Dynamic Multileaf Collimator Research Articles

Markerless EPID image guided dynamic multi-leaf collimator tracking for lung tumors

Compensation of target motion during the delivery of radiotherapy has the potential to improve treatment accuracy, dose conformity and sparing of healthy tissue. We implement an online image guided therapy system based on soft tissue localization (STiL) of the target from electronic portal images and treatment aperture adaptation with a dynamic multi-leaf collimator (DMLC). The treatment aperture is moved synchronously and in real time with the tumor during the entire breathing cycle. The system is implemented and tested on a Varian TX clinical linear accelerator featuring an AS-1000 electronic portal imaging device (EPID) acquiring images at a frame rate of 12.86 Hz throughout the treatment. A position update cycle for the treatment aperture consists of four steps: in the first step at time t = t0 a frame is grabbed, in the second step the frame is processed with the STiL algorithm to get the tumor position at t = t0, in a third step the tumor position at t = ti + δt is predicted to overcome system latencies and in the fourth step, the DMLC control software calculates the required leaf motions and applies them at time t = ti + δt. The prediction model is trained before the start of the treatment with data representing the tumor motion. We analyze the system latency with a dynamic chest phantom (4D motion phantom, Washington University). We estimate the average planar position deviation between target and treatment aperture in a clinical setting by driving the phantom with several lung tumor trajectories (recorded from fiducial tracking during radiotherapy delivery to the lung). DMLC tracking for lung stereotactic body radiation therapy without fiducial markers was successfully demonstrated. The inherent system latency is found to be δt = (230 ± 11) ms for a MV portal image acquisition frame rate of 12.86 Hz. The root mean square deviation between tumor and aperture position is smaller than 1 mm. We demonstrate the feasibility of real-time markerless DMLC tracking with a standard LINAC-mounted (EPID).

VMAT monthly QA using two techniques: 2D ion chamber array with an isocentric gantry mount and an in vivo dosimetric device attached to gantry

AbstractPurposeVarian RapidArc is a volumetric modulated arc therapy (VMAT) that obtains a conformal dose around the desired structure by employing variable gantry speed, dose rate and dynamic multileaf collimator (DMLC) speed as the gantry rotates about machine isocenter. This study is meant to build upon previous research by Ling et al. by completing the tests with an in vivo dosimetric device attached to the linac gantry and a 2D ionisation chamber array with an isocentric gantry mount.Materials and methodsTwo PTW detectors, seven29 array with gantry mount and DAVID, were attached to the linear accelerator gantry, allowing each device to remain perpendicular to the beam at all gantry angles. Three tests for RapidArc evaluation were performed on these devices including: dose rate and gantry speed variation, DMLC speed and dose rate variation and DMLC position accuracy. The reproducibility of the arc data was also reported.ResultsA picket fence plan varying dose rates (111 to 600 MU/minute) and gantry speeds (5·5 to 4·3°/second) was delivered consisting of seven sections of different combinations. These measurements were compared with static gantry, open field measurements and found to be within 2·39% for the DAVID device and 0·84% for the seven29. A four-section picket fence of varying DMLC speeds (0·46, 0·92, 1·84 and 2·76 cm/second) was similarly evaluated and found to be within 1·99% and 3·66% for the DAVID and seven29, respectively. For DMLC position accuracy, a picket fence arc plan was compared with a static picket fence and found to agree within 0.38% and 2.91%. Reproducibility for these three RapidArc plans was found to be within 0·30% and 2·70% for the DAVID and seven29.ConclusionThe DAVID and seven29 detectors were able to perform the RapidArc quality assurance tests efficiently and accurately and the results were reproducible. Periodic verification of DMLC movement, dose rate variation and gantry speed variation relating to RapidArc delivery can be completed in a timelier manner using this equipment.

