Intensity Modulated Arc Therapy Delivery Research Articles

Intensity modulated arc therapy implementation in a three phase adaptive (18)F-FDG-PET voxel intensity-based planning strategy for head-and-neck cancer.

BackgroundThis study investigates the implementation of a new intensity modulated arc therapy (IMAT) class solution in comparison to a 6-static beam step-and-shoot intensity modulated radiotherapy (s-IMRT) for three-phase adaptive 18F-FDG-PET-voxel-based dose-painting-by-numbers (DPBN) for head-and-neck cancer.MethodsWe developed 18F-FDG-PET-voxel intensity-based IMAT employing multiple arcs and compared it to clinically used s-IMRT DPBN. Three IMAT plans using 18F-FDG-PET/CT acquired before treatment (phase I), after 8 fractions (phase II) and CT acquired after 18 fractions (phase III) were generated for each of 10 patients treated with 3 s-IMRT plans based on the same image sets. Based on deformable image registration (ABAS, version 0.41, Elekta CMS Software, Maryland Heights, MO), doses of the 3 plans were summed on the pretreatment CT using validated in-house developed software. Dosimetric indices in targets and organs-at-risk (OARs), biologic conformity of treatment plans set at ≤5 %, treatment quality and efficiency were compared between IMAT and s-IMRT for the whole group and for individual patients.ResultsDoses to most organs-at-risk (OARs) were significantly better in IMAT plans, while target levels were similar for both types of plans. On average, IMAT ipsilateral and contralateral parotid mean doses were 14.0 % (p = 0.001) and 12.7 % (p < 0.001) lower, respectively. Pharyngeal constrictors D50% levels were similar or reduced with up to 54.9 % for IMAT compared to s-IMRT for individual patient cases. IMAT significantly improved biologic conformity by 2.1 % for treatment phases I and II. 3D phantom measurements reported an agreement of ≥95 % for 3 % and 3 mm criteria for both treatment modalities. IMAT delivery time was significantly shortened on average by 41.1 %.ConclusionsIMAT implementation significantly improved the biologic conformity as compared to s-IMRT in adaptive dose-escalated DPBN treatments. The better OAR sparing and faster delivery highly improved the treatment efficiency.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-016-0629-3) contains supplementary material, which is available to authorized users.

SU‐E‐T‐830: A Plan Comparison Study between Three Different Intensity Modulated Arc Therapy Techniques

Purpose: Intensity Modulated Arc Therapy (IMAT) is gaining widespread attention from various radiation oncology vendors and several centers are implementing them clinically. The aim of this study is to compare Plan quality between Rapid Arc (Varian), Volumetric Modulated Arc Therapy (Elekta) and Smart Arc (Pinnacle) for the same dataset and plan objectives. Methods: 3 anonymized prostate cancer cases with the same DICOM RT structure set were used to compare between the 3 IMAT delivery schemes. The Rapid Arc plans were generated using Eclipse Treatment Planning System (TPS), the VMAT plans were generated using MONACO TPS, and the Smart Arc plans were generated using Pinnacle TPS. The Dose Volume Histograms (DVH) was exported for each plan and plotted together to compare the DVH between 3 different TPS for each patient. In each case the plan objectives were kept the same. Also, the RT dose files were compared from the 3 different delivery methods by importing them into a 3rd party software. Results: To achieve the same plan quality, the Rapid Arc plans were generated using 2 arcs. There was no substantial improvement in plan quality when 2 arcs were used in VMAT plans using MONACO, therefore plans were generated using only one 360 degree arc for both VMAT and Smart Arc plans. The VMAT plans have more dose heterogeneity across the target volume while organs at risk sparing varied depending on the case. The Smart Arc plans had the lowest total monitor units among the 3 delivery methods while the Rapid Arc plans had the largest because 2 arcs were needed to achieve same plan quality.Conclusion: All 3 IMAT methods produced acceptable plans for Prostate IMRT. The relative strengths of each system varied depending on the complexity of the case and there is a trade off between each delivery method.

