Dosimetric Verification Research Articles

Impact of different optimization strategies on the compatibility between planned and delivered doses during radiation therapy of cervical cancer.

To analyse the impact of different optimization strategies on the compatibility between planned and delivered doses during radiotherapy of cervical cancer. Four treatment plans differing in optimisation strategies were prepared for ten cervical cancer cases. These were: volumetric modulated arc therapy with (_OPT) and without optimization of the doses in the bone marrow and for two sets of margins applied to the clinical target volume that arose from image guidance based on the bones (IG(B)) and soft tissues (IG(ST)). The plans were subjected to dosimetric verification by using the ArcCHECK system and 3DVH software. The planned dose distributions were compared with the corresponding measured dose distributions in the light of complexity of the plans and its deliverability. The clinically significant impact of the plans complexity on their deliverability is visible only for the gamma passing rates analysis performed in a local mode and directly in the organs. While more general analyses show statistically significant differences, the clinical relevance of them has not been confirmed. The analysis showed that IG(ST)_OPT and IG(B)_OPT significantly differ from IG(ST) and IG(B). The clinical acceptance of IG(ST)_OPT obtained for hard combinations of gamma acceptance criteria (2%/2 mm) confirm its satisfactory deliverability. In turn, for IG(B)_OPT in the case of the rectum, the combination of 2%/2 mm did not meet the criteria of acceptance. Despite the complexity of the IG(ST)_OPT, the results of analysis confirm the acceptance of its deliverability when 2%/2 mm gamma acceptance criteria are used during the analysis.

Dosimetric Verification of Clinical MV Photon beam Profiles Using by a 2D ion Chamber Array

Dosimetric Verification of Clinical MV Photon beam Profiles Using by a 2D ion Chamber Array

Three-dimensional dose reconstruction-based pretreatment dosimetric verification in volumetric modulated arc therapy for prostate cancer.

PurposeWe performed three-dimensional (3D) dose reconstruction-based pretreatment verification to evaluate gamma analysis acceptance criteria in volumetric modulated arc therapy (VMAT) for prostate cancer.Materials and MethodsPretreatment verification for 28 VMAT plans for prostate cancer was performed using the COMPASS system with a dolphin detector. The 3D reconstructed dose distribution of the treatment planning system calculation (TC) was compared with that of COMPASS independent calculation (CC) and COMPASS reconstruction from the dolphin detector measurement (CR). Gamma results (gamma failure rate and average gamma value [GFR and γAvg]) and dose-volume histogram (DVH) deviations, 98%, 2% and mean dose-volume difference (DD98%, DD2% and DDmean), were evaluated. Gamma analyses were performed with two acceptance criteria, 2%/2 mm and 3%/3 mm.ResultsThe GFR in 2%/2 mm criteria were less than 8%, and those in 3%/3 mm criteria were less than 1% for all structures in comparisons between TC, CC, and CR. In the comparison between TC and CR, GFR and γAvg in 2%/2 mm criteria were significantly higher than those in 3%/3 mm criteria. The DVH deviations were within 2%, except for DDmean (%) for rectum and bladder.ConclusionsThe 3%/3 mm criteria were not strict enough to identify any discrepancies between planned and measured doses, and DVH deviations were less than 2% in most parameters. Therefore, gamma criteria of 2%/2 mm and DVH related parameters could be a useful tool for pretreatment verification for VMAT in prostate cancer.