Dosimetric benefit of DMLC tracking for conventional and sub-volume boosted prostate intensity-modulated arc radiotherapy

This study investigated the dosimetric impact of uncompensated motion and motion compensation with dynamic multileaf collimator (DMLC) tracking for prostate intensity modulated arc therapy. Two treatment approaches were investigated; a conventional approach with a uniform radiation dose to the target volume and an intraprostatic lesion (IPL) boosted approach with an increased dose to a subvolume of the prostate. The impact on plan quality of optimizations with a leaf position constraint, which limited the distance between neighbouring adjacent MLC leaves, was also investigated. Deliveries were done with and without DMLC tracking on a linear acceleration with a high-resolution MLC. A cylindrical phantom containing two orthogonal diode arrays was used for dosimetry. A motion platform reproduced six patient-derived prostate motion traces, with the average displacement ranging from 1.0 to 8.9 mm during the first 75 s. A research DMLC tracking system was used for real-time motion compensation with optical monitoring for position input. The gamma index was used for evaluation, with measurements with a static phantom or the planned dose as reference, using 2% and 2 mm gamma criteria. The average pass rate with DMLC tracking was 99.9% (range 98.7–100%, measurement as reference), whereas the pass rate for untracked deliveries decreased distinctly as the average displacement increased, with an average pass rate of 61.3% (range 32.7–99.3%). Dose–volume histograms showed that DMLC tracking maintained the planned dose distributions in the presence of motion whereas traces with >3 mm average displacement caused clear plan degradation for untracked deliveries. The dose to the rectum and bladder had an evident dependence on the motion direction and amplitude for untracked deliveries, and the dose to the rectum was slightly increased for IPL boosted plans compared to conventional plans for anterior motion with large amplitude. In conclusion, optimization using a leaf position constraint had minimal dosimetric effect, DMLC tracking improved the target and normal tissue dose distributions compared to no tracking for target motion >3 mm, with the DMLC tracking distributions showing generally good agreement between the planned and delivered doses.

PD-0045: Time-resolved dose reconstruction of VMAT delivery to moving targets with and without dynamic MLC tracking

PD-0045: Time-resolved dose reconstruction of VMAT delivery to moving targets with and without dynamic MLC tracking

PD-0044: Respiratory-induced motion of pancreatic cancer: time trends during the course of radiotherapy

Purpose/Objective: Irradiation of pancreatic tumors requires large margins to account for respiratory-induced intrafractional tumor motion. These margins are derived from a single pre-treatment 4DCT scan. This respiratory-induced intrafractional motion can be visualized using implanted fiducial markers. The purpose of the current study is to quantify the respiratoryinduced motion of pancreatic cancer, both on the 4DCT scan as well as on daily cone beam CT (CBCT) scans during a 5 week course of radiation to reveal possible trends during the course of treatment. Materials and Methods: Eight patients (4 male / 4 female) with borderline resectable pancreatic cancer were treated with 25 x 2Gy plus weekly gemcitabine 300 mg/m. All patients received 2 to 4 fiducial markers by endoscopic ultrasound-guided implantation. The amplitude of respiratory-induced motion was measured on the pretreatment 4DCT scan and also on all available daily CBCT scans. Phase reconstructions of the CBCT scans were made by an automated selection of the 2D projection images corresponding to the peak inhale and exhale phases. These reconstructions were matched to the (reference) planning CT. The amplitudes (differences between the 2 extreme positions) were analyzed by matching on the markers (in three directions) and on the diaphragm (superior-inferior (SI) direction only). Rotations and deformations of the tissue were not analyzed. The amplitudes were linearly fit as a function of time (i.e. treatment day). The fits were tested for significance using a Student’s t-test and statistical significance was assumed at p≤0.05. Results: The amplitude of intrafractional motion of the markers was greatest in the SI direction (mean 5.9mm, SD: 2.7mm on CBCT). Differences up to 5.1mm (range: 0.3 5.1 mm) were found between the amplitude on the 4DCT scan and the mean amplitude of all CBCT scans of a single patient. The analysis of daily respiratory-induced motion of the markers revealed significant trends in the amplitude as a function of time (table and figure). The diaphragm did not always show the same trend as the markers. For the markers neither the amplitudes nor the trends were significant in the other 2 directions (data not shown). Conclusions: The 4DCT used for treatment planning is often not representative of the respiratory-induced motion amplitude during treatment. The CBCT projection images showed significant time trends in the amplitude of respiratory-induced motion during a five week course of radiation. These time trends in the amplitudes indicate the usefulness of adaptive treatment procedures. PD-0045 Time-resolved dose reconstruction of VMAT delivery to moving targets with and without dynamic MLC tracking T. Ravkilde, P.J. Keall, C. Grau, M. Hoyer, P.R. Poulsen Aarhus University Hospital, Department of Oncology, Aarhus, Denmark University of Sydney, Sydney Medical School, Sydney, Australia

An experimental comparison of conventional two-bank and novel four-bank dynamic MLC tracking

