Beam Plans Research Articles

Assessing localized dosimetric effects due to unplanned gas cavities during pelvic MR-guided radiotherapy using Monte Carlo simulations.

It has been proposed that beam modulation and opposing beam configurations can cancel effects of the Electron Return Effect (ERE) during MR-guided radiotherapy (MRgRT). However, this may not always be the case for unplanned gas cavities outside of the target in the pelvic region. We evaluate dosimetric effects, including effects in the rectal wall, due to unplanned spherical air cavities during MRgRT. Nine virtual cuboid water phantoms containing spherical air cavities (0.5-7.5cm diameter) and a reference phantom without air were created. Monte Carlo dose calculations of 7MV photons under the influence of a 1.5T transverse magnetic field were produced using Monaco 5.19.02 Treatment Planning System (TPS) (Elekta AB, Stockholm, Sweden). Cavities in the path of a single and multiple beam plans were considered. Dose distributions of phantoms with and without air cavities were compared (ΔD% ) using a spherical coordinate system originating in the center of the cavity. Effects in the rectal wall were quantified by comparing dose volume histogram (DVH) parameters for solid and gaseous filling from simulated rectal wall structures. Max(ΔD% ) of ~70% and 20% were observed around large cavities in the path of a single and multiple beam plans, respectively. Approximately 45cm3 of phantom surrounding the largest cavity in a single beam received dose changes of >10%. Dmean in the rectal wall was unchanged when comparing gaseous and solid filling in the path of a single beam; however, D1cc and Dmax increased by up to ~45% and ~63%, respectively. Unplanned gas cavities in the path of a single beam during pelvic MRgRT with a 1.5T transverse magnetic field cause dose changes which may impact toxicity in the rectal wall, depending on local dose and fractionation. Effects are reduced but not eliminated with a five-beam plan.

Optimizing Efficiency and Safety in External Beam Radiotherapy Using Automated Plan Check (APC) Tool and Six Sigma Methodology

PURPOSE To develop and implement an automated plan check (APC) tool using a Six Sigma methodology with the aim of improving safety and efficiency in external beam radiotherapy. METHODS The Six Sigma define-measure-analyze-improve-control (DMAIC) framework was used by measuring defects stemming from treatment planning that were reported to the departmental incidence learning system (ILS). The common error pathways observed in the reported data were combined with our departmental physics plan check list, and AAPM TG-275 identified items. Prioritized by risk priority number (RPN) and severity values, the check items were added to the APC tool developed using Varian Eclipse Scripting Application Programming Interface (ESAPI). At 9 months post-APC implementation, the tool encompassed 89 check items, and its effectiveness was evaluated by comparing RPN values and rates of reported errors. To test the efficiency gains, physics plan check time and reported error rate were prospectively compared for 20 treatment plans. RESULTS The APC tool was successfully implemented for external beam plan checking. FMEA RPN ranking re-evaluation at 9 months post-APC demonstrated a statistically significant average decrease in RPN values from 129.2 to 83.7 (P < .05). After the introduction of APC, the average frequency of reported treatment-planning errors was reduced from 16.1% to 4.1%. For high-severity errors, the reduction was 82.7% for prescription/plan mismatches and 84.4% for incorrect shift note. The process shifted from 4σ to 5σ quality for isocenter-shift errors. The efficiency study showed a statistically significant decrease in plan check time (10.1 ± 7.3 min, P = .005) and decrease in errors propagating to physics plan check (80%). CONCLUSIONS Incorporation of APC tool has significantly reduced the error rate. The DMAIC framework can provide an iterative and robust workflow to improve the efficiency and quality of treatment planning procedure enabling a safer radiotherapy process.

Longitudinal Review of the White Board Implementation Shows Improved Efficiency in Radiation Therapy Planning