3D dosimetric verification of unity MR-linac treatments by portal dosimetry

Purpose and background3D dosimetric verification of online adaptive workflows is essential as their complexity is unprecedented in radiation oncology. The aim of this work is to demonstrate the feasibility of back-projection portal dosimetry for 3D dosimetric verification of Unity MR-linac treatments. Material and MethodsAn earlier presented 2D back-projection algorithm for the Unity MR-linac geometry was extended for 3D dose reconstruction and comparison against planned dose distributions. ‘In-air’ as well as in-vivo portal EPID images can be used as input. The method was validated using data from treatments of 5 patients (2 rectal, 2 prostate cancer and one oligo metastasis). 3D pre-treatment verification of the reference plan using ‘in-air’ EPID images was performed and compared against measured (with the Octavius 4D system) and planned (in the planning CT) dose distributions. In-vivo EPID dose distributions were compared to the TPS for the first three adaptations of all treatments. For all comparisons, dose difference values at the reference point and γ-parameters were reported. ResultsThe comparison against the OCTAVIUS 4D system (3%, 2 mm, local) showed y-mean = 0.52 ± 0.10 and y-passrate = 91.9%, 95% CI [85.4, 98.4], and ΔDRP = −0.1 ± 1.1%. Pre-treatment verification against TPS data (3%, 2 mm, global) showed y-mean = 0.52 ± 0.04, y-passrate = 93.5%, 95% CI [92.4, 94.6] and ΔDRP = −0.9 ± 1.5%. The averaged y-results for the in-vivo 3D verification were y-mean = 0.52 ± 0.05, y-passrate = 92.5%, 95% CI [90.2, 94.8] and ΔDRP = 0.8 ± 2.1%. Conclusion3D dosimetric verification of Unity MR-linac treatments using portal dosimetry is feasible, pre-treatment as well as in-vivo.

A Deep Learning-based correction to EPID dosimetry for attenuation and scatter in the Unity MR-Linac system

PurposeEPID dosimetry in the Unity MR-Linac system allows for reconstruction of absolute dose distributions within the patient geometry. Dose reconstruction is accurate for the parts of the beam arriving at the EPID through the MRI central unattenuated region, free of gradient coils, resulting in a maximum field size of ~10 × 22 cm2 at isocentre. The purpose of this study is to develop a Deep Learning-based method to improve the accuracy of 2D EPID reconstructed dose distributions outside this central region, accounting for the effects of the extra attenuation and scatter. MethodsA U-Net was trained to correct EPID dose images calculated at the isocenter inside a cylindrical phantom using the corresponding TPS dose images as ground truth for training. The model was evaluated using a 5-fold cross validation procedure. The clinical validity of the U-Net corrected dose images (the so-called DEEPID dose images) was assessed with in vivo verification data of 45 large rectum IMRT fields. The sensitivity of DEEPID to leaf bank position errors (±1.5 mm) and ±5% MU delivery errors was also tested. ResultsCompared to the TPS, in vivo 2D DEEPID dose images showed an average γ-pass rate of 90.2% (72.6%–99.4%) outside the central unattenuated region. Without DEEPID correction, this number was 44.5% (4.0%–78.4%). DEEPID correctly detected the introduced delivery errors. ConclusionsDEEPID allows for accurate dose reconstruction using the entire EPID image, thus enabling dosimetric verification for field sizes up to ~19 × 22 cm2 at isocentre. The method can be used to detect clinically relevant errors.

Feasibility study of the Fricke chemical dosimeter as an independent dosimetric system for the small animal radiation research platform (SARRP)

For the small animal radiation research platform (SARRP) with X-ray beams in the medium energy range (tube operating voltage at 220 kVp), reference dosimetry is based on the AAPM TG-61 recommendations following the in-phantom method. The objective of this study was to evaluate the feasibility of the Fricke solution as a dosimeter to determine the absorbed dose to water. Feasibility studies at this X-ray energy range are not widely available. We evaluated the accuracy, dose linearity and dose rate dependence in a comparison with an NE 2571 Farmer ionization chamber (IC) and measurements in water. The G(Fe3+) factor was calculated from the curve fitting of the chemical yields for two radioactive sources (192Ir and 60Co) and one X-ray system with a tube operating at 150 and 250 kVp. The same methodology was followed for the dependence of the G(Fe3+) value on the energy and the dose agreement assessment for 180 and 200 kVp in the SARRP. The Fricke system exhibits a good linear response over the range of 5–70 Gy and an accuracy better than 2% for a 2 Gy/min dose rate. The dose rate dependence is smaller than 1% for dose rates greater than 1 Gy/min. The dependence of the G(Fe3+) value on the energy is smaller than 0.41%, with dose agreements better than 2%. The feasibility of the dosimeter for measurements at high doses and high dose rates makes it a suitable tool for dosimetric verifications in several preclinical irradiation configurations.