SU-GG-T-114: A Feasibility Study of a 4D Intensity-Modulated Arc Tracking Technique for Treatment of Moving Targets

Purpose: A major challenge of Intensity‐modulated arc therapy(IMAT) is in dealing with moving targets since the conventional gating technique may not be easily applied. The purpose of this work is to develop a 4D IMAT tracking technique for moving targets and investigate its feasibility for clinical use. Method and Materials: 4DCT images at 10 different breathing phases were acquired. The target was contoured for each phase and a 3D RapidArc plan was generated based on the 50%phase CTimages. Each control point in the RapidArc plan was then correlated to the patient's respiration phases with consideration given to changes in gantry speed and dose rate. A novel program was developed to read the RapidArc plan and target contours from all 10 phases. In order to generate a 4D RapidArc plan with the capability of DMLC tracking, the MLC modification algorithm originally developed for on‐line plan modification was used to modify the MLC leaf positions at each control point to correspond to the respiration phases. 4D RapidArc plans for phantom and lung cases were used to evaluate the proposed technique. Experimental measurements for both 4D and 3D plans in a moving phantom were performed and the results were compared with the calculated and measured results for the 3D RapidArc plans in a “static” phantom. Results: Analysis of isodose and gamma distributions indicated that the dose distribution in the moving target delivered using the proposed technique is comparable to that in a static target delivered using a 3D RapidArc plan. The results were much improved using the 4D IMAT delivery compared with the 3D plans for a moving target. Conclusion: A 4D RapidArc tracking technique can be implemented and our results indicated that it is feasible to incorporate the intra‐fraction organ motion into a 4D IMAT planning to track the moving targets.

A method incorporating 4DCT data for evaluating the dosimetric effects of respiratory motion in single-arc IMAT

This study introduces a method incorporating 4DCT data to determine the impact of respiratory motion in single-arc intensity-modulated arc therapy (IMAT). Simulation was done by re-warping the static dose distribution of all phases of a 4DCT image set with a 3D deformation map to reference CT images at end-inspiration and end-expiration. To calculate the dose received during respiration under IMAT, the control points were interpolated and re-distributed into separate IMAT plans corresponding to each respiratory phase. This study also investigated the role that plan complexity may play in the dosimetric impact of the respiratory motion in the delivery of IMAT. The dosimetric impact of organ motion was evaluated by analyzing the degradation of D95, D50 and D05 of the CTV and PTV. From the results shown for the patients in this study who had maximum organ motion displacement ∼15 mm, the dosimetric impact is rather small. Therefore, our preliminary results suggest that respiratory motion of less than 1.5 cm may be ignored for both moderately and highly modulated IMAT, irrespective of the number of fractions. Specifically, highly modulated plans only increased the degradation of D95 of the DVH curves for a single fraction by 2% in the CTV and 9% in the PTV compared to the expected value of the multi-fraction plan.

Dosimetric verification of IMAT delivery with a conventional EPID system and a commercial portal dose image prediction tool

The electronic portal imaging device (EPID) is a system for checking the patient setup; as a result of its integration with the linear accelerator and software customized for dosimetry, it is increasingly used for verification of the delivery of fixed-field intensity-modulated radiation therapy (IMRT). In order to extend such an approach to intensity-modulated arc therapy (IMAT), the combined use of an EPID system and a portal dose image prediction (PDIP) tool has been investigated. The dosimetric behavior of an EPID system, mechanically reinforced to maintain its positional stability during the accelerator gantry rotation, has been studied to assess its ability to measure portal dose distributions for IMAT treatment beams. In addition, the PDIP tool of a commercial treatment planning system, commonly used for static IMRT dosimetry, has been validated for simulating the PDIs of IMAT treatment fields. The method has been applied to the delivery verification of 23 treatment fields that were measured in their dual mode of IMRT and IMAT modalities. The EPID system has proved to be appropriate for measuring the PDIs of IMAT fields; additionally the PDIP tool was able to simulate these accurately. The results are quite similar to those obtained for static IMRT treatment verification, although it was necessary to investigate the dependence of the EPID signal and of the accelerator monitor chamber response on variable dose rate. Our initial tests indicate that the EPID system, together with the PDIP tool, is a suitable device for the verification of IMAT plan delivery; however, additional tests are necessary to confirm these results.

