Verification Of Dose Distributions Research Articles

Dose distribution verification in high-dose-rate brachytherapy using a highly sensitive normoxic N-vinylpyrrolidone polymer gel dosimeter

Rapid technological advances in high-dose-rate brachytherapy have led to a requirement for greater accuracy in treatment planning system calculations and in the verification of dose distributions. In high-dose-rate brachytherapy, it is important to measure the dose distribution in the low-dose region at a position away from the source in addition to the high-dose range in the proximity of the source. The aim of this study was to investigate the accuracy of a treatment plan designed for prostate cancer in the low-dose range using a normoxic N-vinylpyrrolidone-based polymer gel (VIPET gel) dosimeter containing inorganic salt as a sensitizer (iVIPET). The dose response was evaluated on the basis of the transverse relaxation rate (R2) measured by magnetic resonance scanning. In the verification of the treatment plan, gamma analysis showed that the dose distributions obtained from the polymer gel dosimeter were in good agreement with those calculated by the treatment planning system. The gamma passing rate according to the 2%/2 mm criterion was 97.9%. The iVIPET gel dosimeter provided better accuracy for low doses than the normal VIPET gel dosimeter, demonstrating the potential to be a useful tool for quality assurance of the dose distribution delivered by high-dose-rate brachytherapy.

222. Optimization of pretreatment IMRT dose distribution verification: Bidimensional gaussian kernel implementation

Purpose Intensity modulated radiation therapy (IMRT) is a widely used technique delivering a highly conformal dose to the target together with a better sparing of organs at risk with respect to conventional irradiation techniques. Quality assurance (QA) of the generated IMRT treatment plan is of most importance to ensure that the patient receives the planned dose; a two-dimensional array of ionization chambers in a solid phantom can be used for the verification. Different methods exist for comparison of dose distributions and the gamma evaluation is the most commonly used. The aim of this work is to convolve the calculated dose distributions in order to optimize the comparison with the measured data obtained with the MatriXX ionization chamber detector arrays (IBA Dosimetry, Germany). Methods In this study a bidimensional gaussian kernel, which addresses the ionization chamber volume averaging effect, has been implemented to optimize the comparison of dose distributions for pretreatment patient-specific IMRT QA. It was determined by scanning a single chamber with a slit beam. A point spread function was reconstructed with a Gaussian function obtained with a MATLAB script. The kernel has been tested on head&neck and prostate treatment plans. Results It has been found that the convolution of the kernel with the treatment planning system calculated dose distribution can improve the comparison respect to the conventional method of IMRT verification. The gamma index passing rate values found with convolution method are higher in all considered clinical cases. Conclusions Intensity modulated radiation therapy (IMRT) is a widely used technique delivering a highly conformal dose to the target together with a better sparing of organs at risk with respect to conventional irradiation techniques. Quality assurance (QA) of the generated IMRT treatment plan is of most importance to ensure that the patient receives the planned dose; a two-dimensional array of ionization chambers in a solid phantom can be used for the verification. Different methods exist for comparison of dose distributions and the gamma evaluation is the most commonly used. The aim of this work is to convolve the calculated dose distributions in order to optimize the comparison with the measured data obtained with the MatriXX ionization chamber detector arrays (IBA Dosimetry, Germany). In this study a bidimensional gaussian kernel, which addresses the ionization chamber volume averaging effect, has been implemented to optimize the comparison of dose distributions for pretreatment patient-specific IMRT QA. It was determined by scanning a single chamber with a slit beam. A point spread function was reconstructed with a Gaussian function obtained with a MATLAB script. The kernel has been tested on head&neck and prostate treatment plans. It has been found that the convolution of the kernel with the treatment planning system calculated dose distribution can improve the comparison respect to the conventional method of IMRT verification. The gamma index passing rate values found with convolution method are higher in all considered clinical cases.

