bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐347: Dosimetric Effects Of Air Cavity Migration Through The Mammosite Multi‐Lumen Balloon Treatments
Authors: Li W , Liu L , Shapiro A
Published: July 2013

Purpose: Study the dosimetric effects of air cavity migration through the mammosite multi‐lumen balloon treatments. Methods: Using Multi‐lumen mammosite had the advantage of shaping the dose around the balloon to get better coverage and less skin dose/hot spots. However, when there was air pocket around the balloon, the air pocket tended to move/change volume throughout the treatment. Since the dose distribution was usually asymmetric in treatment plans, the effects of air pocket need to be studied.Five(5) patients were selected for this study. For each patient, multi‐lumen mammosite balloon was inserted into the cavity. Patient was then CTed and a treatment plan was created using Varian Eclipse TPS for delivery with Varian Varisource IX. PTV was created by expanding 1cm from balloon surface, 34 Gy were delivered in 10 fractions. The patient was CTed again after the last fraction before the balloon was extracted. The position of the balloon was checked to match the original CT. The treatment plan was recreated in the post‐treatment CT.In order to evaluate the true difference, a new PTV was created in both original CT and post‐treatment CT. The new PTV was 1 cm expansion from the cavity, excluding the entrance pathway, since these tissues were intended to be treated. Coverage and hot spots were evaluated with the new PTVs. Results: Through the treatment, air pocket normally will reduce volume and redistributed more evenly around the balloon, therefore coverage improved through the treatment. Since hot spots in multi‐lumen cases were not symmetric, and during the planning, hot spots were usually targeted at air pocket if possible, at the end of the treatment, more tissue would receive high doses due to the air‐pocket change. Conclusion: Care needs to be taken when there is air‐pocket in treatment planning using multi‐lumen mammosite treatment, especially the hot spots.
SU‐E‐T‐349: An Optimized Dosimetric Study of a Multi‐Channel Vaginal Cylindrical Applicator in Brachytherapy
Authors: Syh J , Syh J , Wu H , White M , Patel B , Rosen L
Published: July 2013

Purpose: Toptimized dosimetry of a CT/MRI multi‐lumen brachytherapy applicator was planned to treat vaginal cancer after hysterectomy. Multichannel design of cylinder includes several peripheral channels isotropically placed 5mm inside cylinder from surface with a separate central channel to allow single lumen brachytherapy as an option. The optimized dosimetry plans were compared between single‐channel versus multi‐channel to determine the sufficient coverage to the vaginal apex and other potential dosimetric advantages. Methods: CT simulation for each treatment of vaginal brachytherapy of postoperative endometrial or vaginal cancer. Seventeen CT image data sets were collected in this study. Standard single channel treatment was planned. Prescribed dose was to the 5mm cuff from the cylinder surface. PTV of 5mm rind with prescribed length was constructed. Doses to PTV and ROIs such as bladder and rectum were documented in DVH. Dose point to the apex of 5mm rind which locates at 5mm from applicator tip was specified. A second brachytherapy plan was generated using a multi‐channel loading. DVH of 5mm rind coverage, doses to apex, bladder and rectum were optimized in Oncentra brachytherapy planning system. Results: Optimized 95% to V95 and lower dose to adjacent structures were reported in multi‐channel plan. Improvements to apex dose by 3‐5% and organ sparing by 7‐14% were achieved. At the apex, inverse planning was further optimized by adjusting unequal contribution selectively from multi‐channel end. It is difficult for single channel to eliminate the cold spot at apex and maintain the dose uniformity to the 5mm cuff simultaneously. Conclusion: Possible optimization of brachytherapy treatment planning of vagina is investigated in this study by using a new multi‐lumen vaginal cylinder. The new device can increase the therapeutic ratio by improving target coverage and at the apex region as well. A reduction in the potential toxicity to the adjacent critical organs is feasible.
SU‐E‐T‐352: MRI‐Based Post Dosimetry for Prostate Seed Implant
Authors: Li J , Lin M , Chen L , Ma C
Published: July 2013

Purpose: To investigate the feasibility of using the MR images only for the post dosimetry evaluation of transrectal‐ultrasound‐guided prostate seed implant. Methods: Post dosimetry evaluation for prostate seed implant is important to ensure the target dose coverage and the normal tissue sparing. CT is primarily used for this procedure and MR image is used in some institutions for target delineation because of its advantages on soft tissue discrimination. It will reduce the cost and increase the efficiency and the accuracy as well to use the MR images alone for the post dosimetry. The barrier for performing MRI based post dosimetry is the accuracy of seed finding. Ten patients with both CT and MR images were recruited for this study. The seeds were found manually based on the MR images only and compared with the CT‐based seed finding for every patient. The dose parameters calculated based on the found seeds were compared. Results: Iodine‐125 seed were used for the implant and the average seed number is 77 for the selected patients. About 92% of the seeds found on the MR images are confirmed by the CT images. The standard deviation of the confirmation rate is 2.9%. The dose parameter V100 and D90 of the target were compared based on the seeds found on MR and CT images. The difference on V100 is 0% with a standard deviation of 4%. The difference on D90 is 2.7% on average with a standard deviation of 11%. Conclusion: Most of the seeds can be found on MR images accurately and the MRI‐based post dosimetry is feasible as proven by the V100 that has no significant difference comparing with the CT‐based dosimetry. Efforts on MR imaging could improve the accuracy of seed detecting and the radioactive seed with MRI marker could fundamentally solve the problem.
SU‐E‐T‐362: Use of a MEMS Accelerometer to Monitor Breathing Motion
Authors: Markovic A
Published: July 2013