Dosimetric Study of an Indigenous and Heterogeneous Pelvic Phantom for Radiotherapy Quality Assurance

Introduction: In vitro dosimetric verification prior to patient treatment plays a key role in accurate and precision radiotherapy treatment delivery. Since the human body is a heterogeneous medium, the aim of this study was to design a heterogeneous pelvic phantom for radiotherapy quality assurance. Material and Methods: A pelvic phantom was designed using wax, pelvic bone, borax powder, and water mimicking different biological tissues. Hounsfield units and relative electron densities were measured. Various intensity-modulated radiotherapy (IMRT) plans were imported to the pelvic phantom for verification and implemented on the Delta 4 phantom. The quantitative evaluation was performed in terms of dose deviation, distance to agreement, and gamma index passing rate. Results: According to the results of the CT images of an actual patient, relative electron densities for bone, fat, air cavity, bladder, and rectum were 1.335, 0.955, 0.158, 1.039, and 1.054, respectively. Moreover, the CT images of a heterogeneous pelvic phantom showed the relative electron densities for bone, fat (wax), air cavity, bladder (water), and rectum (borax powder) as 1.632, 0.896, 0.159, 1.037, and 1.051, respectively.The mean percentage variation between planned and measured doses was found to be 2.13% within the tolerance limit (< ±3%) .In all test cases, the gamma index passing rate was greater than 90%. Conclusion: The findings showed the suitability of the materials used in the design of the heterogeneous phantom. Therefore, it can be concluded that the designed phantom can be used for regular radiotherapy quality assurance

Effect of the rotation errors on the γ pass rate of volume-modulated arc therapy plan in rectal cancer.

To study the effect of rotation errors on the γ pass rate of volume-modulated arc therapy (VMAT) plan in rectal cancer based on the ArcCheck phantom. CT data from 20 rectal cancer patients underwent VMRT were selected randomly for this study. Targeting areas were selected, and clinical radiotherapy and validation plans were formulated. ArcCheck model was selected to validate the radiotherapy plans. The effect of the rotation errors on the dosimetric verification for VMAT in rectal cancer was simulated and analyzed with ArcCheck model software. When there was no rotation errors, the γ pass rate of VMRT plans was more than 95%. When the absolute rotation angle was less than or equal to 1°, the γ pass rate of VMAT plans was more than 90%, meeting the clinical requirements. When the absolute rotation angle was greater than 1°, the γ pass rate was less than 90%, which did not meet clinical requirements. The rotation errors affect the γ pass rate of VMAT plans. The larger the rotation angle, the lower the γ pass rate. It meets clinical requirements when the rotation error is less than or equal to 1°.

A multi-institutional initiative on patient-related quality assurance: Independent computational dose verification of fluence-modulated treatment techniques

PurposeThis multi-institutional study investigates whether computational verification of fluence-modulated treatment plans using independent software with its own Strahlerkopfmodel is an appropriate method for patient-related quality assurance (PRQA) in the context of various combinations of linear accelerators (linacs), treatment techniques and treatment planning systems (TPS). Materials and methodsThe PRQA-software's (Mobius3D) recalculations of 9 institutions’ treatment plans were analyzed for a horseshoe-shaped planning target volume (PTV) inside a phantom. The recomputed dose distributions were compared to a) the dose distributions as calculated by all TPS's and b) the measured dose distributions, which were acquired using the same independent measuring system for all institutions. Furthermore, dose volume histograms were examined. The penumbra deviations and mean gamma values were quantified using Verisoft (PTW). Additionally, workflow requirements for computational verification were discussed. ResultsMobius3D is compatible with all examined TPSs, treatment techniques and linacs. The mean PTV dose differences (Mobius3D-TPS, <3.0%) and 3D gamma passing rates (>95.0%) led to a positive plan acceptance result in all cases. These results are similar to the outcome of the dosimetric measurements with one exception. The mean gamma values (<0.5) show a good agreement between Mobius3D and the TPS dose distributions. ConclusionUsing Mobius3D was proven to be an appropriate computational PRQA method for the tested combinations of linacs, treatment techniques and TPS's. The clinical use of Mobius3D has to be complemented with regular dosimetric measurements and thorough linac and TPS QA. Mobius3D's computational verification reduced measurement effort and personnel needs in comparison to dosimetric verifications.

