bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐394: Effects of Various Encapsulation Materials to the Dose Fall‐Off for Beta‐Emitting Brachytherapy Seed
Authors: Jung J , Podder T , Lee C
Published: November 2011

Purpose: A beta‐emitting glass seed without encapsulation has been proposed for the brachytherapy treatment of prostate cancer. Beta sources have several desirable properties. Beta sources have short range and are easily shielded, lowering extraneous dose to the medical staff as well as the patient. Even though there are several advantages of using beta‐emitting glass seeds, the dose fall‐off is considerable. This study aims at evaluating the effects of dose fall‐off and bremsstrahlung using encapsulation. Methods: MCNPX2.6 Monte Carlo transport code was used to calculate the quantitative dosimetric parameters of the seeds. The radioisotope used for seed was 142Pr. The 142Pr decays by beta emission with a 2.162 MeV endpoint energy is followed by emission of a 1.575 MeV gamma‐ray with 3.7% abundance. The gamma dose was not included in this research except the contributions of bremsstrahlung because the encapsulation does not attenuate the 1.5 MeV gamma much. Various encapsulation materials were investigated including silver, titanium, and tungsten. A simulation model of glass seed with encapsulation was developed within MCNPX2.6. Dosimetry data in water were calculated in radial distances from the glass seed from 0.1 cm to 1.0 cm by using the mesh tally feature in MCNPX2.6. Results: The relative dose of titanium and tungsten to water at 0.2 cm were 84.5 % and 21.9 % while the relative dose at 0.6 cm was 60.4% and 12.4% respectively. Considering the reduction of dose at the target point (0.6 cm), it was found that titanium is the most appropriate material. The contribution of bremsstrahlung was insignificant. Conclusions: The effects of source encapsulation on dose fall‐off were studied. The encapsulation has effectively made the dose profiles uniform. The bremsstrahlung effect was insignificant. From this study, the dosimetric parameters would be derived for clinical use.
SU‐E‐T‐407: Investigation of the Need for Bolus for Superficial Head and Neck Tumors Treated with 3D, RapidArc, and IMRT
Authors: Svoboda A , Sheu R , Lo Y , Dumane V
Published: November 2011

Purpose: To determine whether or not the multiple oblique and tangential beam angles of RapidArc and IMRT treatment plans eliminate the need for bolus when treating superficial tumors. This study uses film measurements to evaluate the efficacy of IMRT and RapidArc planning techniques employed to achieve adequate surface dose without bolus. Methods: Gafchromic EBT2 QD+ film was used to measure skin dose for plans created using Eclipse 8.9 and delivered via a Varian 21iX linac. Dose was delivered to a cylindrical Catphan (CP504) and the film measurements were evaluated using in‐house software. Measurements were compared to results from the treatment planning system calculated using Varianˈs Anisotropic Analytical Algorithm (8.9.08). Results: The Gafchromic film measurements showed that the 7‐field IMRT plan with strategically placed beams tangential to the phantom surface did increase skin dose 63% compared with a non‐bolused 3D plan. RapidArc delivery increased the skin dose by an additional 13% beyond IMRT. However, even with RapidArc, the skin dose obtained was only 44% of the prescribed dose (2Gy) showing that bolus is still required. Attempts to modify planning technique by adding skin flash to lessen the dose falloff and expanding the external contour to match had no effect on skin dose. Incidentally, we found that the 5mm bolus often used for head and neck plans only brought the surface dose to 78% of the prescription dose (in the case of 3D) which may still be clinically inadequate. Conclusions: IMRT and RapidArc treatment plans delivered without bolus do increase skin dose compared with their 3D counterparts, but the effect is not great enough avoid using bolus when prescription surface dose is required. While planning techniques to lessen dose fall off at the skin may be useful for managing setup uncertainty, they do not increase surface dose in any way.
SU‐E‐T‐412: Biological Consequences of Errors in MLC Calibration in the Context of IMRT Delivery and QA
Authors: Moiseenko V , LaPointe V , James K , Yin L , Liu M , Pawlicki T
Published: November 2011

Purpose: To quantify biological consequences of MLC calibration errors in prostate IMRT and relate those biological consequences to IMRT QA action levels. Methods: Ten consecutive prostate cancer patients treated with a 5‐field IMRT (Varian iX with Millenium 120 leaf MLC) were selected. All cases were treated with a prescription dose of 74 Gy. An in‐house program was used to introduce systematic MLC offsets. Combinations of X1 and X2 offsets in the range from −2 to +2mm were explored. Modified files were imported into the treatment planning system (TPS) to recalculate dose for each modified plan. DVHs were exported from the TPS and generalized equivalent uniform doses (gEUD) were calculated for the rectum (n=0.09) and PTV (a=−10). Each plan was also recalculated on a CT scan of a cylindrical phantom with a contoured ion camber. Dose distributions were exported for 3D gamma analysis with dose and distance to agreement set to 3% and 3 mm, respectively. Pass criteria were: at least 95% of voxels have to pass and dose to chamber has to be within 2% of the reference dose. Results: Plans with offsets of 2 mm, single leaf bank or combined, have not passed gamma QA, however 30% of plans passed chamber dose QA. For 1.5 mm offsets 20% and 70% of plans passed gamma and chamber dose QA, respectively. Change in gEUD for PTV and rectum was of the order of 2% for plans passing both QA methods and larger than 3% for those which failed QA. Conclusions: With currently used methods and criteria for prostate IMRT QA, we can only detect changes in gEUD greater than 3% that are resultant from MLC calibration errors. This is important because it provides a lower limit on the ability of current IMRT QA to estimate a biological impact for machine errors.
SU‐E‐T‐413: Optimizing the Delivery of Intensity‐Modulated Stereotactic Body Radiation Therapy to Lung Tumours Influenced by Respiratory Motion
Authors: Gaede S , Hamelin N , Kline M , Goosheh S , Yartsev S , Chen J
Published: November 2011

