Intensity-modulated Arc Therapy Plans Research Articles

Application of IMAT versus fixed-gantry IMRT in cervical esophageal cancer : A comparison in dosimetry and implementation

Objective To compare and analyze the characteristics of intensity-modulated arc therapy (IMAT) versus fixed-gantry intensity-modulated radiotherapy (IMRT) in the treatment of cervical esophageal cancer.Methods Ten patients treated in our radiotherapy center were selected for this study.Based on the identical CT and planning target volume (PTV), two IMAT plans were generated with Eclipse ver8.6 planning system.IMAT1 consisting of a single 359.8° rotation, and IMAT2 consisting of two coplanar 359.8° rotations.PTV were prescribed to 60 Gy in 30 fractions.Planning objectives for PTV,corresponding with the IMRT plans, were V98 larger than 97% and V110 no more than 15%.The maximum dose of spinal-cord was constrained below 45 Gy.One-way ANOVA were applied to dose-volume values for PTV and OAR from DVH.Results There were no significant differences between IMRT and IMAT in PTV D98, V98, CI or total-lung V5, V10, V30, V40, V50 and mean lung dose (all P > 0.05).However, the differences were significant in terms of D2, V110 and HI of PTV, V20 of the total-lung (all P<0.05).On the MU,IMRT = 1174.8 MU,IMAT1 =709.7 MU,and IMAT2 =803.8 MU (F =39.25,P =0.000).On the treatment time,IMRT= 14.9 min,IMAT1 = 1.9 min, and IMAT2 =2.66 min (F=45.14,P=0.000).Conclusions IMAT is equal to IMRT in dosimetric evaluation.Due to much less MU and delivery time,IMAT is an ideal technique in treating patients by reducing the uncomfortable influences which could effect the treatment.However, IMAT1 is slightly inferior to IMAT2. Key words: Esophageal neoplasms/radiotherapy; Radiotherapy, intensity-modulated ; Radiotherapy, intensity-modulated arc; Dosimetry

Electromagnetic-Guided Dynamic Multileaf Collimator Tracking Enables Motion Management for Intensity-Modulated Arc Therapy

Intensity-modulated arc therapy (IMAT) is attractive because of high-dose conformality and efficient delivery. However, managing intrafraction motion is challenging for IMAT. The purpose of this research was to develop and investigate electromagnetically guided dynamic multileaf collimator (DMLC) tracking as an enabling technology to treat moving targets during IMAT. A real-time three-dimensional DMLC-based target tracking system was developed and integrated with a linear accelerator. The DMLC tracking software inputs a real-time electromagnetically measured target position and the IMAT plan, and dynamically creates new leaf positions directed at the moving target. Low- and high-modulation IMAT plans were created for lung and prostate cancer cases. The IMAT plans were delivered to a three-axis motion platform programmed with measured patient motion. Dosimetric measurements were acquired by placing an ion chamber array on the moving platform. Measurements were acquired with tracking, without tracking (current clinical practice), and with the phantom in a static position (reference). Analysis of dose distribution differences from the static reference used a γ-test. On average, 1.6% of dose points for the lung plans and 1.2% of points for the prostate plans failed the 3-mm/3% γ-test with tracking; without tracking, 34% and 14% (respectively) of points failed the γ-test. The delivery time was the same with and without tracking. Electromagnetic-guided DMLC target tracking with IMAT has been investigated for the first time. Dose distributions to moving targets with DMLC tracking were significantly superior to those without tracking. There was no loss of treatment efficiency with DMLC tracking.