The AccuLeaf mMLC featuring four multileaf-collimator (MLC) banks has been used for the first time for an experimental comparison of conventional two-bank with novel four-bank dynamic MLC tracking of a two-dimensional sinusoidal respiratory motion. This comparison was performed for a square aperture, and for three conformal treatment apertures from clinical radiotherapy lung cancer patients. The system latency of this prototype tracking system was evaluated and found to be 1.0 s and the frequency at which MLC positions could be updated, 1 Hz, and therefore accurate MLC tracking of irregular patient motion would be difficult with the system in its current form. The MLC leaf velocity required for two-bank-MLC and four-bank-MLC tracking was evaluated for the apertures studied and a substantial decrease was found in the maximum MLC velocity required when four-banks were used for tracking rather than two. A dosimetric comparison of the two techniques was also performed and minimal difference was found between two-bank-MLC and four-bank-MLC tracking. The use of four MLC banks for dynamic MLC tracking is shown to be potentially advantageous for increasing the delivery efficiency compared with two-bank-MLC tracking where difficulties are encountered if large leaf shifts are required to track motion perpendicular to the direction of leaf travel.

Reduced-order constrained optimization (ROCO): Clinical application to head-and-neck IMRT

The authors present the application of the reduced order constrained optimization (ROCO) method, previously successfully applied to the prostate and lung sites, to the head-and-neck (H&N) site, demonstrating that it can quickly and automatically generate clinically competitive IMRT plans. We provide guidelines for applying ROCO to larynx, oropharynx, and nasopharynx cases, and report the results of a live experiment that demonstrates how an expert planner can save several hours of trial-and-error interaction using the proposed approach. The ROCO method used for H&N IMRT planning consists of three major steps. First, the intensity space of treatment plans is sampled by solving a series of unconstrained optimization problems with a parameter range based on previously treated patient data. Second, the dominant modes in the intensity space are estimated by dimensionality reduction using principal component analysis (PCA). Third, a constrained optimization problem over this basis is quickly solved to find an IMRT plan that meets organ-at-risk (OAR) and target coverage constraints. The quality of the plan is assessed using evaluation tools within Memorial Sloan-Kettering Cancer Center (MSKCC)'s treatment planning system (TPS). The authors generated ten H&N IMRT plans for previously treated patients using the ROCO method and processed them for deliverability by a dynamic multileaf collimator (DMLC). The authors quantitatively compared the ROCO plans to the previously achieved clinical plans using the TPS tools used at MSKCC, including DVH and isodose contour analysis, and concluded that the ROCO plans would be clinically acceptable. In our current implementation, ROCO H&N plans can be generated using about 1.6 h of offline computation followed by 5-15 min of semiautomatic planning time. Additionally, the authors conducted a live session for a plan designated by MSKCC performed together with an expert H&N planner. A technical assistant set up the first two steps, which were performed without further human interaction, and then collaborated in a virtual meeting with the expert planner to perform the third (constrained optimization) step. The expert planner performed in-depth analysis of the resulting ROCO plan and deemed it to be clinically acceptable and in some aspects superior to the clinical plan. This entire process took 135 min including two constrained optimization runs, in comparison to the estimated 4 h that would have been required using traditional clinical planning tools. The H&N site is very challenging for IMRT planning, due to several levels of prescription and a large, variable number (6-20) of OARs that depend on the location of the tumor. ROCO for H&N shows promise in generating clinically acceptable plans both more quickly and with substantially less human interaction.

Treatment Efficiency of Volumetric Modulated Arc Therapy in Comparison With Intensity-Modulated Radiotherapy in the Treatment of Prostate Cancer

