensity, particle energy and type. Irradiations were performed at the Centro Nazionale di Adroterapia Oncologica (CNAO) for both protons and carbon ions. Measurements with Lynx were compared to those made with EBT3 reference films. The detector response dependence on radiation LET was also investigated. Results Lynx basic characterization results have shown a proportional behavior with delivered number of particles. Strong dependence of the acquired signal on beam energy and particle type was found. No significant differences in the investigated beam parameters measured with Lynx and EBT3 film were observed. The detector also showed good performances in term of image uniformity and geometrical distortion. Conclusion The Lynx PT system appears as a promising tool for measurements for both single pencil beam and scanned beam, enabling fast QA and beam optics optimization checks.

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 patient's 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.

SU‐F‐T‐21: A Novel Beam‐Light View, Collimating Applicator for HDR Ocular Conjunctiva Brachytherapy‐Treatments

Purpose:We propose the use of a HDR X‐ray source collimator to apply a conformal, relatively small, radiation suitable for a single fraction with short delivery time. In addition, this technique can be applied using a radioactive source.Methods:We have built a stainless steel 1.5 mm thick applicator, to accommodate the needle applicator of the Intra‐Beam X‐ray source. Additional cavity is created in the applicator to allow the hosting/nesting/positioning of a LED diode. This LED is allowing a pre‐irradiation beam marking on the tissue. The visible light emitted from the opening of the collimated applicator will delineate/verify the aperture of the kV beam to be applied, as well as serve as distance indicator and will assist in the determination of dose to be delivered. For the evaluation of the collimated spatial dose distribution we have performed water tank measurements using (IBA Dosimetry) with a 0.4 cc ion chamber (IBA Dosimetry). We have scanned a two dimensional array with 1mm pitch in depth and 0.3 mm step size laterally. Additional verifications were conducted using Gaf‐Chromic film for PDD measurements and Optical Stimulated Luminescence Dosimetry (OSLD, Landauer inc.) for absolute dosimetry.Results:The collimated applicator enables a conformal irradiated cross‐section of about 3 mm square at the applicator surface was used in this study. A 180 seconds of 50 kVp delivery yielded 29 Gy, 20.6 Gy and 14.5 Gy at 5, 10 and 15 mm depths respectively. These results are in good agreement with the needle applicator depth dose curve published data.Conclusion:We have demonstrated the feasibility of focal HDR brachytherapy for conjunctival and ocular tumors, using the Intra‐Beam needle applicator with in‐house developed collimator. The delivery time was found to be several minutes‐ suitable for an intra‐operative procedure and will allow dose fractionation deliveries.

SU-G-TeP2-07: Dosimetric Characterization of a New HDR Multi-Channel Esophageal Applicator for Brachytherapy

Purpose: To characterize the dose distribution of a new multi-channel esophageal applicator for brachytherapy HDR treatment, and particularly the effect of the presence of air or water in the applicator's expansion balloon. Methods: A new multi-channel (6) inflatable applicator for esophageal HDR has been developed in house and tested in a simple water phantom. CT image sets were obtained under several balloon expansions (80ml of air, 50 cc of water), and channel loadings and used with the Oncentra (Elekta) planning system based on TG43 formalism. 400 cGy was prescribed to a plane 1cm away from the applicator. Planar dose distributions were measured for that plane and one next to the applicator using Gafchromic EBT3 film and scanned by a Vidar VXR-12 film digitizer. Film and TPS generated dose distributions of film were sent to OmniPro I'mRT (iba DOSIMETRY) for analysis. 2D dose profiles in both X and Y directions were compared and gamma analysis performed. Results: Film dose measurement of the air-inflated applicator is lower than the TPS calculated dose by as much as 60%. Only 80.8% of the pixels passed the gamma criteria (3%/3mm). For the water-inflated applicator, the measured film dose is fairly close to the TPS calculated dose (typically within <3%). 99.84% of the pixels passed the gamma criteria (3%/3mm). Conclusion: TG43 based calculations worked well when water was used in the expansion balloon. However, when air is present in that balloon, the neglect of heterogeneity corrections in the TG43 calculation results in large differences between calculated and measured doses. This could result in severe underdosing when used in a patient. This study illustrates the need for a TPS with an advanced algorithm which can account for heterogeneity. Supported by Innovations Department, Cleveland Clinic.