Intensity-Modulated Arc Therapy with Simultaneous Integrated Boost in the Treatment of Primary Irresectable Cervical Cancer

To report on the planning procedure, quality control, and clinical implementation of intensity-modulated arc therapy (IMAT) delivering a simultaneous integrated boost (SIB) in patients with primary irresectable cervix carcinoma. Six patients underwent PET-CT (positron emission tomography-computed tomography) and MRI (magnetic resonance imaging) before treatment planning. Prescription (25 fractions) was (1) a median dose (D(50)) of 62, 58 and 56 Gy to the primary tumor (GTV_cervix), primary clinical target volume (CTV_cervix) and its planning target volume (PTV_cervix), respectively; (2) a D(50) of 60 Gy to the PET-positive lymph nodes (GTV_nodes); (3) a minimal dose (D(98)) of 45 Gy to the planning target volume of the elective lymph nodes (PTV_nodes). IMAT plans were generated using an anatomy-based exclusion tool with the aid of weight and leaf position optimization. The dosimetric delivery of IMAT was validated preclinically using radiochromic film dosimetry. Five to nine arcs were needed to create valid IMAT plans. Dose constraints on D(50) were not met in two patients (both GTV_cervix: 1 Gy and 3 Gy less). D(98) for PTV_nodes was not met in three patients (1 Gy each). Film dosimetry showed excellent gamma evaluation. There were no treatment interruptions. IMAT allows delivering an SIB to the macroscopic tumor without compromising the dose to the elective lymph nodes or the organs at risk. The clinical implementation is feasible.

SU‐FF‐T‐126: Initial Experience with Philips SmartArc On An Elekta Linac

Purpose: The recent introduction of delivery control systems capable of delivering intensity modulated arc therapy (IMAT) has led to renewed interest in arc‐based IMRT delivered on a conventional linac. A key challenge to successful implementation of IMAT is the need for robust inverse planning tools capable of properly accounting for IMAT delivery constraints. In this work, we evaluate a new IMAT planning module, called SmartArc, designed for the Pinnacle3 treatment planning system. Method and Materials: In February 2009, we received an alpha version of the SmartArc planning module for the Pinnacle3 treatment planning system. The SmartArc tools were developed by RaySearch and build on the existing direct machine parameter optimization (DMPO) functionality in Pinnacle3. The SmartArc module was used to create 9 IMAT plans (4 head‐and‐neck, 2 lung, and 3 prostates). Each plan was verified on an Elekta Synergy with the PreciseBeam VMAT delivery control system using the IBA Matrixx 2D ionization chamber array. Each plan was compared with a corresponding fixed‐field IMRT or helical tomotherapy plan in terms of both plan quality and delivery efficiency. Results: A single arc was sufficient to produce highly conformal IMAT treatment plans for the prostate and lung cases. On average, the prostate plans required 536 monitor units and delivered in 1.9 minutes while the lung plans required 750 monitor units and delivered in 2.2 minutes. For the more complex head‐and‐neck cases, the use of two arcs was necessary to meet the treatment goals and an average of 607 monitor units and 5.4 minutes were required. Conclusion: The SmartArc module in Pinnacle3 serves as a robust IMAT planning tool capable of planning both single arc and multiple arc deliveries. 2D gamma analysis showed that all plans verified with greater than 95% of the measured points meeting our 3%/3mm criteria.Research supported by Elekta