Treatment with intensity-modulated radiation therapy (IMRT) is increasingly standard for prostate cancer. Volume-modulated arc therapy (VMAT) to deliver IMRT potentially enables shorter treatment time. The aim of this study was to test this hypothesis by measuring the average patient in-room time with VMAT versus dynamic multileaf collimator (DMLC) IMRT. Custom institutional software (RTMetrix) was used to mine the treatment times from the record-and-verify database. The in-room time (the time between patient entry and exit) was computed for each patient using RTMetrix. Average room time was compared between VMAT patients (n = 44) and IMRT patients (n = 99). Subgroup comparisons (1-arc or 2-arc VMAT, 5-field or 7-field IMRT, and electromagnetic transponder [Calypso] or gold-marker tracking) were performed. For all comparisons, 2-tailed, 2-sample, equal variance Student's t-tests were used. Average room time was significantly shorter for all VMAT versus DMLC IMRT (P = .0014) procedures, along with VMAT versus 7-field DMLC IMRT (P < .001), but not VMAT versus 5-field DMLC IMRT (P = .81). Room time was longer for Calypso versus gold seed patients (P < .001), but VMAT reduced treatment time in Calypso patients (P = .01). This resulted in Calypso VMAT patients' having similar treatment times to non-Calypso DMLC IMRT patients (P = .220). These data show that VMAT can shorten room times and improve patient throughput over 7-field DMLC IMRT. Additionally, the data demonstrate that treatment with VMAT permits the use of advanced prostate tracking (Calypso), resulting in similar room times as with standard 7-field DMLC IMRT with conventional tracking.

Proof of concept of MRI-guided tracked radiation delivery: tracking one-dimensional motion

In radiotherapy one aims to deliver a radiation dose to a tumour with high geometrical accuracy while sparing organs at risk (OARs). Although image guidance decreases geometrical uncertainties, treatment of cancer of abdominal organs is further complicated by respiratory motion, requiring intra-fraction motion compensation to fulfil the treatment intent. With an ideal delivery system, the optimal method of intra-fraction motion compensation is to adapt the beam collimation to the moving target using a dynamic multi-leaf collimator (MLC) aperture. The many guidance strategies for such tracked radiation delivery tested up to now mainly use markers and are therefore invasive and cannot deal with target deformations or adaptations for OAR positions. We propose to address these shortcomings using the online MRI guidance provided by an MRI accelerator and present a first step towards demonstration of the technical feasibility of this proposal. The position of a phantom subjected to one-dimensional (1D) periodic translation was tracked using a fast 1D MR sequence. Real-time communication with the MR scanner and control of the MLC aperture were established. Based on the time-resolved position of the phantom, tracked radiation delivery to the phantom was realized. Dose distributions for various delivery conditions were recorded on a gafchromic film. Without motion a sharply defined dose distribution is obtained, whereas considerable blur occurs for delivery to a moving phantom. With compensation for motion, the sharpness of the dose distribution is nearly restored. The total latency in our motion management architecture is approximately 200 ms. Combination of the recorded phantom and aperture positions with the planned dose distribution enabled the reconstruction of the delivered dose in all cases, which illustrates the promise of online dose accumulation and confirms that latency compensation could further enhance our results. For a simple 1D tracked delivery scenario, the technical feasibility of MRI-guided tracked radiation delivery is confirmed. More generic tracking scenarios require advanced MRI, leading to increased acquisition time and more challenging image processing problems. Latency compensation is therefore an important subject of future investigations.

Updated Results of the Montpellier IMRT Prostate Cancer Cohort: Focus on Elderly Patients

In our institution intensity modulated radiation therapy (IMRT) is routinely used to treat prostate cancer since 2001. We present the updated results of our series with a median follow-up of 47 months (range, 6-101). Elderly patients (EP), aged ≥75 years, are particularly analyzed. Up to July 2008, 373 patients were treated with IMRT using a 6-field technique by dynamic multileaf collimation with 18 MV photons. All patients received the entire treatment course to a prescribed total dose of 80 Gy (minimum PTV dose of 76 Gy, excluding rectum). No pelvic irradiation was applied. Androgen ablation therapy (AAT) was delivered for 6 months and 2-3 years in intermediate and high-risk patients (d'Amico classification), respectively (n = 139). If possible, bilateral ilioobturator lymphadenectomy (n = 37) was performed in high-risk patients. PSA failure was defined as nadir + 2. Toxicity was assessed according to the NCI/CTCAE v.3.0. Multivariate analysis using the Cox model was performed to assess factors that may impact on PSA relapse. Logistic regression was used to correlate clinical and physical parameters with grade ≥ 2 gastro-intestinal (GI) and genitourinary (GU) toxicities. Median follow-up were 47 months (range, 6-101), 49 months and 39 months, for all patients, <75 y and EP, respectively. Median age was 69 years (range, 40-81). Sixty two patients were ≥ 75 years (17%). One hundred thirty-eight, 167, and 68 patients were classified as low (group 1), intermediate (group 2), and high-risk (group 3), respectively. The respective 5-year biochemical-free survival was 94%, 79% and 88%, respectively. Five-year biochemical relapse-free survival was 93% for EP (95% CI, 0.79-0.97) and 82% (95% CI, 0.75-0.87) for the others (p = 0.6). In group 2 (n = 23 recurrences), multivariate analysis showed that the absence of AAT and the number of positive biopsies impact on PSA relapse (p = 0.02, HR 2.4 and p = 0.002, HR 3.25, respectively). For patients without AAT, multivariate analysis showed that the PSA nadir >0.6 (HR: 7.4), the time to PSA nadir ≤ 28.3 months (HR: 13.1) and Gleason score ≥ 8 (HR 8.2) were predictive for biochemical recurrence. The incidence of late grade ≥ 2 rectal and urinary toxicities were 8.8% and 11%, respectively. Five-year ≥ G2 late toxicity free survival was 79% (95% CI, 73%-84%) for <75 y and 80% (95% CI, 64%-89%) for EP (p = 0.79).The dose received by 50% of the rectum was the only factor significantly correlated with late grade ≥ 2 rectal bleeding (p = 0.02). For bladder toxicity, no parameters reach significance. IMRT to 80 Gy can provide good carcinologic results and low late toxicity rates in all prostate cancer subgroups. For EP, biochemical control and tolerance are comparable to younger patients. Intermediate-risk patients seem to benefit from a 6-month AAT combined to high dose IMRT.