SU-GG-T-540: Intensity-Modulated Arc Therapy for Stereotactic Radiotherapy of Spinal &amp; Paraspinal Tumors

Purpose: Stereotactic radiosurgery of metastatic spinal tumor with intensity‐modulated radiotherapy(IMRT) technique requires a long treatment time due to an extensive monitor units (MU) resulting from multiple highly intensity‐modulated beams in order to sparing adjacent spinal cord and other critical structures. This study investigates the feasibility of using intensitymodulated arc therapy (IMAT) as an alternative modality with a shorter treatment time while maintaining a compatible dosimetric performance as IMRT technique. Methods/Materials: 8 patients with spinal or paraspinal tumor were recruited in this study. All those patients were previously treated with IMRT technique, in which 18Gy or 24Gy doses were delivered in a single fraction with 11 to 13 coplanar radiation beams. Single arc and 2‐arc IMAT plans were retrospectively generated for each patients using RapidArcTM treatment planning system (Varian Medical System, Sunnyvale, CA). The previous delivered IMRT plans were chosen as a reference. The differences of following parameters between IMAT and IMRT plans were used to evaluate the plan performance: the volumes of PTV receiving 95% and 100% of prescribed dose(V95, V100), the maximum spinal cord dose (MSPDOSE) and the total monitor units (TMU). Results: For all 8 patients, the differences of V95 and V100 between single arc IMAT and IMRT plans are −5.3%±4.8% and −9.3%±7.8% , while the difference of MSPDOSE is 0.23Gy±0.87Gy. In contrary, the differences of V95 and V100 between 2‐arc IMAT and IMRT plans are −0.67%±2.01%, −1.1%±2.23% , while the difference of MSPDOSE is 0.38Gy±0.47GY. The ratios of TMU of single arc and 2‐arc IMAT plans over IMRT plan are 55%±19% and 65%±17%. Conclusion: For stereotactic radiosurgery of spinal tumor,IMRT plan provide better dose coverage than single arc IMAT plan, but 2‐arc IMAT plan is capable of providing a compatible dosimetric performance as IMRT plan while significantly reducing the treatment time.

SU‐GG‐T‐530: Comparison of Coplanar and Non‐Coplanar Intensity Modulated Arc Techniques for Treatment of Intracranial Multi‐Focal Stereotactic Radiosurgery

Objectives: Intensity modulated arc therapy (IMAT) can potentially be a useful technique for the treatment of multi‐focal intracranial stereotactic radiosurgery (SRS) with a single isocenter setup. This study compares the effects of coplanar and non‐coplanar beams on IMAT SRS plans. Methods: A Novalis TX linear accelerator system with a high‐definition multi‐leaf collimator (HDMLC, lead width 2.5 mm at isocenter) was used for treatment. RapidArc with beam modulation using HDMLC at each position along the arcs was used for treatment planning. Cone‐beam CT (CBCT) images were used for precise localization prior to treatment delivery. Patients with 2 to 5 lesions that were treated with conventional SRS plans using dynamic arc and conformal beams were retrospectively planned with RapidArc. Single‐arc IMAT plans are compared to 5‐arc IMAT plans for target coverage, conformity index (CI), and the volume within the low dose (5 Gy) isodose line. Results: The conformality index of non‐coplanar 5‐arc IMAT plans is better than that of the single‐arc IMAT plans. The CI's for a four lesion SRS case are 1.38, 2.29, 1.33, 1.51 for the non‐coplanar 5‐arc IMAT plan and 1.82, 2.83, 1.62, 1.97 for the single‐arc IMAT plan respectively. Figure 1 shows the comparison of the isodose distribution for non‐coplanar 5‐arc and single‐arc IMAT plans. The volume within the 5 Gy isodose line of non‐coplanar 5‐arc IMAT plans is smaller than that of single‐arc IMAT plans. Conclusion: IMAT can be used for stereotactic radiosurgery of multiple intracranial lesions with a single isocenter setup to significantly reduce the treatment time. Non‐coplanar IMAT plans are better than coplanar IMAT plans in terms of conformality and dose to the normal tissue.