Radiation therapy has a planning process that begins when a patient decides to undergo radiation therapy and spans to the time when the patient starts his/her treatments. This process can be lengthy due to the complex work flow that takes place between CT simulation and treatment delivery. The process also requires detailed communication between physicians, physicists, planners and other support staff. Any delays or mistakes in the work flow can cause safety concerns, patient anxiety and/or decrease the efficiency of the center. Our growing department uses a system of Smarter Radiation Oncology tools, which, among other things, includes the use of our home grown electronic tracking system called “Treatment Planning White Board” to streamline the process of radiation therapy treatment planning. In this study we evaluate the longitudinal efficiency of the Treatment Planning White Board. From 2016 – 2018, we tracked 5860 patient treatments’ (2016:391; 2017: 2494; 2018: 2975) using the Treatment Planning White Board. There are 9 steps that are tracked between CT simulation (CT SIM) to virtual simulation (VSIM) prior to treatment. Each step is assigned a predetermined time limit and personnel responsible for each of the steps are sent reminders throughout the process. We calculated the work hours (WHrs) (excluding nights, weekends and holidays) for each step of the process. We report the overall and prostate cancer external beam planning specific WHrs averages with the standard error of mean. With the implementation of the White Board, there was a significant improvement in WHrs of the treatment planning process (from CT SIM to VSIM): 2016, 155 +/- 8.5 WHrs; 2017, 113 +/- 2.3 WHrs; 2018, 86 +/- 1.2 WHrs. In 2018, there was a 45% and 25% improvement in overall treatment planning WHrs compared to 2016 and 2017, respectively. The most significant changes occurred in the physics and planning parts of the process, including plan generation after contour review and QA aspects. There were no differences in the physician aspects of the process: contour time, MD plan review or MD sign off. When we further evaluated the WHrs for prostate cancer external beam planning, these differences persisted. CT SIM to VSIM WHrs were: 2016, 160 +/- 15.5 WHrs; 2017,127 +/- 6.3 WHrs; 2018, 104.3 +/- 4 WHrs. In 2018, there was a 35% and 18% improvement in prostate cancer treatment planning WHrs compared to 2016 and 2017, respectively. Similar to the overall trends, the improvement were seen in the physics and planning aspects of the process. Treatment Planning White Board implementation has led to longitudinal improvement in efficiency in radiation therapy planning at our busy academic center mainly by improving the physics and planning aspects of the treatment. We hope to use this data to refine planning time and alleviate patient anxiety, by providing them with better estimates and transparency of the planning process.

Optimizing efficiency and safety in external beam radiotherapy using automated plan check (APC) tool and six sigma methodology.

PurposeTo develop and implement an automated plan check (APC) tool using a Six Sigma methodology with the aim of improving safety and efficiency in external beam radiotherapy.MethodsThe Six Sigma define‐measure‐analyze‐improve‐control (DMAIC) framework was used by measuring defects stemming from treatment planning that were reported to the departmental incidence learning system (ILS). The common error pathways observed in the reported data were combined with our departmental physics plan check list, and AAPM TG‐275 identified items. Prioritized by risk priority number (RPN) and severity values, the check items were added to the APC tool developed using Varian Eclipse Scripting Application Programming Interface (ESAPI). At 9 months post‐APC implementation, the tool encompassed 89 check items, and its effectiveness was evaluated by comparing RPN values and rates of reported errors. To test the efficiency gains, physics plan check time and reported error rate were prospectively compared for 20 treatment plans.ResultsThe APC tool was successfully implemented for external beam plan checking. FMEA RPN ranking re‐evaluation at 9 months post‐APC demonstrated a statistically significant average decrease in RPN values from 129.2 to 83.7 (P < .05). After the introduction of APC, the average frequency of reported treatment‐planning errors was reduced from 16.1% to 4.1%. For high‐severity errors, the reduction was 82.7% for prescription/plan mismatches and 84.4% for incorrect shift note. The process shifted from 4σ to 5σ quality for isocenter‐shift errors. The efficiency study showed a statistically significant decrease in plan check time (10.1 ± 7.3 min, P = .005) and decrease in errors propagating to physics plan check (80%).ConclusionsIncorporation of APC tool has significantly reduced the error rate. The DMAIC framework can provide an iterative and robust workflow to improve the efficiency and quality of treatment planning procedure enabling a safer radiotherapy process.

Initial commissioning measurements of respiratory gated liver VMAT stereotactic ablative body radiotherapy

A dosimetric evaluation of a respiratory gated VMAT SABR technique was performed at two different beam energies. Dynamic ion chamber, EBT3 film and Fricke-xylenol orange-gelatin (FXG) gel measurements were acquired using a motion phantom with custom inserts for each dosimeter. Ion chamber and gel dosimeter measurements show good agreement between the measured and calculated plan dose within the planning target volume (PTV). Lower agreement than expected was observed between calculated plan dose and measured film dose, particularly for the 10 MV flattening filter free beam plan, a result that warrants further investigation.

Robust beam orientation optimization for intensity-modulated proton therapy.

