bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐430: Planning and Dosimetric Comparison of the Gamma Knife Convolution and TMR 10 Algorithms
Authors: Cates J , Drzymala R , Izaguirre E , Sun B
Published: July 2013

Purpose: To compare the dose distributions for identical treatment plans calculated by the Gamma Knife TMR 10 and convolution algorithms and measured with film dosimetry. Methods: An anthropomorphic head phantom was CT imaged with EBT2 film placed between each of seven axial sections. The resulting data set was used to plan three 16mm collimated targets on the Gamma Knife Perfexion, with each target centered on a film plane. Target 1 was placed within a homogeneous region while Targets 2 and 3 were placed in heterogeneous regions, i.e. tissue‐air and bone‐tissue interfaces, respectively. Plans using the same targets were made using both the TMR 10 and convolution algorithms. The prescription was delivered to the phantom using the TMR 10 treatment plans after which the convolution treatment plans were adjusted to Result in identical treatment times, thus ensuring identical dose delivery. Film dosimetry was done to determine actual dose delivered at target center and was compared to the predicted dose for each algorithm. Results: While there was strong correlation between both algorithms, the convolution algorithm predicted a higher delivered maximum dose than TMR 10, up to 2.5% higher in homogeneous tissue and up to 7% near an air cavity. Film dosimetry results were consistent with the convolution algorithm predictions, with an error of less than three percent. Conclusion: The Gamma Knife convolution algorithm predicts delivered dose to a clinically acceptable level, which was confirmed by film dosimetry. However, film in an anthropomorphic head phantom may not be adequate to measure the most significant differences between the two algorithms. Precise stereotactic treatments will require precise dosimetry, and a phantom developed specifically with Gamma Knife geometry in mind may be necessary to fully characterize the dosimetry at anatomy interfaces.
SU‐E‐T‐432: Dosimetric Computation of Cyberknife SBRT Plans for Treatment of Kidney and Adrenal Gland
Authors: Albani D , Fabien J , Brindle J , Zhang Y , Dobbins D , Wessels B , Ellis R , Podder T
Published: July 2013

Purpose: It is well‐accepted that Monte Carlo algorithm (MCA) methods are superior to Ray‐Tracing algorithm (RTA), i.e., effective path length (EPL), methods for computing radiation dose, especially for structures with heterogeneous tissue compositions. In this study, the differences of dose distribution in target volumes (kidneys and adrenal glands) and adjacent organs at risk (OARs) were evaluated while the treatment plans were computed using MCA and RTA for Cyberknife™ stereotactic body radiotherapy (SBRT). Methods: A total of 14 renal tumor (10 kidney and 4 adrenal gland) patients who had prescriptions in the range of 24Gy–48Gy (mean=42Gy) were selected. Treatment plans were computed using RTA and MCA in Cyberknife MultiPlan™ (version‐3.5.2). First, dose was calculated using RTA and then was recomputed using MCA, keeping total MU the same. Liver, heart, stomach, spleen, bowel, and contralateral kidney were considered as OARs. Dosimetric parameters considered for the target and OARs were minimum dose, mean dose, and maximum point dose. Additionally, dose delivered to 95% and 5% of the target volumes were evaluated. Dose computed with two algorithms were compared and statistically analyzed using two‐tailed t‐tests. Results: This study revealed that differences (average) in mean dose (0.79Gy), maximum dose (2.02Gy), 95% (0.76Gy) and 5% (0.83Gy) of target coverage dose were not statistically significant (p‐value>0.05). Differences in minimum dose (average=0.04–3.75Gy) to OARs exhibited significance (p‐value< 0.05), though differences in minimum dose to target volumes (average=1.14Gy) showed no significant difference (p‐value>0.05). Conclusion: Although Monte‐Carlo is more accurate compared to Ray‐Tracing for dosimetric computation, especially tumors in heterogeneous tissue composition, dose computation and delivery using MCA are quite involved due to additional computational time and associated patient‐specific quality assurance. This study indicated that both RTA and MCA are almost equally effective as dosimetric computation methods for renal tumors, and RTA can be used without compromising dosimetric accuracy.
SU‐E‐T‐435: 4D MIMVista Workflow Vs 4D Dose Calculations Based On a Monte Carlo Method
Authors: Valdes G , Iwamoto K , Lee P , Low D , Robinson C , Lamb J
Published: July 2013