Design, Fabrication, and Validation of a Polymethyl Methacrylate Head Phantom for Dosimetric Verification of Cranial Radiotherapy Treatment Plans.

Purpose:The present study aims to design and fabricate a novel, versatile, and cost-effective Polymethyl Methacrylate (PMMA) head phantom for the dosimetric pretreatment verification of radiotherapy (RT) treatment plans.Materials and Methods:The head phantom designing involves slice-wise modeling of an adult head using PMMA. The phantom has provisions to hold detectors such as ionization chambers of different sizes, Gafchromic films, gel dosimeter, and optically stimulated luminescence dosimeter. For the point dose verification purpose, 15 volumetric modulated arc therapy patient plans were selected, and doses were measured using a CC13 ionization chamber. The percentage gamma passing rate was calculated for acceptance criteria 3%/3 mm and 2%/2 mm using OmniPro I’mRT film QA software, and Gafchromic EBT3 films were used for 2D planar dose verification.Results:Treatment planning system calculated, and the measured point doses showed a percentage deviation ranged from 0.26 to 1.92. The planar dose fluence measurements, for set acceptance criteria of 3%/3 mm and 2%/2 mm, percentages of points having gamma value <1 were in the range of 99.17 ± 0.25 to 99.88 ± 0.15 and 93.16 ± 0.38 to 98.89 ± 0.23, respectively. Measured dose verification indices were within the acceptable limit.Conclusions:The dosimetric study reveals that head phantom can be used for routine pretreatment verification for the cranial RT, especially for stereotactic radiosurgery/RT as a part of patient-specific quality assurance. The presently fabricated and validated phantom is novel, versatile, and cost-effective, and many institutes can afford it.

Evaluation of transrectal ultrasound-based dosimetry for brachytherapy of prostate cancer: a single-center experience.

PurposeTo explore the possibility of intraoperative transrectal ultrasound (TRUS)-based dose verification in transperineal brachytherapy (BT) with iodine-125 (125I) seeds for prostate cancer.Material and methodsFifteen patients with prostate cancer were treated using BT with 125I seeds. Post-implant TRUS and computed tomography (CT) images were imported into treatment planning system (TPS) for dosimetry. Dosimetry parameters, including minimum dose received by 90% of the volume (D90), percentage of the volume receiving 100% of prescribed dose (V100), and percentage of the volume receiving 200% of prescribed dose (V200) were calculated based on TRUS and CT images, separately. The D90 value of TRUS-based dosimetry was transformed to its expected value. Comparisons of the dosimetric parameters between post-operative verification and preoperative plans were made by paired t-test. One-way ANOVA model was used to assess the differences in preoperative plans. Agreements were evaluated between the preoperative planning and post-operative actual dose parameters using Bland-Altman analysis.ResultsIn total, 825 of 125I seeds were implanted successfully in 15 patients. In TRUS-based dosimetry, 674 seeds (81%) were identified clearly in TRUS-based images, and the expected value of D90 parameter showed no significant differences compared with the preoperative planning and CT post-operation results (p > 0.05). In CT-based dosimetry, 810 seeds (98%) were identified clearly in CT-based images, and there was good consistency of D90, V100, and V200 values (p > 0.05). Post-implant CT-based dosimetry indicated that 125I seed implantation had fulfilled the expected plan.ConclusionsIntraoperative TRUS can be used for dosimetric verification of BT for prostate cancer.