P217] Comprehensive procedure for IMRT QA testing

Purpose To design a versatile procedure for testing the repeatability and accuracy of geometric setup i.e. leaves motion, gantry and collimator position and isocenter stability, as well as dosimetric properties of beam parameters in conditions representative for intensity modulated radiation therapy (IMRT) techniques, with respect to ease for performance and automated analysis. Methods Tests were design for the cubic water-equivalent phantom with the insertion of MatriXX Evolution dosimetric device, consisted of 1024 air ionization chambers arranged in 2D array for dose distribution acquiring. Measurements were analyzed using OmniPro I’mRT software. All tests were checked for phantom setup reproducibility contribution in the final uncertainty of each result. The capability to detect in these tests the gross errors in gantry, collimator and treatment couch setup was checked. The influence of dosimetric device placement with respect to the beam central axis on reconstructed dose values and localization was also investigated, taking into account the limited resolution related to ionization chambers spacing. Results Stability of isocenter position performed for full-arc rotating gantry and 1×1 cm2 field size together with cone beam CT, analyzed against the convergence of MV to kV point of rotation showed accuracy of 0.2 mm whereas 0.5 mm is declared by the linac manufacturer. The test involving the re-loading of the field size for non-zero gantry position showed 0.5% accuracy of leaves positioning. Dose analysis for strip test was appeared to be sensitive to small couch angles often difficult to perceive, resulting in about 2% dose discrepancy for 2–5 deg. angles mismatch. Conclusions It is possible to verify the parameters of linac operation which are crucial for accurate and precise IMRT realization in a simple tests involving the equipment commonly used for dose distribution verification. The advantage of using 2D array of dosimeters is the possibility to compare two subsequent measurements in terms of gamma index, what enables to maintain the single linac operation stability aa well as twin parameters across the set of linacs. For IMRT QA purpose 3% and 1 mm criteria are sensitive enough to detect the mismatch in geometric or dosimetric parameters of linac of 0.2 mm and 1%, respectively.

Implementation and evaluation of an Epson V800 Scanner and Gafchromic EBT3 Film for use in routine Linac QA and Patient Specific QA checks

In order to achieve more conformal dose distributions to the patient forms of radiotherapy delivery have become increasingly complex. With this comes an increased chance of error in delivery and an increased need to verify dose distribution using patient-specific QA. The aim of this project was to evaluate the use of Gafchromic EBT3 film and an Epson V800 scanner in linac QA and patient-specific QA checks. Response of EBT3 was found to be linear and accurate within 3% for a dose range of 0–400 cGy with use of the green channel. Effects of change in orientation showed max variation between two orientations of 1%. Change in response of 6% was shown between scans taken one hour and 24 h post-irradiation confirming that all scans should be taken at the same interval post-irradiation. A 1 h interval was chosen. Variation of position showed max change in response laterally of 6% confirming that scans should be performed in the centre of the scanner bed. Variation in number of warm-up scans showed max deviation of 0.4%. Intra-sheet homogeneity was within 0.8% and inter-sheet homogeneity was within 4%. Using these findings a protocol was devised and three basic fields and three IMRT and VMAT patient plans were measured. After normalisation of dose differences between measured and calculated distributions, EBT3 had similar or higher gamma-index passing rates than those achieved by Octavius detectors. In conclusion the method derived was shown to be a suitable alternative to the 2D arrays and reduced time required for film dosimetry.

P292] Dosimetric verification and clinical evaluation of PRIMO as an independent Monte-Carlo-based dose verification tool

Purpose Radiotherapy treatment depends on the accuracy of the dose delivery in patients. The purpose of this study is to test an independent Monte-Carlo dose calculation program (PRIMO) to verify dose distributions of 6-MV-FF VMAT plans. Methods A Varian TrueBeam accelerator in 6-MV-FF mode was commissioned in PRIMO (V.0.3.1.1577) using Varian’s phase space files. Field sizes from 3 × 3 cm2 up to 40 × 40 cm2 were simulated in PRIMO and compared to the measured basic beam data. Pass rates greater 99% were achieved using a gamma criterion of 2% and 2 mm. Furthermore six VMAT plans (one glioblastoma, two prostate and three head neck plans) were initially calculated with Eclipse (V.13.1) using the Acuros-XB algorithm for dose calculation (dose to medium). These plans were transferred to the Octavius verification phantom (PTW Freiburg) geometry for all verification purposes. Phantom dose calculations with PRIMO were performed using the dose planning mode (DPM). For dose verification the dose distributions from Eclipse were imported in PRIMO via DICOM-RT and γ3mm,3%, γ2mm,2% and γ1mm,1% were applied. Additionally the results were compared to 2D ionisation chamber array (PTW Octavius 729) measurements and portal dosimetry measurements using the MV imaging unit of the linac (aS1200). The predicted portal dose was calculated with the Portal Dose Image Prediction algorithm (V.13.0.26). Results The comparison of the calculated 2D dose distributions in the ionization chamber plane of the Octavius detector 729 resulted in an average accordance between PRIMO and Eclipse of (99.9 ± 0.1)%, (98.3 ± 1.9)% and (82.5 ± 7.0)%, γ3mm,3%, γ2mm,2% and γ1mm,1% respectively. The 2D array dose measurements showed an average agreement to the Acuros-XB calculations of (98.1 ± 0.6)%, (89.9 ± 2.2)% and (53.7 ± 4.9)% (same gamma criteria as previously). The portal dosimetry analysis showed an average compliance between simulations and measurements of (99.0 ± 1.5)%, (96.4 ± 2.8)% and (67.3 ± 10.0)% (gamma criteria as mentioned above). Conclusions PRIMO is a Monte-Carlo-based dose simulation program which gives the possibility of independent dose verification in clinics. In future cumbersome array measurements could be replaced by simulations. Further investigations on the sensitivity of PRIMO for MLC misalignment and dose errors have to be done.