SU-GG-T-237: Impact of Modeling the Treatment Couch for IMAT Treatments

Purpose: Intensity modulated arc therapy (IMAT) can result in superior plan quality and delivery efficiency. As part of the treatment planning optimization process the impact of the treatment couch is not always considered leading to treatments not being delivered as planned. In this work the dosimetric differences between plans calculated with and without the couch are studied using a phantom and then the clinical impact of delivering IMAT plans without the treatment couch outlined assessed. Method and Materials: Single and dual arc IMAT plans, obtained using Oncentra MasterPlan v3.3sp1 (Nucletron, BV), were planned and recalculated on a CT scan set of a 2D ionization chamber array (IBA MatriXX Evolution) positioned between slabs of 30×30cm solid water. The couch was either not considered or modeled as a 2.2cm thick object of density 1.02. A constant dose-rate was assumed during the planning process and a Varian 600CD with 120 leaf millennium MLC at 6MV used. Calculated and measured plans were compared using a gamma criteria of 3%/3mm. Using data from a prostate cancer patient, single and dual arc plans were created using both constant and variable dose-rate IMAT without the couch outlined and then recalculated with the couch outlined. Results: When not considering the treatment couch, the percentage of pixels passing the 3%/3mm gamma criteria was 77.62% and 70.65% for single and dual arc IMAT plans respectively. When the couch was included, the percentage of pixels passing increased to 96.54% and 99.18% for single and dual arc plans. IMAT prostate plans created without the couch but recalculated with the couch resulted in plans that were not considered to be clinically acceptable. Conclusion: It is essential that the treatment couch is properly accounted for in the IMAT planning process otherwise the delivered treatment will not match the treatment as planned.

SU-GG-T-137: Similarities between Static and Rotational Intensity Modulated Plans

Purpose: To explore similarities between intensity modulated radiotherapy(IMRT) and intensity modulated arc therapy (IMAT) techniques in the context of the number of multi‐leaf collimator(MLC) segments required to achieve plan objectives, the major factor influencing plan quality. Materials and Methods: Three clinical cases with increasing complexity were studied: (a) prostate only, (b) prostate and seminal vesicles, (c) prostate and pelvic lymph nodes. Initial “gold‐standard” plans with the maximum possible organ‐at‐risk sparing were generated for all three cases. For each case, multiple IMRT and IMAT plans were generated with varying intensity levels (IMRT) and arc control points (IMAT), which translates to varying MLC segments in both modalities. The IMAT/IMRT plans were forced to mimic the organ‐at‐risk sparing and target coverage in the gold‐standard plans, thereby only allowing the target dose inhomogeneity to be variable. A higher target dose inhomogeneity (quantified as D5 — dose to the highest 5% of target volume) implies that the plan is less capable of modulation. Results: For each case, given a similar number of MLC segments, both IMRT and IMAT plans exhibit similar target dose inhomogeneity, indicating there is no difference in their ability to provide dose painting. Target dose inhomogeneity remained approximately constant with decreasing segments, but sharply increased below a specific critical number of segments (70, 100, 110 for cases a, b, c, respectively). Conclusions: For the cases studied, IMAT and IMRT plans are similar in their dependence on the number of MLC segments. A minimum critical number of segments is required to ensure adequate plan quality. Future studies are needed to establish the range of minimum critical number of segments for different treatment sites and target‐organ geometries. Acknowledgment: Research supported in part by a master research grant from Varian Medical Systems.

SU-GG-T-217: Can Two Dimensional Diode Arrays Serve as an Effective Tool for VMAT Quality Assurance?