SU‐F‐T‐464: Development of a Secondary Check Procedure to Evaluated Flatness and Symmetry Discrepancies Detected During Daily Morning QA

Purpose:A daily QA device is used to monitor output, flatness and symmetry constancy for all linac photon and electron energies. If large deviations from baseline in flatness or symmetry are reported it becomes necessary to crosscheck the measurements with a second device. Setting up another device such as Matrixx (IBA Dosimetry) can be time consuming, due to its warm‐up time, and trained personnel may not be readily available to analyze the results. Furthermore, this discrepancy is frequently isolated to a single energy. Unaffected energies could still be used, avoiding further patient delays, if a method to gather data for offline analysis could be developed. We find that optically stimulated luminescent dosimeters (OSLDs) provide a quick, simple, and inexpensive solution to this important clinical problem.Methods:The exact geometry of the detectors on the daily tracker (Keithley Therapy Beam Evaluator) was reproduced by placing nanoDot OSLDs (Landauer) on a solid water phantom. A combination of bolus and solid water was placed on top to provide buildup and prevent air gaps. Standard daily measurements of output, flatness and symmetry were taken for 2 photon energies (6x,10x) and 5 electron energies (6e,9e,12e,15e,18e) using the tracker. These measurements were then repeated with the OSLD phantom.Results:The time it took to set up the OSLD phantom was comparable to that of the tracker. The inline and crossline OSLD phantom measurements of flatness and symmetry agreed with the tracker results to within 2%.Conclusion:OSLDs provide a good solution for a quick second check when questionable flatness and symmetry results are detected with the tracker during daily QA.

SU-F-T-344: Commissioning Constant Dose Rate VMAT in the Raystation Treatment Planning System for a Varian Clinac IX

Purpose: To demonstrate the commissioning of constant dose rate volumetric modulated arc therapy (VMAT) in the Raystation treatment planning system for a Varian Clinac iX with Exact couch. Methods: Constant dose rate (CDR) VMAT is an option in the Raystation treatment planning system, enabling VMAT delivery on Varian linacs without a RapidArc upgrade. Raystation 4.7 was used to commission CDR-VMAT for a Varian Clinac iX. Raystation arc model parameters were selected to match machine deliverability characteristics. A Varian Exact couch model was added to Raystation 4.7 and commissioned for use in VMAT optimization. CDR-VMAT commissioning checks were performed on the linac, including patient-specific QA measurements for 10 test patients using both the ArcCHECK from Sun Nuclear Corporation and COMPASS from IBA Dosimetry. Multi-criteria optimization (MCO) in Raystation was used for CDR-VMAT planning. Results: Raystation 4.7 generated clinically acceptable and deliverable CDR-VMAT plans for the Varian Clinac. VMAT plans were optimized including a model of the Exact couch with both rails in the out positions. CDR-VMAT plans generated with MCO in Raystation were dosimetrically comparable to Raystation MCO-generated IMRT plans. Patient-specific QA measurements with the ArcCHECK on the couch showed good agreement with the treatment planning system prediction. Patient-specific, structure-specific, multi-statistical parameter 3D QA measurements with gantry-mounted COMPASS also showed good agreement. Conclusion: Constant dose rate VMAT was successfully modeled in Raystation 4.7 for a Varian Clinac iX, and Raystation's multicriteria optimization generated constant dose rate VMAT plans which were deliverable and dosimetrically comparable to IMRT plans.

