Gamma Index Evaluation Research Articles

Patient-specific deep learning for 3D protoacoustic image reconstruction and dose verification in proton therapy.

Protoacoustic (PA) imaging has the potential to provide real-time 3D dose verification of proton therapy. However, PA images are susceptible to severe distortion due to limited angle acquisition. Our previous studies showed the potential of using deep learning to enhance PA images. As the model was trained using a limited number of patients' data, its efficacy was limited when applied to individualpatients. In this study, we developed a patient-specific deep learning method for protoacoustic imaging to improve the reconstruction quality of protoacoustic imaging and the accuracy of dose verification for individualpatients. Our method consists of two stages: in the first stage, a group model is trained from a diverse training set containing all patients, where a novel deep learning network is employed to directly reconstruct the initial pressure maps from the radiofrequency (RF) signals; in the second stage, we apply transfer learning on the pre-trained group model using patient-specific dataset derived from a novel data augmentation method to tune it into a patient-specific model. Raw PA signals were simulated based on computed tomography (CT) images and the pressure map derived from the planned dose. The reconstructed PA images were evaluated against the ground truth by using the root mean squared errors (RMSE), structural similarity index measure (SSIM) and gamma index on 10 specific prostate cancer patients. The significance level was evaluated by t-test with the p-value threshold of 0.05 compared with the results from the groupmodel. The patient-specific model achieved an average RMSE of 0.014 ( ), and an average SSIM of 0.981 ( ), out-performing the group model. Qualitative results also demonstrated that our patient-specific approach acquired better imaging quality with more details reconstructed when comparing with the group model. Dose verification achieved an average RMSE of 0.011 ( ), and an average SSIM of 0.995 ( ). Gamma index evaluation demonstrated a high agreement (97.4% [ ] and 97.9% [ ] for 1%/3 and 1%/5mm) between the predicted and the ground truth dose maps. Our approach approximately took 6 s to reconstruct PA images for each patient, demonstrating its feasibility for online 3D dose verification for prostate protontherapy. Our method demonstrated the feasibility of achieving 3D high-precision PA-based dose verification using patient-specific deep-learning approaches, which can potentially be used to guide the treatment to mitigate the impact of range uncertainty and improve the precision. Further studies are needed to validate the clinical impact of thetechnique.

Dosimetry with the TruView gel on a 0.35 T MR-Linac: A feasibility study

The purpose of this work is to assess the feasibility of performing dosimetry with the TruView (ModusQA, London, Ontario, Canada) MTB Fricke gel by MRI reading on a low field MR-Linac (0.35 T). Radiochromic TruView gels were manufactured at IRSN. A 4 echoes time (TE) single-shot fast spin echo sequence (HASTE) was used for 2D images acquisitions. Raw MRI images were denoised (MP-PCA) and transversal relaxation rates (R2) maps were extracted using a homemade Matlab routine. Temperature influences on R2 as well as gel homogeneity were investigated. A dosimetric evaluation of the TruView gel was conducted implying four gels made from the same batch, with three of them dedicated to calibration, and the fourth one received a simple dosimetric plan. Results showed the TruView gel to be sensitive to temperature drift. A protocol was thus developed to ensure thermal stability with the use of a dedicated temperature-controlled water bath. An inhomogeneity that manifests as a R2 increase near the air/gel interface was found to cause a 4% amplitude deviation in R2 along unirradiated gels. Air sealing drastically reduced the inhomogeneity. Dose versus R2 calibration was established. TruView gel sensitivity was found to be low (0.0093 Gy−1 s−1). 1D and 2D comparison to planned dose distribution displayed overall agreement with global gamma index (5%/3 mm) evaluation with an 87 % passing rate. Main discrepancies in the low and gradient dose regions were attributable to the calibration range and diffusion effects. This study demonstrated the feasibility of using the TruView gel for dosimetric evaluation on a low field MR-Linac. The protocol developed needs to be extended to a full 3D evaluation to enable patient QA and may require modifications to the gel formulation to increase its overall sensitivity and ensure limited dependence on temperature drift.