Purpose: Breathing motion is routinely monitored for purposes of gating a linear accelerator beam during radiotherapy treatment or for binning CT images during a 4D CT scan. This study proposes the use of a MEMS (micoelectromechanical system) accelerometer placed on a patient's chest to measure the timing and magnitude of regular breathing motion. A small package of electronics (Fig 2.) can be assembled with a battery which can unobtrusively be placed on a patient under their gown precluding the need to expose their skin as required by other camera‐based monitoring systems. The accelerometer communicates with a computer running specialized software that displays a breathing pattern viewable by the radiation therapists at the control console. Methods: A 12‐bit ST Microelectronics accelerometer (LIS3DH) of dimensions 3×3×1 mm connected to a 5×6 cm breadboard (Fig. 1) was used to measure breathing motion under simulated treatment conditions for this pilot study. The accelerometer can transfer data at rates ranging from 100 to 1600 Hz via either a USB cable or Bluetooth that relays acceleration data to a computer running a MATLAB program. The high‐speed data rate allows data averaging which aids in the reduction of signal noise. The accelerometer provides an acceleration resolution of 9.8×10−3 m/sec2. We are currently investigating the use of a new low‐noise 16‐bit accelerometer which will improve signal to noise response. Results: Fig. 3 shows accelerometer data acquired at a rate of 100Hz during regular breathing while placed on a subject's chest. High speed data rates allow for averaging (20 points, in this case) which reduces noise but does not affect the breathing trace period or amplitude. Conclusion: This pilot study used an accelerometer placed on a subject's chest to measure breathing motion during radiotherapy treatments. The small size of the electronics package can easily be placed on a patient under their gown.
SU‐E‐T‐364: Investigation of the PMLC Delivery Accuracy for MERT On An Artiste Linac
Authors: Jin L , Eldib A , Li J , Fan J , Wang L , Lin T , Ma C
Published: July 2013

Purpose: To verify the delivery accuracy of energy‐intensity modulated electron radiotherapy (MERT) using the existing photon multileaf collimator (pMLC) on the Artiste linac. Methods: Phase space data for 6, 9, 12 and 15 MeV electron beams of the Artiste accelerator, as an input source for our in‐house Monte Carlo (MC)‐based dose calculation/optimization planning code for MERT, were first commissioned by comparing measured PDD and dose profiles for pMLC‐shaped fields of 3 × 3 cm2and 10 × 10 cm2. MC calculated dose was then calibrated for each of the energies by comparing with chamber measurements for pMLC‐delivered electron beam and further verified at a different SSD setup. An energy‐intensity modulated electron plan was generated for a breast treatment on a breast phantom at a 60 cm SSD and recalculated with a solid water phantom and later delivered to the phantom through ten pMLC segments ‐ ‐each associated one of above energies and a specific monitor unit. Film measurement for the plan was done at the depth of 1.5 cm. The measured 2D dose distribution was then compared with calculations to verify the plan. Results: Comparisons between measured PDDs and dose profiles, and MC calculations for the various energies showed good agreement (within 2%/2mm). The verification measurement for MC dose calculation calibration at different SSDs was within 2% of the calculated values for all energies. The measured 2D dose distribution of the MERT plan was consistent with the calculations in low‐medium dose regions while in high dose regions, there existed some discrepancy. Conclusion: With pMLC shaped fields, MC dose calculations agreed with measured PDDs and dose profiles at various depths. The measured dose distribution of a breast plan showed good agreement in low‐medium dose regions while the discrepancy in high dose regions remains to be investigated.
SU‐E‐T‐366: Time‐Resolved EPID Dosimetry for Validating ArcIMRT
Authors: Yeo I , Jung J , Kim J , Patyal B
Published: July 2013

Purpose: The purpose of this study is to demonstrate time‐resolved dosimetry for verifying arc intensity modulated radiation therapy (arcIMRT) delivery, utilizing the continuous image acquisition of an electronic portal imaging device (EPID). Methods: An arcIMRT field was made of sliding, multi‐leaf collimator (MLC) motion of X1 leaves traveling from −5 cm to +5 cm and back, while Y opening was kept at 10 cm. For this field, this travel was repeated three times during the gantry rotation of 180°. The images were continuously acquired while the EPID was irradiated with 240 MU at a constant dose rate of 300 MU/min. By summing 10 frames, and thus reducing the temporal resolution to 1 second, a longitudinal non‐uniformity in the images was reduced to less than 3%. Dose images in EPID were also calculated by the XVMC code for MLC positions at 1 mm interval using the EPID model developed in our previous study. The calculated images at 1 sec resolution were then correlated to and compared with the images, validating the time‐resolved dosimetry. Results: Over the period of 48 seconds, 481 EPID images were acquired. The MLCs traveled at 1.25 cm/sec. Thus, 10 images were assigned to 1.25 cm of MLC travel for the correlation and for dose evaluation. For all gantry angles, the agreement of the gamma tests was above 90%, given 3 mm distance to agreement and 3% dose difference, when MLCs moved forwardly to the X2 side. However, when they travel back, the pass rates were below 90% due to MLC lagging which was detected in time‐resolved EPID dose imaging. Conclusion: Time‐resolved, four‐dimensional dose validation of arcIMRT was demonstrated, showing the temporal information of dynamic radiation delivery. This can be used for the validation of 4D treatment delivery techniques.
SU‐E‐T‐367: Dosimetric Comparison of VMAT and S&S IMRT Regarding the Dose Delivered to the Skin in Different Cancer Sites
Authors: Defoor D , Stathakis S , Regan M , Esquivel C , Mavroidis P , Papanikolaou N
Published: July 2013