Quality assurance of carbon ion and proton beams: A feasibility study for using the 2D MatriXX detector

PurposeThe quality assurance (QA) procedures in particle therapy centers with active beam scanning make extensive use of films, which do not provide immediate results. The purpose of this work is to verify whether the 2D MatriXX detector by IBA Dosimetry has enough sensitivity to replace films in some of the measurements. MethodsMatriXX is a commercial detector composed of 32×32 parallel plate ionization chambers designed for pre-treatment dose verification in conventional radiation therapy. The detector and GAFCHROMIC® films were exposed simultaneously to a 131.44MeV proton and a 221.45MeV/u carbon-ion therapeutic beam at the CNAO therapy center of Pavia – Italy, and the results were analyzed and compared. ResultsThe sensitivity MatriXX on the beam position, beam width and field flatness was investigated. For the first two quantities, a method for correcting systematic uncertainties, dependent on the beam size, was developed allowing to achieve a position resolution equal to 230μm for carbon ions and less than 100μm for protons. The beam size and the field flatness measured using MatriXX were compared with the same quantities measured with the irradiated film, showing a good agreement. ConclusionsThe results indicate that a 2D detector such as MatriXX can be used to measure several parameters of a scanned ion beam quickly and precisely and suggest that the QA would benefit from a new protocol where the MatriXX detector is added to the existing systems.

Pencil beam proton radiography using a multilayer ionization chamber

A pencil beam proton radiography (PR) method, using a commercial multilayer ionization chamber (MLIC) integrated with a treatment planning system (TPS) was developed.A Giraffe (IBA Dosimetry) MLIC (±0.5 mm accuracy) was used to obtain pencil beam PR by delivering spots uniformly positioned at a 5.0 mm distance in a 9 × 9 square of spots. PRs of an electron-density (with tissue-equivalent inserts) phantom and a head phantom were acquired. The integral depth dose (IDD) curves of the delivered spots were computed by the TPS in a volume of water simulating the MLIC, and virtually added to the CT at the exit side of the phantoms. For each spot, measured and calculated IDD were overlapped in order to compute a map of range errors. On the head-phantom, the maximum dose from PR acquisition was estimated. Additionally, on the head phantom the impact on the range errors map was estimated in case of a 1 mm position misalignment.In the electron-density phantom, range errors were within 1 mm in the soft-tissue rods, but greater in the dense-rod. In the head-phantom the range errors were −0.9 ± 2.7 mm on the whole map and within 1 mm in the brain area. On both phantoms greater errors were observed at inhomogeneity interfaces, due to sensitivity to small misalignment, and inaccurate TPS dose computation. The effect of the 1 mm misalignment was clearly visible on the range error map and produced an increased spread of range errors (−1.0 ± 3.8 mm on the whole map). The dose to the patient for such PR acquisitions would be acceptable as the maximum dose to the head phantom was <2cGyE.By the described 2D method, allowing to discriminate misalignments, range verification can be performed in selected areas to implement an in vivo quality assurance program.

Practical Assessment of Bronchoscopically Inserted Fiducial Markers for Image Guidance in Stereotactic Lung Radiotherapy

IntroductionStereotactic radiotherapy is a high-dose precision technique necessitating accurate target visualization through either cone beam computed tomography (CBCT) or planar imaging with implanted fiducial markers. We have investigated the properties for image guidance using fiducial markers implanted through minimally invasive bronchoscopy. MethodsTwo fiducial marker types were implanted endobronchially in 10 patients undergoing radical radiation treatment for non–small cell lung cancer (eight using Visicoil linear fiducial markers [IBA Dosimetry GmbH, Schwarzenbruck Germany] and two using superDimension and superLock two-band markers [Covidien Inc., Minneapolis, MN]). Patients underwent four-dimensional computed tomography imaging for treatment planning and after completion of treatment to investigate marker movement. As part of the image guidance assessment, megavolt electronic portal images (EPIs) were acquired in addition to kilovolt planar and CBCT (Varian Medical Systems, Palo Alto, CA) images. ResultsIn two of 10 patients (both receiving Visicoil markers), marker migration was observed before treatment. In patients with stable markers, both types were clearly visible in planar kilovolt imaging; however, in EPIs the markers could be detected only in selected beam directions in which bony interference was minimal. Diagnostic computed tomography scanning was able to demonstrate the markers with clarity, but significant starring artifacts were observed in CBCT. This was particularly problematic in patients with some lateral component of tumor motion during breathing. ConclusionsThe potential for fiducial migration must be considered and investigated if bronchoscopic implantation of fiducial markers is performed. The choice of marker is a compromise between trying to minimize CBCT artifacts while enabling visualization in EPI imaging, which is an ideal tool to verify gated radiotherapy delivery.