Purpose: To quantify the interplay effect for various IMRT techniques used to deliver Stereotactic‐Body Radiation Therapy (SBRT) to early‐stage lung cancer. Methods: Five lung cancer patients who received SBRT were retrospectively planned (54Gy/3fx) on the average 4D‐CT dataset with eight different IMRT techniques: three fixed‐beam IMRT plans (simple step‐and‐shoot (S/S), complex S/S, and sliding‐window (S/W)), Tomotherapy plans using three different beam sizes (1cm, 2.5cm, and 5cm), and two Volumetric‐Modulated Arc Therapy (VMAT) plans (one planned with Rapid Arc (Eclipse v8.9, Varian Medical Systems, Palo Alto, USA) and one planned with Smart Arc (Pinnacle v9.0, Philips Medical Systems, Cleveland, USA)). All treatment plans were calculated on a CT of the ArcCHECK phantom by Sun Nuclear (Gland, Switzerland) that was mounted on the QUASAR™ Programmable Respiratory Motion Platform (Modus Medical Devices Inc., London, Ontario). Each plan was delivered under the following motion conditions: 1) Static mode; 2) Sinusoidal mode (4s period) with S/I motion ranging from 5mm–20mm peak‐to‐peak in 5 mm increments; 3) Real‐patient waveforms ranging from 5mm–30mm peak‐to‐peak. A standard 3%/3mm gamma analysis compared each delivery to their corresponding calculated plan. Results: All methods were acceptable on average (>90% points pass) up to 15mm peak‐to‐peak motion except for the Rapid Arc plan, which had a significantly less pass rate (75.6+/−3.9%) than all other techniques (p<.02). Tomotherapy plans using either a 2.5cm beam (98.7+/−2.1%) or 5cm beam (96.8+/−3.5%) were still acceptable for motion up to 20 mm peak‐to‐peak. For irregular motion greater than 15mm, only the VMAT techniques failed ((74.9+/−16.6)% for Rapid Arc and (89.3+/−8.9)% for Smart Arc on average. Conclusions: Interplay effect in SBRT delivery was dependent on the IMRT technique for peak‐to‐peak motions greater than 15 mm. Tomotherapy was the least sensitive for motion up to 20mm. Rapid Arc was the most sensitive for motion greater than 15mm peak‐to‐peak.
SU‐E‐T‐416: Geometric and Dosimetric Effects of Residual Setup Error on the Cord and Larynx in Split‐Field Head and Neck Radiation Therapy
Authors: Yock A , Zhang L , Garden A , Dong L
Published: November 2011

Purpose: In split‐field head and neck radiation therapy, image guided alignment of the superior IMRT field may result in large setup errors in the supraclavicular field. The goal of this study was to quantify geometric and dosimetric effects of residual setup error on the cord and larynx using the split‐field technique. Methods: Cord and larynx contours of 27 patients were propagated from a planning CT onto daily in‐room CTs using deformable registration. Cord contours were divided into superior and inferior structures at the match plane. Interfractional variations in the position of the larynx, superior cord, and inferior cord centroids were measured. Deformable registration allowed the calculation of cumulative dose delivered for each patient. The discrepancy between planned and delivered doses was compared for the maximum dose to the superior and inferior cord, mean dose to the larynx, larynx V50 and D60. Results: The group mean error, systematic error, and random error of the larynx centroid positions were 0.12cm, 0.06cm, and 0.08cm, laterally, and 0.15cm, 0.06cm, and 0.08cm in the anterior‐posterior direction. The lateral values for the superior cord were 0.04cm, 0.01cm, and 0.03cm, and for the inferior cord were 0.17cm, 0.06cm, and 0.12cm. Lateral and anterior‐posterior margins were 0.22cm and 0.20cm (larynx), 0.05cm and 0.08cm (superior cord), and 0.22cm and 0.35cm (inferior cord). Maximum cord dose was 43.8Gy. The average change in maximum dose was 0.3Gy (max=1.8Gy) for the superior cord, and −0.6Gy (max=1.2Gy) for the inferior cord. The increases in larynx mean dose, V50, and D60 averaged 1.6Gy (SD=2.5Gy), 3.24% (SD=3.52%), and 4.7Gy (SD=6.1Gy). Conclusions: While the inferior cord demonstrated the largest motion, it remained within the cord block. The increase in maximum cord dose and mean larynx dose were no more than 1.8Gy and 5.1Gy, respectively. The split‐field technique remains clinically viable.