Purpose: The risk for radiation induced skin complications can limit the effectiveness of radiation therapy when large areas of skin are involved in the treatment. Therefore, the accurate determination of skin dose may be critical in preventing post‐therapy side effects. The purpose of this study is to quantify the skin dose received by the Volumetric Modulated Arc Therapy (VMAT) and a 7 field MLC‐based IMRT step and shoot (S&S) delivery modalities using the same dose prescription. Methods: 5 prostate, 5 head‐and‐neck, and 5 brain cancer patients were used in this study to quantify skin dose for the examined radiation modalities. 30 treatment plans were produced (15 VMAT and 15 IMRT S&S) and for each patient the skin dose was measured using ultra‐thin TLDs. Doses were measured at 11 points on the surface of a cheese phantom at 25 degree intervals using ultra‐thin lithium‐fluoride TLDs with surface dimensions of 0.3cm × 0.3cm and a thickness of 0.01cm, comparable to the thickness of the skin. For each patient, the corresponding doses were also calculated using the Pinnacle treatment planning system. Results: A 36% increase in skin dose was measured on the brain patients using VMAT vs. IMRT S&S while a 38% decrease in skin dose was measured on the H&N patients using VMAT vs. IMRT S&S. The prostate patients showed just a 12% increase in skin dose using VMAT vs. IMRT S&S. The skin doses from Pinnacle showed small differences between the two modalities. The absolute differences of the dose values between the plan and the measurements were within an order of magnitude of the measured doses. Conclusion: The skin dose differences between the VMAT vs. IMRT S&S modalities were found to be significant. However, the direction of the differences was found to be treatment site specific and its magnitude patient specific.
SU‐E‐T‐368: Effect of Oblique Beam Incidence On 3D Dose Reconstruction for Small Field IMRT QA
Authors: Zhang J , Laub W
Published: July 2013

Purpose: To compare dose reconstruction accuracy with different oblique beam angles when 2d detector arrays are used for small field IMRT QA. Methods: 2D detector arrays are convenient to use but their low resolution limits the number of sampling points inside small radiation fields. In this study, we demonstrate that with oblique beam incidence, more points can be measured and 3D dose volume can be reconstructed more accurately. QA plans were simulated and measured using four oblique angles: 90 degree (straight incidence), 60 degree, 30 degree, and 15 degree. 3D dose volume was reconstructed based on dose measurements through back‐projection and inverse attenuation correction. Total delivering dose was calculated after rotating individual beam doses into actual beam angles. Two IMRT QA devices were used in our study, including a PTW /Octavius phantom and a Delta4 QA device. A spine SBRT plan was chosen as the study case. Gamma passing rate, mean PTV dose, and maximum dose received 2cc of spinal cord were calculated to compare reconstructed dose to original planning dose. Effects of measurement error and detector resolution were also investigated. Results: Oblique beam incidence will improve the sampling frequency of dose measurments. When the detector spacing is 10mm, 30 degree is the optimal angle for the Delta4 phantom and 15 degree is the opitmal angle for the PTW/Octavius phantom. Compared with standard straight incidence setup, QA with the optimal angle has 10% higher gamma passing rate (with 2mm/2% criteria) in both simulation and actual measurement. However, oblique incidence didn' t improve QA accuracy consistently when the detector spacing is 5mm. Conclusion: Oblique incidence can be used to improve IMRT QA accuracy for 2d‐array devices which don' t have sufficient spatial resolution for small field measurement.
SU‐E‐T‐369: Dosimetric Accuracy of Delivering Cu‐ATSM PET Dose Painting Plans with Tomotherapy
Authors: Bradshaw T , Henzler M , Forrest L , Jeraj R
Published: July 2013

Purpose: Unlike conventional IMRT plans, PET‐based dose painting plans can include high dose modulation and steep dose gradients within target volumes; these can be challenging for some therapy systems to deliver. The purpose of this study was to assess tomotherapy's ability to accurately deliver PET‐based dose painting plans. Methods: Five canine patients with sinonasal tumors were imaged with [61Cu]Cu‐ATSM PET/CT prior to treatment. Dose painting prescriptions were created with the following parameters: 1) PTV = 42 Gy, 2) GTV voxel doses were linearly related to Cu‐ATSM SUV, and 3) GTV voxel doses had a lower limit of 42 Gy, an upper limit of 100 Gy, and a mean GTV dose of 50 Gy. Plans were split into 10 fractions. Voxel dose prescriptions were converted into 8 contours representing dose levels and imported into tomotherapy TPS for optimization. One fraction from each optimized plan was delivered to the tomotherapy phantom. Absolute and relative measurements were performed with ion chamber and EDR2 film, respectively. Results: Maximum voxel doses varied between patients, ranging from 69 Gy to 100 Gy, depending on Cu‐ATSM uptake patterns. Good conformity was obtained between optimized plans and voxel prescriptions: on average 89% of voxels received +/−10% of their prescribed dose. Absolute dose measurements were within 3% of dose plans for all patients, with an average difference of −1.4%. Film dosimetry showed tomotherapy was capable of delivering the highly modulated plans. Gamma analysis (3%/3mm) showed that plans on average had only 0.1% of gamma values greater than 1. Conclusion: Tomotherapy was able to deliver dose painting treatment plans for all five of the test patients with a high degree of accuracy. Despite high dose modulation within planning structures, PET‐based dose painting is feasible with tomotherapy.
SU‐E‐T‐371: Can Dose Matrix Mapping to Kilo‐Voltage Cone Beam CT Images Be Reliably Used to Monitor Weekly Dose Changes for Head and Neck Cancer Patients?
Authors: Liu H , Shang Q , Greskovich J , Koyfman S , Lotey R , Xia P
Published: July 2013