Dose conformality and robustness are equally important in intensity modulated proton therapy (IMPT). Despite the obvious implication of beam orientation on both dosimetry and robustness, an automated, robust beam orientation optimization algorithm has not been incorporated due to the problem complexity and paramount computational challenge. In this study, we developed a novel IMPT framework that integrates robust beam orientation optimization (BOO) and robust fluence map optimization (FMO) in a unified framework. The unified framework is formulated to include a dose fidelity term, a heterogeneity-weighted group sparsity term, and a sensitivity regularization term. The L2, 1/2-norm group sparsity is used to reduce the number of active beams from the initial 1162 evenly distributed noncoplanar candidate beams, to between two and four. A heterogeneity index, which evaluates the lateral tissue heterogeneity of a beam, is used to weigh the group sparsity term. With this index, beams more resilient to setup uncertainties are encouraged. There is a symbiotic relationship between the heterogeneity index and the sensitivity regularization; the integrated optimization framework further improves beam robustness against both range and setup uncertainties. This Sensitivity regularization and Heterogeneity weighting based BOO and FMO framework (SHBOO-FMO) was tested on two skull-base tumor (SBT) patients and two bilateral head-and-neck (H&N) patients. The conventional CTV-based optimized plans (Conv) with SHBOO-FMO beams (SHBOO-Conv) and manual beams (MAN-Conv) were compared to investigate the beam robustness of the proposed method. The dosimetry and robustness of SHBOO-FMO plan were compared against the manual beam plan with CTV-based voxel-wise worst-case scenario approach (MAN-WC). With SHBOO-FMO method, the beams with superior range robustness over manual beams were selected while the setup robustness was maintained or improved. On average, the lowest [D95%, V95%, V100%] of CTV were increased from [93.85%, 91.06%, 70.64%] in MAN-Conv plans, to [98.62%, 98.61%, 96.17%] in SHBOO-Conv plans with range uncertainties. With setup uncertainties, the average lowest [D98%, D95%, V95%, V100%] of CTV were increased from [92.06%, 94.83%, 94.31%, 78.93%] in MAN-Conv plans, to [93.54%, 96.61%, 97.01%, 91.98%] in SHBOO-Conv plans. Compared with the MAN-WC plans, the final SHBOO-FMO plans achieved comparable plan robustness and better OAR sparing, with an average reduction of [Dmean, Dmax] of [6.31, 6.55] GyRBE for the SBT cases and [1.89, 5.08] GyRBE for the H&N cases from the MAN-WC plans. We developed a novel method to integrate robust BOO and robust FMO into IMPT optimization for a unified solution of both BOO and FMO, generating plans with superior dosimetry and good robustness.

Patient organ doses from megavoltage computed tomography delivery with a helical tomotherapy unit using a general treatment planning system.

The purpose of this study was to quantify actual patient organ doses from megavoltage computed tomography (MVCT) using an MVCT beam model of a helical tomotherapy unit in a general treatment planning system (TPS). Dosimetric parameters (percentage depth dose, lateral beam profile, and longitudinal beam profile) of the MVCT beam were measured using Gafchromic EBT3 films (ISP Corporation, Wayne, NJ, USA) and used for beam modeling in a Pinnacle3 TPS (Philips, Amsterdam, Netherlands); this TPS is widely used with linear accelerators. The created beam model was adjusted and validated by assessing point doses in a cylindrical phantom in static and helical beam plans with fine, normal and coarse pitches. Maximum doses delivered to important organs from MVCT delivery for five clinical cases were calculated using the created beam model. The difference (average ± one standard deviation for all evaluation points) between calculated and measured doses was −0.69 ± 1.20% in the static beam plan. In the helical beam plan, the differences were 1.83 ± 2.65%, 1.35 ± 5.94% and −0.66 ± 8.48% for fine, normal and coarse pitches, respectively. The average maximum additional dose to important organs from MVCT in clinical cases was 0.82% of the prescribed dose. In conclusion, we investigated a method for quantifying patient organ dose from MVCT delivery on helical tomotherapy using an MVCT beam model in a general TPS. This technique enables estimation of the patient-specific organ dose from MVCT delivery, without the need for additional equipment.

Polymorph segmentation representation for medical image computing

Background and objectiveSegmentation is a ubiquitous operation in medical image computing. Various data representations can describe segmentation results, such as labelmap volumes or surface models. Conversions between them are often required, which typically include complex data processing steps. We identified four challenges related to managing multiple representations: conversion method selection, data provenance, data consistency, and coherence of in-memory objects. MethodsA complex data container preserves identity and provenance of the contained representations and ensures data coherence. Conversions are executed automatically on-demand. A graph containing the implemented conversion algorithms determines each execution, ensuring consistency between various representations. The design and implementation of a software library are proposed, in order to provide a readily usable software tool to manage segmentation data in multiple data representations. A low-level core library called PolySeg implemented in the Visualization Toolkit (VTK) manages the data objects and conversions. It is used by a high-level application layer, which has been implemented in the medical image visualization and analysis platform 3D Slicer. The application layer provides advanced visualization, transformation, interoperability, and other functions. ResultsThe core conversion algorithms comprising the graph were validated. Several applications were implemented based on the library, demonstrating advantages in terms of usability and ease of software development in each case. The Segment Editor application provides fast, comprehensive, and easy-to-use manual and semi-automatic segmentation workflows. Clinical applications for gel dosimetry, external beam planning, and MRI-ultrasound image fusion in brachytherapy were rapidly prototyped resulting robust applications that are already in use in clinical research. The conversion algorithms were found to be accurate and reliable using these applications. ConclusionsA generic software library has been designed and developed for automatic management of multiple data formats in segmentation tasks. It enhances both user and developer experience, enabling fast and convenient manual workflows and quicker and more robust software prototyping. The software’s BSD-style open-source license allows complete freedom of use of the library.