A Feasibility Study of IMRT of Lung Cancer Using Gafchromic EBT3 Film

Intensity modulated radiation therapy (IMRT) is an advanced method for delivery of three dimensional therapies, which provides optimal dose distribution with giving multiple nonuniform fluency to the patient. The complex dose distribution of IMRT should be checked to ensure that the accurate dose is delivered. Today, film dosimetry is a powerful tool for radiotherapy treatment Quality Assurance (QA) and a good method to verify dose distribution in phantoms. This study aimed to evaluate the accuracy of IMRT treatment planning system, Prowess Panther® software, with Gafchromic EBT3 films in a inhomogeneity phantom. The IMRT plan was generated by Prowess Panther® treatment planning system (TPS) version 5.2 on a inhomogeneity phantom, then it was irradiated by ONCOR linear accelerator (Linac) with 6 (MV) photon beam energy. The Gafchromic EBT3 film located between the phantom has measured the dose distribution. To compare between TPS calculated doses and film measured doses, Gamma criteria 3%/3 mm, 4%/4 mm, 5%/5 mm, 6%/6 mm and 7%/7 mm Dose Difference (DD) and Distance to Agreement (DTA), respectively were used. Gammas passing rates for PTV are obtained 67.5% for 3%/3mm, 78.8% for 4%/4mm, 86.3% for 5%/5mm, 91.2% for 6%/6mm and 94.3% for 7%/7mm and for organs at risk is 72.4% for 3%/3mm, 82.8% for 4%/4mm, 89.8% for 5%/5mm, 93.3% for 6%/6mm and 95.4% for 7%/7mm (respectively DD/DTA). By increasing the range of criteria the capability increased. The results show that the use of EBT3 film in a inhomogeneity phantoms allows us to evaluate the dose differences between the EBT3 measured dose distribution and TPS calculated dose distribution .Hence, a result Prowess Panther® TPS can be used for IMRT technique treatment.

Dosimetric feasibility of an anthropomorphic three-dimensional PRESAGE® dosimeter for verification of single entry hybrid catheter accelerated partial breast brachytherapy

AbstractPurposeTo determine the feasibility of an anthropomorphic breast polyurethane-based three-dimensional (3D) dosimeter with cavity to measure dose distributions and skin dose for a commercial strut-based applicator strut-adjusted volume implant (SAVI™) 6–1.Materials and methodsAn anthropomorphic breast 3D dosimeter was created with a cavity to accommodate the SAVI™ strut-based device. 2 Gy was prescribed to the breast dosimeter having D95 to planning target volume evaluation (PTV_EVAL) while limiting 125% of the prescribed dose to the skin. Independent dose distribution verification was performed with GAFCHROMIC® EBT2 film. The dose distribution from the 3D dosimeter was compared to the distributions from commercial brachytherapy treatment planning system (TPS) and film. Point skin doses, line profiles and dose–volume histogram (DVHs) for the skin and PTV_EVAL were compared.ResultsThe maximum difference in skin dose for TPS and the 3D dosimeter was 4% whereas 41% between the TPS and EBT2 film. The maximum dose difference for line profiles between TPS, 3D dosimeter, and film was 4·1%. DVHs of skin and PTV_EVAL for TPS and 3D dosimeter differed by a maximum of 4% at 5 mm depth and skin differed by a maximum 1·5% between TPS and 3D dosimeter. The criterion for gamma analysis comparison was 92·5% at ±5%±3 mm criterion. The TPS demonstrated at least ±5% comparability in predicting dose to the skin, PTV_EVAL and normal breast tissue.Conclusions3D anthropomorphic polyurethane dosimeter with cavity gives comparable results to the TPS dose predictions and GAFCHROMIC® EBT2 film results in the context of HDR brachytherapy.