Funding provided by the University of Texas Graduate School of Biomedical Sciences at Houston.
SU‐E‐T‐417: A Planning and Delivery Study of a Rotational IMRT Technique with Burst Delivery
Authors: Kainz K , Chen G , Chang Y , Prah D , Shukla H , Stahl J , Li X
Published: November 2011

Purpose: A novel rotational IMRT (rIMRT) technique using burst delivery (continuous gantry rotation with beam off during MLC repositioning) is investigated. We evaluate the plan quality and delivery efficiency and accuracy of this dynamic technique with a conventional flat 6MV photon beam. Methods: Burst‐delivery rIMRT was implemented in a planning system (Panther, Prowess) and delivered with a 160‐MLC linac (Artiste, Siemens). Ten rIMRT plans were generated for five anonymized patient cases encompassing head and neck, brain, prostate, and prone breast. All plans were analyzed retrospectively and not used for treatment. Among the varied plan parameters were the number of optimization points, number of arcs, gantry speed, and gantry angle range (alpha) over which the beam is turned on at each optimization point. Combined rotational/step‐and‐shoot rIMRT plans were also created by superimposing multiple‐segment static fields at several optimization points. The rIMRT trial plans were compared with each other and with plans generated using helical tomotherapy. Plans were delivered and verified using a diode array, ionization chambers, and thermoluminescent dosimeters. Results: Burst‐mode rIMRT can achieve plan quality comparable to helical tomotherapy, while the former may lead to slightly better OAR sparing for certain cases and the latter generally achieves slightly lower hot spots. Few instances were found in which increasing the number of optimization points above 36, or superimposing step‐and‐shoot IMRT segments, led to statistically significant improvements in OAR sparing. Using an additional rIMRT partial arc yielded substantial OAR dose improvements for the brain case. Measured doses from the rIMRT plan delivery were within 4% of the plan calculation in low dose gradient regions. Delivery time range was 228–375 seconds for single‐arc rIMRT 200‐cGy prescription with a 300 MU/min dose rate. Conclusions: Rotational IMRT with burst delivery, whether combined with static fields or not, yields clinically acceptable and deliverable treatment plans.
SU‐E‐T‐418: Characteristics of MLC Leaf Positioning Errors in VMAT Delivery for SBRT Plans
Authors: Xiong L , Iftimia I , Kazi M , Mower H
Published: November 2011

Purpose: Volumetric modulated arc therapy (VMAT) is a favored modality for SRS/SBRT treatment due to the efficiency in delivery and high conformality to targets achievable by this technique. Due to the dynamic characteristics of the MLC movement throughout the delivery process, tolerance is necessary in the system configuration for the deviation of MLC leaf position from the planned position. This work examines the characteristics of the MLC positioning errors in VMAT delivery. Methods: Five RapidArc fields with a mean MU=892 planned for SBRT lung cases were selected for this study. Each field was delivered 8 times using a Varian Trilogy machine. The MLC leaf positions were recorded at 50ms intervals by the MLC controller into the system dynalog files. The analysis of the dynalog files yields the statistics of the MLC position error and leaf bank gap error. The cumulative distribution functions (CDF) for the errors were calculated. These distribution functions were compared with the configured limit of 2 mm at the isocenter for dynamic leaf tolerance. Results: The distribution function of the leaf positioning error peaked at 0.5mm for each delivery. No violation on the system limitation was observed. The standard deviation from repeated delivery of the same field is very small. For all the fields, 90%, 95%, and 99% of the leaf positioning errors (E90, E95, E98) are within 0.50, 0.69, and 0.94 mm. The leaf gap error has similar overall characteristics as the leaf positioning error. E90, E95, E98 for the gap errors are respectively 0.69, 0.87 and 1.3 mm. Conclusions: The MLC leaf positioning errors are within the system tolerance throughout VMAT delivery. Cumulative distribution function is a useful method for analyzing these errors. The statistical characteristics and some key statistical parameters for the MLC positioning error and the leaf gap errors were obtained.
SU‐E‐T‐423: Feasibility and Parameter Study on the Radiation Therapy with Laser‐Accelerated Electrons Using a Sharp Density Transition Scheme
Authors: Yoo S , Kim D , Min B , Jeong H , Shin D , Lee S , Park S
Published: November 2011

Purpose: In the cancer therapy using laser‐accelerated VHEE (very high energy electron) beams, stability of the beam generation is a very important issue. Generation of stable mono‐energetic electron beam is affected by the conditions of electron injection. We used the sharp density transition scheme for the stable generation of the VHEE beams and investigate the characteristics of the mono‐energetic electron beams with a laser focal spot size. Methods: To generate sharp density transition structure experimentally, a study on the generation of plasma shock structure was performed at Korea Electrotechnology Research Institute. The characteristics of VHEE beams generated from laser‐plasma interaction via linearly polarized intense laser pulses are investigated using two‐dimensional particle‐in‐cell simulations. To investigate the feasibility of the cancer therapy with VHEE beams with a sharp density transition scheme, Monte‐Carlo simulations are performed for calculating the dosimetric properties. Results: The sharp density transition structure can be generated using a pre‐pulse propagating orthogonal to the main laser pulse. 200 ps and 400 mJ pre‐pulse can generate the plasma shock structure with a sharp density transition under 10㎛. The charge of the electron bunch is related to the area of the phase‐mixing region. The electron bunch with the maximum charge was produced in the maximum area of the mixing region for the specific laser focal size. Energy spread of electron bunch increases with the increasing focal spot size and transverse bunch size decrease with increasing focal spot size. Dose distributions have also been calculated with the GEANT4 code and we present the feasibility of the particle therapy with sharp density transition scheme based VHEE beams for the deep seated tumor. Conclusions: In this paper, we present the superior feasibility of the cancer therapy using laser‐accelerated VHEE beams with a sharp density transition scheme.
SU‐E‐T‐431: Dosimetric Uncertainties in RapidArc Treatment
Authors: Li J S , Lin M H , Wang L , Ma C‐M
Published: November 2011