Purpose: We investigated the accuracy of a commercially‐available software algorithm for 4D dose accumulation that consists of rigidly projecting the 3D dose calculated in the reference respiratory phase onto the remaining phases, followed by deformable registration, versus 4D dose calculation involving Monte Carlo recalculations of the original treatment plan applied to each of the phases. Methods: Radiotherapy plans for stage I–II lung cancer were created for 8 patients who had respiratory‐correlated computed tomography (4D‐CT) performed as part of an IRB‐approved research protocol. Plans were generated and evaluated using the dose calculated by a Monte Carlo algorithm on the end‐exhale 4D‐CT phase. 3D doses for all other phases and the 4D dose were calculated using a rigid dose‐projection workflow as part of a commercially‐available software package. The resulting DVHs were compared with those obtained using Monte Carlo calculations after the initial plans were recalculated on each of the respiratory phases. In both cases the final dose accumulation was performed using the same commercially available deformable registration algorithm. The calculated dose distributions were compared according to the principal dosimetric indices used in our clinic to evaluate gross tumor volume (GTV) coverage: dose received by 99% of the gross tumor volume (D99(GTV)), minimum dose received by the GTV (Dmin(GTV)), and mean dose received by the GTV (Dmean(GTV)). Results: Mean differences between the commercial dose projection and the Monte Carlo recalculation algorithms were 0.1 Gy, 0.3 Gy, and 0.2 Gy for D99(GTV), Dmin(GTV) and Dmean(GTV). Maximum differences were 2.1 Gy, 2.9 Gy, and 2.2 Gy, respectively, between the two methods. Conclusion: The commercial dose accumulation workflow using rigid dose projection is much faster than 4D accumulation with Monte Carlo dose recalculation, and no large dosimetric differences were found between the methods. This study lends support to its use in a clinical setting.
SU‐E‐T‐442: Characteristics of Prompt Gamma Compton Interactions in Detectors
Authors: Mackin D , Polf J , Avery S , Lee E , Dolney D , Beddar S
Published: July 2013

Purpose: Prompt gamma radiation produced from non‐elastic scattering during proton therapy provides a signal that can be used to verify the beam range and possibly the dose in vivo. The purpose of this study is to highlight how the Compton interaction characteristics change based on the gamma energy and to determine how these characteristics will affect the detection system. Methods: The prompt gamma rays are produced at discrete energy levels ranging from 511 keV to 7.12 MeV. Over this energy range the interaction characteristics are known to vary widely. Thus, when designing a detection system, it may be advantageous to consider individual characteristic gamma energies (.511, 1.33, 2.2, 4.44 and 6.13 MeV) rather than all prompt gammas as a whole. We simulated 106 Compton scatter events for the 5 energies. For each event we randomly selected a scattering angle based on the Klien‐Nishina scattering formula. We then calculated the corresponding Compton electron scattering angle, energy, and CSDA range. Results: The average Compton electron CSDA range in CdZnTe for the events from the lowest energy gammas in our study is 0.2 mm and for the highest energy gammas is as high as 7 mm. Over 80% of the Compton electrons from the highest energy events have a CSDA range greater than 3 mm. Thus, the stopping power of the detector material should be taken into consideration when designing a detector for the high energy gammas. The average scattering angle of the Compton photons ranged from 61° to 39° for low to high energies, respectively. Therefore, the high energy gammas, the stages of multistage detectors can be placed further apart with a lower impact on detection efficiency. Conclusion: The designers of gamma imaging systems for protons may benefit by considering each gamma energy independently rather than focusing on the entire spectrum.
SU‐E‐T‐443: Measuring Prompt Gamma Emission During Passive Beam Proton Radiotherapy for Real Time Treatment Verification
Authors: Jeyasugiththan J , Peterson S
Published: July 2013