Purpose: Two dimensional diode arrays have been widely adopted in the verification of fixed field IMRT deliveries. With increasing adoption of volume modulated arc therapy (VMAT), an immersed question is whether these diode arrays can be used for quality assurance for rotational therapy. The major concerns are the angular dependence of diode arrays and the detector to detector variance within the array. In this work, we studied the angular dependence of each detector and investigated the tools for correcting this dependency. Material and Methods: A MapCheck-I (Sun Nuclear Inc.) was used as the diode array for this study. The two dimensional (2D) array was inserted in a multi-cube solid water phantom and was irradiated on an Elekta Synergy Linac using open square fields from different gantry angles. The measured 2D intensity map was then compared to theoretical map generated in the treatment planning system (TPS), Pinnacle3. A 2D correction ratio map for each gantry angle was produced. Correction was implemented to static beams and VMAT plans of prostate cases. Results: The correction maps for all gantry angles were obtained and the angular response of each individual detector was determined. The correction map was found be dependent on the field size that was used to irradiate the diode array. The correction ratio, as defined as measure/planned, is higher for larger field size. The correction to prostate VMAT cases has no significant improvement. The verification of individual segment reveals that the irregular MLC shapes has impact on the diode response. Conclusion: Cautious should be taken to use 2D diode array for rotational arc therapy quality assurance. More studies need to be done to correct the complex dependence of the correction ratio on field size and aperture shape. Research supported by Elekta.

TU‐E‐204B‐01: First Demonstration of Image‐Based Dynamic MLC Tracking of a Moving Target during Intensity Modulated Arc Therapy

Purpose:Intensity modulated arc therapy (IMAT) enables efficient and highly conformai dose delivery. However, the dose distribution can be compromised by intrafraction target motion. Dynamic MLC (DMLC) tracking can potentially maintain the planned target dose distribution. The purpose of this study was to use a single kV imager for DMLC tracking during IMAT and to investigate the ability of this tracking to maintain the planned dose distribution. Method and Materials:A motion phantom carrying a 2D ion chamber array and build‐up material with an embedded gold marker was programmed to reproduce eight representative tumor trajectories (four prostate, four lung tumors). For each trajectory, a low and high intensity modulated IMAT plan were delivered with and without DMLC tracking, and the resulting 2D dose distributions were measured. The 3D real‐time target position signal for tracking was provided by fluoroscopic kV images acquired immediately before and during treatment. Each kV image was used together with all previous images to estimate the 3D target position and then the MLC leaves were refitted to this estimated target position. For lung, a prediction algorithm was used to compensate for the tracking latency. A 3%/3mm γ‐test was used to quantify the discrepancy between the measured dose distributions and a reference dose distribution delivered without target motion. Results:For the low modulated plans, tracking reduced the mean 3%/3mm γ‐failure rate from 32% to 0.2% for prostate and from 14% to 1% for lung. For the high modulated plans, the γ‐failure rate reduction was from 17% to 6% (prostate) and from 36% to 11% (lung). The finite MLC adaptation speed was a major cause for the residual γ‐failures during tracking. Conclusion:Image‐based tracking with IMAT was demonstrated for the first time. The tracking greatly improved the dose distributions to moving targets. Conflict of Interest: Supported by NTH/NCI R01‐93626 and Varian Medical Systems.

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.

Comparing Radiation Treatments Using Intensity-Modulated Beams, Multiple Arcs, and Single Arcs

A dosimetric comparison of multiple static-field intensity-modulated radiation therapy (IMRT), multiarc intensity-modulated arc therapy (IMAT), and single-arc arc-modulated radiation therapy (AMRT) was performed to evaluate their clinical advantages and shortcomings. Twelve cases were selected for this study, including three head-and-neck, three brain, three lung, and three prostate cases. An IMRT, IMAT, and AMRT plan was generated for each of the cases, with clinically relevant planning constraints. For a fair comparison, the same parameters were used for the IMRT, IMAT, and AMRT planning for each patient. Multiarc IMAT provided the best plan quality, while single-arc AMRT achieved dose distributions comparable to those of IMRT, especially in the complicated head-and-neck and brain cases. Both AMRT and IMAT showed effective normal tissue sparing without compromising target coverage and delivered a lower total dose to the surrounding normal tissues in some cases. IMAT provides the most uniform and conformal dose distributions, especially for the cases with large and complex targets, but with a delivery time similar to that of IMRT; whereas AMRT achieves results comparable to IMRT with significantly faster treatment delivery.

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.