Purpose: The purpose of this study is to investigate the dosimetric effects of the beam delivery system on the delivered dose distributions of RapidArc treatments and their impact on RapidArc patient‐specific quality assurance. Methods: Monte Carlo dose calculations based on intensity maps that were built from the actual deliverable leaf sequences with and without considering the characteristics of the beam delivery system were performed in this study using our in‐house Monte Carlo software. The resolution of the intensity map is 0.2×0.2mm2 at the isocenter plane. The characteristics studied in this work include the source distribution, MLC leaf thickness, leakage, tongue‐and‐groove structure, effective leaf offset and dynamic delivery. The mean dose of the treatment target volume and the voxel doses in the high dose region of 5 RapidArc plans were used for comparison and evaluation. Results: The extra‐focal source, MLC leaf thickness, leakage, tongue‐and‐groove structure, effective leaf offset and dynamic delivery can affect the mean dose of a RapidArc treatment by up to 1.0%, 4.8%, 3.2%, 6.7%, 9.4% and 0.4%, respectively, when these factors are considered separately. They also cause significant uncertainties to the voxel dose with standard deviations up to 1.0%, 0.6%, 0.2%, 1.0%, 1.6% and 1.1%, respectively. The overall effect of all these characteristics on the mean dose is up to 2.2% and the standard deviation of the voxel dose uncertainty is up to 1.8%. The maximum voxel dose difference caused by these factors is up to 7.6%, 6.5%, 4.8%, 11.5%, 16.8% and 21.5% in the high dose region. Conclusions: The characteristics of the beam delivery system have great contribution to point dose uncertainty of a RapidArc treatment. And the effects are case‐dependent and location‐dependent. Treatment planning system should consider all these effects more precisely in the final dose calculation to reduce the measured dose uncertainty.
SU‐E‐T‐433: VMAT Arc Delivery with Gantry Acceleration Constraints
Authors: Papiez L , Daescu O , Behra S , Rangaraj SD.
Published: November 2011

Purpose: To evaluate the impact of gantry acceleration on the VMAT arc therapy delivery. Methods and Materials: The rapid delivery of VMAT arc therapy requires fast motion of the gantry around the patient. The difficulty is that the large inertia of the accelerator gantry limits rotation speed changes restricting the ability of the accelerator to arc as fast as possible without violating the integrity of the plan (plan delivery requires preserving mutual relations between gantry speed, MLC leaves speeds and beam dose rate). This work examines arc therapy deliveries for static and moving target in presence of acceleration limitation for gantry angular speed. We compute the shortest time of arc therapy delivery, provided different acceleration limitations are imposed. The problem of arc delivery to moving target is a complex optimal control due to dependence of motion constraints on the state of the evolution. As may be expected more stringent limits on the acceleration lead to longer delivery time and the successive decrease of the mechanical strain on the gantry rotation mechanism. Results. The static prostate VMAT arc treatment investigated sets the acceleration limitations for gantry speed from 2deg/sec2 to 1.5deg/sec2 to 1.0deg/sec2 to 0.5deg /dsec2 to 0.0deg sec2. For subsequent deliveries the optimal time of therapy delivery increases appropriately from a just above 1 minute to above 3 minutes. Conclusions. The ultimate limitation imposed on the acceleration of the gantry (constant gantry speed delivery) leads to approximately threefold increase in most efficient delivery times for the example. This time increase is accompanied, by complete removal of mechanical stress on the system of the gantry motion. The constant gantry angular speed delivery is not realistically achievable for moving target when the motion is measured in real time if the planned dose delivery integrity is to be preserved.
SU‐E‐T‐435: Comparison in Dose Distributions Between 2D‐Arrays for Pre‐Treatment IMRT QA
Authors: Nakaguchi Y , Araki F
Published: November 2011

Purpose: In recent years, many 2D‐arrays have been developed as dose verification. Therefore, the comparison in the dose accuracy between the 2D‐arrays is important especially for complex intensity modulated radiation therapy (IMRT). In this study, we compared dosimetric properties between various 2D‐arrays and also evaluated the accuracy of absolute dose and dose profile measurements for clinical IMRT fields. Methods: In this study, the following detectors were used to check the dose accuracy: MapCHECK (Sun Nuclear, Melbourne, FL), EPID (Varian Medical Systems, Palo Alto, CA), EPID‐based dosimetry (EPIDose, Sun Nuclear, Melbourne, FL), COMPASS (IBA Dosimetry, Germany) and EDR2 film (Eastman Kodak, Rochester, NY), Exradin A‐14SL ion chamber (0.016 cc, Standard Imaging, Middleton, WI). First the dosimetric properties of the 2D‐arrays were compared with dose linearity, dose rate, and output factor for 6 and 15 MV photon beams. Next the accuracy of the absolute dose and dose profile was evaluated for 21 clinical IMRT fields. Results: All detector responses for the dose linear were within 1%, and for the dose rates agreed within a standard deviation of ±1.2%. Except for EPID, the output factors were insignificant deviation. This is because EPID is the fluence measurement. In almost 2D‐arrays, the point dose agreed with treatment planning system (TPS) in less than 3% in the clinical fields. As for EPID, the accuracy that is equal to other detectors is obtained by some correction factors. Pass rates of each detector for TPS were more than 95% in the gamma analysis (criteria: 3 mm/3%). EPIDose is in a good agreement with TPS. This is because EPIDose has high resolution. Conclusions: All 2D arrays used in this study showed almost the same accuracy for clinical IMRT fields. EPIDose has better resolution than other 2D arrays and thus is expected for dose distributions with a small field.
SU‐E‐T‐437: Dosimetry Study of Setup Errors in Intensity‐Modulated Radiotherapy of Cervical Cancer Post‐Surgery
Authors: Li Y , Chi Z , Liu D
Published: November 2011