Dosimetric comparison of VMAT with integrated skin flash to 3D field-in-field tangents for left breast irradiation.

Volumetric modulated arc therapy (VMAT) has been implemented for left breast irradiation to reduce prescription dose to the heart and improve dose homogeneity across the targeted breast. Our in‐house method requires application of a bolus during the optimization process with a target outside of the body, then removing the bolus during the final calculation in order to incorporate skin flash in VMAT plans. To quantify the dosimetric trade‐offs between traditional 3D field‐in‐field tangents and VMAT with integrated skin flash for these patients, we compared nine consecutive patients who recently received radiation to their entire left breast but not their regional lymphatics. Tangent plans used non‐divergent tangents of mixed energies and VMAT plans utilized four 6 MV arcs of roughly 260°. Mean dose to the heart, contralateral lung, and contralateral breast and their volume receiving 5%, 10%, and 20% of the prescription dose were higher in all nine VMAT plans than in the static tangential beam plans. For all critical structures, the mean VMAT DVH was higher in the low‐dose region and crossed the 3D field‐in‐field DVH between 23.13% and 34.18% of the prescription dose (984.75‐1454.70 cGy). However, the volume of the contralateral breast and heart receiving the prescription dose was slightly lower in the VMAT plans, but not statistically significant. VMAT provided superior homogeneity, with a mean homogeneity index of 9.41 ± 1.64 compared to 11.05 ± 1.82 for 3D tangents. Results indicate that VMAT spares the heart, contralateral lung, and contralateral breast from prescription dose at the cost of increasing their mean and low‐dose volume and delivers a more homogenous dose distribution to the breast. For these reasons, VMAT is selectively applied at the request of the physician for left breast radiation without respiratory gating to spare the heart from prescription dose in cases of poor anatomical geometry.

Cherenkov imaging for linac beam shape analysis as a remote electronic quality assessment verification tool.

A remote imaging system tracking Cherenkov emission was analyzed to verify that the linear accelerator (linac) beam shape could be quantitatively measured at the irradiation surface for Quality Audit (QA). The Cherenkov camera recorded 2D dose images delivered on a solid acrylonitrile butadiene styrene (ABS) plastic phantom surface for a range of square beam sizes, and 6MV photons. Imaging was done at source to surface distance (SSD) of 100cm and compared to GaF film images and linac light fields of the same beam sizes, ranging over 5×5cm2 up to 20×20cm2 . Line profiles of each field were compared in both X and Y jaw directions. Each measurement was repeated on two different Clinac2100 machines. An interreader comparison of the beam width interpretation was completed using procedures commonly employed for beam to light field coincidence verification. Cherenkov measurements are also done for beams of complex treatment plan and isocenter QA. The Cherenkov image widths matched with the measured GaF images and light field images, with accuracy in the range of ±1mm standard deviation. The differences between the measurements were minor and within tolerance of geometrical requirement of standard linac QA procedures conducted by human setup verification, which had a similar error range. The measurement made by the remote imaging system allowed for beam shape extraction of radiation fields at the SSD location of the beam. The proposed Cherenkov image acquisition system provides a valid way to remotely confirm radiation field sizes and provides similar information to that obtained from the linac light field or GaF film estimates of the beam size. The major benefit of this approach is that with a fixed installation of the camera, testing could be done completely under software control with automated image analysis, potentially simplifying conventional QA procedures with appropriate calibration of boundary definitions, and the natural extension to capturing dynamic treatment beamlets at SSD could have future value, such as verification of beam plans with complex beam shapes, like IMRT or "star-shot" QA for the isocenter.

An orthotopic non-small cell lung cancer model for image-guided small animal radiotherapy platforms.

An orthotopic non-small cell lung cancer model in NMRI-nude mice was established to investigate the complementary information acquired from 80 kVp microcone-beam CT (micro-CBCT) and bioluminescence imaging (BLI) using different angles and filter settings. Different micro-CBCT-based radiation-delivery plans were evaluated based on their dose-volume histogram metrics of tumor and organs at risk to select the optimal treatment plan. H1299 cell suspensions injected directly into the lung render exponentially growing single tumor nodules whose CBCT-based volume quantification strongly correlated with BLI-integrated intensity. Parallel-opposed single angle beam plans through a single lung are preferred for smaller tumors, whereas for larger tumors, plans that spread the radiation dose across healthy tissues are favored. Closely mimicking a clinical setting for lung cancer with highly advanced preclinical radiation treatment planning is possible in mice developing orthotopic lung tumors. BLI and CBCT imaging of orthotopic lung tumors provide complementary information in a temporal manner. The optimal radiotherapy plan is tumor volume-dependent.