Similarities between static and rotational intensity-modulated plans

The aim of this study was to explore similarities between intensity-modulated radiotherapy (IMRT) and intensity-modulated arc therapy (IMAT) techniques in the context of the number of multi-leaf collimator (MLC) segments required to achieve plan objectives, the major factor influencing plan quality. Three clinical cases with increasing complexity were studied: (a) prostate only, (b) prostate and seminal vesicles and (c) prostate and pelvic lymph nodes. Initial ‘gold-standard’ plans with the maximum possible organ-at-risk sparing were generated for all three cases. For each case, multiple IMRT and IMAT plans were generated with varying intensity levels (IMRT) and arc control points (IMAT), which translate into varying MLC segments in both modalities. The IMAT/IMRT plans were forced to mimic the organ-at-risk sparing and target coverage in the gold-standard plans, thereby only allowing the target dose inhomogeneity to be variable. A higher target dose inhomogeneity (quantified as D5—dose to the highest 5% of target volume) implies that the plan is less capable of modulation. For each case, given a similar number of MLC segments, both IMRT and IMAT plans exhibit similar target dose inhomogeneity, indicating that there is no difference in their ability to provide dose painting. Target dose inhomogeneity remained approximately constant with decreasing segments, but sharply increased below a specific critical number of segments (70, 100, 110 for cases a, b, c, respectively). For the cases studied, IMAT and IMRT plans are similar in their dependence on the number of MLC segments. A minimum critical number of segments are required to ensure adequate plan quality. Future studies are needed to establish the range of minimum critical number of segments for different treatment sites and target-organ geometries.

Prostate Treatment using Intensity Modulated Arc Therapy: Clinical Experience and Comparison with DMLC IMRT

Prostate Treatment using Intensity Modulated Arc Therapy: Clinical Experience and Comparison with DMLC IMRT

Poster - Wed Eve-51: Geometric Plan Adaptation for Rotational Radiation Therapy

Inter-fractional position uncertainty of patient anatomy limits the radiation dose we can deliver, requiring us to compromise the margins of the planning target volume (PTV) and/or the prescription dose to maintain a low risk of severe complications. Spinal cord is often the critical organ and the spine is difficult to position consistently even with vac-loc bags. A difference in curvature of the spine between planning and each treatment can lead to geometric changes that cannot be corrected by couch movements, requiring an adaptation of the aperture shapes. Intensity Modulated Arc Therapy (IMAT) has stronger geometrical influences than IMRT, and therefore, potentially more amenable for geometric adaptive strategies and has not been studied. We propose to use online imaging to locate the spinal canal and use geometric and algebraic adaptive algorithms to adapt an IMAT plan in real time to better shield the spinal cord while preserving the required dose to the PTV. Test results on a C-shaped phantom demonstrated that the maximum dose to the cord (originally 114cGy per fraction) became 119cGy and 141cGy with and without plan adaptation. A new plan created on the shifted cord geometry had a maximum cord dose of 134 cGy. The maximum and minimum PTV doses respectively were 210 cGy and 149 cGy in the initial plan, 214 cGy and 109 cGy after adaptation, while the new plan was 210 cGy and 157 cGy. In summary, geometric and algebraic adaptive algorithms maintains the original plan better than a de-novo plan based on shifted geometries.