Development of an iterative reconstruction method to overcome 2D detector low resolution limitations in MLC leaf position error detection for 3D dose verification in IMRT

The objective of this study was to introduce a new iterative method to reconstruct multi leaf collimator (MLC) positions based on low resolution ionization detector array measurements and to evaluate its error detection performance. The iterative reconstruction method consists of a fluence model, a detector model and an optimizer. Expected detector response was calculated using a radiotherapy treatment plan in combination with the fluence model and detector model. MLC leaf positions were reconstructed by minimizing differences between expected and measured detector response. The iterative reconstruction method was evaluated for an Elekta SLi with 10.0 mm MLC leafs in combination with the COMPASS system and the MatriXX Evolution (IBA Dosimetry) detector with a spacing of 7.62 mm. The detector was positioned in such a way that each leaf pair of the MLC was aligned with one row of ionization chambers. Known leaf displacements were introduced in various field geometries ranging from −10.0 mm to 10.0 mm. Error detection performance was tested for MLC leaf position dependency relative to the detector position, gantry angle dependency, monitor unit dependency, and for ten clinical intensity modulated radiotherapy (IMRT) treatment beams. For one clinical head and neck IMRT treatment beam, influence of the iterative reconstruction method on existing 3D dose reconstruction artifacts was evaluated. The described iterative reconstruction method was capable of individual MLC leaf position reconstruction with millimeter accuracy, independent of the relative detector position within the range of clinically applied MU’s for IMRT. Dose reconstruction artifacts in a clinical IMRT treatment beam were considerably reduced as compared to the current dose verification procedure. The iterative reconstruction method allows high accuracy 3D dose verification by including actual MLC leaf positions reconstructed from low resolution 2D measurements.

Patient-specific online dose verification based on transmission detector measurements

Background and purposeSince IMRT-techniques lead to an increasingly complicated environment, a patient specific IMRT-plan verification is recommended. Furthermore, verifications during patient irradiation and 3D dose reconstruction have the potential to improve treatment delivery, accuracy and safety. This study provides a detailed investigation of the new transmission detector (DTD) Dolphin (IBA Dosimetry, Germany) for online dosimetry. Materials and methodsThe clinical performance of the DTD was tested by dosimetric plan verification in 2D and 3D for 18 IMRT-sequences. In 2D, DTD measurements were compared to a pre-treatment verification method and a treatment planning system by gamma index and dose difference evaluations. In 3D, dose–volume-histogram (DVH) indices and gamma analysis were evaluated. Furthermore, the error detection ability was tested with leaf position uncertainties and deviations in the linear accelerator (LINAC) output. ResultsThe DTD measurements were in excellent agreement to reference measurements in both 2D (γ3%,3mm=(99.7±0.6)% <1, ΔD±5%=(99.5±0.5)%) and 3D. Only a small dose underestimation (<2%) within the target volume was observed when analyzing DVH-indices. Positional errors of the leaf banks larger than 1mm and errors in LINAC output larger than 2% were identified with the DTD. ConclusionsThe DTD measures the delivered dose with sufficient accuracy and is therefore suitable for clinical routine.

Polarity correction factor for flattening filter free photon beams in several cylindrical ionization chambers.

In this study, we aimed to compare the polarity correction factor in ionization chambers for flattening filter free (FFF) photon beams and flattening filter (FF) beams. Measurements were performed with both 6 and 10 MV FFF and FF beams. Five commercial ionization chambers were evaluated: PTW TN30013; IBA Dosimetry CC01, CC04, and CC13; and Exradin A12S. Except for the CC01 ionization chamber, the other four chambers showed less than a 0.3% difference in the polarity effect between the FFF and the FF beams. The CC01 chamber showed a strong field-size-dependence, unlike the other chambers. The polarity effect for all chambers with FFF beams did not change with the dose rate. Except in the case of the CC01 chamber, the difference in the polarity effect between FFF and FF beams was not significant.