Purpose: In external beam radiotherapy for head‐and‐neck cancer, the actual dose delivered to the patients could considerably deviate from the planned dose due to volumetric and anatomic variations. The purpose of this study was to investigate whether direct dose matrix mapping can be reliably used to monitor weekly dose changes when compared to recalculation method on weekly kilo‐voltage CBCT images. Methods: Ten head‐and‐neck patients, each with a planning CT, a mid‐term replanning CT, and seven weekly CBCTs, were selected for this study. Contours of tumor volumes and radiosensitive normal structures were deformed from the planning CT to the CBCTs using a commercial program. These deformed contours were validated using the same day CT and CBCT. The daily doses were obtained by two Methods: (1)mapping the planned dose matrix to the corresponding CBCTs after rigid image registration; (2)recalculating dose on the CBCT images with the same planning beam configurations. For a fair comparison, density override of the external contours was performed for both methods. Results: No significant dose differences were seen comparing the dose matrix mapping and recalculation methods for both the target and sensitive structures(p≫0.05). From both dose mapping and recalculation methods, the difference in D95 between the dose of the day and planned dose was observed for the PTV with 1.7 ± 2.2% variations. Although no significant changes in the mean dose were observed for the other sensitive organs, the mean dose of the parotid glands increased 14.1 ± 14.9% for the right parotid and 7.5 ± 9.8% for the left parotid. Conclusion: Dose matrix mapping is an effective and practical method to monitor weekly dose changes for patients with HN cancer. With the deformed contours from the planning CT, both the dose matrix mapping and recalculation methods can provide quantitative dose analysis and physician guidance for decisions on the adaptive replanning.
SU‐E‐T‐373: Feasibility of Treatment Adaptation Using MLC Leaf Shifting Algorithm to Account for Neck Position Changes for Patients with Head and Neck Cancer
Authors: Shang Q , Liu H , Koyfman S , Greskovich J , Xia P
Published: July 2013

Purpose: For patients with head and neck cancer, patient's neck position is difficult to reproduce during daily treatment due to neck deflection, despite of daily image guidance. We investigated the feasibility of a novel treatment adaptation method using an MLC leaf shifting algorithm to account for neck position variations. Methods: A patient with nasopharyngeal cancer, who received intensity modulated radiotherapy, was retrospectively selected. The prescription dose was 70 Gy to the gross tumor volume (GTV) and 56 Gy to the elective nodal region (CTV) in 35 fractions. A second planning CT was acquired after 22 fractions. A dual image registration was performed to determine the neck position relative to the skull. The contours of the GTV and the superior portion of the CTV were transferred from the initial planning CT to the second CT after rigidly aligning to the skull. The contours of the inferior part of the CTV were transferred after rigidly aligning to the vertebral body of the lower neck region. An MLC shifted plan was created by adjusting each IMRT segment to accommodate neck position changes using an in‐house program. D99s of the GTV and CTV from the MLC shifted plan were calculated and compared to those from the isocenter shifted plan aligned to the skull. Results: The shifts of the neck position relative to the skull were 5 mm, 8 mm, and 1 mm in the left, posterior, and superior directions, respectively. D99s of the GTV were identical at 67.6 Gy for both the MLC shifted and the isocenter shifted plans. D99 of the CTV was 53.1 Gy for the MLC shifted plan and 48.3 Gy for the isocenter shifted plan. Conclusions: Treatment adaptation using an MLC leaf shifting algorithm is feasible to account for neck deflection. This method will be tested for additional 20 patients.
SU‐E‐T‐374: A Simple Virtual Source Model for Monte Carlo Simulation of Helical TomoTherapy
Authors: Yuan J , Chen Q
Published: July 2013

Purpose: To present a methodology based on a simplified source, jaw, and MLC model to simulate helical TomoTherapy with the MC method without using detailed phase‐space files (PSFs).Methods and Materials: Current studies on Monte Carlo (MC) simulation for helical TomoTherapy adopt a full MC model and use PSFs at different scoring planes to facilitate patient dose calculations. The purpose of this work is to develop a simple virtual source model (VSM) to calculate the patient dose for helical TomoTherapy with the MC method. The VSM is based on the commissioning data of helical TomoTherapy unit, which can be exported from the treatment planning station (TPS). To calculate the patient dose distribution, the TPS‐generated sinogram is first extracted from an archived patient XML file. Next, a fluence map is created by incorporating the open‐close leaf time contained in the sinogram file with leaf filter, jaw penumbra, and leaf latency. The VSM was validated for various geometrical set‐ups including open fields, heterogeneous media, and clinical DQA cases. Results: Good agreement was achieved for both transverse and longitudinal profiles between MC calculations and measurements with differences within 2%/2 mm. For MLC validation that compared MC calculations with film measurement, the passing rate was 96.4% using a gamma index analysis tool (2%/1 mm acceptance criteria). A DQA plan was simulated for a clinical head‐and‐neck plan. The patient dose from the MC simulation agrees with the TPS Result very well (γavg [1%, 2mm] = 0.7). Good agreements between the planar patient dose distributions from the MC simulations and the TPS were observed as well (γavg [3%, 2mm] = 0.4, passing rate = 94.7%). Conclusion: A simple VSM can be used in the MC method to accurately simulate the patient dose distributions for helical TomoTherapy.
SU‐E‐T‐375: Evaluation of the Ability of Three Commercially Available Dosimeters to Detect Systematic Delivery Errors in IMRT Plans
Authors: Gray A , Bawazeer O , Arumugam S , Vial P , Thwaites D , Holloway L
Published: July 2013