Dosimetry of ruthenium-106 ophthalmic applicators with thin layer thermoluminescence dosimeters - Clinical quality control

Ruthenium-106 ophthalmic applicators have proven to be effective when using beta emitters in brachytherapy. For dose calculations , typically, the dosimetric reference data given by the manufacturer are used. An additional check of the applicators is usually not provided. However, the medical physicist is responsible for correct dosimetry in the clinic; therefore dosimetric verification is desirable. Despite the fact that the use of beta-ray emitting sealed brachytherapy sources is a safe treatment method, errors can occur (Kaulich et al., 2004). Hence, a method for absolute dose measurements based on the use of thin layer MCP-N-thermoluminescence detectors (TLD Poland, Krakow, Poland) is described in this study. A custom-made polymethyl methacrylate (PMMA)- based phantom was developed for this study. The surface of the phantom was designed to fit with spherical shells of ruthenium-106 ophthalmic applicators (e.g. applicator type CCA, CCB and CIA by Eckert & Ziegler BEBIG GmbH, Berlin, Germany) studied in this work. To verify the reference data from the source certificates, absolute point dose values were measured at different phantom depths with the thermoluminescence detectors and compared to the certificate values.Calibrations of the thermoluminescence detectors were performed in a water phantom with a 6 MV CLINAC (Artiste, Siemens Medical, Erlangen, Germany) before. A comparison with scintillator measurement results given by the manufacturer in the applicator certificate shows the measurement series of absolute dose using MCP-N thin layer detectors being in good accordance with the values of the applicator certificate. The dose values calculated with the source certificate can be confirmed with a maximum deviation of 6.5%. Further, it can be shown that compared to TLD-100, the use of MCP-N thermoluminescence detectors is an advantage, when calibrating with 6 MV photons. The phantom measuring procedure presented in this study provides a practice-oriented realization for quality control of ruthenium-106 ophthalmic applicators in clinical routine The phantom seems capable of performing periodic system tests, as well as controlling the introduction of new applicators delivered by the manufacturer.

Dosimetric Verification of Hybrid Treatment Plan by TLD in the Treatment of Nasopharyngeal Cancer Radiotherapy

Nazofarenks kanserinde geleneksel tedavi planlama, günümüzde yerini optimizasyon tekniklerinin kullanıldığı tersten planlamaya Yoğunluk Ayarlı Radyoterapi – Yoğunluk Ayarlı Arc Terapi (IMRT-IMAT) ve ikisinin birlikte kullanıldığı Hibrit tedavi planlamalara bırakmıştır. Özellikle IMAT veya IMRT tedavi planlarında düşürülemeyen kritik organ dozları veya iyi kapsanamayan PTV hacimleri için Hibrit planlara ihtiyaç duyulmaktadır. Bu karmaşık hesaplamaları yapan Tedavi Planlama Sistemlerinin (TPS) doz hesaplarının dozimetrik yöntemlerle doğrulanması gerekmektedir. Bu çalışmada Termolüminesans dozimetre (TLD) yöntemi ile TPS doğruluğu test edilmiştir. Alderson® Rando Fantom’un nazofarenks kanseri tedavi planlaması için tedavi şartlarında bilgisayarlı tomografi görüntüleri çekilmiştir. Planlanan Hedef Hacim 70 Gy (PTV70) ve kritik organlar (sağ parotis, sol parotis, medulla ve beyin sapı) tespit edilirken standart bir nazofarenks tümörlü hastanın Bilgisayarlı tomografi kesitlerinden füzyon yapılarak yararlanılmıştır. IMRT, IMAT ve Hibrit tedavi planları Eclipse (version 8.9.17) TPS kullanılarak fantom üzerinde aynı doz reçetesi ile oluşturulmuştur. Alderson® Rando Fantomda sağ ve sol parotise, medulla spinalise ve beyin sapına denk gelecek şekilde yerleştirilen TLD’lerin okuma değerleri toplam doza dönüştürülerek tedavi planlama bilgisayarındaki doz değerleriyle karşılaştırılmıştır. Eclipse TPS ile TLD ortalama okuma değerleri arasında en fazla fark IMRT planında Sol Parotiste % 3.5, IMAT planında Beyin Sapında % 2.78, Hibrit planda Beyin Sapında % 3.16 olarak bulunmuştur. Bu çalışma ile Nazofarenks için yapılan Hibrit tedavi planının TPS de doğruluğunun % 4 ten az olduğunu dozimetrik olarak tespit ettik.