A Method of Dose Reconstruction for Moving Targets With Dynamic Treatments

Organ motion during radiotherapy treatment delivery may lead to target dose distributions that differ markedly from the planned dose. The aim of this study was to develop a method that allows a treatment planning system (TPS) to perform accurate dose reconstruction for moving targets with any dynamic treatment type. The method was validated in phantom measurements for a range of treatments including IMRT, VMAT, and dynamic MLC tracking. Conclusions

Experimental investigation of a general real-time 3D target localization method using sequential kV imaging combined with respiratory monitoring

The goal of this work was to experimentally quantify the geometric accuracy of a novel real-time 3D target localization method using sequential kV imaging combined with respiratory monitoring for clinically realistic arc and static field treatment delivery and target motion conditions. A general method for real-time target localization using kV imaging and respiratory monitoring was developed. Each dimension of internal target motion T(x, y, z; t) was estimated from the external respiratory signal R(t) through the correlation between R(ti) and the projected marker positions p(xp, yp; ti) on kV images by a state-augmented linear model: T(x, y, z; t) = aR(t) + bR(t − τ) + c. The model parameters, a, b, c, were determined by minimizing the squared fitting error ∑‖p(xp, yp; ti) − P(θi) · (aR(ti) + bR(ti − τ) + c)‖2 with the projection operator P(θi). The model parameters were first initialized based on acquired kV arc images prior to MV beam delivery. This method was implemented on a trilogy linear accelerator consisting of an OBI x-ray imager (operating at 1 Hz) and real-time position monitoring (RPM) system (30 Hz). Arc and static field plans were delivered to a moving phantom programmed with measured lung tumour motion from ten patients. During delivery, the localization method determined the target position and the beam was adjusted in real time via dynamic multileaf collimator (DMLC) adaptation. The beam-target alignment error was quantified by segmenting the beam aperture and a phantom-embedded fiducial marker on MV images and analysing their relative position. With the localization method, the root-mean-squared errors of the ten lung tumour traces ranged from 0.7–1.3 mm and 0.8–1.4 mm during the single arc and five-field static beam delivery, respectively. Without the localization method, these errors ranged from 3.1–7.3 mm. In summary, a general method for real-time target localization using kV imaging and respiratory monitoring has been experimentally investigated for arc and static field delivery. The average beam-target error was 1 mm.

2D lag and signal nonlinearity correction in an amorphous silicon EPID and their impact on pretreatment dosimetric verification