Purpose: To evaluate the sensitivity of three patient‐specific IMRT QA devices to systematic delivery errors. Methods: Four clinical step‐and‐shoot IMRT plans were selected, two simple prostate, one medium complexity head and neck (H&N) and one complex H&N. Twelve plan variations with errors introduced were created: Multi‐Leaf Collimator (MLC) positional errors (all leaf pairs shifted in the same direction (1, 2, 3 mm) and opposite direction (1, 2, 3 mm)); collimator rotation offsets (1, 2, 3 degrees) and gantry rotation offsets (0.5, 1, 2 degrees). The total dose for each plan was measured using an ArcCHECK helical diode array. Each field for each plan, excluding those with gantry offsets, was measured using an Electronic Portal Imager and a MatriXX Evolution 2D ionisation chamber array. The measured dose was compared to the calculated dose for the no error plan using Gamma analysis with 3%/3mm, 3%/2mm, and 2%/2mm criteria. Results: The ability of the different devices to detect errors was similar. In most cases pass rates reduced with increasing errors and/or stricter gamma criteria as expected. The ArcCHECK demonstrated sensitivity to 2 degree gantry errors, with a reduction in pass rate of 6–14% between the 1 and 2 degree errors in all plans for all gamma criteria. Even for the 2%/2mm criteria, no device detected a 3 degree collimator error in either prostate plan, however collimator errors were detected for the H&N plans which had elongated fields. For all criteria, there were different scenarios for each detector where MLC position errors of 3 mm were not detected. Conclusion: None of the devices could reliably detect MLC position or collimator rotation errors that were not within recommended linear accelerator monthly QA tolerances. There were no consistent differences between the ability of the devices to detect systematic delivery errors. Financial support has been provided by the Saudi Arabian Cultural Mission towards this work.
SU‐E‐T‐376: Incorporating Photon Beamlet Energy in Optimization of Intensity Modulated Beams
Authors: McGeachy P , Khan R
Published: July 2013

Purpose: To develop an energy dependent optimization criteria for incorporation into the intensity modulated radiation therapy (IMRT) optimization problem and perform simultaneous optimization of photon energy and intensity for X‐ray modulated radiotherapy (XMRT). Methods: For a simple spherical tumor model, energy dependent optimization criteria (EDOC) were formulated based on three objectives: maximize distal edge dose of the tumor, minimize healthy tissue entrance dose, and maximize uniformity of the tumor dose distribution. This was implemented in MATLAB to test on a simplified scenario of a 10 cm wide 1D photon beam incident on a spherical tumor (10cm radius) centered within a 5600 cm3 water phantom. The 1D beam was further partitioned into 20 beamlets of equal width. From the MATLAB program, an optimal assignment of monoenergetic photons (ranging from 1 MeV to 18 MeV) for each beamlet based on the EDOC was obtained. The dose calculation for each monoenergetic photon beamlet was restricted to primary dose only. Results: The beamlet distributions showed the mid tumor region assigned higher energies due to the large amount of tumor tissue traversed. Outer edge beamlets were assigned lower energies to spare the large portion of healthy tissue traversed. Changing priorities for each objective effected the optimal distribution in an intuitive manner (i.e. larger emphasis on sparing healthy tissue results in lower energies assigned to each beamlet). Conclusion: A viable set of energy dependent optimization criteria has been developed that can be used in reformulating the inverse optimization problem for radiotherapy, allowing optimization in terms of both beam energy and intensity. Currently, geant4 is being used to generate and benchmark dose profiles for 1 MeV to 18 MeV photons energies to improve the accuracy of the dose calculation. A rigorous testing of the optimization criteria through added complexity of radiotherapy is currently being investigated.
SU‐E‐T‐379: Clinical Implementation of VMAT
Authors: Kapur P , Kahler D , Liu C , Kapoor R
Published: July 2013

Purpose: The goal of this study is to evaluate the impact of VMAT planning parameters on plan quality and delivery efficiency on linear accelerators. Methods: VMAT technology provide a number of variable on treatment delivery such as MLC motion, dose rate (DR) and gantry rotation speed. Three parameters‐Leaf Speed (LS), Minimum DR and Control points (CP) were chosen to evaluate their impact on the VMAT plan quality and delivery efficiency. Five prostate cases were selected to generate several VMAT plans on Pinnacle TPS (v9.2) with Elekta LINAC (Synergy‐S). Leaf Speed: Pinnacle provides options for maximum LS while optimizing a treatment plan. We chose 3 LS for a comparative study 2cm/sec (maximum LS allowed at LINAC), 1.5cm/sec & 1cm/sec. Minimum DR: Due to varying DR we evaluated the minimum DR that can be used to perform and deliver a treatment plan on the LINAC.Control points: TPS provides 3 options for choosing the number of CP 91, 121 & 181 i.e. CP generated with gantry spacing 4°, 3°, 2° in a single arc. Results: Leaf Speed: Due to LS impact of treatment delivery time, it is recommended to stay below the maximum LS of LINAC to avoid beam cut off. The optimum LS was found as 1.5cm/sec.Minimum DR: Beam cut off issue occurred on LINAC when the DR fell below 30MU/sec. So it becomes important to choose a higher value of DR while optimizing a plan in TPS.Control points: The number of CP used to generate a treatment plan has a minimal impact on plan quality and huge impact on plan delivery because it increases the optimization and delivery time. Conclusion: Each parameter in the above study has different impact on the plan quality & delivery efficiency. Hence it is important to evaluate all the parameters before clinical VMAT implementation.
SU‐E‐T‐380: Modeling MLC Leaf End in 2D for Sliding Window IMRT and Arc Therapy
Authors: Liang X , Zhu T
Published: July 2013