Hybrid-supervised deep learning for domain transfer 3D protoacoustic image reconstruction

Objective. Protoacoustic imaging showed great promise in providing real-time 3D dose verification of proton therapy. However, the limited acquisition angle in protoacoustic imaging induces severe artifacts, which impairs its accuracy for dose verification. In this study, we developed a hybrid-supervised deep learning method for protoacoustic imaging to address the limited view issue. Approach. We proposed a Recon-Enhance two-stage deep learning method. In the Recon-stage, a transformer-based network was developed to reconstruct initial pressure maps from raw acoustic signals. The network is trained in a hybrid-supervised approach, where it is first trained using supervision by the iteratively reconstructed pressure map and then fine-tuned using transfer learning and self-supervision based on the data fidelity constraint. In the enhance-stage, a 3D U-net is applied to further enhance the image quality with supervision from the ground truth pressure map. The final protoacoustic images are then converted to dose for proton verification. Main results. The results evaluated on a dataset of 126 prostate cancer patients achieved an average root mean squared errors (RMSE) of 0.0292, and an average structural similarity index measure (SSIM) of 0.9618, out-performing related start-of-the-art methods. Qualitative results also demonstrated that our approach addressed the limit-view issue with more details reconstructed. Dose verification achieved an average RMSE of 0.018, and an average SSIM of 0.9891. Gamma index evaluation demonstrated a high agreement (94.7% and 95.7% for 1%/3 mm and 1%/5 mm) between the predicted and the ground truth dose maps. Notably, the processing time was reduced to 6 s, demonstrating its feasibility for online 3D dose verification for prostate proton therapy. Significance. Our study achieved start-of-the-art performance in the challenging task of direct reconstruction from radiofrequency signals, demonstrating the great promise of PA imaging as a highly efficient and accurate tool for in vivo 3D proton dose verification to minimize the range uncertainties of proton therapy to improve its precision and outcomes.

Helical tomotherapy and two types of volumetric modulated arc therapy: dosimetric and clinical comparison for several cancer sites

Radiotherapy accelerators have undergone continuous technological developments. We investigated the differences between Radixact™ and VMAT treatment plans. Sixty patients were included in this study. Dosimetric comparison between the Radixact™ and VMAT plans was performed for six cancer sites: whole-brain, head and neck, lymphoma, lung, prostate, and rectum. The VMAT plans were generated with two Elekta linear accelerators (Synergy®and Versa HD™). The planning target volume (PTV) coverage, organs-at-risk dose constraints, and four dosimetric indexes were considered. The deliverability of the plans was assessed using quality assurance (gamma index evaluation) measurements; clinical judgment was included in the assessment. The mean AAPM TG218 (3%-2mm, global normalization) gamma index valueswere 99.4%, 97.8%, and 96.6% forRadixact™,Versa HD™, andSynergy®, respectively. Radixact™ performed better than Versa HD™ in terms of dosimetric indexes, hippocampi D100%, spinal cord Dmax, rectum V38.4 Gy, bladder V30 Gy,and V40 Gy. Versa HD™saved more of the (lungs-PTV) V5 Gyand (lungs-PTV) Dmean,heartDmean, breasts V4 Gy, and bowel V45 Gy. RegardingSynergy®, the head and neckRadixact™plan saved more of the parotid gland, oral cavity, and supraglottic larynx. From a clinical point of view, for the head and neck, prostate, and rectal sites, theRadixact™ and Versa HD™ plans were similar;Radixact™ plans were preferable for the head and neck and rectum toSynergy®plans. The quality of linac plans has improved, and differences with tomotherapy have decreased. However, tomotherapy continues to be an essential add-on in multi-machine departments.

PO-1099 To plan and deliver adjuvant breast radiotherapy over 1 week: 1-week breast workflow implementation