This investigation provides measurements of signal lag and nonlinearity separately for the Varian aS500 electronic portal imaging device (EPID), and an algorithm to correct for these effects in 2D; their potential impact on intensity modulated radiation therapy (IMRT) verification is also investigated. The authors quantify lag, as a function of both delivered monitor units (MU) and time, by using a range of MUs delivered at a clinically used rate of 400 MU∕min. Explicit cumulative lag curves are thus determined for a range of MUs and times between the end of irradiation and the end of image acquisition. Signal nonlinearity is also investigated as a function of total MUs delivered. The family of cumulative lag curves and signal nonlinearity are then used to determine their effects on dynamic multileaf collimator (MLC) (IMRT) deliveries, and to correct for theses effects in 2D. Images acquired with an aS500 EPID and Varis Portal-Vision software were used to quantify detector lag and signal-nonlinearity. For the signal lag investigation, Portal-Vision's service monitor was used to acquire EPID images at a rate of 8 frames/s. The images were acquired during irradiation and 66 s thereafter, by inhibiting the M-holdoff-In signal of the Linac for a range of 4.5-198.5 MUs. Relative cumulative lag was calculated by integrating the EPID signal for a time after beam-off, and normalizing this to the integrated EPID signal accumulated during radiation. Signal nonlinearity was studied by acquiring 10 × 10 cm(2) open-field EPID images in "integrated image" mode for a range of 2-500 MUs, and normalized to the 100 MU case. All data were incorporated into in-house written software to create a 2D correction map for these effects, using the field's MLC file and a field-specific calculated 2D "time-map," which keeps track of the time elapsed from the last fluence delivered at each given point in the image to the end of the beam delivery. Relative cumulative lag curves reveal that the lag alone can deviate the EPID's perceived dose by as large as 6% (1 MU delivery, 60 s postirradiation). For signal nonlinearity relative to 100 MU, EPID signals per MU of 0.84 and 1.01 were observed for 2 and 500 MUs, respectively. Correction maps were applied to a 1 cm sweeping-window 14 × 14 cm(2) field and clinical head-and-neck IMRT field. A mean correction of 1.028 was implemented in the head-and-neck field, which significantly reduced lag-related asymmetries in the EPID images, and restored linearity to the EPID imager's dose response. Corrections made to the sweeping-field showed good agreement with the treatment planning system-predicted field, yielding an average percent difference of 0.05% ± 0.91%, compared to the -1.32% ± 1.02% before corrections, or 1.75% ± 1.04% when only a signal nonlinearity correction is made. Lag and signal-nonlinearity have been quantified for an aS500 EPID imager, and an effective 2D correction method has been developed which effectively removes nonlinearity and lag effects. Both of these effects were shown to negatively impact IMRT verifications. Especially fields that involve prolonged irradiation and small overall MUs should be corrected for in 2D.

A method of dose reconstruction for moving targets compatible with dynamic treatments

To develop a method that allows a commercial treatment planning system (TPS) to perform accurate dose reconstruction for rigidly moving targets and to validate the method in phantom measurements for a range of treatments including intensity modulated radiation therapy (IMRT), volumetric arc therapy (VMAT), and dynamic multileaf collimator (DMLC) tracking. An in-house computer program was developed to manipulate Dicom treatment plans exported from a TPS (Eclipse, Varian Medical Systems) such that target motion during treatment delivery was incorporated into the plans. For each treatment, a motion including plan was generated by dividing the intratreatment target motion into 1 mm position bins and construct sub-beams that represented the parts of the treatment that were delivered, while the target was located within each position bin. For each sub-beam, the target shift was modeled by a corresponding isocenter shift. The motion incorporating Dicom plans were reimported into the TPS, where dose calculation resulted in motion including target dose distributions. For experimental validation of the dose reconstruction a thorax phantom with a moveable lung equivalent rod with a tumor insert of solid water was first CT scanned. The tumor insert was delineated as a gross tumor volume (GTV), and a planning target volume (PTV) was formed by adding margins. A conformal plan, two IMRT plans (step-and-shoot and sliding windows), and a VMAT plan were generated giving minimum target doses of 95% (GTV) and 67% (PTV) of the prescription dose (3 Gy). Two conformal fields with MLC leaves perpendicular and parallel to the tumor motion, respectively, were generated for DMLC tracking. All treatment plans were delivered to the thorax phantom without tumor motion and with a sinusoidal tumor motion. The two conformal fields were delivered with and without portal image guided DMLC tracking based on an embedded gold marker. The target dose distribution was measured with a radiochromic film in the moving rod and compared with the reconstructed doses using gamma tests. Considerable interplay effects between machine motion and target motion were observed for the treatments without tracking. For nontracking experiments, the mean 2 mm∕2% gamma pass rate over all investigated scenarios was 99.6% between calculated and measured doses. For tracking experiments, the mean gamma pass rate was 99.4%. A method for accurate dose reconstruction for moving targets with dynamic treatments was developed and experimentally validated in a variety of delivery scenarios. The method is suitable for integration into TPSs, e.g., for reconstruction of the dose delivered to moving tumors or calculation of target doses delivered with DMLC tracking.

