IMRT Treatment Research Articles

PO-1779: The feasibility study of MLC-based IMRT treatment planning on a Cobalt-60 machine

PO-1779: The feasibility study of MLC-based IMRT treatment planning on a Cobalt-60 machine

Accuracy of the Treatment Plan Dose Preview Functionality of a Newly Commercially-Available, CBCT-Based, Online Adaptive Radiotherapy Treatment System

The treatment planning module of a recently introduced, CBCT-based, online adaptive radiotherapy system includes a dose preview functionality to provide clinicians feedback regarding the set and priority of desired dose objectives. Informed clinical decision making requires that this prediction is accurate and not dependent on the specific technique and optimization solution selected for treatment. This work evaluates the accuracy of this dose preview functionality with respect to different IMRT and VMAT treatment configurations. A software emulator of the online adaptive radiotherapy system was used to retrospectively create alternative treatment plans for 10 cervical cancer and 10 rectal cancer patients. The dose preview functionality was used to predict the value of dosimetric variables optimized and calculated for five different treatment configurations, including a 7-, 9-, and 12-field IMRT plan, as well as a 2- and 3-arc VMAT plan. The dosimetric variables evaluated were the volume of the PTV receiving 95% of the 45 Gy prescription dose (PTV V95%) and the maximum dose (Dmax) to the bladder, bowel, and femoral heads. The accuracy of the dose preview functionality was comparable for all treatment field configurations. Across all configurations for the rectal patients, the average magnitude of the dose preview error was 1.0% (σ = 0.7%) for the PTV V95%, and 0.6 Gy (σ = 0.5 Gy), 0.9 Gy (σ = 1.4 Gy), and 1.7 Gy (σ = 1.4 Gy) for the bladder, bowel, and femoral head Dmax, respectively. For the cervical patients, the values were 0.7% (σ = 0.3%) for the PTV V95%, and 0.9 Gy (σ = 0.6 Gy), 1.2 Gy (σ = 1.1 Gy), and 2.6 Gy (σ = 2.5 Gy) for the bladder, bowel, and femoral head Dmax, respectively. Initial experience suggests that the dose preview functionality of the online adaptive radiotherapy system is accurate for both IMRT and VMAT plans to within a couple percent and a couple Gray. While the dose of the actual treatment plan should be known to greater accuracy, this may be sufficient for the purpose of informing clinicians as to the appropriateness of dose objectives prior to optimization of the actual treatment plan.

Placing a Bolus Helps to Lower the RT Dose to Pacemaker in IMRT Treatment of Head and Neck Cancer.

Planning IMRT for Head and Neck cancer (HNC) patients can be challenging if a pacemaker is in the vicinity of treatment field. This study investigated the use of bolus on the pacemaker to reduce RT dose. Three patients with HNC with pacemaker were included in this study. All subjects were planned to receive RT to neck nodes on the same side as pacemaker. Patients were simulated in the supine position with an aquaplast mask for immobilization. Patients were treated with a dose painting technique. Prescriptions were as follows: Patient 1, 70Gy, 63Gy, and 56Gy and bilateral neck coverage, patient 2 to 66Gy, 59.4Gy, and 56Gy to ipsilateral neck, and patient 3, 70Gy, 63Gy, and 58.10Gy and bilateral necks to high, intermediate and low risk regions respectively. Plans were generated with and without 1cm bolus on the skin over the pacemaker. Currently, the commonly accepted dose for a pacemaker is <2Gy. All patients were planned with a volumetric modulated arc therapy (VMAT) technique using 6MV energy and 3 full arcs with varying collimator angles. The use of bolus over the pacemaker throughout the treatment course reduces the dose received by the pacemaker without affecting PTVs coverage. There was a 33.4%, 19.6%, 34.9% reduction in dose to the pacemaker in patients 1, 2 and 3 respectively when bolus was used and placed on the device. Conclusions: This novel technique of using bolus while planning IMRT for HNC patients with pacemakers yields significant dosimetric advantages and helps to reduce the dose to the pacemaker.Tabled 1Abstract 2800; TablePacemaker Patientskin to pacemaker distanceDistance PTV to pacemakerMax dose w/o 1cm bolusMax dose w 1cm bolus19mm6cm311.1cGy207.3cGy27mm2cm401.4cGy322.8cGy36mm6cm248.8cGy162cGy Open table in a new tab