Monte Carlo dose verification of VMAT treatment plans using Elekta Agility 160-leaf MLC

In this study, we verified volumetric modulated arc therapy (VMAT) plans in an Elekta Synergy system with an integrated Agility 160-leaf multileaf collimator (MLC) by comparing them with Monte Carlo (MC)-calculated dose distributions using the AAPM TG-119 structure sets. The head configuration of the linear accelerator with the integrated MLC was simulated with the EGSnrc/BEAMnrc code. Firstly, the dosimetric properties of the MLC were evaluated with the MC technique and film measurements. Next, VMAT plans were created with the Pinnacle3 treatment planning system (TPS) for four regions in the AAPM TG-119 structures. They were then verified by comparing them with MC-calculated dose distributions using dose volume histograms (DVHs) and three-dimensional (3D) gamma analysis. The MC simulations for the Agility MLC dosimetric properties were in acceptable agreement with measurements. TPS-VMAT plans using TG-119 structure sets agreed with MC dose distributions within 2% in the comparison of D95 in planning target volumes (PTVs) evaluated from DVHs. In contrast, higher dose regions such as D20, D10, and D5 in PTVs for TPS tended to be smaller than MC values. This tendency was particularly noticeable for mock head and neck with complicated structures. In 3D gamma analysis, the passing rates with 3%/3mm criteria in PTVs were ≥99%, except for mock head and neck (89.5%). All passing rates for organs at risk (OARs) were in acceptable agreement of >96%. It is useful to verify dose distributions of PTVs and OARs in TPS-VMAT plans by using MC dose calculations and 3D gamma analysis.

Evaluation of two-dimensional dose distributions for pre-treatment patient-specific IMRT dosimetry.

BackgroundThe accuracy of dose calculation is crucial for success of the radiotherapy treatment. One of the methods that represent the current standard for patient-specific dosimetry is the evaluation of dose distributions measured with an ionization chamber array inside a homogeneous phantom using gamma method. Nevertheless, this method does not replicate the realistic conditions present when a patient is undergoing therapy. Therefore, to more accurately evaluate the treatment planning system (TPS) capabilities, gamma passing rates were examined for beams of different complexity passing through inhomogeneous phantoms.Materials and methodsThe research was performed using Siemens Oncor Expression linear accelerator, Siemens Somatom Open CT simulator and Elekta Monaco TPS. A 2D detector array was used to evaluate dose distribution accuracy in homogeneous, semi-anthropomorphic and anthropomorphic phantoms. Validation was based on gamma analysis with 3%/3mm and 2%/2mm criteria, respectively.ResultsPassing rates of the complex dose distributions degrade depending on the thickness of non-water equivalent material. They also depend on dose reporting mode used. It is observed that the passing rate decreases with plan complexity. Comparison of the data for all set-ups of semi-anthropomorphic and anthropomorphic phantoms shows that passing rates are higher in the anthropomorphic phantom.ConclusionsPresented results raise a question of possible limits of dose distribution verification in assessment of plan delivery quality. Consequently, good results obtained using standard patient specific dosimetry methodology do not guarantee the accuracy of delivered dose distribution in real clinical cases.

Development and clinical characterization of a novel 2041 liquid-filled ionization chambers array for high-resolution verification of radiotherapy treatments.