Characterization of a dynamic multi-leaf collimator for stereotactic radiotherapy applications

The Apex® dynamic mini-multileaf collimator has recently been released by Elekta and attaches directly to the linear accelerator head. This paper details the work and results obtained in characterizing this mini-MLC for stereotactic usage within our department. A range of mechanical and dosimetric characteristics were investigated which included inter and intra leaf leakage, light/radiation field congruence, leaf position reproducibility, radiation penumbra, total scatter factors and mechanical rotational stability with the additional mini-MLC weight.

Quality assurance of dynamic parameters in volumetric modulated arc therapy

The purpose of this study was to demonstrate quality assurance checks for accuracy of gantry speed and position, dose rate and multileaf collimator (MLC) speed and position for a volumetric modulated arc treatment (VMAT) modality (Synergy S; Elekta, Stockholm, Sweden), and to check that all the necessary variables and parameters were synchronous. Three tests (for gantry position-dose delivery synchronisation, gantry speed-dose delivery synchronisation and MLC leaf speed and positions) were performed. The average error in gantry position was 0.5° and the average difference was 3 MU for a linear and a parabolic relationship between gantry position and delivered dose. In the third part of this test (sawtooth variation), the maximum difference was 9.3 MU, with a gantry position difference of 1.2°. In the sweeping field method test, a linear relationship was observed between recorded doses and distance from the central axis, as expected. In the open field method, errors were encountered at the beginning and at the end of the delivery arc, termed the "beginning" and "end" errors. For MLC position verification, the maximum error was -2.46 mm and the mean error was 0.0153 ±0.4668 mm, and 3.4% of leaves analysed showed errors of >±1 mm. This experiment demonstrates that the variables and parameters of the Synergy S are synchronous and that the system is suitable for delivering VMAT using a dynamic MLC.

Poster - Thur Eve - 10: Long term stability of VMAT quality assurance parameters using an EPID.

The rapidly growing use of volumetric modulated arc therapy (VMAT) treatments in radiation therapy calls for a quantitative, automated, and reliable quality assurance (QA) procedure that can be used routinely in the clinical setting. In this work, we present a series VMAT QA procedures used to assess dynamic multi-leaf collimator (MLC) positional accuracy, variable dose-rate accuracy, and MLC leaf speed accuracy. The QA procedures were performed using amorphous silicon electronic portal imaging devices (EPID) to determine the long term stability of the measured parameters on two Varian linear accelerators. The measurements were repeated weekly on both linear accelerators for a period of three months and the EPID images were analyzed using custom Matlab software. The results of the picket fence tests indicate that MLC leaf positions can be identified to within 0.11 mm and 0.15 mm for static gantry delivery and VMAT delivery respectively. In addition, the dose-rate, gantry speed and MLC leaf speed tests both show very good stability over the measurement period. The measurements thus far, suggest that a number of the dosimetry tests may be suitable for quarterly QA for Varian iX and Trilogy linacs. However, additional measurements are required to confirm the frequency with which each test is required for safe and reliable VMAT delivery at our centre.

TU‐A‐BRA‐10: Real‐Time Markerless Tumor Tracking with MV Imaging and a Dynamic Multi‐Leaf Collimator (DMLC)

Purpose: The implementation of a real‐time adaptive therapy system. Automatic soft tissue localization (STiL) is used to drive DMLC adaptation maintaining the position of the dynamic treatment aperture relative to the tumor location during the entire breathing cycle. The STiL component utilizes electronic portal images and operates without the need for fiducial markers. The proposed system has the potential to improve treatment accuracy, dose conformity and sparing of healthy tissue. Methods: The system is implemented and tested on a clinical linear accelerator featuring an electronic portal imaging device (EPID) and a DMLC control system. EPID images are continuously acquired at a frame rate of 12.86 Hz. The STiL component processes the images in real‐time, sending its output ‐ the current tumor position ‐ to the DMLC component, which moves the aperture to that position. Image transfer, tumor position calculation and DMLC motion introduce a time lag between tumor position at acquisition time and at the time the treatment aperture reaches this position. We analyze this latency with a dynamic chest phantom driving a 1D sinusoidal motion (20mm superior‐inferior motion range and 4.5s period). We estimate the resulting average geometric systematic error in a clinical setting by driving the phantom with several patient traces (recorded from fiducial tracking during lung SBRT). Results: The individual geometric errors of the StiL and the DMLC component are each smaller than 1 mm. The overall system latency was found to be 210 ms. The average rms‐error of 11 patients traces (172 beams) was found to be (1.8 ± 0.8) mm with this latency. Conclusions: We have implemented a real‐time adaptive therapy system integrating automatic soft tissue tumor localization with DMLC adaptation of the treatment aperture. The functionality of the combined system was tested successfully and the systemic latency and resulting rms error measured. Varian Medical Systems Inc., NCI grant CA93626

