IMRT Segments Research Articles

MO‐D‐M100J‐06: Reducing Intra‐Fraction Organ Motion Effects Using Segment Size Constraint in Direct Aperture Optimization

Purpose: In IMRT delivery, an important issue is intra‐fraction organ motion, which causes significant degradation of the delivered dose. Some simulation researches have showed that organ motion effects could be significantly reduced by increasing the segment size. In this study, a direct aperture optimization based commercial inverse planning system was modified to assess the clinical impact of creating optimized plans with segment size constraints (SSC). Our study seeks to answer what price in static plan quality one has to pay to avoid dose degradation in moving targets? Method and Materials: IMRT plans with and without SSC were optimized for two abdominal cases using static CT images. The MLC travel direction is aligned with the projected motion direction and SSC penalizes all MLC leaf openings smaller than twice the projected motion amplitude. Static plans were recalculated with a sinusoidal target motion with 1cm amplitude and 4second period. Results: In both cases with and without SSC, PTV volume covered by 95% of the prescribed dose (V95) was less than 1%, indicating that SSC had little effect on static plan quality. After incorporating target motion, V95 was improved by applying SSC, with the degree of improvement depending on the particular case. For case 1, V95 improved from 84.5% to 93.3% by adding SSC. Further study shows that dose coverage of peripheral PTV regions improved more significantly with SSC than central regions. In all plans, differences of the doses to the critical structures were within a few percent. By adding SSC, the treatment plan was more tolerant to target motion. Conclusions: Our study showed that when segmental IMRT plans were delivered to a moving target, SSC improves delivered dose conformity (V95) as much as 9% without significantly sacrificing static plan quality. Peripheral PTV shows more improvement in dose coverage with SSC than central regions.

Effects of organ motion on IMRT treatments with segments of few monitor units

Interplay between organ (breathing) motion and leaf motion has been shown in the literature to have a small dosimetric impact for clinical conditions (over a 30 fraction treatment). However, previous studies did not consider the case of treatment beams made up of many few-monitor-unit (MU) segments, where the segment delivery time (1-2 s) is of the order of the breathing period (3-5 s). In this study we assess if breathing compromises the radiotherapy treatment with IMRT segments of low number of MUs. We assess (i) how delivered dose varies, from patient to patient, with the number of MU per segment, (ii) if this delivered dose is identical to the average dose calculated without motion over the path of the motion, and (iii) the impact of the daily variation of the delivered dose as a function of MU per segment. The organ motion was studied along two orthogonal directions, representing the left-right and cranial-caudal directions of organ movement for a patient setup in the supine position. Breathing motion was modeled as sin(x), sin4(x), and sin6(x), based on functions used in the literature to represent organ motion. Measurements were performed with an ionization chamber and films. For a systematic study of motion effects, a MATLAB simulation was written to model organ movement and dose delivery. In the case of a single beam made up of one single segment, the dose delivered to point in a moving target over 30 fractions can vary up to 20% and 10% for segments of 10 MU and 20 MU, respectively. This dose error occurs because the tumor spends most of the time near the edges of the radiation beam. In the case of a single beam made of multiple segments with low MU, we observed 2.4%, 3.3%, and 4.3% differences, respectively, for sin(x), sin4(x), and sin6(x) motion, between delivered dose and motion-averaged dose for points in the penumbra region of the beam and over 30 fractions. In approximately 5-10% of the cases, differences between the motion-averaged dose and the delivered 30-fraction dose could reach 6%, 8% and 10-12%, respectively for sin(x), sin4(x), and sin6(x) motion. To analyze a clinical IMRT beam, two patient plans were randomly selected. For one of the patients, the beams showed a likelihood of up to 25.6% that the delivered dose would deviate from the motion-averaged dose by more than 1%. For the second patient, there was a likelihood of up to 62.8% of delivering a dose that differs by more than 1% from the motion-averaged dose and a likelihood of up to approximately 30% for a 2% dose error. For the entire five-beam IMRT plan, statistical averaging over the beams reduces the overall dose error between the delivered dose and the motion-averaged dose. For both patients there was a likelihood of up to 7.0% and 33.9% that the dose error was greater than 1%, respectively. For one of the patients, there was a 12.6% likelihood of a 2% dose error. Daily intrafraction variation of the delivered dose of more than 10% is non-negligible and can potentially lead to biological effects. We observed [for sin(x), sin4(x), and sin6(x)] that below 10-15 MU leads to large daily variations of the order of 15-35%. Therefore, for small MU segments, non-negligible biological effects can be incurred. We conclude that for most clinical cases the effects may be small because of the use of many beams, it is desirable to avoid low-MU segments when treating moving targets. In addition, dose averaging may not work well for hypo-fractionation, where fewer fractions are used. For hypo-fractionation, PDF modeling of the tumor motion in IMRT optimization may not be adequate.