The aim of this study was to present a novel 2041 liquid-filled ionization chamber array for high-resolution verification of radiotherapy treatments. The prototype has 2041 ionization chambers of 2.5 × 2.5 mm2 area filled with isooctane. The detection elements are arranged in a central square grid of 43 × 43, totally covering an area of 107.5 × 107.5 mm2 . The central inline and cross-line are extended to 227 mm and the diagonals to 321 mm to be able to perform profile measurements of large fields. We have studied stability, pixel response uniformity, dose rate dependence, depth and field size dependence and anisotropy. We present results for output factors, tongue-and-groove, garden fence, small field profiles, irregular fields, and verification of dose planes of patient treatments. Comparison with other detectors used for small field dosimetry (SFD, CC13, microDiamond) has shown good agreement. Output factors measured with the device for square fields ranging from 10 × 10 to 100 × 100 mm2 showed relative differences within 1%. The response of the detector shows a strong dependence on the angle of incident radiation that needs to be corrected for. On the other hand, inter-pixel relative response variations in the 0.95-1.08 range have been found and corrected for. The application of the device for the verification of dose planes of several treatments has shown gamma passing rates above 97% for tolerances of 2% and 2 mm. The verification of other clinical fields, like small fields and irregular fields used in the commissioning of the TPS, also showed large passing rates. The verification of garden fence and tongue-and-groove fields was affected by volume-averaging effects. The results show that the liquid filled ionization chamber prototype here presented is appropriate for the verification of radiotherapy treatments with high spatial resolution. Recombination effects do not affect very much the verification of relative dose distributions. However, verification of absolute dose distributions may require normalization to a radiation field which is representative of the dose rate of the treatment delivered.

DOSIMETRIC CHARACTERIZATION OF COLLIMATORS FOR SPATIALLY FRACTIONATED PROTON THERAPY OF THE EYE.

The aim of these studies was to investigate depth dose distribution of a 60 MeV proton beam formed by a set of dedicated mesh-formed collimators. The set of mini-beams formed by the mesh is scattered in the tissue and lead to uniform energy deposition in the target volume. The rationale is to protect the eyelid skin from complications after hypofractionated radiation therapy. The experimental verification of the depth dose distribution for collimators of 1-3 mm mesh size was performed in solid state and water phantoms on 60 MeV proton beam from the AIC-144 cyclotron. The measurements were accompanied by Monte Carlo calculations of proton transport using the FLUKA code. The result demonstrate that the smallest mesh diameter of 1 mm leads to fast broadening of the resulting mini-beams due to multiple proton scattering in the collimator. The effect is diminishing with growing collimator diameter.

Real-time volumetric relative dosimetry for magnetic resonance—image-guided radiation therapy (MR-IGRT)

The integration of magnetic resonance imaging (MRI) with linear accelerators (linac) has enabled the use of 3D MR-visible gel dosimeters for real-time verification of volumetric dose distributions. Several iron-based radiochromic 3D gels were created in-house then imaged and irradiated in a pre-clinical 1.5 T–7 MV MR-Linac. MR images were acquired using a range of balanced-fast field echo (b-FFE) sequences during irradiation to assess the contrast and dose response in irradiated regions and to minimize the presence of MR artifacts. Out of four radiochromic 3D gel formulations, the FOX 3D gel was found to provide superior MR contrast in the irradiated regions. The FOX gels responded linearly with respect to real-time dose and the signal remained stable post-irradiation for at least 20 min. The response of the FOX gel also was found to be unaffected by the radiofrequency and gradient fields created by the b-FFE sequence during irradiation. A reusable version of the FOX gel was used for b-FFE sequence optimization to reduce artifacts by increasing the number of averages at the expense of temporal resolution. Regardless of the real-time MR sequence used, the FOX 3D gels responded linearly to dose with minimal magnetic field effects due to the strong 1.5 T field or gradient fields present during imaging. These gels can easily be made in-house using non-reusable and reusable formulations depending on the needs of the clinic, and the results of this study encourage further applications of 3D gels for MR-IGRT applications.

Water equivalent PRESAGE® for synchrotron radiation therapy dosimetry.

Synchrotron Radiation Therapy techniques are currently being trialed and commissioned at synchrotrons around the world. The patient treatment planning systems (TPS) developed for these treatments use simulated data of the synchrotron x-ray beam to produce the dosimetry in the treatment plan. The purpose of this study was to investigate a water equivalent PRESAGE® dosimeter capable of 3D dosimetry over an energy range suitable for synchrotron x-ray beams. Water equivalent PRESAGE® dosimeters were fabricated with a radiological effective atomic number similar to water over an energy range of 10 keV to 10 MeV. The dosimeters were irradiated at various energies, scanned using optical CT (OCT) scanning and compared to ion chamber measurements. Percentage depth dose and beam profiles of the synchrotron beam were compared to Monte Carlo (MC) model simulations. The PDD profiles of the water equivalent PRESAGE® agreed with ion chamber measurements and MC calculations within 2% for all keV energies investigated. The PRESAGE® also showed good agreement to the MC model for depths below 5 mm of the synchrotron beam where ion chamber data do not exist. The spatial resolution of the OCT was not sufficient to accurately measure the penumbra of the synchrotron beams compared to MC calculations or EBT3 film; however, the water equivalent PRESAGE® was able to verify dose profile characteristics of the MC model. The radiological response of a water equivalent PRESAGE® dosimeter has been validated for synchrotron x-ray beam energies along with the ability to independently verify dose distributions of a MC model.