Exploring the feasibility of a clinical proton beam with an adaptive aperture for pre-clinical research.

To investigate whether the Mevion S250i with HYPERSCAN clinical proton system could be used for pre-clinical research with millimetric beams. The nozzle of the proton beam line, consisting of an energy modulation system (EMS) and an adaptive aperture (AA), was modelled with the TOPAS Monte Carlo Simulation Toolkit. With the EMS, the 230 MeV beam nominal range can be decreased in multiples of 2.1 mm. Monte Carlo dose calculations were performed in a mouse lung tumour CT image. The AA allows fields as small as 5 × 1 mm2 to be used for irradiation. The best plans to give 2 Gy to the tumour were derived from a set of discrete energies allowed by the EMS, different field sizes and beam directions. The final proton plans were compared to a precision photon irradiation plan. Treatment times were also assessed. Seven different proton beam plans were investigated, with a good coverage to the tumour (D95 > 1.95 Gy, D5 < 2.3 Gy) and with potentially less damage to the organs at risk than the photon plan. For very small fields and low energies, the number of protons arriving to the target drops to 1-3%, nevertheless the treatment times would be below 5 s. The proton plans made in this study, collimated by an AA, could be used for animal irradiation. This is one of the first study to demonstrate the feasibility of pre-clinical research with a clinical proton beam with an adaptive aperture used to create small fields.

Investigations of line scanning proton therapy with dynamic multi-leaf collimator

PurposeScanning proton therapy has dosimetric advantage over passive treatment, but has a large penumbra in low-energy region. This study investigates the penumbra reduction when multi-leaf collimators (MLCs) are used for line scanning proton beams and secondary neutron production from MLCs. MethodsScanning beam plans with and without MLC shaping were devised. Line scanning proton plan of 36 energy layers between 71.2 and 155.2 MeV was generated. The MLCs were shaped according to the cross-sectional target shape for each energy layer. The two-dimensional doses were measured through an ion-chamber array, depending on the presence of MLC field, and Monte Carlo (MC) simulations were performed. The plan, measurement, and MC data, with and without MLC, were compared at each depth. The secondary neutron dose was simulated with MC. Ambient neutron dose equivalents were computed for the line scanning with 10 × 10 × 5 cm3 volume and maximum proton energy of 150 MeV, with and without MLCs, at lateral distances of 25–200 cm from the isocenter. The neutron dose for a wobbling plan with 10 × 10 × 5 cm3 volume was also evaluated. ResultsThe lateral penumbra width using MLC was reduced by 23.2% on average, up to a maximum of 32.2%, over the four depths evaluated. The ambient neutron dose equivalent was 18.52% of that of the wobbling beam but was 353.1% larger than the scanning open field. ConclusionsMLC field shaping with line scanning reduced the lateral penumbra and should be effective in sparing normal tissue. However, it is important to investigate the increase in neutron dose.

FMEA of MR-Only Treatment Planning in the Pelvis

PurposeTo evaluate the implementation of a magnetic resonance (MR)-only workflow (ie, implementing MR simulation as the primary planning modality) using failure mode and effects analysis (FMEA) in comparison with a conventional multimodality (MR simulation in conjunction with computed tomography simulation) workflow for pelvis external beam planning. Methods and MaterialsTo perform the FMEA, a multidisciplinary 9-member team was assembled and developed process maps, identified potential failure modes (FMs), and assigned numerical values to the severity (S), frequency of occurrence (O), and detectability (D) of those FMs. Risk priority numbers (RPNs) were calculated via the product of S, O, and D as a metric for evaluating relative patient risk. An alternative 3-digit composite number (SOD) was computed to emphasize high-severity FMs. Fault tree analysis identified the causality chain leading to the highest-severity FM. ResultsSeven processes were identified, 3 of which were shared between workflows. Image fusion and target delineation subprocesses using the conventional workflow added 9 and 10 FMs, respectively, with 6 RPNs >100. By contrast, synthetic computed tomography generation introduced 3 major subprocesses and propagated 46 unique FMs, 15 with RPNs >100. For the conventional workflow, the largest RPN scores were introduced by image fusion (RPN range, 120-192). For the MR-only workflow, the highest RPN scores were from inaccuracies in target delineation resulting from misinterpretation of MR images (RPN = 240) and insufficient management of patient- and system-level distortions (RPN = 210 and 168, respectively). Underestimation (RPN = 140) or overestimation (RPN = 192) of bone volume produced higher RPN scores. The highest SODs for both workflows were related to changes in target location because of internal anatomy changes (conventional = 961, MR-only = 822). ConclusionsFMEA identified areas for mitigating risk in MR-only pelvis RTP, and SODs identified high-severity process modes. Efforts to develop a quality management program to mitigate high FMs are underway.