Experimental measurements and Monte Carlo simulations for dosimetric evaluations of intrafraction motion for gated and ungated intensity modulated arc therapy deliveries

Respiratory gated radiation therapy allows for a smaller margin expansion for the planning target volume (PTV) to account for respiratory induced motion and is emerging as a common method to treat lung and liver tumors. We investigated the dosimetric effect of free motion and gated delivery for intensity modulated arc therapy (IMAT) with experimental measurements and Monte Carlo simulations. The impact of PTV margin and duty cycle for gated delivery is studied with Monte Carlo simulations. A motion phantom is used for this study. Two sets of contours were drawn on the mid-inspiration CT scan of this motion phantom. For each set of contours, an IMAT plan to be delivered with constant dose rate was created. The plans were generated on a CT scan of the phantom in the static condition with 3 mm PTV margin and applied to the motion phantom under four conditions: static, full superior–inferior (SI) motion (A = 1 cm, T = 4 s) and gating conditions (25% and 50% duty cycles) with full SI motion. A 6 by 15 cm piece of radiographic film was placed in the sagittal plane of the phantom and then irradiated under all measurement conditions. Film calibration was performed with a step-wedge method to convert optical density to dose. Gated IMAT delivery was first validated in 2D by comparing static film with that from gating and full motion. A previously verified simulation tool for IMRT that takes the log files from the multileaf collimator (MLC) controller and the gating system were adapted to simulate the delivered IMAT treatment for full 3D dosimetric analysis. The IMAT simulations were validated against the 2D film measurements. The resultant IMAT simulations were evaluated with dose criteria, dose-volume histograms and 3D gamma analysis. We validated gated IMAT deliveries when we compared the static film with the one from gating using 25% duty cycle using 2D gamma analysis. Within experimental and setup uncertainties, film measurements agreed with their corresponding simulated plans using 2D gamma analysis. Finally, when planning with margins designed for gating with 25% duty cycle and applying 50% or no gating during treatment, the dose differences in Dmin, D99% and D95% of the clinical target volume can be up to 27 cGy, 20 cGy and 18 cGy, respectively, for a plan with 200 cGy prescription dose. We have experimentally delivered gated IMAT with constant dose rate to a motion phantom and assessed their accuracies with film dosimetry and Monte Carlo simulations. Film dosimetry demonstrated that 25% gating and static plans are within 2%, 2 mm. The Monte Carlo simulation method was employed to generate dose delivered in 3D to a motion phantom, and the dosimetric results were reported. Since our film measurements agreed well with Monte Carlo simulations, we can reliably use this simulation tool to further study the dosimetric effects of target motion and effectiveness of gating for IMAT deliveries.

Sci-Fri PM: Planning-03: Fundamental understanding of the inter-relation of arc range, angular dose rate and MLC leaf position optimization of Intensity Modulated Arc Therapy for a concave target.

Intensity Modulated Arc Therapy (IMAT) is a rotational variant of Intensity Modulated Radiation Therapy (IMRT) that can be implemented with or without angular dose rate optimization. The purpose of this study is to determine the inter-relationship among arc range, Multi-Leaf Collimator (MLC) leaf positions and angular dose rate optimizations for IMAT delivery of a concave target. A concave planning target volume (PTV) with central cylindrical organ at risk (OAR) was used in dissecting the inter-relationships. Plans with full and limited arc range were generated using leaf position optimization (LPO) alone, angular dose rate optimization (ADRO) alone or LPO followed by ADRO. Two initial IMAT arcs were conformal avoidance arcs created with 5° angular increments where MLC leaf positions were determined from the beams eye view to irradiate the PTV but avoid the OAR. The objective function value (evaluating dose to PTV and OAR), a conformity index, dose homogeneity index, mean dose to OAR and normal tissues were computed and used to evaluate the treatment plans. Dose rate variations and MLC leaf movements as a function of gantry angle were examined in this approach. The results demonstrate that LPO followed by ADRO provided the lowest objective function values, best conformity and dose homogeneity indices, and third in mean dose to OAR and normal tissues for the complete arc range. Future work will address different strategies for simultaneously optimizing both the MLC leaf positions and the variable angular dose rate for IMAT delivery and compare single and multiple arc plans.