Purpose or Objective To limit hospital visits and transportation of the patients with early breast cancer (BC) especially during the covid-19 pandemic, we aimed to implement a workflow of treatment to prepare and deliver adjuvant breast radiotherapy (RT) within 1 week. Materials and Methods Based on the Fast Forward RT schedule of 5 fractions of 5.2 Gy over 5 days for adjuvant BC, we designed a workflow to perform BC RT over 1 week from the first RT consultation to the end of treatment (figure 1). Patients were seen in consultation on Monday morning. The planning CT was carried out immediately after the consultation. A deep inspiration breath-hold technique was systematically offered to all patients. Automatic delineation using an artificial Intelligence (AI) based model with ART-PlanTM software was first corrected by a junior radiation oncologist (RO) then, independently by a senior RO. As 3D conformal RT failed to achieve dose constraints of the fast forward trial in many cases (due to breast hotspots), we developed a restricted IMRT (rIMRT) technique based on 2 tangential beams (internal and external) using inverse planning restricted optimization parameters to improve the dose homogeneity. Treatment planning was semi-automated by scripting the creation and optimization steps of rIMRT plans. After validation of the reliability of rIMRT treatment delivery by patient-specific quality assurance (PSQA) with gamma index evaluation on 20 patients, we decided to abandon systematic PSQAs. In-vivo dosimetry was validated by end-to-end controls and performed for each patient during the first fraction on Monday afternoon. Daily repositioning was controlled by portal images of the largest segment of each beam. Structured baseline, end of treatment and follow up evaluation forms were used to prospectively collect toxicities and oncological outcomes at each consultation. Patient agreement for data collection and analysis was prospectively obtained. $Φg Results From February 2021 to March 2021, the 1-week breast procedure was proposed to up to 3 patients/week, >65 years, in complete resection after conservative surgery, with pT1-T3 N0 BC, without tumor bed boost or regional lymph nodes irradiation. Six out of seven patients (85.7%) accepted the 1-week breast procedure. All procedures were successfully conducted over 5 days with complete patient and RT team satisfaction. A followup of acute toxicities by online consultation on day 10, structured evaluation forms and management decision trees has been implemented for these patients. No grade >2 acute toxicities (CTACE V4.0) have been reported so far. Conclusion The successful implementation of this comprehensive 1-week breast workflow with AI based delineation and semi-automated rIMRT without PSQA demonstrates the practical feasibility of the whole procedure over 5 days reducing drastically the coming-and-going to the hospital and the overall RT management time per patient. Our work opens the way for further development of comprehensive compact workflows in various settings.

Accuracy assessment of the CNAO dose delivery system in the initial period of clinical activity and impact of later improvements on delivered dose distributions.

A retrospective analysis of the dose delivery system (DDS) performances of the initial clinical operation at CNAO (Centro Nazionale di Adroterapia Oncologica) is reported, and compared with the dose delivery accuracy following the implementation of a position feedback control. Log files and raw data of the DDS were analyzed for every field of patients treated with protons and carbon ions between January 2012 and April 2013 (~3800 fields). To investigate the DDS accuracy, the spot positions and the number of particles per spot measured by the DDS and prescribed by the treatment planning system were compared for each field. The impact of deviations on dose distributions was studied by comparing, through the gamma-index method, 2 three-dimensional (3D) physical dose maps (one for prescribed, one for measured data), generated by a validated dose computation software. The maximum gamma and the percentage of points with gamma≤1 (passing volume) were studied as a function of the treatment day, and correlated with the deviations from the prescription in the measured number of particles and spot positions. Finally, delivered dose distributions of same treatment plans were compared before and after the implementation of a feedback algorithm for the correction of small position deviations, to study the effect on the delivery quality. A double comparison of prescribed and measured 3D maps, before and after feedback implementation, is reported and studied for a representative treatment delivered in 2012, redelivered on a polymethyl methacrylate (PMMA) block in 2018. Systematic deviations of spot positions, mainly due to beam lateral offsets, were always found within 1.5mm, with the exception of the initial clinical period. The number of particles was very stable, as possible deviations are exclusively related to the quantization error in the conversion from monitor counts to particles. For the chosen representative patient treatment, the gamma-index evaluation of prescribed and measured dose maps, before and after feedback implementation, showed a higher variability of maximum gamma for the 2012 irradiation, with respect to the reirradiation of 2018. However, the 2012 passing volume is >99.8% for the sum of all fields, which is comparable to the value of 2018, with the exception of one day with 98.2% passing volume, probably related to an instability of the accelerating system. A detailed retrospective analysis of the DDS performances in the initial period of CNAO clinical activity is reported. The spot position deviations are referable to beam lateral offset fluctuations, while almost no deviation was found in the number of particles. The impact of deviations on dose distributions showed that the position feedback implementation and the increased beam control capability acquired after the first years of clinical experience led to an evident improvement in the DDS stability, evaluated in terms of gamma-index as a measure of the impact on dose distributions. However, the clinical effect of the maximum gamma variability found in the 2012 representative irradiation is mitigated by averaging along the number of fractions, and the high percentage of passing volumes confirmed the accuracy of the delivery even before the feedback implementation.