Second cancer risk after radiation therapy of ependymoma using the flattening filter free irradiation mode of a linear accelerator.

Pediatric patients suffering from ependymoma are usually treated with cranial or craniospinal three‐dimensional (3D) conformal radiotherapy (3DCRT). Intensity‐modulated techniques spare dose to the surrounding tissue, but the risk for second malignancies may be increased due to the increase in low‐dose volume. The aim of this study is to investigate if the flattening filter free (FFF) mode allows reducing the risk for second malignancies compared to the mode with flattening filter (FF) for intensity‐modulated techniques and to 3DCRT. A reduction of the risk would be advantageous for treating pediatric ependymoma. 3DCRT was compared to intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT) with and without flattening filter. Dose–volume histograms (DVHs) were compared to evaluate the plan quality and used to calculate the excess absolute risk (EAR) to develop second cancer in the brain. Dose verification was performed with a two‐dimensional (2D) ionization chamber array and the out‐of‐field dose was measured with an ionization chamber to determine the EAR in peripheral organs. Delivery times were measured. Both VMAT and IMRT achieved similar plan quality in terms of dose sparing in the OAR and higher PTV coverage as compared to 3DCRT. Peripheral dose in low‐dose region, which is proportional to the EAR in organs located in this region, for example, gonads, bladder, or bowel, could be significantly reduced using FFF. The lowest peripheral EAR and lowest delivery times were hereby achieved with VMATFFF. The EAR calculated based on DVH in the brain could not be reduced using FFF mode. VMATFFF improved the target coverage and homogeneity and kept the dose in the OAR similar compared to 3DCRT. In addition, delivery times were significantly reduced using VMATFFF. Therefore, for radiotherapy of ependymoma patients, VMATFFF may be considered advantageous for the combination of Elekta Synergy linac and Oncentra External Beam planning system used in this study.

Effects of spot parameters in pencil beam scanning treatment planning.

Spot size σ (in air at isocenter), interspot spacing d, and spot charge q influence dose delivery efficiency and plan quality in Intensity Modulated Proton Therapy (IMPT) treatment planning. The choice and range of parameters varies among different manufacturers. The goal of this work is to demonstrate the influence of the spot parameters on dose quality and delivery in IMPT treatment plans, to show their interdependence, and to make practitioners aware of the spot parameter values for a certain facility. Our study could help as a guideline to make the trade-off between treatment quality and time in existing PBS centers and in future systems. We created plans for seven patients and a phantom, with different tumor sites and volumes, and compared the effect of small-, medium-, and large-spot widths (σ = 2.5, 5, and 10 mm) and interspot distances (1σ, 1.5σ, and 1.75σ) on dose, spot charge, and treatment time. Moreover, we quantified how postplanning charge threshold cuts affect plan quality and the total number of spots to deliver, for different spot widths and interspot distances. We show the effect of a minimum charge (or MU) cutoff value for a given proton delivery system. Spot size had a strong influence on dose: larger spots resulted in more protons delivered outside the target region. We observed dose differences of 2-13 Gy (RBE) between 2.5 mm and 10 mm spots, where the amount of extra dose was due to dose penumbra around the target region. Interspot distance had little influence on dose quality for our patient group. Both parameters strongly influence spot charge in the plans and thus the possible impact of postplanning charge threshold cuts. If such charge thresholds are not included in the treatment planning system (TPS), it is important that the practitioner validates that a given combination of lower charge threshold, interspot spacing, and spot size does not result in a plan degradation. Low average spot charge occurs for small spots, small interspot distances, many beam directions, and low fractional dose values. The choice of spot parameters values is a trade-off between accelerator and beam line design, plan quality, and treatment efficiency. We recommend the use of small spot sizes for better organ-at-risk sparing and lateral interspot distances of 1.5σ to avoid long treatment times. We note that plan quality is influenced by the charge cutoff. Our results show that the charge cutoff can be sufficiently large (i.e., 106 protons) to accommodate limitations on beam delivery systems. It is, therefore, not necessary per se to include the charge cutoff in the treatment planning optimization such that Pareto navigation (e.g., as practiced at our institution) is not excluded and optimal plans can be obtained without, perhaps, a bias from the charge cutoff. We recommend that the impact of a minimum charge cut impact is carefully verified for the spot sizes and spot distances applied or that it is accommodated in the TPS.