Reducing IMRT QA Workload by 95% and Keeping the Same Level of Quality Control

Reducing the maximum possible error (max-error) in a quality assurance program is the natural metric in contrast to reducing the mean value of the expected error. Current machine learning (ML) models used to predict IMRT QA, however, minimize the later and not the former. Here, we propose a novel ML algorithm for predicting QA plan passing rate that minimizes the max-error. From a single institution, 498 patient IMRT QA plans were measured using a commercial 2D diode array (Sun Nuclear’s MapCheck®) and delivered on five isocentric linear accelerators. Passing rates were determined based on 3%/3mm local dose and distance-to-agreement. Each treatment plan was characterized by 78 features to quantify complexity, such as MLC position, field area, X/Y jaw position, etc. Two ML models were used to predict passing rate: (1) an ordinary least squares (OLS) linear model that minimize the mean of the expected error; and (2) Chebyshev minimax (MM) linear solution that minimizes max-error. An 80% training dataset was used for fitting and optimization, and a 20% testing dataset was used to compare predictions. Model fitting and optimization (using Ridge, Lasso and Lasso-like regularization) was performed in a computer algorithm. When using all 498 plans for training, the max-error of IMRT QA passing rate predictions was 7.6% for the OLS model and 3.0% for the MM model. The mean square error was, however, 1.4% and 2.8% for the OLS and MM models respectively showing the tradeoff between both optimization methods. Following optimization and testing with a dataset that the model had not seen before (20% plans), the OLS and MM model max-errors were 7.0% and 3.8% respectively. To reach the clinical goal that all plans have at least a 90% passing rate, using the OLS we would have to QA all plans predicted to be 97.0% passing rate or lower (79 out of 497, 16.0%). Whereas with the MM model, we would only need to QA all plans predicted to be 93.8% or lower (24 out of 497, 4.8%) which is a 4x reduction compared to the OLS model. Efficient and safe QA programs for IMRT treatments require accurate identification of plans that are likely to not meet passing criteria. Such a program would enable relocation of clinical resources to higher priority tasks, which is in alignment with TG100 guidelines. Using Chebyshev minimax optimization, we could reduce resources for IMRT QA by 95% while guaranteeing a safe clinical QA program that meets current passing criteria.

Dosimetric comparison of three-dimensional conformal radiotherapy and static and dynamic intensity-modulated radiotherapy for the treatment of early-stage glottic cancer

The aim of this study was to compare dosimetric variations using the three-dimensional conformal radiotherapy (3DCRT), dynamic intensity-modulated radiation therapy (D-IMRT), and static intensity-modulated radiation therapy (S-IMRT) techniques for glottic cancer. Ten patients with early-stage glottic cancer were retrospectively selected and evaluated. The 3DCRT and IMRT treatment plans were performed using the solution commercialized by Varian with the Eclipse treatment planning system (TPS). For each patient, five different treatment plans were created and compared with respect to the doses received by the organs at risk (OARs) including the carotid arteries, thyroid gland, and spinal cord; the dose homogeneity index (DHI); conformity indexes (CI); and total monitor unit (MU) counts required for the treatment. The Mann-Whitney U test was used for statistical analyses. Statistically significant differences for the 3DCRT, D-IMRT, and S-IMRT techniques were observed for the planning target volume (PTV) mean and maximum doses. The results of this study indicated an increase in DHI for 3DCRT compared with D-IMRT and S-IMRT. Furthermore, the S-IMRT technique led to the superior decreased dose to the OAR. The 3DCRT plans performed better at the Dmax of the spinal cord and MU counts. The D-IMRT and S-IMRT techniques allowed more homogeneous dose distributions in PTV. Considering the dose to OAR, S-IMRT was more appropriate rather than 3DCRT and D-IMRT.

Assessment of robustness of institutional applied clinical target volume (CTV) to planning target volume (PTV) margin in cervical cancer using biological models

The aim of this study is to investigate the robustness of our institutionally applied clinical target volume (CTV)-to-planning target volume (PTV) margins in cervical cancer patients in terms of an equivalent uniform dose (EUD) based on tumor control probability (TCP). We simulated target motion using 25 IMRT cervical cancer plans to demonstrate the effect of geometrical uncertainties on the EUD and TCP. The different components of the total geometrical uncertainties budget were estimated. The biological effects were compared by calculating the EUDs from the trial DVHs. The impact of geometric uncertainties was calculated as a percentage of the difference between 〖EUD〗_static and 〖EUD〗_motion, where the 〖EUD〗_static is the EUD calculated from the target DVHs and 〖EUD〗_motion is averaged, over a 1000 calculated EUDs for each of the analyzed IMRT treatment plans. The multivariate nonlinear regression was used to find the predicted difference between the static and motion EUD. The estimate of the systematic and random motion errors were Σ_(total(SI,LR,AP)) (mm)=(2.6; 2.5; 1.8) and σ_(total(SI,LR,AP)) (mm)=(3.4; 1.4; 3.4). For average 〈EUD〉_motion=44.3 Gy (over 25 patients) we have found a TCP decrease of about 1%, %(ΔTCP)≈1% for predefined PTV margin. According to the calculated EUD motion-distributions, for particular patients, the CTV does receive the prescribed EUD of 45 Gy. The predicted difference in EUD showed that our isotropic margin of 10 mm is large enough to absorb geometric uncertainties and ensure dose coverage of the moving CTV in the cervical cancer patients.