SU‐E‐T‐356: Dose Response of a 2D Diode Array for Non‐Primary Photon Radiation from a Dynamic MLC

Purpose: To quantify the dose responses of a 2D‐diode array for ‘non‐primary’ radiations (scatter and transmission from MLC and gantry‐head) under dynamic MLC dose deliveries. The ‘non‐primary doses' have been overlooked by the 2D‐diode dose‐calibration because the primary radiation dominates over a large calibration field (10×10cm2). If non‐primary dose responses of diode are significant, it would cause systematic errors when non‐primary radiations become important (in small field). Methods: The dose response is the diode‐dose normalized by chamber‐dose. Three setups under a 6MeV photon beam (Trilogy) were measured by detector (diode and chamber) at central‐axis (CAX): (1) the CAX diode was accurately located at places: either within MLC‐leaf collimation (2.5 mm) or between leaves. This measurement field is entirely blocked by MLC. (2) Dynamic MLC exposures are tested by 6 different MLC‐gaps (1, 5, 10, 20, 50 and 100 mm) sweeping over the CAX detector. (3) To estimate the scatter dose but without MLC involved, the CAX detector is blocked from primary beam by jaws only but received scatter doses from narrow‐slit apertures with distances from CAX. Results: (1) Results showed the diodes tends to underestimate dose of lower energy radiation, e.g. scatters. Experiment (3) confirmed this by showing its decreased dose response by scatter source away from CAX; i.e. more lower‐energy scatters in larger lateral scattered angles. As the slit is narrowed, less scatters and higher diode response were observed. (2) 3% difference in diode response to MLC‐leakage vs. non‐leakage was detected. However the impact of MLC leakage is not substantial in dynamic MLC delivery unless MLC‐gap is narrow enough (e.g. 1 mm). (3) Overall non‐primary radiations still significantly present as gap&lt;10mm by 3% criterion. Conclusions: The diode dose responses to non‐primary radiations can cause more than 3% systematic errors for small‐aperture dynamic MLC QA unless appropriate corrections are made.

SU‐C‐BRCD‐01: Spatial Variation of Dosimetric Leaf Gap and the Impact in Dose Delivery

Purpose: To evaluate the variation of dosimetric leaf‐gap (DLG) along the travel path of each MLC leaf pairs. Furthermore, dosimetric implication of this variation is studied for several dynamic MLC fields. Methods: During dose calculation, the treatment planning systems (TPS) retracts the MLC leaf positions by half of the DLG value (measured at CA) for all leaf positions in a Dynamic MLC plan to accurately model the rounded leaf ends. The 6 MV DLG values were measured, for all leaf pairs, in increment of 0.5 cm at Dmax, in the direction of leaf motion. The measurements were performed on three Varian Linacs, employing the Millennium 120‐leaf MLCs. Measurements were performed using both the Sun Nuclear MapCheck device and the PTW pinpoint ion chamber. Measurements with an ion chamber were also performed with varying leaf gaps to determine % change in dose with leaf gap discrepancy. Results: The measured DLGs for all leaf pairs were very similar along the in‐plane direction; varying maximally only by 0.3 mm. But in the in the leaf travel direction, the DLG increases gradually from CA to 6.0 cm off‐axis, reaching to a maximum variation of 1.6 mm for a particular leaf pair. Varying gap measurements indicate that DLG deviation of about 1 mm can cause a dose difference of about 5.0 % for a fluence with 1.5 cm leaf width sweeps and significantly increases to about 12.5 % difference for 0.5 cm sweeps. Conclusions: The spatial variation in DLG is caused by the variation in beam spectrum as well as MLC motion across the field. Fields with dynamic MLC centered on the CAX would not be affected since DLG does not vary as much; but any field residing significantly off‐axis with narrow sweeping leafs can exhibit significant dose differences due to large DLG differences.