Field-In-Field Plan Versus Tangential Wedged Beam Plan in Chest Wall Radiotherapy of Post-Mastectomy Patients: Treatment Planning Study

Introduction: In this study, dose distribution of the chest wall in post-mastectomy breast cancer patients was evaluated and compared in the tangential wedged beam (TWB) and field-in-field (FIF) plans. Materials and Methods: Thirty-six patients with left-sided breast cancer were enrolled in this study. The FIF and TWB plans were generated for each patient to compare dosimetric parameters of the chest wall. The maximum dose (Dmax), homogeneity index (HI), conformity index (CI), and uniformity index (UI) were defined and used for comparison of the dosimetric parameters of the planning target volume (PTV) in both FIF and TWB plans. The percentage of volumes receiving at least 10, 20, 30, and 40 Gy of the left lung and 5, 10, 20, 25 and 30 Gy of the heart were used to compare the dosimetric results of the organs at risk. Statistical analysis was performed using SPSS, version 20. Results: The FIF plan had significantly lower HI (P=0.000) than the TWB plan, indicating that the FIF plan was better than the TWB plan in PTV. The V40lung (15.36±4.35 vs. 18.37±4.42) and V30heart (8.15±3.75 vs. 10.94±3.94; P=0.000) were significantly lower in the FIF plan than in the TWB plan. In addition, the monitor unit (MU) was significantly lower in the FIF plan than in the TWB plan (227.76 vs. 323.59; P=0.000). Conclusion: The FIF plan significantly reduced the dose volume of the left lung and heart in post-mastectomy radiotherapy compared to the TWB plan. Therefore, the FIF plan is recommended for this purpose.

Radiation therapy of synchronous bilateral breast carcinoma (SBBC) using multiple techniques

The purpose of this study was to establish intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) treatment plans for synchronous bilateral breast cancer (SBBC) and to compare those plans with the previous treatment plans using 3D conformal radiation therapy (3DCRT). The differences among the treatments were also statistically compared regarding dosimetry distribution and treatment efficiency. The research was conducted with 10 SBBC patients. The study established IMRT (12 fields with a single isocenter) and VMAT (2 partial arcs with a single isocenter) treatment plans for SBBC patients and then compared those plans with 3DCRT (8 fields with multiple isocenters). The plans were evaluated based on a dose-volume histogram analysis. For planning target volumes (PTVs), the mean doses and the values of V95%, V105%, conformity index, and homogeneity index were reported. For the organs at risk, the analysis included the mean dose, maximum dose, and VXGy, depending on the organs (lungs, heart, and liver). To objectively evaluate the efficiency of the treatment plans, each plan's beam times, treatment times (including set-up time), and monitor units were compared. Tukey test and one-way analysis of variance were used to compare the PTV and organs at risk values of the 3 techniques. Additionally, the independent-samples t-test was used to compare the 2 techniques (IMRT and VMAT) based on the values of Rt. PTV and Lt. PTV (p < 0.05). For PTV dose distribution, IMRT showed increases of approximately 1.2% in Dmean and of approximately 5.7% in V95% dose distribution compared with 3DCRT. In comparison to VMAT, 3DCRT showed about 3.0% higher dose distribution in Dmean and V95%. IMRT was the best in terms of conformity index and homogeneity index (p < 0.05), whereas 3DCRT and VMAT did not significantly differ from each other. In terms of dose distribution on lungs, heart, and liver, the percentage of volume at high doses such as V30Gy and V40Gy was approximately 70% lower for IMRT and approximately 40% lower for VMAT than for 3DCRT. For distribution volumes of low doses such as V5% and V10%, that for 3DCRT was approximately 60% smaller than for IMRT and approximately 70% smaller than for VMAT. Comparison between IMRT and VMAT showed that the IMRT was superior in all distribution factors. VMAT showed better treatment efficiency than 3DCRT or IMRT. Among the SBBC radiotherapy treatment plans, IMRT was superior to 3DCRT and VMAT in terms of PTV dose distribution, whereas VMAT showed the most outstanding treatment efficiency.

Evaluation of Proton Arc Therapy Using a Double Scattering System with a Single, Pre-Cut Aperture with no Compensator