Purpose: Recent studies demonstrate the feasibility of prompt‐gamma imaging for pencil beam proton therapy. In contrast to passive‐scatter proton radiotherapy, the existence of high background signal from secondary radiation produced by protons in the beam line components makes the detection of prompt‐gammas challenging. Our primary goal is to produce a more complete measurement of the prompt‐gammas produced in tissue for clinical treatment conditions. Methods: The proton treatment facility at iThemba Labs was studied in detail and all the nozzle components that interact with the proton beam were built and positioned at the locations specified by the manufacturer using the Geant4 Monte Carlo toolkit. The treatment nozzle was validated against depth dose and lateral profiles in a water phantom at therapeutic energies. The model included NaI detectors of different dimensions to detect the prompt‐gamma spectra, and standard gamma emitting sources in the energy range 0.661 to 4.438 MeV were used to determine the Gaussian broadening parameters of the detectors. Finally, prompt‐gamma spectra from the tissue phantom were simulated over a range of energies in order to make comparisons with the experimental spectra currently being measured. Results: The prompt‐gamma energy spectra show prominent peaks caused by collisions of proton with nuclei in the tissue phantom. Single and double escape peaks of 4.44 MeV were clearly seen, while the 6.129 MeV peak was unresolved due to the Doppler broadening effect, peaks at 6.916 and 7.115 MeV were not observed, but the single and double escape peaks of 6.129 MeV were visible. Conclusion: Our study confirms the feasibility of prompt‐gamma imaging during passive proton radiotherapy to assist for on‐line treatment verification. The energy range from 3.0 MeV to 5.0 MeV is a suitable candidate for further investigation and the presence of background radiation can be minimized by introducing proper shielding and collimation.
SU‐E‐T‐444: A Comparison of Single‐Field Vs Multi‐Field Delivery Uncertainty in Proton Therapy
Authors: Grantham K , Klein E
Published: July 2013

Purpose: To evaluate of the impact of uncertainty on proton plan delivery when one field per fraction treatment regimes (SFPF) are used rather than delivering all planned fields for every fraction (MFPF). Methods: A nominal proton treatment plan (NP) was created in Eclipse (Varian) to treat a spherical target to 54Gy using parallel‐opposed fields. The fields created in the NP were applied to geometries with uncertainties introduced. Uncertainty was simulated for four scenarios: a 1cm shift in one of the fields for 10 of 30 fractions, a 5° rotation of the patient for 10 of 30 fractions, and unilateral introduction of 3cm of bone or air for 10 of 30 fractions. The full treatment was calculated using summed plans for the SFPF case is delivered and the MFPF case. The results of the SFPF and MFPF plan sums were compared to each other and the NP by calculating Dmax/min/mean along with V90,95,98,99,100%. Dmax/min/mean were also compared for an OAR placed contralateral to the introduced uncertainty. Results: The cases for 5° rotations introduced, showed no significant difference from the NP. In all other cases, the introduced uncertainty degraded the treatment delivery with the most significant difference in the target Dmin. The SFPF treatments consistently showed a decrease in Dmin compared to the MFPF treatment. For the introduction of air and bone the SFPF treatments exhibit increases of 1.5% and 20% respectively. With bone introduced, V95 and V90 suffered an additional 3% and 4% for SFPF treatments. With air introduced, dose to the Dmax, Dmin, and Dmean were increased by 14, 16, and 20% respectively for SFPF treatments. Conclusion: Tumor response and minimum target dose can be correlated. In the presence of uncertainty, SFPF treatment regimes may Result in a decreased tumor response and increased OAR dose.
SU‐E‐T‐455: A Treatment Planning Feasibility Study for Prostate LDR Brachytherapy Treatments Using the New 103‐Pd CivaString Source. Comparison with Clinical Cases Using the TheraSeed Model 200 103‐Pd Source
Authors: Ververs J , Anscher M , Rivard M , Todor D
Published: July 2013

Purpose: This is a comparative dosimetric study using a new 103Pd CivaString source for prostate brachytherapy seed implants. Clinical plans using TheraSeed Model 200 103Pd seeds were used as the standard. The aims were to explore the feasibility and planning advantages with, and define guidelines for optimization and clinical implementation of, CivaString linear sources. Methods: Treatment plans from twelve patients implanted with Model 200 seeds were re‐planned using the CivaString source. Prostate volumes ranged from 23 to 61 cm3. CivaString dosimetric characterization was based on TG‐43 parameters derived from Monte Carlo simulations. Two CivaString plans were generated for each patient, first with a uniform source strength, then using higher‐strength sources peripherally and lower‐strength sources interiorly. The planning objective was matching the prostate D90 of the analogous Model 200‐based plan and was achieved by scaling CivaString source strength for a fixed but patient‐specific needle placement. Dosimetric quality was assessed in reference to prostate D90, V100, and V150, rectum V100, and urethra V150. Results: For virtually the same prostate D90, CivaString plans matched all dosimetric parameters yet provided a slight reduction in prostate V150. CivaString plans permitted reduction in the number of needles required to deliver implants, ranging from 55% to 68% compared with Model 200 plans. This allowed large prostate 103Pd source implants with 12–14 needles. In large‐volume prostate implants, use of two source strengths appeared beneficial to reduce V150 for prostate and urethra. Conclusion: Based on clinical scenarios, planning with the CivaString source significantly reduced the number of required needles while delivering similar dose distributions to the prostate, urethra, and rectum. Planning was dramatically simplified, and optimization was replaced by simple guidelines that allowed creation of high‐quality treatment plans within minutes.
SU‐E‐T‐459: Distance Matters — the Minimum CTV‐To‐Bladder Distance Tells All in HDR Cervical Cancer Treatment
Authors: Li M , Wong J , Karim M , Gao Z , Goldberg Y
Published: July 2013