Optimization of MLC parameters for TPS calculation and dosimetric verification: application to single isocenter radiosurgery of multiple brain lesions using VMAT

Linac and MLC-based stereotactic radiosurgery (SRS) using single-isocenter-multiple-target (SIMT) VMAT has become increasingly popular in the management of multi-focal cranial metastases. However, significant geometrical and dosimetric challenges exist due to the typically small target volumes and in most cases, non-isocentric locations. To the best of our knowledge, there hasn’t been a study in the optimization of MLC parameters, in the context of SIMT SRS, to ensure TPS calculation accuracy. In this work, we set out to optimize the dosimetric leaf gap (DLG) for the HD MLC installed on dedicated stereotactic Varian STx systems using a diverse group of 21 clinical SRS and SBRT plans. These plans featured a broad range of target sizes and target-to-isocenter distances that are typical of the stereotactic cases treated on these systems. Dose discrepancies between TPS calculations and verification measurements using a previously validated diode array Delta4 (ScandiDos) were minimized in a balanced manner to accommodate the variety of stereotactic plans. A DLG of 0.6 mm was found to be ‘optimal’ for the HD MLC and for the ‘typical’ plans treated on our STx systems. The finding was independently verified using commercially available 3D polymer gel dosimeter CrystalBallTM (MGS Research Inc.). 3D verification for 6 SIMT SRS plans, consisted of 5 to 15 targets, achieved an average gamma score of 97.3% (σ = 2.0%) on 3%/2 mm criteria with a cutoff isodose level of 20%. We further examined the practice of routine dosimetric verifications including the selection of appropriate detectors and optimal gamma parameters. We found that the commonly used standard 3%/3 mm criteria would have resulted in all but 4 (out of 2840) clinical plans achieving a gamma score of 95% or better, and therefore, losing sensitivity to detect potential dosimetric discrepancies. Based on the characteristics of stereotactic plans, a more stringent distance-to-agreement parameter is needed.

Safety and efficacy of CT-guided radioactive iodine-125 seed implantation assisted by a 3D printing template for the treatment of thoracic malignancies.

To ascertain the safety and efficacy of radioactive iodine-125 seed implantation (RISI) for the treatment of thoracic tumors. Clinical patients with primary or metastatic tumors in the chest treated with RISI were analyzed. The RISI process included the following stages: preoperative planning, template design and 3D printing, CT-guided RISI assisted by a template, and postoperative dosimetric verification. The prescribed dose was ≥ 80Gy. The main analytic measures were the local control (LC) rate and toxicity. From April 2015 to July 2018, a total of 92 patients, including 41 with lung cancer and 51 with lung metastases, were analyzed. The median lesion diameter was 5cm. The median postoperative D90 was 142.6Gy. The median follow-up was 10.7months. The overall survival rates at 1year and 3years were 59.7% and 22.2%, respectively. The LC rates at 1year and 3years were 64.9% and 32.8%, respectively. The LC rates at 3years for patients with D90 < 140Gy and D90 > 140Gy were 23.1% and 54.3%, respectively (P = 0.014). The LC rate of metastatic lung cancer was more favorable than that of primary lung cancer. The multivariate analyses showed that the dose and lesion type were independent factors for LC (P < 0.05). No factors were related to OS. The incidence of pneumothorax and hemoptysis was 35.8% and 3.2%, respectively. Few cases of radiotherapy-related toxicity effects were observed. RISI may be safe and efficacious and is associated with few complications during the treatment of thoracic tumors. If patients need local treatment and surgery or radiotherapy is not available, RISI could be considered.