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Target motion during treatment with IMRT can create unexpected hot or cold region due to adjacent static segments. To understand the effects of intra-fraction organ motion when delivering segmental IMRT so that plans can be designed to avoid the undesirable motion effects.

TH‐E‐ValA‐01: On the Dose Delivered to a Moving Target When Employing Different IMRT Delivery Mechanisms

Purpose: To investigate the influence of target motion on dose distributions generated using unmodulated open fields, solid intensity modulator (SIM), Step and Shoot MLC (SMLC) and dynamic MLC (DMLC). Method and Materials: For two lung cancer cases, four treatment plans were generated using Pinnacle3 7.9t consisting of an open field, SIM, SMLC and DMLC delivery on a Varian Clinac 600C/D equipped with a 120 leaf Millennium MLC. The coordinates (x, y, z, t) of the 4D motion trace for each of the tumors were determined using 4D‐CT from which a 4D motion kernel was generated. For each beam used in the experiment, the beams‐eye view tumor motion due to breathing was simulated using a computerized 2D tabletop apparatus. A MAPcheck diode array was incorporated into the apparatus for dose distribution analysis. Each of the four static treatment plans was delivered to the breathing MAPcheck ten times at various points of the breathing cycle. Results: The variation in diode dose readings within the tumor motion envelope was compared for the open field, solid, segmented, and dynamic IMRT deliveries. The open field provided the most uniform dose to the entire set of tumor mimicking diodes followed by SIM, SMLC, and DMLC IMRT, respectively. On an individual diode by diode basis over ten trials, the open field had the smallest average coefficient of variation of 0.122% followed by SIM (0.98%), SMLC (2.22%) and DMLC (3.88%) IMRT delivery, respectively. Conclusion: For the three IMRT delivery methods (SIM, SMLC, and DMLC), SIM consistently provided a more uniform dose to the tumor over many trials. SMLC performed as well as the solid modulators in many cases or was slightly out performed by SIM. DMLC consistently delivered the least uniform dose to the tumor over many trials.

SU-FF-T-205: Effects of Intra-Fraction Motion On IMRT Treatment with Segments of Few Monitor Units

Introduction: Intra‐fraction motion can affect the delivered dose distribution in radiotherapy. This is a concern in IMRT because of potential interplay between the delivered fluence pattern and the patient's breathing pattern. A widely applied approach to model organ motion is the use of a probability density function (pdf) in IMRT‐optimization. We assess if breathing compromises treatment with IMRT segments delivering only few monitor units (MU), where the delivery time of the segments will be of the order of the breathing period. Further, we assess the limitation of IMRT‐optimization based on pdfs for incorporating organ motion in treatment planning.Methods:A motor‐driven platform, which moves sinusoidally with a user‐specified amplitude and a period of 4s for tumor motion was used. The measurements were performed for motion amplitude of 4cm in 4×4cm2–10×10cm2 open fields and dose‐rate of 500MUmin−1. The MUs delivered were 8 to 183MUs, corresponding to delivery times of 1s to 22s. The measurements were repeated for ten different initial phases. Results: The delivered dose to a moving target varies with initial phase and with segment delivery time. For very long segment times the delivered dose has a 1–2% spread with the initial phase. Therefore, over 30 fractions, the average dose delivered to the moving tumor will converge to the mean dose delivered over the breathing‐period. For short delivery times, the delivered dose varies significantly with the initial phase. This is because the delivered dose becomes dependent on non‐uniform dose and penumbra effects. Conclusion: Motion can compromise an IMRT treatment if the segment delivery time (1–12s) is of the order of the breathing period and motion occurs in a region of non‐uniform dose. Since the delivered dose also depends strongly on the initial phase, for short delivery times, care is needed when modeling organ motion as a pdf in IMRT optimization.