Purpose: To measure the setup errors by using cone beam computed tomography (CBCT) and to analyze the effect of the setup errors on the physics dose of targets and peripheral organs at risk (OARs) for cervical Cancer undergoing the intensity modu1ated radiation therapy (IMRT). Methods: Ten cervical Cancer cases were chosen at random. CBCT was performed before radiotherapy．The acquired CBCT were co‐ registered with the planning CT for online set‐up correction and errors of isocenter positions on x、y、z axes were obtained .Those were simulated in actual therapy on the treatment planning system (TPS)，and recalculated their dose distribution．A series of relative targets and OAR dose parameters were analyzed，and obtained the impact of the setup errors on the physics dose. Results: (1)A total of 126 CBCT scans were performed on 10 patients．The detection rates of deviation of ≤0.3cm were 92.86％ in left‐ to‐right (X) direction，83.33％ in superior‐to‐inferior (Y) direction， and 91.26％ in anterior‐to ‐posterior (Z)direction；The setup errors on X，Y and Z axes direction were(0.24±0.21)cm 2289;(0.28±0.32)cm 2289;(0.10士0.23)c m. (2) Dose and volumes index of CTV were no significant difference between simulated plans and actual plans. (3) V45、V50 of bladder and V30、V40、V45、V50 of rectum in simulated plans were significantly larger than those in actual plans，especially V45、V50 of rectum(P<0.005). Conclusions: The CBCT plays key ro1e in the guarantee of accurate delivery of IMRT for cervical Cancer,especially peripheral OARs.
SU‐E‐T‐438: Motion Induced Dose Artifact of Multi‐Fractional Tomotheapy
Authors: Shin E , Han Y , Park H , Shin J , Ju S , Kim J , Choi D
Published: November 2011

Purpose: Treating moving oragan has been an issue due to the dynamic nature of Tomotheapy. Non of study has investigated throughly the multi‐ fractional effects of treatments. Therefore, we designed a study to evaluate the cumulative error in moving target and nearby normal tissues. Methods: A moving phantom whose motion pattern could be rogrammed by a user was produced. Four plans which used different jaw width (1.05 cm, 2.5 cm), pitch (0.660, 0,287) and modulation factor (1.5, 2.5) to deliver 1.49 Gy to 95% of PTV in each fraction were made. For each plan, 5 different motions (amplitude 1–3cm, Period 3–5sec) and irregular motion were tested. Film measurements for accumulated dose were made with Gafchromic@EBT films from 1 to 5 fractions. Dose distribution on each film was compared with that measured in static phantom. Profile shapes, DoseArea Histogram (DAH)s and gamma index were used for comparison Results: The dose distortion increased up to 3rd fractions and, after 3rd fractions dose distributions in the target and OAR converged to some constant distribution. The distortion level inside the target was affected by motion parameters; larger motion amplitude and larger motion period resulted in increased dose. However dose at the center of the critical organ was rather affected by the motion amplitude than the motion period . The irregularity of the motion was not thought to cause large dose artifact. More complicated plan with larger modulation factor (2.5) resulted in larger dose distortin than the plan with modulation factor 1.5. The observed phenomena is thought to be reproducible since 3 different measurements of accumulated 5 fraction of treatments showed very similar dose distributions Conclusions: When treating moving organ in small number of fractions, verifying the dose artifact is necessary. (This work was supported by Korea Government (MEST, Grant No2010‐0011771)
SU‐E‐T‐443: Dosimetric Verification for Volumetric Modulated Arc Therapy (VMAT) with Gating
Authors: Qian J , Xing L , Luxton G
Published: November 2011

Purpose: To establish a dose verification technique to evaluate the fidelity of gated VMAT delivery under a clinic‐relevant range of simulated periodic breathing. Methods: A Varian TrueBeam system and VMAT plans for three cancer patients (lung, pancreas and abdominal metastasis) were used in this study. Each patient plan was delivered four times, namely, once without gating and then with gating to a single open‐gate window (25%–70% phase cycle) for three simulated respiratory periods (3, 4.5 and 6 seconds). After that, trajectory log‐files that record actual delivery parameters of the system during treatment were retrieved. MLC leaf positions, gantry angles and cumulative MUs for each control point were extracted from the log‐files and used to reconstitute the DICOM‐radiotherapy plan file. This file was then imported into the Eclipse treatment planning system and dose was recalculated on the planning CT of the patient, as well as on the corresponding hybrid verification plans. An ion chamber array, PTW‐Seven29 was used as an independent method to verify the reconstructed dose. Gamma‐index analyses, maximum, minimum and mean doses, and dose‐volume histograms were compared to evaluate the fidelity of gated VMAT delivery under clinic‐relevant respiratory conditions. Results: Comparison of Seven29 measurements with and without gating showed agreement with 100% passing rate for standard 3%/3mm criteria. Reconstructed dose of deliveries with gating for the simulated respiratory periods was essentially identical with the original plan. Gamma‐index passing rates between the original and reconstituted plans and between the original plans and Seven29 measurements of gated deliveries were 100% and 91.7%–98.3%, respectively. Conclusions: A dose verification technique was demonstrated for respiratory‐gated VMAT delivery. This technique showed that under clinical operation, the TrueBeam system faithfully realized VMAT plans with gated delivery. This methodology affords a useful tool for dosimetric assessment of the newly available respiratory‐gated VMAT.
SU‐E‐T‐444: Feasibility Study of In‐Vivo Transit Dosimetry for IMRT Patients
Authors: Kim G , Rice R , Detorie N , Dragojevic I , Pawlicki T
Published: November 2011