Purpose: To develop a 2D geometric model for MLC accounting for leaf end dose leakage for IMRT and arc therapy. Methods: Leaf‐end dose leakage is one of the problems for MLC dose calculation and modeling. Dosimetric leaf gap used to model the MLC and to count for leakage in dose calculation, but may not be accurate for smaller leaf gaps. We propose another geometric modeling method to compensate for the MLC round‐shape leaf ends dose leakage, and improve the accuracy of dose calculation and dose verification. A triangular function is used to geometrically model the MLC leaf end leakage in the leaf motion direction, and a step function is used in the perpendicular direction. Dose measurements with different leaf gap, different window width, and different window height were conducted, and the results were used to fit the analytical model to get the model parameters. Results: Analytical models have been obtained for stop‐and‐shoot and dynamic modes for MLC motion. Parameters a=0.4, lw=5.0 mm for 6X and a=0.54, lw=4.1 mm for 15x were obtained from the fitting process. The proposed MLC leaf end model improves the dose profile at the two ends of the sliding window opening. This improvement is especially significant for smaller sliding window openings, which are commonly used for highly modulated IMRT plans and arc therapy plans. Conclusion: This work models the MLC round leaf end shape and movement pattern for IMRT dose calculation. The theory, as well as the results in this work provides a useful tool for photon beam IMRT dose calculation and verification.
SU‐E‐T‐382: Investigation of the Characteristics of the OCTAVIUS Detector 1000 SRS for Verification of the Small Field Size IMRT and SRS Beams
Authors: Markovic M , Stathakis S , Ha C , Poppe B , Gutierrez A , Papanikolaou N
Published: July 2013

Purpose: The aim for the study was to investigate characteristics of the OCTAVIUS Detector 1000 SRS for verification of the small field size IMRT and SRS beams. Methods: Several different treatments sites have been chosen. High modulation steep gradient small field size beams were delivered in two different setup settings. In the first setup virtual water phantom has been placed on the detector and irradiated. In the same setup EDR film has been irradiated. Measured data has been analyzed. In the second setup detector has been placed in the PTW Octavius phantom and irradiated. In the same setup EDR film and PTW 729 have been irradiated. Measured data has been analyzed. Results: In the first setup detector has been irradiated with virtual water phantom and measured Result showed excellent agreement with the film measurements. Along the compared profiles, absolute dose measured was in ±1 % of the film measurements in the high dose region and ±3 % in low dose region. Measured results showed also very good agreement in region of high gradients. Second set of measurements showed excellent agreement with the film measurements in high dose region with absolute dose difference within ±1% and ±3% in low dose region. Agreement with PTW 729 was in ±3 % in high and low dose region along the compared profiles. Region of high gradient showed better agreement with the film than with PTW 729 due to the inferior detector resolution. Conclusion: Broad range of measurements performed in the investigation showed that OCTAVIUS Detector 1000 SRS is a dosimetrically accurate and sensitive device and it can be used for clinical verification of the highly modulated small field size beams used in the IMRT and SRS treatments.
SU‐E‐T‐384: Combining Multiple Arcs Into a Single Arc for Efficient Delivery
Authors: Oddiraju S , Rangaraj D , Papiez L
Published: July 2013

Purpose: Linear accelerators have introduced delivery control systems with the ability to deliver Volumetric Modulated Arc therapy (VMAT) either by a single arc or by multiple‐arc. VMAT delivery for complex treatments such as head and neck requires providing more number of segments for a given plan. The primary goal of this study is to combine these multiple arcs into a single arc and compare the dose distribution and the treatment time for the two plans. Methods: Five Head and Neck plans and five prostate plans were considered for this study. VMAT plans were generated using Varian SmartArc treatment planning system version 8.5. A combined DICOM plan with single arc is created which has half the spacing as the original multiple arc plan and have all the segments of the multiple arc plan. For the combined arc plan and the multiple arc plans, the delivery time for optimal delivery under delivery constraints is determined using the delivery algorithm. A fluence map comparison between the combined arc and multiple arcs is made to prove the dosimetric similarity of the arc plans. Results: The delivery time for the Head & Neck plans for the combined arc is improved by 35% and for the prostate plan is improved by 37% as compared to the multiple arc plans. Difference fluence is calculated from the multiple arc fluence and the combined arc fluence for all the plans. There is less than 5% dosimetric difference between the multiple arc plans and the combined arc plans for both prostate and head and neck plans. Conclusion: Combined arc delivery provides comparable plan quality and improved treatment delivery time. Hence, for more complex sites combined arc delivery is preferable due to the improvement in the treatment time while maintaining similar dose distribution with that of the multiple arc plans.
SU‐E‐T‐386: Does the Couch Need to Be Included for Accurate IMRT Planning?
Authors: Gulam M , Gopal A , Chin K , Kim J , Orfali A , Gordon J , Chetty I , Nurushev T
Published: July 2013