Development and Validation of Single Field Multi-Ion Particle Therapy Treatments

To develop and validate combined ion-beam with constant relative biological effectiveness (RBE) (CICR) particle therapy in single field arrangements for improved treatment efficacy, robustness, and normal tissue sparing. The PRECISE (PaRticle thErapy using single and Combined Ion optimization StratEgies) treatment planning system was developed to investigate clinical viability of CICR treatments. Single-field uniform dose (SFUD) with a single ion (proton [p], helium [He], or carbon [C]) and CICR (C-p and C-He) treatments were generated for 3 patient cases with a clinically prescribed dose of 3 Gy (RBE) per fraction. Spread-out Bragg peak plans were irradiated in homogenous and clinical-like settings using an anthropomorphic head phantom. A dosimetric and biological verification of CICRC-p treatments using a murine glioma cell line (GL261) was performed. CICR treatment plans for the 3 patients presented highly uniform physical dose while reducing high dose-averaged linear energy transfer gradients compared with carbon ions alone. When considering uncertainty in tissue parameter (α/β)x assignment and RBE modeling, the CICRC-p treatment exhibited enhanced biophysical stability within the target volume, similar to protons alone. CICR treatments reduced dose to normal tissue surrounding the target, exhibiting similar or improved dosimetric features compared with SFUDHe. For both CICRC-p and SFUD treatments, measurements verified the planned dose in the target within ∼3%. Planned versus measured target RBE values were 1.38 ± 0.02 and 1.39 ± 0.07 (<1% deviation), respectively, for the CICRC-p treatment in heterogenous settings. Here, we demonstrate that by combining 2 (or more) ions in a single field arrangement, more robust biological and more conformal dose distributions can be delivered compared with conventional particle therapy treatment planning. This work constitutes the first dosimetric and biological verification of multi-ion particle therapy in homogeneous as well as heterogenous settings.

Bulk Anatomical Density Based Dose Calculation for Patient-Specific Quality Assurance of MRI-Only Prostate Radiotherapy.

Prostate cancer treatment planning can be performed using magnetic resonance imaging (MRI) only with sCT scans. However, sCT scans are computer generated from MRI data and therefore robust, efficient, and accurate patient-specific quality assurance methods for dosimetric verification are required. Bulk anatomical density (BAD) maps can be generated based on anatomical contours derived from the MRI image. This study investigates and optimizes the BAD map approach for sCT quality assurance with a large patient CT and MRI dataset. 3D T2-weighted MRI and full density CT images of 54 patients were used to create BAD maps with different tissue class combinations. Mean Hounsfield units (HU) of Fat (F: below −30 HU), the entire Tissue [T: excluding bone (B)], and Muscle (M: excluding bone and fat) were derived from the CT scans. CT based BAD maps (BADBT,CT and BADBMF,CT) and a conventional bone and water bulk-density method (BADBW,CT) were compared to full CT calculations with bone assignments to 366 HU (measured) and 288 HU (obtained from literature). Optimal bulk densities of Tissue for BADBT,CT and Bone for BADBMF,CT were derived to provide zero mean isocenter dose agreement to the CT plan. Using the optimal densities, the dose agreement of BADBT,CT and BADBMF,CT to CT was redetermined. These maps were then created for the MRI dataset using auto-generated contours and dose calculations compared to CT. The average mean density of Bone, Fat, Muscle, and Tissue were 365.5 ± 62.2, −109.5 ± 12.9, 23.3 ± 9.7, and −46.3 ± 15.2 HU, respectively. Comparing to other bulk-density maps, BADBMF,CT maps provided the closest dose to CT. Calculated optimal mean densities of Tissue and Bone were −32.7 and 323.7 HU, respectively. The isocenter dose agreement of the optimal density assigned BADBT,CT and BADBMF,CT to full density CT were 0.10 ± 0.65% and 0.01 ± 0.45%, respectively. The isocenter dose agreement of MRI generated BADBT,MR and BADBMF,MR to full density CT were −0.15 ± 0.90% and −0.16 ± 0.65%, respectively. The BAD method with optimal bulk densities can provide robust, accurate and efficient patient-specific quality assurance for dose calculations in MRI-only radiotherapy.

Considerations for the verification of volumetric modulated arc therapy-planned dose distributions