Purpose: The ultimate goal of IMRT QA should be to verify that the correct dose has been delivered to the patient. The purpose of this study is to evaluate for feasibility and reliability of volumetric transit dosimetry as a routine in‐vivo dosimetry method during the actual patient treatment for different sites. Methods: To evaluate volumetric transit dosimetry for IMRT delivery, prostate, H&N and lung IMRT cases were selected. During patient treatment, the EPID imager was extended to collect transit radiation through the patient. Collected transit dose images are imported and reconstructed as a volumetric dose distribution using the Dosimetry Check (Math Resolutions LLC, Maryland, USA) Exit Dose module. Dosimetry Check program to compare to the reconstructed volumetric dose including tissue heterogeneous correction from the transit EPID images. Also, to investigate the influence of daily setup or anatomy change, multiple transit dosimetry measurements were performed on different treatment days up to 5 fractions for prostate case. In addition, patientˈs verification plan was generated on a homogeneous CIRS IMRT phantom to verify the absolute dose and transit dosimetry algorithm check. Collected QA results were evaluated with 1) gamma passing rate (3 mm DTA and 3% dose) comparing calculated dose from exit dosimetry and treatment planning system (TPS) dose prediction, 2) difference of absolute point dose from exit dosimetry module compared to ion chamber measurements in the phantom. Results: The current implementation shows promise at predicting point doses from exit dose images acquired during patient treatment. However, 3D dose distributions calculated from exit dose images are not consistent in their agreement with the planned dose distribution. Conclusions: Further study is needed to explain the differences. Areas to consider include offsets in imager position, differences in dose calculation algorithm, couch attenuation effects and anatomy changes.
SU‐E‐T‐448: A Dual‐Plan Superposition and Adaptive Setup Strategy for the Reduction of Planning Margins in Whole‐Pelvis and Prostate IMRT
Authors: Yi B , Yu C , Lerma F , Prado K , Amin P
Published: November 2011

Purpose: Increased global setup margins are needed to account for prostate motion within the pelvis, for alignment of the prostate and associated pelvic lymph node (PLN) volumes. This leads to increased rectum and bladder doses. We present a combined treatment planning and image guidance strategy wherein both the prostate and PLNs can be independently targeted, using a dual treatment plan (DP) that superimposes prostate and PLN dose prescriptions. Methods: Two IMRT plans of prescription dose 45 Gy, a conventional whole‐pelvis and prostate plan (WP) and a DP are developed. The WP possesses a 1‐cm setup margin for the prostate and PLN. It is assumed that pelvic bony anatomy will be used for image guidance during treatment. In the DP, a first prescription assigns 100% of prescription dose to the PLN volume and 80% to the prostate. Setup margins remain same as that of WP. In a second prescription, the additional 20% is assigned to the prostate volume only. Setup margin for the prostate is 3 mm. Here it assumed that bony‐anatomy image guidance is used for prescription 1 and that soft‐tissue (prostate) image guidance is used for prescription 2. Isocenter‐shift plans (Iso_shift DP) are developed in which the prostate‐only isocenters are moved 5mm in all directions: the Anterior, Posterior, Superior, Inferior and Lateral. Results: Rectum and bladder doses are significantly reduced in the DP relative to the WP. The rectum V35 is reduced to 21% from 72%; the bladder V35 is reduced to 15% from 26%. PTV coverage is essentially unchanged. The Iso_shift DP shows only very small differences from the DP. Conclusions: Significant reductions in rectum and bladder doses are achievable when this dual‐plan (DP) and dual image‐guidance strategy is employed. Even under significant prostate‐ motion conditions, PTV coverage is maintained with better critical structure sparing.
SU‐E‐T‐459: Measurement of Gated Rapidarc Accuracy
Authors: Kim D , Chung W , Gwak H , Shin D , Kang J , Hong S
Published: November 2011

Purpose: Recently, Kyung Hee university hospital at Gangdong installed the Varian 21iX linac which has the volumetric imrt (RapidArc) option and a real‐time position management (RPM) system. One of advantage of rapidarc is a short treatment time comparing with other treatments. Thus, the rapidarc treatment with the RPM system for the lung and liver cases, can reduce the treatment time comparing with the intensity modulation radiotherapy (IMRT) and 3 dimensional radiotherapy (3DRT) with gating system. Therefore, we estimate the accuracy of the gated rapidarc treatment. Methods: 10 patients data are used for this study. Each patient was planned by rapidarc technique using varian ECLISPE v8.6 planning machine. For the patient Quality Assurence (QA), a MATRIXX detector and IˈM RT software are used. At bottom and top of MATRIXX detector, 5×30×30cm̂3 of solid water phantoms are install for build up. A MATRIXX detector is fixed but the infrared reflecting box is periodically moved to provide the gating signal to the RPM system. Each patients dosimetric distribution is measured and the treatment summary which is provided by linac operating system is recoded to estimate the accuracy of the gated rapidarc treatment. Results: The dosimetric comparison between the measurement and the treatment plan is matched well for the 10 patients. With 3% of delta dose and 3mm of delta distance, more than 98% of area gives the gamma value as less than 1. Conclusions: We found that the patient dose measurement is matched well with the RTP planˈs dose distribution for the gated rapidarc treatment cases in the mechanical point of view. This study shows the gated rapidarcˈs dose reproducibility during the patient QA but this result has a limitation for the dose confirming for the real patients which has more soft and flexible body than the solid phantom.
SU‐E‐T‐465: Assessment of the Effect of Respiration Motion and Setup Error during Whole Breast Irradiation : Comparison of Standard Physical Wedges, Electric Compensation, and Field in Field Technique
Authors: Furuya T , Sugimoto S , Kurokawa C , Ozawa S , Karasawa K , Sasai K
Published: November 2011