Purpose: To evaluate the effect of using couch structures in IMRT planningMethods: Three TG‐119 IMRT plans (Head&Neck, Prostate, C‐shape hard) developed for commissioning the Varian Eclipse AAA 10.0.28 algorithm were delivered on a Varian 21EX with Exact Couch. This algorithm allows accounting for the effects of the treatment couch via a separate structure outside the external contour. Each plan was calculated twice (with and without couch), keeping monitor units and leaf motions the same to facilitate comparison. Plans were delivered to an ion chamber and an EBT2 film in a 30×3×20 cm3 phantom at two locations reflecting high and low dose regions per TG‐119. Additionally, 15 clinical cases (five each of 6MV H&N, 6MV prostate and 18MV prostate were delivered; four of the ten prostate cases included pelvic lymph node involvement. The film analysis (using in‐house software) required >95% points to pass a gamma criterion of 3%,3mm. Results: For the TG‐119 plans 99.1%±2.7% and 94.6%±19.9% of the points met the gamma 3%,3mm with and without the couch model respectively; the major contribution for the lower value (94.6%) was from the H&N plan for which only 85% of the points passed. Gamma results for clinical cases were: H&N (99.7%±0.9% and 97.4±7.9%), Prostate 6MV: (99.6%±1.8% and 94.7%±12.7%), Prostate 18MV results with and without the couch were nearly identical (99.8%±0.9% vs. 99.7%±1.1%, respectively). We also found the largest differences for posterior oblique beams traversing the couch rails: For 6MV prostate, there was no relative difference for beams striking the phantom only, 0.7% when treating entering through the couch, and 4.5% for beams traversing the rails. Conclusion: Including the couch structures is important for lower energies (6MV), especially for oblique beams traversing the couch rails. However, adjusting rail positions during delivery should improve agreement for algorithms which do not account for the couch.
SU‐E‐T‐387: Evaluation of VMAT Dosimetric Effect Caused by Slowing MLC Leaves Using MapCHECK
Authors: Xu Z , Wang IZ
Published: July 2013

Purpose: This study is to report the effects on the quality of volumetric modulated arc therapy (VMAT) dose delivery by simulating multileaf collimator (MLC) leaf speed errors that do not trigger any MLC motion interlocks (i.e. <2 mm leaf position error). Method and Materials: Four VMAT plans (two one‐arc brain cases and two four‐arc anal cases) were modified using MATLAB. Within each arc, five consecutive MLC leafs on the same bank were selected for modification. Leaf speed at each control point was calculated. Only those control points with highest speed (average ∼4.6% of total control points) were rise to the Varian MLC speed limit (2.5 cm/s), which gave a leaf position difference of less than 2 mm. No changes in MU per control point and gantry speed were introduced. The modified plans were now considered as “standard” plans and the original plans were treated as “wrong” plans with leafs moving at a lower speed. Modified DICOM plans were imported back into Eclipse for dose calculations and DVH comparisons. The recalculated planar dose was compared with original MapCHECK measurement using the standard patient‐specific VMAT quality assurance (QA) approach, with a gamma‐index of 3%/3mm, 3%/2mm, and 2%/2mm. Results: All cases passed the >85% criterion for a gamma‐index of 3%/3mm. There was only a small passing rate difference (<1%) between the original and the modified plans. The DVH calculations of the modified plans demonstrated very minimal changes in PTV and OAR doses. Conclusions: By introducing the leaf speed error without triggering the MLC motion interlock, the dose changes cannot be detected by MapCHECK. However, the consequential effects on target and OAR doses are also negligible, which supports the reliability of current patient IMRT QA method for VMAT plan verification.
SU‐E‐T‐389: Treatment Interruptions in Stereotactic Body Radiotherapy (SBRT) and Stereotactic Radiosurgery (SRS): Influence and Compensation
Authors: Guo FQ , Chiang V , Yu J , Chang B , Bond J , Chen Z , Nath R , Deng J
Published: July 2013

Purpose: Unscheduled treatment interruptions in SBRT/SRS can occur for a variety of reasons. As the fractional dose is much larger and the total treatment time much shorter, treatment prolongation may have larger influence in SBRT/SRS than in conventional radiotherapy. Methods: A biologically effective dose (BED) model was used to quantify the impact of interruptions with a daily equivalent repopulation constant K. The BEDs were calculated for regular daily treatment and interrupted treatments with various interruption schemes and K values. The dose required to compensate for the BED reduction due to interruption was then calculated. Results: For a typical five‐fraction SBRT assuming (α/β) 10Gy for tumor and 2Gy for normal tissue, K 0.5Gy/day, and a seven‐day gap, the dose per fraction required for the remainder of the treatments to compensate for treatment breaks (dnew) would be 103%, 104%, 106%, or 111% of the originally prescribed dose per fraction (d) with the gap after 1, 2, 3, or 4 fraction(s). For a four‐fraction SBRT, the dnew would be 104%, 106%, or 111% with the gap after 1, 2, or 3 fraction(s). For a three‐fraction SBRT, the dnew would be 106% or 111% with the gap after 1 or 2 fraction(s). With a fourteen‐day gap and same K, the increase Δd=[(dnew‐d)/d]×100% would double for the various scenarios respectively. If K increases from 0.5Gy/day to 1.0Gy/day, the Δd would roughly double for the various scenarios accordingly. Conclusion: According to our BED model, significantly higher dnew would be needed to compensate for the reduction in local control with interruptions, particularly for the large gaps and the late occurrings. Therefore, for anticipated interruptions, it is better to interrupt the treatment sooner than later. As our model depends on K, more clinical data would be needed to substantiate its validity for various lesions treated by SBRT/SRS.
SU‐E‐T‐393: A Study of Varian Linac Beam Data Equivalency for IMRT
Authors: Dimofte A , Zhu T
Published: July 2013