IMAT‐SIM: A new method for the clinical dosimetry of intensity‐modulated arc therapy (IMAT)

Dynamic-gantry multi-leaf collimator (MLC)-based, intensity-modulated radiotherapy (IMAT) has been proposed as an alternative to tomotherapy. In contrast to fixed-gantry, MLC-based intensity-modulated radiotherapy (IMRT), where commercial treatment planning systems (TPS) or dosimetric analysis software currently provide many automatic tools enabling two-dimensional (2D) detectors (matrix or electronic portal imaging devices) to be used as measurement systems, for the planning and delivery of IMAT these tools are generally not available. A new dosimetric method is proposed to overcome some of these limitations. By converting the MLC files of IMAT beams from arc to fixed gantry-angle modality, while keeping the leaf trajectories equal, IMAT plans can be both simulated in the TPS and executed as fixed-gantry, sliding-window DMLC treatments. In support of this idea, measurements of six IMAT plans, in their double form of original arcs and converted fixed-gantry DMLC beams (IMAT-SIM), have been compared among themselves and with their corresponding IMAT-SIM TPS calculations. Radiographic films and a 2D matrix ionization chamber detector rigidly attached to the accelerator gantry and set into a cubic plastic phantom have been used for these measurements. Finally, the TPS calculation-algorithm implementations of both conformal dynamic MLC arc (CD-ARC) modalities, used for clinical IMAT calculations, and DMLC modalities (IMAT-SIM), proposed as references for validating IMAT plan dose-distributions, have been compared. The comparisons between IMAT and IMAT-SIM delivered beams have shown very good agreement with similar shapes of the measured dose profiles which can achieve a mean deviation (+/-2sigma) of (0.35+/-0.16) mm and (0.37+/-0.14)%, with maximum deviations of 1.5 mm and 3%. Matching the IMAT measurements with their corresponding IMAT-SIM data calculated by the TPS, these deviations remain in the range of (1.01+/-0.28) mm and (-1.76+/-0.42)%, with maximums of 3 mm and 5%, limits generally accepted for IMRT plan dose validation. Differences in the algorithm implementations have been found, but by correcting CD-ARC calculations for the leaf-end transmission offset (LTO) effect the IMAT and IMAT-SIM simulations agree well in terms of final dose distributions. The differences found between IMAT and the IMAT-SIM beam measurements are due to the different controls of leaf motion (via electron gun delay in the latter) that cannot be used in the former to correct possible speed variations in the rotation of the gantry. As the IMAT delivered beams are identical to what the patient will receive during the treatment, and the IMAT-SIM beam calculations made by the TPS reproduce exactly the treatment plans of that patient, the accuracy of this new dosimetric method is comparable to that which is currently used for static IMRT. This new approach of 2D-detector dosimetry, together with the commissioning, quality-assurance, and preclinical dosimetric procedures currently used for IMRT techniques, can be applied and extended to any kind of dynamic-gantry MLC-based treatment modality either CD-ARC or IMAT.

SU‐FF‐T‐104: Arc Sequencing — An Inverse Planning Technique for Intensity Modulated Arc Therapy