SU‐FF‐T‐394: Start‐Up Characteristics of Elekta SLi‐Linacs: Minimum Number of MU Required for IMRT

Purpose: On Elekta SLi‐linacs steering of flatness and symmetry starts after a user‐definable delay of 0.5 to 1 s after beam‐on, which equals 5 to 10 MU at 600 MU/min. The impact of start‐up characteristics of Elekta SLi linacs on dose delivery with small segments during IMRT was investigated and the minimum number of monitor units needed per segment was quantified. Method and Materials: Linearity of the ionisation monitor was quantified by taking ionisation chamber readings for 10×10 cm2 6MV and 10 MV photon beams over the range 1–200 MU. By using the Wellhöfer Beam Imaging System (BIS) dosimetric images of 820×820 pixels of the beam were acquired during the first 10 monitor units for 5 gantry angles (−180°, −90°, 0°, 90°, 180°). Every 120ms images were taken of a 40×40 cm2 field. All data are compared to the reference value after delivery of 10 MU, which is regarded to be the steady‐state value. Results: Deviations in ionisation monitor readings were less than 0.5% for segments larger than 2 MU. For segments of 1MU readings were 1.5% less than expected. Values for symmetry and flatness of the beams were very large during the first MU but decreased within 2MU to twice the steady‐state value at 10 MU. The integrated symmetry (defined as the symmetry of the integrated dose distribution) and integrated flatness decreased within 4MU to twice the steady‐state value. Neither gantry angle nor energy had influence on these results. Conclusion: During IMRT segments of 4MU can be safely used for all gantry angles. Errors in dose are smaller than 1% and errors in symmetry and flatness are less than twice that of 10 MU segments.

Constrained segment shapes in direct‐aperture optimization for step‐and‐shoot IMRT

Previous studies have shown that, by optimizing segment shapes and weights directly, without explicitly optimizing fluence profiles, effective IMRT plans can be generated with fewer segments. This study proposes a method of direct-aperture optimization with aperture shape constraints, which is designed to provide segmental IMRT plans using a minimum of simple, regular segments. The method uses a cubic function to create smoothly curving multileaf collimator shapes. Constraints on segment dimension and equivalent square are applied, and each segment can be constrained to lie within the previous one, for easy generation of fluence profiles with a single maximum. To simply optimize the segment shapes and reject any shapes which violate the constraints is too inefficient, so an innovative method of feedback optimization is used to ensure in advance that viable aperture shapes are generated. The algorithm is demonstrated using a simple cylindrical phantom consisting of a hemi-annular planning target volume and a central cylindrical organ-at-risk. A simple IMRT rectum case is presented, where segments are used to replace a wedge. More complex cases of prostate and seminal vesicles and prostate and pelvic nodes are also shown. The algorithm produces effective plans in each case with three to five segments per beam. For the simple plans, the constraint that each segment should be contained within the previous one adds additional simplicity to the plan, for a small reduction in plan quality. This study confirms that direct-aperture optimization gives efficient solutions to the segmental IMRT inverse problem and provides a method for generating simple apertures. By using such a method, the workload of IMRT verification may be reduced and simplified, as verification of fluence profiles from individual beams may be eliminated.