Verification of CyberKnife Dose Distributions with Gafchromic EBT2 Film

Verification of CyberKnife Dose Distributions with Gafchromic EBT2 Film

Verification of Mechanical Leaf Gap Error and VMAT Dose Distribution on Varian VitalBeam™ Linear Accelerator

Verification of Mechanical Leaf Gap Error and VMAT Dose Distribution on Varian VitalBeam™ Linear Accelerator

Dosimetric verification of small fields in the lung using lung-equivalent polymer gel and Monte Carlo simulation.

The main purpose of this study was evaluate a polymer-gel-dosimeter (PGD) for three-dimensional verification of dose distributions in the lung that is called lung-equivalent gel (LEG) and then to compare its result with Monte Carlo (MC) method. In the present study, to achieve a lung density for PGD, gel is beaten until foam is obtained, and then sodium dodecyl sulfate is added as a surfactant to increase the surface tension of the gel. The foam gel was irradiated with 1 cm × 1 cm field size in the 6 MV photon beams of ONCOR SIEMENS LINAC, along the central axis of the gel. The LEG was then scanned on a 1.5 Tesla magnetic resonance imaging scanner after irradiation using a multiple-spin echo sequence. Least-square fitting the pixel values from 32 consecutive images using a single exponential decay function derived the R2 relaxation rates. Moreover, 6 and 18 MV photon beams of ONCOR SIEMENS LINAC are simulated using MCNPX MC Code. The MC model is used to calculate the depth dose water and low-density water resembling the soft tissue and lung, respectively. Percentages of dose reduction in the lung region relative to homogeneous phantom for 6 MV photon beam were 44.6%, 39%, 13%, and 7% for 0.5 cm × 0.5 cm, 1 cm × 1 cm, 2 cm × 2 cm, and 3 cm × 3 cm fields, respectively. For 18 MV photon beam, the results were found to be 82%, 69%, 46%, and 25.8% for the same field sizes, respectively. Preliminary results show good agreement between depth dose measured with the LEG and the depth dose calculated using MCNP code. Our study showed that the dose reduction with small fields in the lung was very high. Thus, inaccurate prediction of absorbed dose inside the lung and also lung/soft-tissue interfaces with small photon beams may lead to critical consequences for treatment outcome.

53. Characterisation of TruView™: A new 3-D reusable radiochromic MethylThymol Blue based gel dosimeter for ionising radiations

Introduction Cutting-edge radiotherapy techniques deliver highly conformal treatments with steep dose gradients around the target volume. Thus, the verification of the complex resultant dose distributions presents a real dosimetric challenge requiring accurate and spatially precise dosimeters. Gel dosimetry is a promising technique as it can inform about the whole 3-D dose distribution in one irradiation and without conventional dosimeters issues [1] . This study focuses on the characterisation of the new TruView™ gel dosimeter (Modus Medical Devices Inc.). This dosimeter is based on Fricke-MTB composition [2] but presents a new MethylThymol Blue (MTB) reactive dye and a reusable structure. The dosimetric properties of this gel dosimeter and the influence of different parameters on its response are presented. Methods The key dosimetric aspects (dose response, dependence with energy and dose rate) and the TruView™ dosimeter variability with different parameters (gel temperature, light exposure, time between irradiation and reading, etc.) were studied thanks to spectrophotometric measurements. Finally, by mean of optical CT scanning (Vista™, Modus Medical Devices Inc.), its performances to measure relative depth dose (PDD) and cross profiles have been analysed. Results TruView™ presents linear response with dose up to 20 Gy independent of both photon beam energy and dose rate on the studied ranges. The study of the numerous parameters influencing its dose response enabled to determine the optimal conditions for using this gel. In those conditions, the comparison of TruView™ relative depth dose (PDD) and of cross profiles with ion chamber measurements shows a good dosimetric and spatial precision with 90% of the points passing the 3 mm 3% gamma test for the PDD and 87% for the cross profiles. Seven days after irradiation, ion diffusion inside the gel leads to return to homogeneity allowing a new irradiation to be performed. The TruView™ response presents an excellent reproducibility as 100% of the points pass the gamma test for the comparison between the PDD of the two uses and 94% for the cross profiles. Conclusions This study provides important knowledge regarding the 3-D gel dosimeter TruView™. This dosimeter presents interesting dosimetric properties but remains delicate to use due to the great response sensitivity to numerous parameters. A further study is currently ongoing to benchmark this promising candidate for clinical dose verification.