Radio-induced cardiotoxicity: From physiopathology and risk factors to adaptation of radiotherapy treatment planning and recommended cardiac follow-up

Cancer and cardiovascular disease (CVD) are the leading cause of mortality worldwide, and breast cancer (BC) the most common malignancy affecting women worldwide. Radiotherapy is an important component of BC treatment and participates in CVD occurrence. It seems, therefore, crucial to gather both radiation oncology and cardiology medical fields to improve the follow-up quality of our BC patients. This review aims at updating our knowledge regarding cardiotoxicities risk factors, and consequently, doses constraints in case of 3D-conformal and IMRT treatment planning. Then we will develop how to reduce cardiac exposure and what kind of cardiac follow-up we could recommend to our breast cancer patients.

Quality assurance of IMRT treatment plans for a 1.5 T MR-linac using a 2D ionization chamber array and a static solid phantom

MR-guided radiotherapy requires novel quality assurance (QA) methods for intensity-modulated radiotherapy treatment plans (TPs). Here, an optimized method for TPs for a 1.5 T MR-linac was developed and implemented clinically.A static solid phantom and an MR-compatible 2D ionization chamber array were used. The array’s response with respect to the incident beam gantry angles was characterized for four different orientations of the array relative to the beam. A lookup table was created identifying the optimum orientation for each gantry angle. For the QA of clinical MR-linac TPs, beams were grouped according to their gantry angles and measured with up to four setups. The method was applied to n = 106 clinical TPs of 54 patients for different tumour entities. Reference plans and plans created in the online adaptive workflow were analysed, using a local 3%/3 mm gamma criterion for dose values larger than 30% of the maximum. Pass rates were averaged over all beam groups.The array’s response strongly depends on the beam incidence angle. Optimum angles typically range from −10° to 80° around the phantom setup angle. Consequently, plan verification required up to four setups. For clinical MR-linac TPs, the overall median pass rate was 98.5% (range 88.6%–100%). Pass rates depended on the tumour entity. Median pass rates were for liver metastases stereotactic body radiotherapy 99.2%, prostate cancer 99%, pancreatic cancer 98.9%, lymph node metastases 98.7%, partial breast irradiation (PBI) 98%, head-and-neck cancer 97.7%, rectal cancer 94% and others 96.6%. 85% of plans were accepted straightaway, with pass rates above 95%. A single plan with a pass rate below 90% was subsequently verified with a modified method. Off-axis target volumes, e.g. PBI, were verified successfully using a lateral shift of the phantom.The method is suitable to verify reference and online adapted TPs for a 1.5 T MR-linac, including plans for off-axis target volumes.

Using deep learning to predict beam-tunable Pareto optimal dose distribution for intensity-modulated radiation therapy.

Many researchers have developed deep learning models for predicting clinical dose distributions and Pareto optimal dose distributions. Models for predicting Pareto optimal dose distributions have generated optimal plans in real time using anatomical structures and static beam orientations. However, Pareto optimal dose prediction for intensity-modulated radiation therapy (IMRT) prostate planning with variable beam numbers and orientations has not yet been investigated. We propose to develop a deep learning model that can predict Pareto optimal dose distributions by using any given set of beam angles, along with patient anatomy, as input to train the deep neural networks. We implement and compare two deep learning networks that predict with two different beam configuration modalities. We generated Pareto optimal plans for 70 patients with prostate cancer. We used fluence map optimization to generate 500 IMRT plans that sampled the Pareto surface for each patient, for a total of 35000 plans. We studied and compared two different models, Models I and II. Although they both used the same anatomical structures - including the planning target volume (PTV), organs at risk (OARs), and body - these models were designed with two different methods for representing beam angles. Model I directly uses beam angles as a second input to the network as a binary vector. Model II converts the beam angles into beam doses that are conformal to the PTV. We divided the 70 patients into 54 training, 6 validation, and 10 testing patients, thus yielding 27000 training, 3000 validation, and 5000 testing plans. Mean square loss (MSE) was taken as the loss function. We used the Adam optimizer with a default learning rate of 0.01 to optimize the network's performance. We evaluated the models' performance by comparing their predicted dose distributions with the ground truth (Pareto optimal) dose distribution, in terms of dose volume histogram (DVH) plots and evaluation metrics such as PTV D98 , D95 , D50 , D2 , Dmax , Dmean , Paddick Conformation Number, R50, and Homogeneity index. Our deep learning models predicted voxel-level dose distributions that precisely matched the ground truth dose distributions. The DVHs generated also precisely matched the ground truth. Evaluation metrics such as PTV statistics, dose conformity, dose spillage (R50), and homogeneity index also confirmed the accuracy of PTV curves on the DVH. Quantitatively, Model I's prediction error of 0.043 (confirmation), 0.043 (homogeneity), 0.327 (R50), 2.80% (D95), 3.90% (D98), 0.6% (D50), and 1.10% (D2) was lower than that of Model II, which obtained 0.076 (confirmation), 0.058 (homogeneity), 0.626 (R50), 7.10% (D95), 6.50% (D98), 8.40% (D50), and 6.30% (D2). Model I also outperformed Model II in terms of the mean dose error and the max dose error on the PTV, bladder, rectum, left femoral head, and right femoral head. Treatment planners who use our models will be able to use deep learning to control the trade-offs between the PTV and OAR weights, as well as the beam number and configurations in real time. Our dose prediction methods provide a stepping stone to building automatic IMRT treatment planning.