TU‐E‐T‐6C‐01: Intensity Modulated Radiation Therapy for the Treatment of Breast Cancer

Intensity modulated radiation therapy for the treatment of Stage 0, I and II breast cancer has been in clinical use at William Beaumont Hospital since March 1999. The goals of IMRT for breast cancer delivered through static, multileaf collimator segments include optimizing dose homogeneity, avoiding unnecessary normal tissue irradiation and standardizing target volume coverage without significantly increasing the time required to plan and treat patients. The IMRT process can be divided into four different phases, simulation, treatment planning, treatment delivery and quality assurance. During the initial simulation procedure an immobilization device is constructed, level marks are tattooed and the treatment borders are defined. A CT is done in the treatment position in order to define the treatment area as well as delineate normal tissues. In addition to the CT scan the use of MR is becoming a useful tool to define nodal regions of interest. The treatment plan is generated to deliver a homogeneous dose throughout the breast volume and minimize dose to the lungs and in the case of left sided breast patients the heart. The beam arrangement of two tangential fields is adjusted for depth, gantry angle and collimator angle in order to provide full coverage to the breast and lumpectomy cavity with sufficient margin and avoid unnecessary irradiation of normal tissue. The IMRT segments are created based on 5% increments of isodose surfaces; these forward planned segments are then optimized in order to deliver a homogeneous dose throughout the breast volume. The plan is evaluated with respect to percent of breast volume receiving doses in excess of 105% and 110% of the prescribed dose. The treatment process includes patient positioning and setup, electronic portal imaging and treatment delivery. This is accomplished in a conventional treatment time slot of 12 minutes. As part of the overall patient quality assurance patients are imaged everyday for every port during their treatment. On weekly review the physicians have ten images, five medial and five lateral, with which to make clinical treatment decisions as to patient setup and treatment port placement. In addition to patient quality assurance plan quality assurance is assessed by completion of an independent calculation check as well as analysis of treatment delivered to the MapCheck device as compared to the planned treatment. Both the fluence and absolute dose measurements can be analyzed with this system. Using intensity modulated radiation therapy to deliver whole breast irradiation has proven to be an efficient method to deliver uniform dose throughout the entire breast volume which can be seen to reduce the acute and chronic toxicity's associated with radiation therapy for the treatment of breast cancer. Research supported by Philips Medical Systems and Elekta Corporation.Educational Objectives:1. To understand the goals and use of IMRT in the treatment of breast cancer.2. To understand the IMRT process for simulation, planning, treatment and quality assurance in the treatment of breast cancer.

SU‐DD‐A1‐02: Commissioning and Clinical Implementation of Elekta MLC IMRT with Corvus Inverse Treatment Planning

Purpose: This presentation details commissioning and clinical implementation of IMRT with the Elekta MLC and Corvus inverse treatment planning system. Particular attention is paid to features unique to this hardware and software that are not addressed in general IMRT guidance literature. Method and Materials: An Elekta Precise accelerator, equipped with 6 and 15MV photon beams and 80 leaf MLC, was commissioned for clinical usage IMRT with Corvus version 5.0. Data required by the planning system was collected using equipment and techniques consistent with current recommendations. A series of user defined intensity shapes, were used for initial testing of both energies by comparing calculated and film measured dose distributions. Five multifield clinical plans for each energy were compared to film in axial, coronal and sagittal planes, with an additional ionization chamber measurement at the intersection point of the three film planes. Results: When a dual off axis segment is defined by Corvus, a common occurrence in IMRT plans, the segment boundaries may be delineated by up to five different penumbra forming hardware combinations, 1) curved end MLC alone, 2) curved end MLC with backup jaw, 3) backup jaw alone, 4) divergent MLC side face alone and 5) divergent main jaw alone. This implies that both sets of jaws, in addition to the MLCs, must be calibrated to a relative accuracy of 0.2mm. When penumbra 1 and 5 were used in the Corvus pencil beam model, optimum agreement was found between plan and measurements. Conclusion: The physicist must be aware of leaf and jaw motion constraints that are unique to the Elekta Accelerator and how they are used in IMRT segment delineation by Corvus. When these factors are considered in hardware calibration and beam modeling, dose accuracy of 3% or 2mm is achievable over a range of stringent tests and clinical plans.