Evaluation of MLC errors of LINAC based on log file

This study aimed to investigate the Multi Leaf Collimator (MLC) quality assurance (QA) and fluence monitor unit (MU) reconstruction based on log files of Varian Linear Accelerator. The graphical user interface (GUI) was developed using MATLAB to analyze the Log files. The MLC QA was analyzed using the error root mean square (RMS) of leaf parameters. Meanwhile, the fluence MU were reconstructed based on the information of leaf position, jaw position, and fractional MU for each 50 ms. The gamma index evaluation between actual and expected fluence MU was calculated to analyze the segment potential error caused by the MLC position. The average of mean error RMS leaves for Bank A and Bank B are 0.213 ± 0.098 mm and 0.207 ± 0.073 mm, whereas the average of maximum error RMS leaves for Bank A and Bank B are 0.491 ± 0.056 mm and 0.48 ± 0.061 mm, respectively. The mean gamma index results of 2%, 2mm and 3%, 3mm criteria between actual and expected fluence MU for the composite beam for all case are 98.2 ± 1.0% and 99.4 ± 0.3%, respectively. The error of MLC based on the log file analysis is within the tolerance level. The results showed that parameters irradiation such as MLC position, jaw position and fractional MU proper with planning.

Dosimetric variations for high-risk prostate cancer by VMAT plans due to patient’s weight changes

AbstractPurpose:The aim of this study is to investigate the impact of anatomical changes in prostate cancer patients on the target coverage when using 6 MV beams-VMAT therapy and to propose strategies that allow us to evaluate the dose or correct it by normalization without having to perform a new simulation.Methods and materials:Ten patients of high-risk prostate cancer were chosen for the study. All test plans were delivered using the same isocenter and monitor units as the original plan and compared against the original unedited plan. The expansion and contraction of body contours due to size changes was mimicked by increasing and decreasing the body contour with depths of −2, −1·5, …, 1·5, 2 cm, in the anterior, and both lateral directions of the patient. A total of 90 plans were evaluated, 9 for each patient. Dose-volume histogram statistics were extracted from each plan and normalized to prescription dose.Results:Weight changes resulted in considerable dose modifications to the target and critical structures. Plans were found to be varied with 2·9% ± 0·3% per cm SSD change for VMAT treatment with a correlation index close to one. Therefore, doses variations were linear to the changes of depth. Gamma index evaluation was performed for the 10 renormalized plans. All of them passed criteria of 3%/3 mm in at least 98.2% of points. Eight of them passed criteria in 99% points. Gamma index 4%/4 mm passed 100% points in all patients for the chosen region of interest.Conclusions:The dosimetry estimation presented in this study shows important data for the radiation oncology staff to justify whether a CT rescan is necessary or not when a patient experiences weight changes during treatment. Based on the results of our study, discrepancies between real dose and planned dose were >5% for 1·7 cm of difference in external contour in the anterior and both lateral directions of the patient.

The effect of breast phantom, and geometry on dose distribution in breast brachytherapy using the strut-adjusted volume implant and contura applicators

Context: Accelerated partial breast irradiation using brachytherapy is a postlumpectomy treatment to reduce cancer recurrence and a choice for people in the early-stage breast cancer. Although accurate dosimetry is necessary to obtain successful clinical outcomes, the usual commercial treatment planning systems use a simple water phantom to simulate the patient. Hence, the precise attenuated radiation and also the scattering effects occurred in real situations may be dosimetrically ignored. Aims: The purpose of this study is to use Monte Carlo simulation to obtain the effect of phantom material and geometry corrections on dose distribution of the strut-adjusted volume implant (SAVI) and Contura high-dose-rate brachytherapy applicators of breast cancer. Settings and Design: Contura with four lumens surrounding the central one and also SAVI with eight peripheral source channels are separately simulated into the breast phantoms. 192Ir high dose rate sources are located on dwell positions in each applicator. Subjects and Methods: The applicators were simulated inside three different phantom geometry and materials. The dose distribution and dose-volume histograms for each phantom were obtained for typical treatment. Gamma index evaluation is performed to examine the dose distribution according to the water phantom for each trial. Results: According to the results for SAVI and Contura applicators, breast material correction shows about 1% deviations from the calculations for water in most points of the breast. Maximum differences are not >3% that are found near the skin. Conclusion: Deviations from the water phantom in both SAVI and Contura treatments show good conformity especially in Contura and it seems that no serious dosimetric correction is necessary for simple water phantom. Although the results for SAVI were not observed with great deviations from water, areas with high-gradient dose need to be precisely considered.