Edge effects in 3D dosimetry: characterisation and correction of the non-uniform dose response of PRESAGE®

Previous work has shown that PRESAGE® can be used successfully to perform 3D dosimetric measurements of complex radiotherapy treatments. However, measurements near the sample edges are known to be difficult to achieve. This is an issue when the doses at air-material interfaces are of interest, for example when investigating the electron return effect (ERE) present in treatments delivered by magnetic resonance (MR)-linac systems.To study this effect, a set of 3.5 cm-diameter cylindrical PRESAGE® samples was uniformly irradiated with multiple dose fractions, using either a conventional linac or an MR-linac. The samples were imaged between fractions using an optical-CT, to read out the corresponding accumulated doses. A calibration between TPS-predicted dose and optical-CT pixel value was determined for individual dosimeters as a function of radial distance from the axis of rotation. This data was used to develop a correction that was applied to four additional samples of PRESAGE® of the same formulation, irradiated with 3D-CRT and IMRT treatment plans, to recover significantly improved 3D measurements of dose. An alternative strategy was also tested, in which the outer surface of the sample was physically removed prior to irradiation.Results show that for the formulation studied here, PRESAGE® samples have a central region that responds uniformly and an edge region of 6–7 mm where there is gradual increase in dosimeter response, rising to an over-response of 24%–36% at the outer boundary. This non-uniform dose response increases in both extent and magnitude over time. Both mitigation strategies investigated were successful. In our four exemplar studies, we show how discrepancies at edges are reduced from 13%–37% of the maximum dose to between 2 and 8%. Quantitative analysis shows that the 3D gamma passing rates rise from 90.4, 69.3, 63.7 and 43.6% to 97.3, 99.9, 96.7 and 98.9% respectively.

Automated Hypofractionated IMRT treatment planning for early-stage breast Cancer

BackgroundHypofractionated whole-breast irradiation is a standard adjuvant therapy for early-stage breast cancer. This study evaluates the plan quality and efficacy of an in-house-developed automated radiotherapy treatment planning algorithm for hypofractionated whole-breast radiotherapy.MethodsA cohort of 99 node-negative left-sided breast cancer patients completed hypofractionated whole-breast irradiation with six-field IMRT for 42.56 Gy in 16 daily fractions from year 2016 to 2018 at a tertiary center were re-planned with an in-house-developed algorithm. The automated plan-generating C#-based program is developed in a Varian ESAPI research mode. The dose-volume histogram (DVH) and other dosimetric parameters of the automated and manual plans were directly compared.ResultsThe average time for generating an autoplan was 5 to 6 min, while the manual planning time ranged from 1 to 1.5 h. There was only a small difference in both the gantry angles and the collimator angles between the autoplans and the manual plans (ranging from 2.2 to 5.3 degrees). Autoplans and manual plans performed similarly well in hotspot volume and PTV coverage, with the autoplans performing slightly better in the ipsilateral-lung-sparing dose parameters but were inferior in contralateral-breast-sparing. The autoplan dosimetric quality did not vary with different breast sizes, but for manual plans, there was worse ipsilateral-lung-sparing (V4Gy) in larger or medium-sized breasts than in smaller breasts. Autoplans were generally superior than manual plans in CI (1.24 ± 0.06 vs. 1.30 ± 0.09, p < 0.01) and MU (1010 ± 46 vs. 1205 ± 187, p < 0.01).ConclusionsOur study presents a well-designed standardized fully automated planning algorithm for optimized whole-breast radiotherapy treatment plan generation. A large cohort of 99 patients were re-planned and retrospectively analyzed. The automated plans demonstrated similar or even better dosimetric quality and efficacy in comparison with the manual plans. Our result suggested that the autoplanning algorithm has great clinical applicability potential.