Purpose: Intensity modulated arc therapy (IMAT) is a rotational approach to IMRT in which the leaves of the multileaf collimator move continuously during arced beam delivery. Overlapping arcs are used to deliver optimized intensity patterns from each beam direction. Despite the promising nature of IMAT, its potential has gone largely unrealized due to a lack of robust inverse planning tools. To address this, we have developed an IMAT arc sequencing algorithm that translates optimized intensity maps into deliverable IMAT plans. Material and Methods: The arc‐sequencing algorithm uses simulated annealing to simultaneously optimize the aperture shapes and weights throughout each arc. The sequencer enforces the delivery constraints while minimizing the discrepancies between the optimized and sequenced intensity maps. The performance of the arc sequencer has been tested for ten patient cases. An IMRT plan was developed for each case using the Pinnacle3 treatment planning system, and the arc sequencer translated the optimized plans into deliverable IMAT plans. Results: Ten IMAT patient plans were developed covering the following sites: 3 prostate, 3 brain, 2 head‐and‐neck, 1 lung, and 1 pancreas. Seven coplanar IMAT plans were created using an average of 4.6 arcs and 685 monitor units. Additionally, three noncoplanar plans were created using an average of 16 arcs and 498 monitor units. The results demonstrate that efficient IMAT delivery plans can be developed that combine the dosimetric advantages of arc therapy with the dose painting capabilities of IMRT. Only modest degradation was seen between the pre and post sequenced plans. Plan accuracy was verified using Monte Carlo dose calculations with each arc approximated as static beams separated by two degrees. An average sequencing time of under 25 minutes was observed. Conclusions: An arc‐sequencing algorithm has been developed that can serve as the first robust inverse planning tool for IMAT.

Clinical implementation of intensity-modulated arc therapy (IMAT) for rectal cancer

Purpose In rectal cancer, combined radiotherapy and chemotherapy, either pre- or postoperatively, is an accepted treatment. Late small bowel (SB) toxicity is a feared side effect and limits radiation-dose escalation in a volume-dependent way. A planning strategy for intensity- modulated arc therapy (IMAT) was developed, and IMAT was clinically implemented with the aim to reduce the volume of SB irradiated at high doses and thus reduce SB toxicity. We report on the treatment plans of the first 7 patients, on the comparison of IMAT with conventional 3D planning (3D), and on the feasibility of IMAT delivery. Methods and materials Seven patients, who were referred to our department for preoperative ( n = 4) or postoperative ( n = 3) radiotherapy for rectal cancer, gave written consent for IMAT treatment. All patients had a planning CT in prone position. The delineation of the clinical target volume was done after fusion of CT and MRI, with the help of a radiologist. For the IMAT plan, arcs were generated using an anatomy-based segmentation tool. The optimization of the arcs was done by weight optimization (WO) and leaf position optimization (LPO), both of which were adapted for IMAT purposes. The 3D plans used one posterior and two lateral wedged beams, of which the outlines were shaped to the beam's-eye view projection of the planning target volume (PTV). Beam WO was done by constrained matrix inversion. For dose–volume histogram analysis, all plans were normalized to 45 Gy as median PTV dose. Polymer gel dosimetry (PGD) on a humanoid phantom was used for the validation of the total chain (planning to delivery). IMAT treatments were delivered by an Elekta Sl i Plus linear accelerator using prototype software with the same interlock class as in clinical mode. Results The IMAT plan resulted in 3 to 6 arcs, with a mean delivery time of 6.3 min and a mean of 456 monitor units (MU) for a 180 cGy fraction. The minimal dose in the PTV was not significantly different between 3D and IMAT plans. Inhomogeneity was highest for the IMAT plans (14.1%) and lowest for the 3D plans (9.9%). Mean dose to the SB was significantly lower for the IMAT plans (12.4 Gy) than for the 3D plans (17.0 Gy). The volume of SB receiving less than any dose level was lower for the IMAT plans than for 3D plans. Integral dose was lower in the IMAT plans than for the 3D plans (respectively 244 J and 262 J to deliver 45 Gy). Differences between the PGD measured dose and the calculated dose were as small for IMAT as for 3D treatments. Conclusion IMAT plans are deliverable within a 5–10-minute time slot, and result in a lower dose to the SB than 3D plans, without creating significant underdosages in the PTV. PGD showed that IMAT delivery is as accurate as 3D delivery.