Characterization and clinical evaluation of a novel 2D detector array for conventional and flattening filter free (FFF) IMRT pre-treatment verification

Background and purposeThe novel MatriXXFFF (IBA Dosimetry, Germany) detector is a new 2D ionization chamber detector array designed for patient specific IMRT-plan verification including flattening-filter-free (FFF) beams. This study provides a detailed analysis of the characterization and clinical evaluation of the new detector array. Material and methodsThe verification of the MatriXXFFF was subdivided into (i) physical dosimetric tests including dose linearity, dose rate dependency and output factor measurements and (ii) patient specific IMRT pre-treatment plan verifications. The MatriXXFFF measurements were compared to the calculated dose distribution of a commissioned treatment planning system by gamma index and dose difference evaluations for 18 IMRT-sequences. All IMRT-sequences were measured with original gantry angles and with collapsing all beams to 0° gantry angle to exclude the influence of the detector's angle dependency. ResultsThe MatriXXFFF was found to be linear and dose rate independent for all investigated modalities (deviations ≤0.6%). Furthermore, the output measurements of the MatriXXFFF were in very good agreement to reference measurements (deviations ≤1.8%). For the clinical evaluation an average pixel passing rate for γ(3%,3mm) of (98.5±1.5)% was achieved when applying a gantry angle correction. Also, with collapsing all beams to 0° gantry angle an excellent agreement to the calculated dose distribution was observed (γ(3%,3mm)=(99.1±1.1)%). ConclusionsThe MatriXXFFF fulfills all physical requirements in terms of dosimetric accuracy. Furthermore, the evaluation of the IMRT-plan measurements showed that the detector particularly together with the gantry angle correction is a reliable device for IMRT-plan verification including FFF.

Homogeneous and inhomogeneous material effect in gamma index evaluation of IMRT technique based on fan beam and Cone Beam CT patient images

Patient-specific Quality Assurance (QA) technique in lung case Intensity-Modulated Radiation Therapy (IMRT) is traditionally limited to homogeneous material, although the fact that the planning is carried out with inhomogeneous material present. Moreover, the chest area has many of inhomogeneous material, such as lung, soft tissue, and bone, which inhomogeneous material requires special attention to avoid inaccuracies in dose calculation in the Treatment Planning System (TPS). Recent preliminary studies shown that the role of Cone Beam CT (CBCT) can be used not only to position the patient at the time prior to irradiation but also to serve as planning modality. Our study presented the influence of a homogeneous and inhomogeneous materials using Fan Beam CT and Cone Beam CT modalities in IMRT technique on the Gamma Index (GI) value. We used a variation of the segment and Calculation Grid Resolution (CGR). The results showed the deviation of averaged GI value to be between CGR 0.2 cm and 0.4 cm with homogeneous material ranging from −0.44% to 1.46%. For inhomogeneous material, the value was range from −1.74% to 0.98%. In performing patient-specific IMRT QA techniques for lung cancer, homogeneous material can be implemented in evaluating the gamma index.

26. Evaluation of the EpiDream method for quality assurance (QA) of electron beams using a Varian electronic portal imaging device (EPID)