Adaptation of Unified Dosimetry Index (UDI) and Critical Organ Scoring Index (COSI) for Target and Organ at Risk (OAR) in Prostate IMRT Technique

The aim of the study is to formulate a dosimetry index that can be used for Quality assessment of IMRT treatment plans of prostate site. To compare different radiotherapy plans different indices are utilized. The present study will attempt to combine all the indices into one unified quality index which can define the quality of a treatment plan and serve as a quality indicator of treatment plan developed among similar and different modalities. This quality index will incorporate homogeneity index, conformity index, dose gradient and coverage index resulting in a direct comparison to an ideal plan. We developed another index in this study related to sparing of organs i.e. UDI (OARs) that incorporate the doses of OARs to compare which plan is optimum in terms of target coverage and OARs sparing. In comparison to this UDI (OARs) another work was done to analyze the dosimetry of overall plan including target coverage and organ at risk sparing i.e. critical organ scoring index (COSI). To validate UDI (Target), UDI (OARs) and COSI, 22 patients were studied which were planned on IMRT. We also define the ranking of the plan according to their target coverage and organ sparing quantities as ‘Excellent’, ‘Good’, ‘Average’ or ‘Poor’ plan. UDI index for the target and organ at risk is found to be a useful tool to quantify a treatment plan quality when volumes with different dose prescription are treated. Keywords: Treatment planning system, Conformity index, Homogeneity index, Dose coverage, Dose gradient, Critical organs DOI : 10.7176/CMR/12-1-04 Publication date: January 31 st 2020

Evaluating the predictive value of quantec rectum tolerance dose suggestions on acute rectal toxicity in prostate carcinoma patients treated with IMRT.

To investigate the predictive value of convenience of rectum dosimetry with Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC) dose limits, maximum rectum dose (Dmax), total rectal volume (TVrectum), rectal volume included in PTV (VrectumPTV) on Grade 2-3 acute rectal toxicity for utilization in clinical practice. Numerous previous data have reported frequent acute proctitis after external-beam RT of prostate cancer. Predicting toxicity limited with dose information is inadequate in clinical practice due to comorbidities and medications used. Sixty-four non-metastatic prostate cancer patients treated with IMRT were enrolled. Patients were treated to a total dose of 70-76Gy. Rectal dose volume histograms (DVH) of all patients were evaluated retrospectively, and a QUANTEC Score between 0 and 5 was calculated for each patient. The correlation between the rectal DVH data, QUANTEC score, TVrectum, VrectumPTV, rectum Dmax and Grade 2-3 rectal toxicity was investigated. In the whole group grade 1, 2 and 3 acute rectal toxicities were 25%, 18.8% and 3.1%, respectively. In the DVH data, rectum doses of all patients were under RTOG dose limits. Statistically significant correlation was found between grade 2-3 rectal toxicity and TVrectum (p=0,043); however. It was not correlated with QUANTEC score, VrectumPTV and Dmax. Our results were not able to show any significant correlation between increasing convenience with QUANTEC limits and lower rectal toxicity. Conclusively, new dosimetric definitions are warranted to predict acute rectal toxicity more accurately in prostate cancer patients during IMRT treatment.

An investigation of a novel reusable radiochromic sheet for 2D dose measurement.

Radiochromic film remains a useful and versatile clinical dosimetry tool. Current film options are single use. Here, we introduce a novel prototype two-dimensional (2D) radiochromic sheet, which optically clears naturally at room temperature after irradiation and can be reused. We evaluate the sheets for potential as a 2D dosimeter and as a radiochromic bolus with capability for dose measurement. A novel derivative of reusable Presage® was manufactured into thin sheets of 5mm thickness. The sheets contained 2% cumin-leucomalachitegreen-diethylamine (LMG-DEA) and plasticizer (up to 25% by weight). Irradiation experiments were performed to characterize the response to megavoltage radiation, including dose sensitivity, temporal decay rate, consistency of repeat irradiations, intra and inter-sheet reproducibility, multi-modality response (electrons and photons), and temperature sensitivity (22°C to 36°C). The local change in optical-density (ΔOD), before and after radiation, was obtained with a flat-bed film scanner and extracting the red channel. Repeat scanning enabled investigation of the temporal decay of ΔOD. Additional studies investigated clinical utility of the sheets through application to IMRT treatment plans (prostate and a TG119 commissioning plan), and a chest wall electron boost treatment. In the latter test, the sheet performed as a radiochromic bolus. The radiation induced OD change in the sheets was found to be proportional to dose and to exponentially decay to baseline in ~24h (R2=0.9986). The sheet could be reused and had similar sensitivity (within 1% after the first irradiation) for at least eight irradiations. Importantly, no memory of previous irradiations was observed within measurement uncertainty. The consistency of dose response from photons (6 and 15MV) and electrons (6-20MeV) was found to be within calibration uncertainty (~1%). The dose sensitivity of the sheets had a temperature dependence of 0.0012 ΔOD/°C. For the short (1min) single field IMRT QA verification, good agreement was observed between the Presage sheet and EBT film (gamma pass rate 97% at 3% 3mm dose-difference and distance-to-agreement tolerance, with a 10% threshold). For the longer (~13min) TG-119 9-field IMRT verification the gamma agreement was lower at 93% pass rate at 5% 3mm, 10% threshold, when compared with Eclipse. The lower rate is attributed to uncertainty arising from signal decay during irradiation and indicates a current limitation. For the electron cutout treatment, both Presage and EBT agreed well (within 2% RMS difference) but differed from the Eclipse treatment plan (~7% RMS difference) indicating some limitations to the Eclipse modeling in this case. The worst case estimates of uncertainty introduced by the signal decay for deliveries of 2, 5, and 10min are 0.6%, 1.4%, and 2.8% respectively. Reusable Presage sheets show promise for 2D dose measurement and as a radiochromic bolus for in vivo dose measurement. The current prototype is suitable for deliveries of length up to 5min, where the uncertainty introduced by signal decay is anticipated to be ~1% (worst case 1.4%), or for longer deliveries where there is no temporal modulation (e.g. physical compensators, or open beams). Additionally, spatial resolution is limited by sheet thickness and scanner resolution, resulting in a practical resolution of 0.8mm.