A.198 - Dosimetric characterization of a commercial 2-D scintillation detector for quality assurance tests in scanned proton and carbon ion beams

Introduction: Pencil beam scanning technique used at CNAO requires beam characteristics to be carefully assessed and periodically checked to guarantee patient safety. Radiochromic films are widely used within daily QA checks for pencil beam measurements and determination of dose homogeneity in scanned fields. This study aimed at characterizing the Lynx PT system (IBA Dosimetry) and assessing its suitability for QA tests as compared to EBT3 films, for proton and carbon ion beams.

Dose distributions verification for High dose rate brachytherapy plans by using ionization chambers 2D array

The purpose of this paper is to perform dosimetric verification (in-phantom) of dose distributions calculated with treatment planning system (TPS) by using 2D chamber array device in HDR brachytherapy. HDR brachytherapy treatment plans’dose distribution verification is performed using the two-dimensional (2D) ionization chamber array MatriXX Evolution developed by IBA Dosimetry (IBA Dosimetry, Germany) whose detector area is covered with the Nucletron Freiburg Flap Applicator Set (Nucletron BV,Veenendaal, the Netherlands) with catheters such that the detector plane was set to 0.86 cm from the catheter plane. Fixed slabs of RW3 (Perspex) were added below the 2D-ARRAY to provide full scattering conditions. The phantom was scanned on computed tomography (CT) for treatment planning with 2.5-mm slice thickness. Based on the CT data of the phantom, three different plans were calculated by the planning system (Oncentrabrachy version 4.3, Nucletron BV) and then are exported to the VERISOFT software for comparison with measured data. For comparison of dose distributions, both dose planes – measured &amp; calculated – were normalized  to the global maximum dose of the reference matrix (measured data set) and compared using the Gamma index method. Gamma indexes were evaluated using a dose-difference criterion of 3% and a distance criterion of 3 mm (γ≤1).The total number of evaluated dose points for the vault test case is 9755, 98.6 % of them (9623 point) passed the criteria of acceptability (3% delta dose and 3-mm distance criteria) and 1.4% of them (132 point) failed it. The total number of evaluated dose points for the full test case is 19964, 93.6 % of them (18683 point) passed  the criteria of acceptability and 6.4% of them (1281 point) failed it. And the total number of evaluated dose points for the cylinder test case is 19871, 96.9 % of them (19258 point) passed the criteria of acceptability and 3.1% of them (613 point) failed it. By thisThe use of the two-dimensional (2D) ionization chamber array MatriXX Evolution for brachytherapy applications has been successfully demonstrated. The array measurements in these planes have shown acceptable agreement with the TPS, generally within 3% delta dose and 3-mm distance agreement criteria within each plane. The comparisons made led to make a relation between the passing percentage values to the number of the evaluated dose points for each test case and it was found that as the number of these pixels increases the possibility of having more failing points increases.

Dose accuracy research on air cavity interface in treatment planning system

Objective To study the accuracy of collapsed cone convolution (CCC) and anisotropic analytical algorithm (AAA) in dosimetric calculation on the air cavity interface. Methods A BEAMnrc/EGSnrc Monte Carlo (MC) simulation was performed on a Varian Trilogy linear accelerator. The IBA Dosimetry blue 3D scanning system was used to verify the accuracy and reliability of the MC simulation. Central axis depth dose distribution and lateral dose profile in a water-equivalent phantom with variously sized air cavities were calculated by CCC and AAA. The obtained depth dose distribution and lateral dose profile were compared with those by MC simulation and EBT2 film, respectively. Results Both CCC and AAA overestimated the dose on the air cavity interface. In spite of some errors, CCC had a higher accuracy than AAA. The errors were mainly related to computational grid, field size, photon energy, cavity size, and the number of fields. Conclusion Electronic disequilibrium on the air cavity interface should be taken into account when CCC and AAA are used for dosimetric calculation in treatment planning system. Key words: Electron disequilibrium; Collapsed cone convolution; Anisotropic analytical algorithm; Monte Carlo algorithm; EBT2 film

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.