Introduction The aim of the study is to evaluate the use of EPID for electron beams QA defined by the 07/27/2007 French decision with a new approach in absorbed dose to water reconstruction: the EpiDream method. Methods The study was performed on two Varian LINACs, equipped with EPID (either with the AS500 or AS1000) for electron beams of 6, 9, 12, 15 and 18 MeV. Each EPID was calibrated with EpiDream application in order to obtain from the Grey Level (GL) image a dose matrix computed in water at the reference depth(zref) of the energy studied. For each energy, GL integrated images were corrected of dark field and acquired using AM Maintenance system(Varian) at the detector source distance (DSD) = 100 cm and DSD = (100 + zref) cm for different parameters : field sizes(6 × 6–20 × 20 cm2, delivered MU (3-300 MU), dose rate(100–300 MU/min). For each irradiation condition, a gamma index evaluation was performed between dose matrix computed by Eclipse TPS algorithm (eMC v13.6, Varian) and dose matrix computed by EpiDream method (DM EpiDream). For a 15 × 15 cm2 field size, a comparison was performed between radial and transverse dose profiles measured with a 0.125 cm3 ionization chamber(PTW31010) in water at zref for SSD = 100 cm and dose profiles obtained from DM EpiDream, considering: homogeneity and symmetry parameters, penumbra and correlation coefficient. Results Data analysis is in progress. However, first results are encouraging: At 9 MeV and for a DSD = 100 + zref, the mean gamma index value for 6 × 6, 10 × 10, and 15 × 15 cm2 field sizes is respectively: 99.9% (σ = 0.1%), 99.8% (σ = 0.32%), 96.3% (σ = 2.2%). Conclusions The EpidDream method would permit to perform electrons beams QA using EPID for all gantry angles by maximizing measurement reproducibility. The aim of the study is to evaluate the use of EPID for electron beams QA defined by the 07/27/2007 French decision with a new approach in absorbed dose to water reconstruction: the EpiDream method. The study was performed on two Varian LINACs, equipped with EPID (either with the AS500 or AS1000) for electron beams of 6, 9, 12, 15 and 18 MeV. Each EPID was calibrated with EpiDream application in order to obtain from the Grey Level (GL) image a dose matrix computed in water at the reference depth(zref) of the energy studied. For each energy, GL integrated images were corrected of dark field and acquired using AM Maintenance system(Varian) at the detector source distance (DSD) = 100 cm and DSD = (100 + zref) cm for different parameters : field sizes(6 × 6–20 × 20 cm2, delivered MU (3-300 MU), dose rate(100–300 MU/min). For each irradiation condition, a gamma index evaluation was performed between dose matrix computed by Eclipse TPS algorithm (eMC v13.6, Varian) and dose matrix computed by EpiDream method (DM EpiDream). For a 15 × 15 cm2 field size, a comparison was performed between radial and transverse dose profiles measured with a 0.125 cm3 ionization chamber(PTW31010) in water at zref for SSD = 100 cm and dose profiles obtained from DM EpiDream, considering: homogeneity and symmetry parameters, penumbra and correlation coefficient. Data analysis is in progress. However, first results are encouraging: At 9 MeV and for a DSD = 100 + zref, the mean gamma index value for 6 × 6, 10 × 10, and 15 × 15 cm2 field sizes is respectively: 99.9% (σ = 0.1%), 99.8% (σ = 0.32%), 96.3% (σ = 2.2%). The EpidDream method would permit to perform electrons beams QA using EPID for all gantry angles by maximizing measurement reproducibility.

27. Intra cranial hypo-fractionated radiotherapy end-to-end QA

Introduction Intra cranial hypo-fractionated radiotherapy concerns small targets which need a complex delivery to optimize the dose distribution. Current techniques combine multiple arcs and intensity modulated beams. Such very precise irradiations need an efficient dosimetric validation. Many detectors enable the comparison between the calculation and the irradiation. They are variably efficient to get a real proof of a good match. We investigated dosimetric validations from 2D detector to 3D anthropomorphic configurations. Material and methods A patient head Dicom CT image files were sent to Rt-Safe company. They 3D-printed the head with a plastic material and filled the brain cavity with an MRI dosimetric gel. The head phantom was placed in the same patient position using the original immobilization devices. The patient plan was imported in the head phantom CT-data to be calculated and then to deliver the irradiation. An MRI acquisition of the head phantom was obtained with a specific configuration. The MRI Dicom file was used to provide a 3D measured dose distribution. The same plan was used with the portal dosimetry PDIP/Varian and the Delta4/Scandidos phantom. The three dosimetric systems provided profiles comparisons and gamma index evaluations. Results The measurements and analyses concern a very small target which PTV diameter is 1cm. The profiles comparisons show a perfect matching with the Delta4 system. The PDIP measurement is slightly compromised with the continuous detector position face to the beam. The gel dosimetry contains uncertainties linked to the complex process and particularly to the MRI acquisition. Delta4 and MRI-gel display respectively a percentage of gamma index passing 3%/3 mm tolerances, of 100% and 99.2%. Table 1 displays the results with different parameters in %, mm for a global Gamma index. Download : Download high-res image (79KB) Download : Download full-size image Conclusion The way to validate and secure an irradiation depends on its complexity. The PDIP is well adapted to a routine check. The delta 4 gives a high level of validation but cannot be irradiated with non-coplanar beams or arcs. Concerning the highest dose per session treatments it is relevant to rely on an end-to-end QA process based on an anthropomorphic phantom close to the real treatment conditions. The 3D-printed head and the gel dosimetry reproduce the exact patient conditions and give a fully satisfactory validation.