Development and validation of a 1.5T MR-Linac full accelerator head and cryostat model for Monte Carlo dose simulations.

To develop, implement, and validate a full 1.5T/7MV magnetic resonance (MR)-Linac accelerator head and cryostat model in EGSnrc for high precision dose calculations accounting for magnetic field effects that are independent from the vendor treatment planning system. Primary electron beam parameters for the implemented model were adapted to be in accordance with measured dose profiles of the Elekta Unity (Elekta AB, Stockholm, Sweden). Parameters to be investigated were the mean electron energy as well as the Gaussian radial intensity and energy distributions. Energy tuning was done comparing depth dose profiles simulated with monoenergetic beams of varying energies to measurements. The optimum radial intensity distribution was found by varying the radial full width at half maximum (FWHM) and comparing simulated and measured lateral profiles. The influence of the energy distribution was investigated by comparing simulated lateral and depth dose profiles with varying energy spreads to measured data. Comparison of simulations and measurements was performed by calculating average and maximum local dose deviations. The model was validated recalculating a clinical intensity-modulated radiation therapy plan for the MR-Linac and comparing the resulting dose distribution with simulations from the commercial treatment planning system Monaco using the gamma criterion. Comparison of simulated and measured data showed that the optimum initial electron beam for MR-Linac simulations was monoenergetic with an electron energy of (7.4±0.2)MeV. The optimum Gaussian radial intensity distribution has a FWHM of (2.2±0.3)mm. The average relative deviations were smaller than 1% for all simulated profiles with optimum electron parameters, whereas the largest maximum deviation of 2.07% was found for the 22×22cm cross-plane profile. Profiles were insensitive to energy spread variations. The IMRT plan recalculated with the final MR-Linac model with optimized initial electron beam parameters showed a gamma pass rate of 99.83 % using a gamma criterion of 3%/3mm. The EGSnrc MR-Linac model developed in this study showed good accordance with measurements and was successfully used to recalculate a first full clinical IMRT treatment plan. Thus, it shows the general possibility for future secondary dose calculations of full IMRT plans with EGSnrc, which needs further detailed investigations before clinical use.

MRIgRT dynamic lung motion thorax anthropomorphic QA phantom: Design, development, reproducibility, and feasibility study.

To design, manufacture, and evaluate a dynamic magnetic resonance imaging/computed tomography (MRI/CT)-compatible anthropomorphic thorax phantom used to credential MR image-guided radiotherapy (MRIgRT) systems participating in NCI-sponsored clinical trials. The dynamic anthropomorphic thorax phantom was constructed from a water-fillable acrylic shell that contained several internal structures representing radiation-sensitive organs within the thoracic region. A custom MR/CT visible cylindrical insert was designed to simulate the left lung with a centrally located tumor target. The surrounding lung tissue was constructed from a heterogeneous in-house mixture using petroleum jelly and miniature (2-4mm diameter) styrofoam balls and the tumor structure was manufactured from liquid PVC plastic. An MR conditional pneumatic system was developed to allow the MRIgRT insert to move in similar inhale/exhale motions. TLDs and radiochromic EBT3 film were inserted into the phantom to measure absolute point doses and dose distributions, respectively. The dynamic MRIgRT thorax phantom was evaluated through a reproducibility study and a feasibility study. Comprehensive end-to-end examinations were done where the phantom was imaged on a CT, an IMRT treatment plan was created and an MR image was captured to verify treatment setup. Then, the phantom was treated on an MRIgRT system. The reproducibility study evaluated how well the phantom could be reproduced in an MRIgRT system by irradiating three times on an Elekta's 1.5T Unity system. The phantom was shipped to three independent institutions and was irradiated on either an MRIdian cobalt-60 (60 Co) or an MRIdian linear accelerator system. Treatment evaluations used TLDs and radiochromic film to compare the planned treatment reported on the treatment planning software against the measured dose on the dosimeters. The phantom on the Unity system had reproducible TLD doses measurements (SD<1.5%). The measured TLD to calculated dose ratios from the reproducibility and feasibility studies ranged from 0.93 to 1.01 and 0.96 to 1.03, respectively. Using a 7%/5mm gamma analysis criteria, the reproducibility and feasibility studies resulted in an average passing rate of 93.3% and 96.8%, respectively. No difference was noted in the results between the MRIdian 60 Co and MRIdian 6MV linac delivery to the phantom and all treatment evaluations were within IROC-Houston's acceptable criterion. A dosimetrically tissue equivalent, CT/MR visible, motion-enabled anthropomorphic MRIgRT thorax phantom was constructed to simulate a lung cancer patient and was evaluated as an appropriate NIH credentialing tool used for MRIgRT systems.