Purpose: We studied the dosimetric impact of respiration motion and setup error on tangential whole breast irradiation for the Japanese patient. We compared between three different techniques; standard physical wedges (SW), electric compensator (EC), and field in field technique (FF). Methods: We performed treatment planning for sixteen patients using each technique. To take the respiration motion and setup error into account, we made plans with the isocenters shifted by ±5mm vertically, longitudinally, and laterally. For the vertical direction, we added ±10mm isocenter displacement plan, because respiration movement in the anterior — posterior (AP) direction is the largest. Mean dose, D95, V95, homogeneity index for CTV, V20 for lung, and V105 for body were evaluated for these plans. For AP respiratory motion, we assumed that the isocenter moves as sine function and DVH parameters were averaged over one period. For the daily setup error, we assumed the distribution of isocenter shifts have the Gaussian form, and calculated setup frequency weighted DVH parameters. Results: In all irradiation techniques, AP movement has a significant impact on the dose distribution. The effect of respiration motion and setup error on dose distribution is the largest for EC. Especially CTV V95 decrease by 4.5 % from original plan. But even if these movement effects are considered, target dose coverage using EC (CTV V95:94.8%), or FF (95.3%) is enough in clinically. For lung dose, the moving effect is almost same between these techniques. Conclusions: In this study, EC and FF irradiation techniques is more sensitive for respiration and setup movement than SW. But these deviations from original plan are clinically negligible. Furthermore, EC or FF decreased hotspots dramatically. As a result, we conclude that EC or FF technique is more meaningful than SW in terms of dosimetric impact.
SU‐E‐T‐467: The Trick in MLC Calibration to Correct Underdose Region in IMRT Delivery
Authors: Lin C , Lin C , Huang Y
Published: November 2011

Purpose: By verifying the consistency between the planning dose and the delivered dose for each segment, we figure out the cause of the low dose regions inside the treatment field which only occur in verification images, but not in the treatment plan. Methods: The treatment planning system is ADAC Pinnacle version 9.0 with SmartArc. The IMRT plan is transferred to IMPAC oncology management system version 8.3 to drive the LINACs. Instead of taking an overall two or three dimensional dose distribution, we take the fluence maps for each beam by using Kodak XV films on two ELEKTA Precise LINACs and one ELEKTA Synergy LINAC. Results: Low dose regions are discovered in only Synergy LINAC and always in gun‐to‐target (G‐T) direction. By comparing the fluence maps between treatment plan and verification film segment by segment, we find these low dose regions result from the jaw position, i.e. the MLC directional backup jaw is too close to cause these unplanned underdoses in the treatment volume. We recheck the light‐field and the radiation field again; there is coincidence between the light‐field and the radiation field and just on the correct position. According to the statute, each radiation field edge is to be aligned with the displayed diaphragm within ±1 mm and each light‐field edge is to be aligned with the radiation field edge within ±1 mm. We recalibrate the MLC directional backup jaws and MLCs position to 0.5 mm larger than display and recheck the fluence maps, the low dose regions disappear eventually Conclusions: When doing the radiation field size calibration, one should calibrate the MLC moving directional radiation field size 0.5 mm larger than display each side on ELEKTA machine to prevent the unanticipated low dose regions inside the tumor in IMRT delivery.
SU‐E‐T‐667: On the Monte Carlo Simulation of Electron Transport in the Sub‐1 KeV Energy Range
Authors: Thomson RM , Kawrakow I
Published: November 2011

Purpose: The validity of ‘classic’ Monte Carlo simulations of electron and positron transport at sub‐1 keV energies is investigated in the context of quantum theory.Methods: Quantum theory dictates that uncertainties on the position and energy‐momentum four vectors of radiation quanta obey Heisenbergˈs uncertainty relation; however, these uncertainties are neglected in ‘classical’ MC simulations of radiation transport in which position and momentum are known precisely. Using the quantum uncertainty relation and electron mean free path, the magnitude of uncertainties on electron position and momentum are calculated for different kinetic energies; a validity bound on the classical simulation of electron transport is derived. Results: In order to satisfy the Heisenberg uncertainty principle, uncertainties of 4% or greater must be assigned to position and momentum for 1 keV electrons in water; at 100 eV, these uncertainties are 15% to 22% and are even larger at lower energies. In gaseous media such as air, these uncertainties are much smaller (less than 1% for electrons with energy 20 eV or greater). Conclusions: The classical Monte Carlo transport treatment is incorrect for sub‐1 keV electrons in water as uncertainties on position and momentum must be large (relative to electron momentum and the mean free path) to satisfy the quantum uncertainty principle. Simulations in condensed media (e.g., water) which do not reflect the quantum nature of electrons and positrons are not faithful representations of the physical reality at these low energies, calling into question the results of MC track structure codes simulating sub‐1 keV electron transport. Further, the large differences in the scale at which quantum effects are important in gaseous and condensed media suggest that track structure measurements in gases are not necessarily representative of track structure in condensed materials on a micrometer or nanometer scale.
SU‐E‐T‐700: PMMC ‐ a High‐Performance Monte Carlo Code for Proton Beam Dose Calculation
Authors: Jacqmin D
Published: November 2011