51. Comparison of ORL and prostate VMAT treatment plans verifications by ArcCheck® Sun Nuclear™ 3D detector and Varian™ PDIP® module: assessment of the errors detection sensitivity and the capability of each process

Introduction The complexity of intensity modulation radiation requires a planned dose distribution verification prior treatment. Delivery a patient treatment plan quality assurance can be carried out by EPID, 2D/3D detector or by ionization chamber. In our department, VMAT treatment plans are checked by ArcCheck® Sun Nuclear™ 3D detector and Varian™ PDIP® module. In order to assess the feasibility of reducing the number of verification processes, we studied the detection sensitivity of different errors [1] and the capability of each process. Methods Treatment plans are checked by ArcCheck® Sun Nuclear™ 3D detector + SNC patient™ v6.7.1 software (AC) and Varian™ As 1000 EPID + Varian™ PDIP® v13.0 module (PDIP). For 5 ORL and prostate treatment plans, errors of collimator angulation (1–5°), dose rate (2–3%) and multi-leaf collimator (opening leaves banks from 0.5–1–2 mm) are simulated. Treatment plans are assessed by γ index (3%/3 mm, 2%/2 mm) with a respective passage criteria of 95% and 90% of the total pixels in global and local mode. A database of 80 ORL treatment plans and 30 prostate treatment plans is analyzed in order to check the capability of each process according to the different γ index analysis criterias used. Results As regards the detection sensitivity of errors, according to the analysis criterias of the γ index, the AC and the PDIP respectively make it possible to detect on average: collimator errors from 2° and 3° for the ORL plans and 4° and above 5° for prostate plans; Dose rate errors of 2% for ORL plans and 3% for prostate plans; MLC banks opening errors of 0.5 mm for ORL plans and 0.5 mm and 1 mm for prostate plans. With a fixed analysis criteria, the error detection sensitivity of the AC appears higher than the PDIP. Concerning the database analysis, for the AC, the mean γ indexes with 3%/3 mm global/local mode, and 2%/2 mm global mode are respectively 99,7 [96,3–100]; 95.8 [83.0–100]; 96.8 [81.0–100] for ORL treatment plans and 99.8 [98.8–100]; 98.2 [93.9–100]; 97.6 [90.0–99.8] for prostate treatment plans. The respective CPM capability indices are 5.2; 0.1; 0.9 for ORL treatment plans and 9.2; 1.0; 1.5 for prostate treatment plans. For the PDIP, the mean γ indexes with 3%/3 mm global/local mode and 2%/2 mm global mode are respectively 99.9 [98.8–100]; 97.6 [92.1–100]; 99.0 [93.6–100] for ORL treatment plans and 99.8 [99.0–100]; 99.0 [93.6–99.9]; 98.6 [95.8–100] for prostate treatment plans. The respective CPM capability indices are 17.4; 0.6; 1.5 for ORL treatment plans and 9.8; 1.6; 3.1 for prostate treatment plans. Conclusions The detection sensitivity of the errors appears equivalent between the two processes AC and PDIP by selecting a suited γ index analysis criteria. Analysis with 3%/3 mm local mode appears the most efficient in the detection of error but also leads to a risk of false positive. On a case by case basis, none of the processes can systematically detect the simulated errors.