To simulate and quantitatively evaluate dose from proton arc therapy using a double scattering proton therapy system with a single aperture and no compensator. Proton arc therapy was simulated with 5, 10 and 15 fields using the Mevion S250 double scattering proton therapy system and the Eclipse treatment planning system (TPS). A single aperture that provided the best conformal circular or rectangular apertures covering the tumor from the different angular views was used. No compensator was used to modulate the intensity of the fields. TPS calculated for each field the most efficient range and modulation providing the most conformal dose coverage of the target. To account for dose delivery uncertainties without a compensator, a planning-target-volume (PTV) was defined as the target. The PTV dose coverage from the arc plans were compared with the conventional proton plans prepared with 2 fields using conformal aperture and compensator for each field. Several dosimetric parameters (Dmin, Dmax, Dmean, D95, D90, D50) were used to evaluate the dose coverage of the PTV and sparing of normal tissues. The proton arc plans for spherical or rectangular regular shape targets provided superior conformal plans with lower skin and critical structure doses compared to conventional proton plans. This was due to the use of multiple beams that delivered low doses at the entrance depths and paths through normal tissues. In contrast, conventional plans with only 2 beams had relatively larger entrance and path doses, while the PTV minimum, maximum and mean doses were nearly equal in both arc and conventional plans (Table 1). The patient setup and dose delivery times were comparable with both arc and conventional plans. This has been achievable by only having to position and image the patient once for setup. Furthermore, the use of no field-specific compensator and using of one aperture size eliminated the need to enter the room and replace these devices for each field. Although, this approach of using proton arc plans had several dosimetric and delivery advantages for regular-shaped targets, the advantages has been limited for PTV with complicated shapes. Simulation of proton arc therapy shows that regular shaped targets can be treated with superior dose coverage of the PTV and sparing of critical structures using a single aperture, no compensator proton arc plans compared to conventional 2 beam proton plans with patient specific apertures and compensators. This proton arc therapy approach is superior for regular spherical or rectangular shaped targets.Abstract 3511; Table 1Comparison of different dose parameters between 2 fields conventional beam plan and proton arc beam plans simulated with 5, 10 and 15 fields with a single circular aperture and no compensatorBeamsStructureMinMaxMeanD95D90D502PTV (%)92105103100101103591106104101104109310610310110415921061041011072Lt Femur (cGy)108131692977-29123026551264712171052121410532595128768912381549248713599671303 Open table in a new tab

Use of Synthetic CT for Magnetic Resonance–Only Based External Beam Pelvic Radiation Therapy of Cervical Cancer

The superiority of T2 MRI over CT for cervical cancer tumor delineation is established by GEC-ESTRO. With the emergence of integrated MRI/LINAC technology, MR-only pelvic radiation needs to be studied in the definitive treatment of cervical cancer. In this study, we evaluate the impact of MR on pelvic targets and organs at risk (OARs) demarcation and dose distribution. Eight patients treated for cervical cancer staged FIGO IIA2 to IIIB for which a supine CT simulation and supine MRI of pelvis was performed were reviewed. All patient data was de-identified in accordance with institutional IRB requirements. Diagnostic MR Images were acquired on a 1.5T scanner. The MR images were manually co-registered with CT using Velocity software. CT body was segmented into 3 tissue types known as Dixon map: bone, fat, muscle/organs, by thresholding of CT number, and assigned HU numbers of 300, -100, and 0, respectively, to generate synCT. Treatment planning from CT was recalculated on the synCTs. Contouring of the CTV was performed by a radiologist on CT and T2MR to ensure accuracy. OARs were contoured on both CT and registered T2-weighted MR images. Four-field box conformal 3D-CRT planning to 45 Gy was used for plan generation. Treatment plans from CT was recalculated on the synCTs using same beam arrangements and monitor units. Dosimetric statistical analysis of the targets and OARs was performed using t-test with an alpha of 0.05. The volumetric difference between CT-CTV (mean 292.70 cm3, range 68.40-1139.90 cm3) and MR-CTV (mean 88.96 cm3, range 30.30-331.70 cm3) was not statistically significant (SS) (p=0.06). The mean D99 for CT and MR uterocervix was 45.32 and 44.99 Gy, respectively, and V100 for CT and MR uterocervix was 226.00 and 176.96, respectively, with no statistical difference between D99 or V100 for CT and MRI uterocervix (p=0.26 and 0.24, respectively). All dosimetric data can be found in Table 1 demonstrating non-significant difference on max doses for all OARs.Abstract 3614; Table 1Mean dosimetry metrics and statistical analysis using student's T-test comparing CT to MRICT CTVMRCTVCT UtcxMRUtcxCT BladMR BladCT RectMR RectCT L FHMRL FHCT R FHMR R FHMean Vol cm3292.790.0292.7238.3Mean Max dose Gy47.747.747.947.341.642.340.242.2t-Test P-Value0.060.240.360.060.750.26 Open table in a new tab The use of MRI for external beam planning provides optimal tumor visualization, providing potential clinical impact by improving target coverage. Our findings suggest that defining the CTV on MRI may be more accurate given the inherent MR advantage for pelvic organ visualization. The average CTV volume was smaller on MRI, which could ultimately lead to improved OAR sparing and reduced toxicities. We feel that this small pilot study warrants further investigation in order to fully evaluate the advantage of MR only based external beam planning for cervical cancer.