Purpose: The proton macro Monte Carlo (PMMC) method uses a fast macro Monte Carlo radiation transport algorithm for high‐energy proton transport through an absorbing medium. Methods: PMMC uses results derived from traditional Monte Carlo simulations of proton transport through macroscopic cylinders of varying length, radii and material composition. These traditional Monte Carlo simulations are done for many incident proton energies and the results are used to transport protons through an absorber geometry in a step‐by‐step manner. The absorber geometry consists of a three‐dimensional (3D) density matrix that can represent objects like a water phantom or computed tomography data. The source is specified by the user in terms of gantry angle and beam characteristics like position, energy and size. During proton transport, energy lost by the protons during each step is scored in a 3D dose grid. The creation of secondary neutrons is tracked as well to allow for estimation of the neutron dose. Results: The algorithm, which was written in C++, is currently capable of simulating 1 million protons per minute. The accuracy of the PMMC simulations was assessed by comparing the results to simulations done with the same source parameters in MCNPX. The simulations consisted of monoenergetic pencil beams and Gaussian‐shaped beams directed onto a water phantom. In general, PMMC and MCNPX are in good agreement in the high‐energy proton region before the Bragg peak, differing by less than 3%. PMMC performs worse at the depth of maximum dose, differing by as much as 10%. Conclusions: Forthcoming modifications to PMMC will address the discrepancies in the Bragg peak region and further modifications are underway to allow PMMC to be used with absorbers that are heterogeneous in density and material composition.
SU‐E‐T‐774: on the Effects of Intrafraction Motion on TomoTherapy SIB Treatments of Lung Tumors: A 4D Monte Carlo Study including Non‐Rigid Registration and Beamlet‐Breathing Phase Correlation
Authors: Sterpin E , Orban J , Geets X , Goossens S , Wanet M , Mackie T , Vynckier S , Gregoire V , Janssens G
Published: November 2011

Purpose: In the context of lung tumor TomoTherapy treatments including a boost region based on a PET scan distribution, the clinical impact of neglecting intra‐fraction motion during dose computation was evaluated with 4D Monte Carlo (MC) simulations. Methods: The study includes ten lung tumor patients including a boost region defined using PET data and treated simultaneously with the remaining of the target volume (simultaneous integrated boost (SIB)) and planned according to the guidelines of ICRU Report 83. In this communication, we focus on one patient and vary temporal parameters to evaluate the potential effect of intrafraction motion interplay with sliding jaw and leaf motions. The 4D‐CT scan was acquired and temporally distributed over ten phases according to a breathing signal delivered by a motion detector, providing also an estimation of the breathing period. Dose was computed with the MC model TomoPen. for every projection, simulations were performed on a CT phase selected according to breathing period and initial phase. The 10 dose maps were accumulated using non‐rigid registration and then compared to the approved treatment plan. Parameters were varied as follows 1) variation of breathing period (from 3 to 8 sec) (motion interplay study) 2) variation in the initial phase (multiple fraction averaging effect) and 3) calculation with all the projections for every phase (only intra‐fraction motion).Results: Depending on the breathing period, all the simulations showed acceptable dose distributions on target volumes of the ten patients (0.3 to 1% for gEUDs of the boost tumor volume and the clinical target volume) indicating potential but minimal interplay effect Conclusions: For the patients studied, assuming regular breathing, the results showed that SIB treatments for a lung tumor with TomoTherapy including intra‐fraction motion delivered acceptable dose distributions. This ongoing study will be extended to more patients, including hypo‐fractionated stereotactic treatments.

Tomotherapy Inc. has partially supported this research
SU‐E‐T‐783: The TomoTherapy Thread Effect Revisited
Authors: Chen M , Chen Y , Chen Q , Lu W
Published: November 2011

Purpose: The TomoTherapy thread effect is a longitudinal pitch‐dependent dose‐ripple artifact caused by helical radiation delivery. It has been observed and studied before. However, previously empirically identified optimal pitches, 0.86/n, are only for the jaw width 5 cm and off‐axis distance 5 cm. Through theoretical analysis and simulation, we were able to identify optimal pitches for various jaw widths at various off‐axis distances. We further investigated the effect of optimization in reducing this ripple artifact. Methods: As the ripple in the dose profile is essentially a 1D problem, we set up a 1D model that account for the contributing factors, including profile divergence, the inverse square law, attenuation, and the cone effect, to study the thread effect. Based on the 1D model, we analyzed individual and combined factors theoretically and numerically to identify optimal pitches. We further set up optimization to reduce ripples in the 1D cases and simulate 3D cases using TomoTherapyˈs optimization. Results: The 1D model captures the thread effect found in 3D. At the off‐axis distance of 5 cm, for the jaw width of 5 cm, the derived optimal pitches agree with 0.86/n. However, for other jaw widths or off‐axis distances, the optimal pitches do not follow 0.86/n. For a fixed jaw width, the optimal pitches shift downward as the off‐axis distance increases. Optimization in both 1D and 3D show consistent results, and the ripples are largely suppressed by optimization through intensity modulation. Conclusions: With optimized intensity modulation, the optimal pitches tend to move toward the ones for the profile divergence factor, as optimized modulation is expected to compensate for the intensity‐related factors up to the restriction of modulation factor and optimization iterations. Choosing a good pitch, though further improves the thread effect, may not be as critical, as optimization already bring down the thread effect considerably.