A virtual dosimetry audit – Towards transferability of gamma index analysis between clinical trial QA groups

PurposeQuality assurance (QA) for clinical trials is important. Lack of compliance can affect trial outcome. Clinical trial QA groups have different methods of dose distribution verification and analysis, all with the ultimate aim of ensuring trial compliance. The aim of this study was to gain a better understanding of different processes to inform future dosimetry audit reciprocity. MaterialsSix clinical trial QA groups participated. Intensity modulated treatment plans were generated for three different cases. A range of 17 virtual ‘measurements’ were generated by introducing a variety of simulated perturbations (such as MLC position deviations, dose differences, gantry rotation errors, Gaussian noise) to three different treatment plan cases. Participants were blinded to the ‘measured’ data details. Each group analysed the datasets using their own gamma index (γ) technique and using standardised parameters for passing criteria, lower dose threshold, γ normalisation and global γ. ResultsFor the same virtual ‘measured’ datasets, different results were observed using local techniques. For the standardised γ, differences in the percentage of points passing with γ < 1 were also found, however these differences were less pronounced than for each clinical trial QA group’s analysis. These variations may be due to different software implementations of γ. ConclusionsThis virtual dosimetry audit has been an informative step in understanding differences in the verification of measured dose distributions between different clinical trial QA groups. This work lays the foundations for audit reciprocity between groups, particularly with more clinical trials being open to international recruitment.

Development of a 3D remote dosimetry protocol compatible with MRgIMRT.

To develop a novel remote 3D dosimetry protocol to verify Magnetic Resonance-guided Radiation Therapy (MRgRT) treatments. The protocol was applied to investigate the accuracy of TG-119 IMRT irradiations delivered by the MRIdian® system (ViewRay® , Oakwood Village, OH, USA) allowing for a 48-hour delay between irradiation at a field institution and subsequent readout at a base institution. The 3D dosimetry protocol utilizes a novel formulation of PRESAGE® radiochromic dosimeters developed for high postirradiation stability and compatibility with optical-CT readout. Optical-CT readout was performed with an in-house system utilizing telecentric lenses affording high-resolution scanning. The protocol was developed from preparatory experiments to characterize PRESAGE® response in relevant conditions. First, linearity and sensitivity of PRESAGE® dose-response in the presence of a magnetic field was evaluated in a small volume study (4ml cuvettes) conducted under MRgRT conditions and irradiated with doses 0-15Gy. Temporal and spatial stability of the dose-response were investigated in large volume studies utilizing large field-of-view (FOV) 2kg cylindrical PRESAGE® dosimeters. Dosimeters were imaged at t=1hr and t=48hrs enabling the development of correction terms to model any observed spatial and temporal changes postirradiation. Polynomial correction factors for temporal and spatial changes in PRESAGE® dosimeters (CT and CR respectively) were obtained by numerical fitting to time-point data acquired in six irradiated dosimeters. A remote dosimetry protocol was developed where PRESAGE® change in optical-density (ΔOD) readings at time t=X (the irradiation to return shipment time interval) were corrected back to a convenient standard time t=1hr using the CT and CR corrections. This refined protocol was then applied to TG-119 (American Association of Physicists in Medicine, Task Group 119) plan deliveries on the MRIdian® system to evaluate the accuracy of MRgRT in these conditions. In the small volume study, in the presence of a 0.35T magnetic field, PRESAGE® was observed to respond linearly (R2 =0.9996) to Co-60 irradiation at t=48hrs postirradiation, within the dose ranges of 0 to 15Gy, with a sensitivity of 0.0305(±0.003) ΔODcm-1 Gy-1 . In the large volume studies, at t=1hr postirradiation, consistent linear response was observed, with average sensitivity of 0.0930±0.002 ΔODcm-1 Gy-1 . However, dosimeters gradually darkened with time (OD<5% per day). A small radial dependence to the dosimeter sensitivity was measured (<3% of maximum dose), which is attributed to a spherically symmetric dosimeter artifact arising from exothermic heating legacy in the PRESAGE® polyurethane substrate during curing. When applied to the TG-119 IMRT irradiations, the remote dosimetry protocol (including correction terms) yielded excellent line-profile and 3D gamma agreement for 3%/3mm, 10% threshold (mean passing rate=96.6%±4.0%). A novel 3D remote dosimetry protocol is introduced for validating off-site dosimetrically complex radiotherapy systems, including MRgRT. The protocol involves correcting for temporal and spatially dependent changes in PRESAGE® radiochromic dosimeters readout by optical-CT. Application of the protocol to TG-119 irradiations enabled verification of MRgRT dose distributions with high resolution.