Analysis of OAR Doses in DMPO-Based Prostate IMRT

Radiotherapy plays an eminent role in the treatment of the prostate cancers with rotational radiotherapy taking the leading edge in the recent years. The study aims to find the relevance of the Direct machine parameter optimization (DMPO) based static gantry IMRT treatment planning in the step and shoot environment for ca prostate case with involved nodes in the era of rotational radiotherapy. Also we aim to determine the relation between the femur dose and the rectum dose, and mean rectum dose and rest rectum volume if any. 32 ca prostate patients were selected retrospectively for this study with prescribed dose of 68Gy (EQD2 72.08Gy α/β 10) and 50Gy in 25 fractions to the prostate and nodes respectively treated with the simultaneous integrated boost technique. The plans were done on a treatment planning system with DMPO algorithm and delivered using step and shoot method in LINAC. For PTV, 95% volume covering 100% prescribed dose was achieved with posterior rectum wall receiving less than 34Gy and organs at risk (OARs) within the tolerance limit. Along with the percentage volume of the rest rectum (rectum minus PTV with 0.5cm margin), the mean doses of rectum, rest rectum, rest bladder (bladder minus PTV with 0.5cm margin) and femur were noted. The relation between rectum mean dose and femur mean dose were studied. Also comparison was done between mean rectum dose and percentage (%)rest rectum volume. The doses received by 5% of the femoral heads were also studied. Plot of mean rectum dose versus mean femur dose received by each patients shows the inverse relation with correlation coefficient of -0.401 (p<0.05). Also the mean rectum dose and %rest rectum volume are inversely related with correlation coefficient of -0.81 (p<0.001). The mean of the dose received by 5% volume of right and left femur is 40.39±4.7Gy and 40.2±3.8Gy respectively with p<0.001. The mean dose of rectum was 36.5±5.5Gy with median dose of 37.6Gy. For rest rectum the mean dose was 28.7±5.6Gy with median dose of 27.8Gy. The mean dose of the left and right femoral heads and rest bladder were 24.2±4.5Gy, 23.7±4.5Gy and 31.6±6.2Gy respectively with p value <0.001. For an optimal plan for ca prostate case achieved on a TPS with DMPO algorithm, we could predict the mean dose of rectum to be 36.5±5.5Gy with 95% Confidence interval for mean femoral head dose of 23.9±4.2Gy. We could also infer that the femur dose and mean rectum dose, and the %rest rectum volume and mean rectum dose were inversely correlated with correlation coefficient of -0.401 and -0.810 respectively. Ca Prostate being the fourth most commonly occurring cancers is widely treated using radiotherapy as an important mode of treatment with different treatment techniques all over the world. Although the rotational radiotherapy delivers the treatment in comparably lower beam on time, the DMPO based static gantry IMRT plans still holds the dosimetric relevance with comparable OAR doses.

Predicting Optimal IMRT Dose Objectives by Relating TCP and NTCP to Target Dose Heterogeneity