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

SU-FF-T-175: Examining Dose Gradients in Intensity Modulated Arc Therapy

Purpose: To study dose gradients from various dose distributions to aid inverse planning of intensity modulated arc therapy (IMAT). Method and Materials: Various dose distributions were generated on three phantom cases: 1) spherical planning target volume (PTV), 2) the same spherical PTV with a centralized organ at risk (OAR) and 3) a C-shaped target with an OAR. In particular, an optimized and an un-optimized IMAT plan were studied for the C-shaped target. Upon generating the dose distributions, we computed their dose gradients. We evaluated a line integral of dose gradients I(C) over a circular contour C for all dose distributions with various radii. Under certain idealized assumptions, I(C) can be shown to be constant for a given contour C for different plans. I(C) can be used to evaluate the dose gradient around the PTV for IMAT optimization. Results: The value of the line integral I(C) varied between contours of different radii. However, for a given circular contour C, the line integral I(C) varied 4.5% and 7% from the extremes of a single field to a PTV conformal arc for contours that are 2.5 cm and 0.5 cm from the edge of the PTV respectively. Using a donut dose distribution as a reference for the C-shaped target, we found that the sum of dose gradients for optimized IMAT plan decreased by 3.5% and 5.4% respectively over the two contours, whereas, for the un-optimized IMAT plan, it was decreased by 20% and 25% respectively. Conclusions: We hypothesize that there is an approximate “conservation of dose gradients” for dose distributions that are optimized with the constraint of uniform PTV dose coverage. Such information is useful in assisting IMAT optimization: if high dose gradients are generated in certain regions, this must come at the expense of a reduced dose gradient in some other regions.

Evaluation of optimization strategies and the effect of initial conditions on IMAT optimization using a leaf position optimization algorithm

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 variation. The purpose of this study is to assess optimization strategies and initial conditions using a leaf position optimization (LPO) algorithm altered for variable dose rate IMAT. A concave planning target volume (PTV) with a central cylindrical organ at risk (OAR) was used in this study. The initial IMAT arcs were approximated by multiple static beams at 5° angular increments where multi-leaf collimator (MLC) leaf positions were determined from the beam's eye view to irradiate the PTV but avoid the OAR. For the optimization strategy, two arcs with arc ranges of 280° and 150° were employed and plans were created using LPO alone, variable dose rate optimization (VDRO) alone, simultaneous LPO and VDRO and sequential combinations of these strategies. To assess the MLC initialization effect, three single 360° arc plans with different initial MLC configurations were generated using the simultaneous LPO and VDRO. The effect of changing optimization degrees of freedom was investigated by employing 3°, 5° and 10° angular sampling intervals for the two 280°, two 150° and single arc plans using LPO and VDRO. The objective function value, a conformity index, a dose homogeneity index, mean dose to OAR and normal tissues were computed and used to evaluate the treatment plans. This study shows that the best optimization strategy for a concave target is to use simultaneous MLC LPO and VDRO. We found that the optimization result is sensitive to the choice of initial MLC aperture shapes suggesting that an LPO-based IMAT plan may not be able to overcome local minima for this geometry. In conclusion, simultaneous MLC leaf position and VDRO are needed with the most appropriate initial conditions (MLC positions, arc ranges and number of arcs) for IMAT.

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.

Stochastic versus deterministic kernel-based superposition approaches for dose calculation of intensity-modulated arcs

Dose calculations for radiation arc therapy are traditionally performed by approximating continuous delivery arcs with multiple static beams. For 3D conformal arc treatments, the shape and weight variation per degree is usually small enough to allow arcs to be approximated by static beams separated by 5°–10°. But with intensity-modulated arc therapy (IMAT), the variation in shape and dose per degree can be large enough to require a finer angular spacing. With the increase in the number of beams, a deterministic dose calculation method, such as collapsed-cone convolution/superposition, will require proportionally longer computational times, which may not be practical clinically. We propose to use a homegrown Monte Carlo kernel-superposition technique (MCKS) to compute doses for rotational delivery. The IMAT plans were generated with 36 static beams, which were subsequently interpolated into finer angular intervals for dose calculation to mimic the continuous arc delivery. Since MCKS uses random sampling of photons, the dose computation time only increased insignificantly for the interpolated-static-beam plans that may involve up to 720 beams. Ten past IMRT cases were selected for this study. Each case took approximately 15–30 min to compute on a single CPU running Mac OS X using the MCKS method. The need for a finer beam spacing is dictated by how fast the beam weights and aperture shapes change between the adjacent static planning beam angles. MCKS, however, obviates the concern by allowing hundreds of beams to be calculated in practically the same time as for a few beams. For more than 43 beams, MCKS usually takes less CPU time than the collapsed-cone algorithm used by the Pinnacle3 planning system.