SU‐FF‐T‐135: Complex IMRT Plan Verification Using a Commercial MU Calculation Package

Purpose: The general availability of IMRT treatments is constrained by the need to perform dosimetry measurements as part of the patient plan verification. While secondary IMRT MU calculators have been successfully used to validate some plans for a restricted number of sites, these calculators have suffered from unacceptably large uncertainties when applied to sites confined to the head and neck region. A strategy to reduce these calculational uncertainties has been developed which permits a greater use of IMRT MU calculators and reduces the need for dosimetric measurements, enabling a larger patient population to receive the benefits offered by IMRT treatments. Method and Materials: Segmental IMRT head and neck treatments were developed using the Pinnacle 6.2b inverse planning module. The IMRT treatment plans where then calculated on a CT image set of PMH IMRT phantom, and validation points corresponding to key target and avoidance regions where identified. The dosimetric data corresponding to these points was exported to RadCalc, a commercial IMRT MU calculator, and the calculations were compared to Pinnacle calculations and in‐phantom measurements. Results: A total of 10 clinical patient cases, each containing 7 to 9 gantry angles, were assessed. Agreement was assessed on basis of total dose delivered to the point of calculation. The measured and RadCalc calculated doses were found to agree within 3% at the high dose point and 5% at the low dose point in all cases, while the Pinnacle calculated dose was found to agree with the measured dose within 2.5% at the high dose point, with deviations as large as 13.5% observed at the low dose point. Conclusion: In‐phantom IMRT verification calculations of the total dose yields similar results as in‐phantom measurements. Consequently, a secondary MU calculator can be used to verify IMRT treatment plans and reduce the frequency of validation measurements.

Dose volume histogram comparison between ADAC Pinnacle and Nomos Corvus systems for IMRT

This paper compares dose volume histograms (DVHs) generated by the ADAC Pinnacle and the Nomos Corvus planning systems. Seven prostate cases and seven head and neck cases were selected for review. Plans computed on both systems possessed exactly the same anatomical contours and IMRT segments. The Pinnacle system used the collapsed cone convolution superposition, while Corvus employed a finite size pencil beam (FSPB) convolution. Prostate DVH results demonstrated similar DVH curves from both systems. For each structure, the ratio of Pinnacle dose value divided by Corvus value was calculated. The high dose structures (which might contain tumour) had ratios close to unity, while the low dose structures (the critical organs) had ratios farther away from unity. Almost all ratios were less than unity, indicating a systematic difference that Pinnacle calculated doses were lower than Corvus ones. Head and neck data provided similar findings. A possible cause for this discrepancy could be the beam modelling. The difference in DVH parameters that we discovered between the two systems was about the same order of magnitude as the measurement-computation difference. When low dose is critical, such difference may affect the clinical planning decision.

101 oral Fast, daily QA of segmented IMRT at all commonly available linacs using an EPID

101 oral Fast, daily QA of segmented IMRT at all commonly available linacs using an EPID

A study of interplay between dynamics of breathing motion and segmental MLC-IMRT delivery for motion-pattern-based 4D lung IMRT

There is a known interplay between intra-fractional organ motion and dynamic MLC IMRT delivery for ungated radiation delivery techniques. Motion-pattern-based 4D IMRT can optimize the cumulative dosimetry throughout the motion cycle while ignoring the interplay between the dynamics of MLC treatment and organ motion. A recent work by Jiang et ala on 3D MLC IMRT of lung cancer showed that point doses can vary considerably (18%) in daily treatments. The goal of this study is to identify factors in the segmental MLC IMRT delivery and organ motion, and their threshold values, which can cause significant deviation in delivered CTV dose volume histograms (DVH) for daily treatment of 4D IMRT lung tumor treatment in typical clinical situations.