SU‐F‐T‐396: Impact of Shoulder Deformation for Head and Neck VMAT

Purpose:For head and neck VMAT (HN‐VMAT), variations of position and deformation of patient's shoulders is a concern to affect inaccuracy of dose distribution. It has been reported that the setup error of the shoulders was variable from 5 mm – 1 cm. The beams of the HN‐VMAT pass through the shoulders. We assessed the impact of shoulder deformation to dose distribution for HN‐VMAT.Methods:One HN‐VMAT plan was generated using a patient's CT. The patient's CT was deformed using ImSimQA (Oncology Systems Limited, Shrewsbury, Shropshire, UK) to generate several patterns of the shoulders’ deformations when the right and left humeral heads were shifted with 3, 6, and 15 mm in the superior and inferior directions (SI), 3, 5, and 15 mm in the anterior and posterior directions (AP), and 5 and 15 mm in the right or left direction (LR). DVH comparison was performed in the different deformation patterns. The dosimetric parameters of D95% for CTV70Gy, CTV60Gy and CTV54Gy and dmax for Spinal cord were also measured. Gamma index evaluation (Criteria: 3%/2mm) was performed to exhibit clinically tolerable area in the comparison.Results:DVH comparison shows similar for all structures. As the comparison for the dosimetric parameters, the variations of D95% in the LR and AP were within 1%. There were larger variations in the SI than those in the other directions, however were within 1.5%. In gamma index evaluation, the small spots with higher gamma index values were appeared when the shift was 6 mm, however the pass ratio was 99.13%.Conclusion:HN‐VMAT should be robust for shoulder deformation and geometric accuracy within 6 mm from patient's setup and image‐guided radiotherapy may be clinically acceptable for target dose coverage or normal tissue dose sparing.

SU‐F‐SPS‐06: Implementation of a Back‐Projection Algorithm for 2D in Vivo Dosimetry with An EPID System

Purpose:To implement a back‐projection algorithm for 2D dose reconstructions for in vivo dosimetry in radiation therapy using an Electronic Portal Imaging Device (EPID) based on amorphous silicon.Methods:An EPID system was used to calculate dose‐response function, pixel sensitivity map, exponential scatter kernels and beam hardenig correction for the back‐projection algorithm. All measurements were done with a 6 MV beam. A 2D dose reconstruction for an irradiated water phantom (30×30×30 cm3) was done to verify the algorithm implementation. Gamma index evaluation between the 2D reconstructed dose and the calculated with a treatment planning system (TPS) was done.Results:A linear fit was found for the dose‐response function. The pixel sensitivity map has a radial symmetry and was calculated with a profile of the pixel sensitivity variation. The parameters for the scatter kernels were determined only for a 6 MV beam. The primary dose was estimated applying the scatter kernel within EPID and scatter kernel within the patient. The beam hardening coefficient is σBH= 3.788×10−4 cm2 and the effective linear attenuation coefficient is µAC= 0.06084 cm−1. The 95% of points evaluated had γ values not longer than the unity, with gamma criteria of ΔD = 3% and Δd = 3 mm, and within the 50% isodose surface.Conclusion:The use of EPID systems proved to be a fast tool for in vivo dosimetry, but the implementation is more complex that the elaborated for pre‐treatment dose verification, therefore, a simplest method must be investigated. The accuracy of this method should be improved modifying the algorithm in order to compare lower isodose curves.

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.