Dosimetric comparison of water phantoms, ion chambers, and data acquisition modes for LINAC characterization

PurposeIn this study a dosimetric comparison utilizing continuous data acquisition and discrete data acquisition is examined using IBA Blue Phantom (IBA Dosimetry, Schwarzenbruck, Germany) and PTW (PTW, Freiberg, Germany) MP3-M water tanks. The tanks were compared according to several factors including set up time, ease of use, and data acquisition times. A tertiary objective is to study the response of several ionization chambers in the two tanks examined. MethodsMeasurements made using a Varian 23EX LINAC (Varian Medical Systems, Palo Alto, CA) include PDDs and beam profiles for various field sizes with IBA CC13, PTW Semiflex 31010, PTW Pinpoint N31016, and PTW 31013 ion chambers for photons (6, 18 MV) and electrons (6, 9, 12, 15, and 18 MeV). Radial and transverse profile scans were done at depths of maximum dose, 5 cm, 10 cm, and 20 cm using the same set of tanks and detectors for the photon beams. Radial and transverse profile scans were done at depth of maximum dose for the electron beams on the same tanks and chambers. Data processing and analysis was performed using PTW's MEPHYSTO Navigator software and IBA's OmniPro Accept version 6.6 for the respective water tank systems. ResultsPDD values agree to within 1% and dmax to within 1 mm for the PTW MP3-M tank using PTW 31010 and Blue Phantom using IBA CC13 chamber, respectively and larger discrepancy with the PTW PinPoint N31016 chamber at 6 MV. With respect to setup time the PTW MP3-M and IBA Blue phantom tank took about 20 and 40 min, respectively. Scan times were longer by 5–15 min per field size in the PTW MP3-M tank for the square field sizes from 1 cm to 40 cm as compared to the IBA Blue phantom. However, data processing times were higher by 7 min per field size with the IBA system. ConclusionsTank measurements showed little deviation with the higher energy photons as compared to the lower energy photons with regards to the PDD measurements. Chamber construction as well as tank set up may be causing the slight deviation in data. It is important to identify the exact source of the potential errors to ensure that proper tank usage is performed when making such measurements to ensure that patient safety is in compliance. Beam profiles done with different chambers and tanks showed little to no deviation from one to another. With regards to continuous versus discrete data measurements the main difference was in the data processing technique used. Discrete data obtained required less data processing as compared to the continuous data acquired.

Effect of the collimator angle on dosimetric verification of volumetric modulated arc therapy

The collimator is usually rotated when planning volumetric modulated arc therapy (VMAT) due to the leakage of radiation between the multi-leaf collimator (MLC) leaves. We studied the effect of the collimator angle on the results of dosimetric verification of VMAT plans for head and neck patients. We studied VMAT plans for 10 head and neck patients. We made two sets of VMAT plans for each patient. Each set was composed of 10 plans with collimator angles of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 degrees. Plans in the first set were optimized individually, and plans in the second set shared the 30-degree collimator-angle optimization. The two sets of plans were verified by using the 2-dimensional ion chamber array MatriXX (IBA Dosimetry, Germany). The comparisons between the calculation and the measurements were made by using a γ-index analysis. The γ-index (2%/2 mm) and (3%/3 mm) passing rates had negative correlations with the collimator angle. The maximum difference between γ-index (3%/3 mm) passing rates of different collimator angles for each patient ranged from 1.46% to 5.60% with an average of 3.67%. There were significant differences (maximum 5.6%) in the passing rates for different collimator angles. The results suggested that the accuracy of the delivered dose depended on the collimator angle. These findings are informative when choosing a collimator angle for VMAT plans.