Background: Volumetric modulated arc therapy (VMAT) is the standard of care for many clinical indications, but should only be considered with proper technical support and quality assurance (QA) in place. Despite the high accuracy of VMAT systems, errors can be present and adequate verification is required. Dosimetric VMAT verification systems have a broadly similar analysis philosophy. However, many factors influence the analyses and the subsequent QA outcome, based on which the plan will pass or fail.Aim: This study investigated various factors that influence the dosimetric impact and detectability of known linac component deviations on VMAT QA, including geometries, tissue densities, gamma criteria and dose–volume differences.Setting: Universitas Hospital (Annex), Bloemfontein, South Africa.Methods: Deliberate multi-leaf collimator (MLC)-bank offsets were introduced on four different VMAT plans of the prostate, nasopharynx and brain. Measured reference dose sets were compared to measured QA results, using the IBA Dolphin© detector and Compass© software for three dosimetric scenarios. Gamma pass rates over a range of criteria from 1%/2-mm to 4%/4-mm in the total volumes and per structure, as well as dose–volume differences were studied.Results: Gamma tests in the total patient/phantom did not sufficiently detect errors. The calculation media did not influence the QA outcome greatly. However, the detection geometry affected the results. Per structure gamma analyses provided superior error detection, although still missed some clinically relevant differences. The addition of dose–volume analyses highlighted several important errors.Conclusion: Volumetric modulated arc therapy using only total volume gamma analyses can easily overlook clinically relevant errors. The choice of gamma criterion is crucial. Verification with at least a per structure gamma test in combination with dose–volume checks is recommended, especially in small target volume cases.

3D printable inter cross-linking network (ICN) gels for reversible transparency control with water content

A patient-specific phantoms, which are equivalent to human tissues, is needed in the experimental dosimetric verification before radiotherapy treatments. Most of the shape of gel phantoms have been structurally limited because of the fabrication method using mold technique. The present study focuses on direct freeform fabrication of gel phantom with the customized optical 3D gel printing system. Life-sized gel phantoms were replicated using conventional mold techniques, the 3D printer with two-step sequential free-radical polymerization process and inter cross-linking network gels as 3D printing materials. The gel phantoms are ultraviolet light-sensitive instead of radiation. The ultraviolet sensitivity of the gel phantoms (using mold techniques) with different molar ratios were experimentally evaluated with transmittance measurements, and were theoretically estimated by curve fitting to the observed data. This measurement gave us the insight that more vinyl monomers in the gel phantoms caused rapid polymerization and visibly opaque as well as conventional gel dosimeters. We confirmed that inter cross-linking network gel was 3D printable with the customized 3D printer, with resolution of fabrication 500 μm, as pre-finger-shaped phantom. The cross section of 3D printed gel phantom showed the distribution of opacity, that is, the distribution of UV irradiation. We expect that the 3D printer specialized gel materials and ICN gels are feasible for practical fabrication method of 3D gel phantom.

Dosimetric validation of Monte Carlo and analytical dose engines with raster-scanning 1H, 4He, 12C, and 16O ion-beams using an anthropomorphic phantom

With high-precision radiotherapy on the rise towards mainstream healthcare, comprehensive validation procedures are essential, especially as more sophisticated technologies emerge. In preparation for the upcoming translation of novel ions, case-/disease-specific ion-beam selection and advanced multi-particle treatment modalities at the Heidelberg Ion-beam Therapy Center (HIT), we quantify the accuracy limits in particle therapy treatment planning under complex heterogeneous conditions for the four ions (1H, 4He, 12C, 16O) using a Monte Carlo Treatment Planning platform (MCTP), an independent GPU-accelerated analytical dose engine developed in-house (FRoG) and the clinical treatment planning system (Syngo RT Planning). Attaching an anthropomorphic half-head Alderson RANDO phantom to entrance window of a dosimetric verification water tank, a cubic target spread-out Bragg peak (SOBP) was optimized using the MCTP to best resolve effects of anatomic heterogeneities on dose homogeneity. Subsequent forward calculations were executed in FRoG and Syngo. Absolute and relative dosimetry was performed in the experimental beam room using 1D and 2D array ionization chamber detectors. Mean absolute percent deviation in dose (|%Δ|) between predictions and PinPoint ionization chamber measurements were within ∼2% for all investigated ions for both MCTP and FRoG. For protons and carbon ions, |%Δ| values were ∼4% for Syngo. For the four ions, 3D-γ analysis (3%/3mm criteria) of FLUKA and FRoG presented mean passing rates of 97.0(±2.4)% and 93.6(±4.2)%. FRoG demonstrated satisfactory agreement with gold standard Monte Carlo simulation and measurement, superior to the commercial system. Our pre-clinical trial landmarks the first measurements taken in anthropomorphic settings for helium, carbon and oxygen ion-beam therapy.