Purpose: The purpose of this study was to compare the photon beam data for four Varian (2300iX) accelerators in six categories: beam quality, phantom scatter factor, output ratio in air, primary off axis ratio, electron disequilibrium factor and benchmark measurements including dose per MU ratio at selected points and a few IMRT plans. Methods: We compared the beam quality of these accelerators, characterizing the attenuation and beam hardening coefficients and percent depth dose (PDD) at 10cm depth. The phantom scatter factor (Sp) and output ratio in air (Sc) were measured in a full phantom and a miniphantom, respectively, at depth of 10cm and SAD = 100cm as a function of field sizes. Results: PDD agreed within 1% and the attenuation coefficients agreed within 3% for the same energy. The output ratio in air Sc is almost machine independent but was energy dependent and agreed within 0.3%. For the same nominal energy, the phantom scatter factor (Sp) measured at 10cm depth agreed within 1.4% regardless of the accelerator. We delivered 3 IMRT plans (prostate, H&N and lung) to study the difference in dose delivery among the four linacs. The dose delivered was compared for depths of 5, 10 and 20cm and to the calculated dose. All measurements were performed using a 2D diode array and analyzed using the distance to agreement (DTA) criteria of 3% and 3mm. The agreement among linear accelerators is much better than the agreement between measurement and calculation, and are better than 3.5% for all IMRT cases studied. Conclusion: Accelerators from the Varian manufacturer, with the same nominal energy and model, can be treated as identical to within 1.8% for conventional treatment as well as for IMRT treatment, provided that caution is given to the dosimetrical characteristics for OAR and small field dosimetry.
SU‐E‐T‐396: An Improved Sequencing Algorithm for Dynamic Multileaf Collimator in Intensity Modulated Radiotherapy
Authors: Jia J , Chow J
Published: July 2013

Purpose: In intensity modulated radiotherapy delivery, a multileaf collimator (MLC) is indispensable with its leaf sequencing algorithm being the link between the external beam treatment plan and the linear accelerator hardware control. In this study, a dynamic MLC leaf sequencing algorithm was developed based on the bixel width of intensity map, which can verify the speed of MLC leaf easier. This results in a better hardware variable‐speed performance lessening the workload of the MLC system. Methods: In dynamic MLC sequencing, we assume each pair of leaves forms a window and moves from left to right with the dose rate of photon beam kept constant. The intensity map was normalized and divided into bixels based on the equal‐space clustering method. For each bixel, the actual travelling distance of leaf was determined by the MLC mount position. So in each single bixel the MLC leaf travels, one leaf pair can keep in constant speed and the accumulated dose from all bixels can form the intensity map. Results: From experimental results of validation using various intensity maps in treatment plans of phantoms and patients, it is found that accurate dose distribution or dose‐volume histogram can be obtained. Our new leaf sequencing algorithm was therefore verified. Conclusion: It is concluded that the performance of the proposed dynamic MLC sequencing algorithm is better than previous works, as the algorithm decreases the need for mechanical constraint in MLC. Natural Science Foundation of Anhui Province (11040606M132), Fundamental Research Funds for the Central Universities (Nos. 2012HGXJ0057, 2012HGXJ0062, 2012HGXJ0071, 2012HGBZ0190) and Seed Foundation of Hefei University of Technology(2012HGZY0007), P.R.China
SU‐E‐T‐399: Improving Gated Delivery Efficiency Using Brief Breath Holds at Both Inhale and Exhale
Authors: Geneser S , Fahimian B , Xing L
Published: July 2013

Purpose: Respiration‐gated intensity modulated radiation therapy (IMRT) and volumetric modulated radiation therapy (VMAT) deliver the treatment beam at a single phase to reduce dose uncertainties resulting from respiratory‐induced organ motion. In some cases, gating can significantly prolong treatment times, leading to increased patient discomfort, treatment duration, and dose uncertainty due to patient postural shifts. To decrease treatment duration without increasing dose uncertainty, we propose dual‐gating in conjunction with with alternating coached five to ten second breath holds at inhale and exhale. In this study, we examine the feasibility of maintaining reproducible inhale and exhale RPM marker positions in a healthy volunteer and compare the expected dual‐gated delivery speedup over the free breathing case.Materials/Methods: Varian Real‐Time Position Management (RPM) respiratory signals from a volunteer were recorded during free breathing and during alternating ten second inspiration and exhalation breath hold coaching. The signals were then processed to examine the volunteer's ability to consistently reproduce RPM marker position during subsequent breath holds. Conventionally‐gated and dual‐gated treatment times were simulated for a given treatment plan under both normal and alternating breath hold conditions and compared. Results: Dual‐gated delivery under free breathing increases the duty cycle from 30.3% to 58.3% as compared with gating at exhale only. Alternated breath hold coaching further improves the duty cycle to 85.1%. Treatment delivery requiring 29.05 minutes to complete under normal exhale gating conditions is reduced to 16.58 minutes for dual‐gating during free breathing and only 5.88 minutes with dual‐gated alternated breath holds. RPM marker reproducibility was within 8% over the course of the delivery time. Conclusion: While additional improvements and testing are necessary, dual‐gated treatment delivery during alternated breath hold coaching is a feasible method for increasing duty cycle and thus improving treatment efficiency for tumors undergoing respiratory‐motion.