269 poster Automatic creation of IMRT segments for 2-field breast irradiation

269 poster Automatic creation of IMRT segments for 2-field breast irradiation

Dosimetric considerations for validation of segmental multileaf collimator IMRT with a commercial treatment planning system

Purpose/Objective: Commercial multileaf collimator (MLC) systems can employ leaves with rounded ends in an attempt to keep the radiation penumbra relatively constant over the range of leaf travel. Partial transmission through rounded leaf ends result in an offset of the 50% dose level to a point inside the physical leaf end position, producing an effective widening of the MLC leaf gap. The effective gap widening can have significant dosimetric consequences for IMRT treatments delivered with an MLC in dynamic mode (DMLC), but this effect is usually considered negligible for IMRT treatments delivered with an MLC in segmental mode (SMLC). In this work, we investigate the dosimetric modeling of Varian MLC leaves by a commercial treatment planning system (Philips ADAC Pinnacle P3) and the effect on IMRT treatments delivered using SMLC.

Verification of dynamic and segmental IMRT delivery by dynamic log file analysis

A program has been developed to evaluate the delivered fluence of step-and-shoot segmental and sliding window dynamic multileaf collimator (MLC) fields. To automate these checks, a number of tools have been developed using data available from the dynamic log files that can be created each time a dynamic delivery occurs. Experiments were performed with a Varian 2100EX with a 120 leaf MLC equipped with dynamic capabilities. A dynamic leaf sequence is delivered and measured with film or an amorphous silicon imager. After delivery, the dynamic log file is written by the accelerator control system. The file reports the expected and actual position for each leaf and the dose fraction every 0.055 seconds. Leaf trajectories are calculated from this data and expected and actual fluence images are created from the difference of opposing leaf trajectories. These images can be compared with the expected delivery, measurements, and calculations of fluence. Tools have been developed to investigate other aspects of the delivery, such as specific leaf errors, beam hold-off flags sent by the control system to the MLC, and gap widths. This program is part of a semi-automated quality assurance (QA) system for pretreatment fluence verification and daily treatment verification of dynamic multileaf collimation (DMLC) delivery. PACS number(s): 87.53.–j, 87.52.–g

Forward or inversely planned segmental multileaf collimator IMRT and sequential tomotherapy to treat multiple dominant intraprostatic lesions of prostate cancer to 90 Gy

Purpose: To investigate the technical feasibility of using forward or inversely planned segmental multileaf collimator (SMLC) intensity-modulated radiotherapy and sequential tomotherapy (ST) to escalate to a dose of 90 Gy to multiple dominant intraprostatic lesions within the prostate gland while delivering a dose of 75.6 Gy to the remaining prostate. Methods and Materials: A selected case with one dominant intraprostatic lesion located at the left base and a second dominant intraprostatic lesion at the right apex of the prostate was planned using three different intensity modulation techniques. Two plans were generated with inverse treatment planning, using either SMLC or ST with a special multivane collimator. The third plan also employed SMLC but was generated using forward planning. All three plans were compared based on dose-volume histograms, isodose distributions, and doses to sensitive normal structures. Results: All three plans meet and exceed the desired dose constraints, limiting doses to the rectum and bladder to an estimated RTOG Grade 2 complication rate of <10%. The ST plan achieved the best dose conformality, whereas the inverse SMLC plan gave the lowest dose to the rectal wall, and the forward SMLC plan obtained the best dose homogeneity inside the targets. Conclusions: Using any of the three intensity-modulated techniques, it is technically feasible to concurrently treat multiple selected high-risk regions within the prostate to 90 Gy and the remaining prostate to 75.6 Gy, while keeping the doses to the rectum and the bladder significantly lower than those associated with a Grade 2 complication rate of 10%.