Simulation of pseudo‐CT images based on deformable image registration of ultrasound images: A proof of concept for transabdominal ultrasound imaging of the prostate during radiotherapy

Imaging of patient anatomy during treatment is a necessity for position verification and for adaptive radiotherapy based on daily dose recalculation. Ultrasound (US) image guided radiotherapy systems are currently available to collect US images at the simulation stage (USsim), coregistered with the simulation computed tomography (CT), and during all treatment fractions. The authors hypothesize that a deformation field derived from US-based deformable image registration can be used to create a daily pseudo-CT (CTps) image that is more representative of the patients' geometry during treatment than the CT acquired at simulation stage (CTsim). The three prostate patients, considered to evaluate this hypothesis, had coregistered CT and US scans on various days. In particular, two patients had two US-CT datasets each and the third one had five US-CT datasets. Deformation fields were computed between pairs of US images of the same patient and then applied to the corresponding USsim scan to yield a new deformed CTps scan. The original treatment plans were used to recalculate dose distributions in the simulation, deformed and ground truth CT (CTgt) images to compare dice similarity coefficients, maximum absolute distance, and mean absolute distance on CT delineations and gamma index (γ) evaluations on both the Hounsfield units (HUs) and the dose. In the majority, deformation did improve the results for all three evaluation methods. The change in gamma failure for dose (γDose, 3%, 3 mm) ranged from an improvement of 11.2% in the prostate volume to a deterioration of 1.3% in the prostate and bladder. The change in gamma failure for the CT images (γCT, 50 HU, 3 mm) ranged from an improvement of 20.5% in the anus and rectum to a deterioration of 3.2% in the prostate. This new technique may generate CTps images that are more representative of the actual patient anatomy than the CTsim scan.

A.211 - Comparison of three different systems for gamma index evaluation in IMRT treatments

Introduction: The Gamma Index (GI) method has become a gold standard for the comparison between measured and calculated dose distributions in inverse radiotherapy planning. However, the methods to verify GI are several, depending on the needs of the radiotherapy unit. This work involves the comparison of three different instruments used for the GI verification: an array of diodes, EBT3 films and a software that uses the EPID system of the radiotherapy LINAC.

Dose Impact of Systemic MLC Position Error for Esophagus Cancer Plan: Volumetric Modulated Arc Therapy Versus Step and Shoot Modulated Therapy

Dose Impact of Systemic MLC Position Error for Esophagus Cancer Plan: Volumetric Modulated Arc Therapy Versus Step and Shoot Modulated Therapy

SU‐E‐T‐602: Patient‐Specific Online Dose Verification Based On Transmission Detector Measurements

Purpose:Intensity modulated radiotherapy requires a comprehensive quality assurance program in general and ideally independent verification of dose delivery. Since conventional 2D detector arrays allow only pre‐treatment verification, there is a debate concerning the need of online dose verification. This study presents the clinical performance, including dosimetric plan verification in 2D as well as in 3D and the error detection abilities of a new transmission detector (TD) for online dose verification of 6MV photon beam.Methods:To validate the dosimetric performance of the new device, dose reconstruction based on TD measurements were compared to a conventional pre‐treatment verification method (reference) and treatment planning system (TPS) for 18 IMRT and VMAT treatment plans. Furthermore, dose reconstruction inside the patient based on TD read‐out was evaluated by comparing various dose volume indices and 3D gamma evaluations against independent dose computation and TPS. To investigate the sensitivity of the new device, different types of systematic and random errors for leaf positions and linac output were introduced in IMRT treatment sequences.Results:The 2D gamma index evaluation of transmission detector based dose reconstruction showed an excellent agreement for all IMRT and VMAT plans compared to reference measurements (99.3±1.2)% and TPS (99.1±0.7)%. Good agreement was also obtained for 3D dose reconstruction based on TD read‐out compared to dose computation (mean gamma value of PTV = 0.27±0.04). Only a minimal dose underestimation within the target volume was observed when analyzing DVH indices (&lt;1%). Positional errors in leaf banks larger than 1mm and errors in linac output larger than 2% could clearly identified with the TD.Conclusion:Since 2D and 3D evaluations for all IMRT and VMAT treatment plans were in excellent agreement with reference measurements and dose computation, the new TD is suitable to qualify for routine treatment plan verification.Funding Support, Disclosures, and Conflict of Interest: COIs: Frank Lohr: Elekta: research grant, travel grants, teaching honoraria IBA: research grant, travel grants, teaching honoraria, advisory board C‐Rad: board honoraria, travel grants Frederik Wenz: Elekta: research grant, teaching honoraria, consultant, advisory board Zeiss: research grant, teaching honoraria, patent Hansjoerg Wertz: Elekta: research grant, teaching honoraria IBA: research grant