There is growing interest in relaxing homogeneity constraints in IMRT treatments for solid tumors that do not contain normal tissue. Knowledge-based dose predictions can improve plan quality, but depend on historical data typically incorporating homogeneity constraints. An analytical TCP and NTCP model could be used to estimate optimal target dose heterogeneity prior to plan optimization by assuming basic properties of the dose distribution, thereby improving prospective plan quality. We derived and validated an analytical model of TCP and NTCP based on the properties of co-planar VMAT including target dose heterogeneity, beam penumbra, and dose spillage outside of the target. We derived the TCP/NTCP model with input parameters including tumor size, tumor edge dose (prescription dose), tumor center dose, fractionation, and estimates of standard radio-biological parameters such as tumor clonogen density, radio-sensitivity mean and variance, and Lyman-Kutcher-Burman (LKB) NTCP model parameters. To validate the model, we created a virtual phantom incorporating a spherical tumor with 2 cm diameter and produced a series of VMAT plans in a treatment planning system with varying central target dose between 108% and 262% of the prescription while minimizing dose spillage outside of the target. Differential DVHs were exported and used to estimate TCP and NTCP using the Marsden and LKB models assuming a radio-resistant tumor prescribed 60 Gy in 30 fractions completely surrounded by a single cylindrical parallel organ. The TCP and NTCP were also calculated for these test cases using the analytical model, and compared to the values calculated using the treatment planning system data. The target heterogeneity and corresponding TCP and NTCP values estimated by the analytical model and planning system based approaches are summarized in Table 1. The errors in the TCP and NTCP estimates of the model were within 0.04 and 0.01, respectively, for all plans, despite not requiring a dose calculation. The analysis also demonstrated a steady increase in TCP with increased central target dose, and a transient decrease in NTCP. A central target dose of 111% minimized NTCP, and a central target dose of 124% maximized TCP while providing the same or lower NTCP as a homogeneous target dose. We have derived an analytical model of TCP and NTCP based on simple tumor and dose characteristics that can be applied immediately following contouring to estimate optimal target dose heterogeneity. This tool can be used to improve the dose objectives used in prospective clinical trials. We will present results following model validation in multiple clinical scenarios and demonstrate patient use cases.Abstract 3822; Table 1Central Target Dose (% of Rx)TCPNTCPPinnacleModelPinnacleModel1080.680.640.0500.0481110.720.680.0490.0471240.800.790.0500.0471610.920.930.0530.0522050.960.970.0590.0592340.980.980.0570.0632490.990.980.0580.0642621.000.980.0560.065 Open table in a new tab

Prostate Intra-fraction Motion Monitoring Using an Integrated Linear Accelerator kV Imaging System

To assess the outcomes of intra-fraction prostate motion monitoring using an integrated linear accelerator based KV imaging system. Between June 2017 and December 2018, 1516 radiation treatment fractions were delivered to 35 prostate cancer patients. Each patient had 3 gold fiducial markers placed in the prostate prior to initiation of therapy and underwent IMRT treatment planning using an arc therapy technique. A commercially available integrated kV imaging system (capable of 2D and 3D cone beam imaging) with localization software was used for each treatment. Prior to initiation of treatment, a cone beam CT was obtained to align the fiducial markers and assess bladder and rectal filling. Once treatment was initiated, a planar kV image was obtained every 40 degrees in the treatment arc (approximately every 5-7 seconds) and analyzed for fiducial marker position. If the fiducial marker position varied by greater than 3.5mm, the treatment was stopped with less than 1 MU delivered. Fiducial marker position was reassessed with another kV image within 10 seconds and if the markers were still outside of the 3.5mm threshold, the patient was repositioned and treatment resumed. All treatment fractions were delivered successfully without imaging system failure. The average time for a treatment fraction was 9.4 minutes ± 2.7 minutes (6.8 - 28.8 minutes). 672 of the treatments (44.3%) had at least one beam interruption due to fiducial marker variation over 3.5 mm from anticipated location. 171 (11.3%) treatments required at least 2 treatment interruptions, 74 (4.9%) treatments required at least 3 treatment interruptions and 35 (2.3%) treatments required 4 or more interruptions. Patient repositioning was required in 365 (24.1%) treatments. An integrated linear accelerator based kV imaging system can be used to successfully and efficiently to monitor intra-fraction prostate motion.

IMRT national audit in Portugal

PurposeThe IAEA newly developed “end-to-end” audit methodology for on-site verification of IMRT dose delivery has been carried out in Portugal in 2018. The main goal was to evaluate the physical aspects of the head and neck (H&N) cancer IMRT treatments. This paper presents the national results. MethodsAll institutions performing IMRT treatments in Portugal, 20 out of 24, have voluntarily participated in this audit. Following the adopted methodology, a Shoulder, Head and Neck End-to-End phantom (SHANE) – that mimics an H&N region, underwent all steps of an IMRT treatment, according to the local practices. The measurements using an ionization chamber placed inside the SHANE phantom at four reference locations (three in PTVs and one in the spinal cord) and an EBT3 film positioned in a coronal plane were compared with calculated doses. FilmQA Pro software was used for film analysis. ResultsFor ionization chamber measurements, the percent difference was within the specified tolerances of ±5% for PTVs and ±7% for the spinal cord in all participating institutions. Considering film analysis, gamma passing rates were on average 96.9%±2.9% for a criterion of 3%/3 mm, 20% threshold, all above the acceptance limit of 90%. ConclusionsThe national results of the H&N IMRT audit showed a compliance between the planned and the delivered doses within the specified tolerances, confirming no major reasons for concern. At the same time the audit identified factors that contributed to increased uncertainties in the IMRT dose delivery in some institutions resulting in recommendations for quality improvement.