Purpose: Bladder/rectum sparing is always the challenge in HDR 3D CT‐guided brachytherapy treatment planning for cervical cancer due to the tight separation between them and High Risk Clinical Target Volume (HRCTV). In this study, we analyzed the minimum CTV‐to‐Bladder distance (MCBD), the minimum CTV‐to‐Rectum distance (MCRD), dose to 2cc of bladder (BD(2cc)) and rectum (RD(2cc)) respectively. The potential application of MCBD and MCRD as plan optimization indicators was investigated. Methods: Total 49 consecutive HDR plans for 11 patients with stage I–IIB cervical cancer were retrospectively reviewed. The prescribed dose (PD) was either 6 Gy/fraction × 5 or 7 Gy/fraction × 4 fractions. For each plan, MCBD and MCRD were measured on relevant transverse CT slices. BD(2cc), RD(2cc), the minimum dose cover 90% HRCTV (D90) were recorded. All plans were optimized with dose constrains BD(2cc)<80%PD, RD(2cc)<70%PD, D90>90%PD. Results: For 49 insertions, MCRD was 14±7mm. MCBD was 2±1mm. D90 was 100±15%PD. RD(2cc) and BD(2cc) were 57±12%, 74±10% of PD respectively. The correlation coefficient of MCBD and BD(2cc) was 0.31(p=0.03), indicating that bladder dose was significantly impacted by MCBD.Among 49 insertions, 19 cases had MCBD<=1mm with BD(2cc)>75% PD and D90 96±13%PD. 13 cases had 1mm<MCBD<=2mm with D90 103±15%PD. 4/13 cases had BD(2cc)>75% PD. 17 out of 49 insertions had MCBD>2mm with D90 105±15%PD. Only 2/17 cases had BD(2cc)>75% PD. For 46/49 insertions, MCRD was >=5mm, which caused RD(2cc) extremely low. Conclusion: Our study shows that bladder sparing is correlated with MCBD. 65% of 49 insertions had MCBD<=2mm. Among them, 72% cases had BD(2cc)>75%PD. We believe that MCBD and MCRD can be used as plan optimization indicators. When any of them is <2mm, the target coverage would be comprised in order to spare bladder or rectum. This allows physicians have a reasonable expectation before plan is completed and guide further insertions.
SU‐E‐T‐464: Dosimetric Assessment of a New Multi‐Center Protocol for Radiotherapy of Multiple Ipsilateral Breast Cancers
Authors: Chen G , Currey A , Yen T , Li X
Published: July 2013

Purpose: To assess dosimetric planning feasibility for enrolling patients into a new prospective multi‐center protocol (ACOSOG Protocol Z11102) of breast conservation surgery with radiotherapy (RT) of multiple ipsilateral breast cancers (MIBC) including multicentric and/or multifocal breast cancers. Methods: CT data for 9 representative breast patients with two lumpectomy cavities in a breast treated in prone position were retrospectively analyzed. The boost PTV for lumpectomy cavity was delineated based on the seroma and/or surgical clips with 1 to 2 cm non‐uniform 3D expansion, not including chestwall, skin, and bone. A whole breast irradiation (WBI) of 50 Gy in 25 fractions was planned using 3D tangential beams of 6 or 15 MV photons with necessary wedges and/or field‐in‐field techniques to provide uniform dose coverage. No nodal irradiation was planned. The dose was calculated with heterogeneity correction using a treatment planning system (Xio v4.80, Elekta). For each boost PTV, additional 10 Gy in 5 fractions was planned by using one single photon beam, either 6 or 15 MV depending on the depth of the PTV, oriented in the direction with shortest depth to the PTV. All plans were evaluated to check whether all the dose‐volume criteria required by the protocol can be met. Results: All plans generated for patients in our study meet the dose‐volume criteria required by the protocol. For all dose‐volume parameters, the discrepancies between the patient averages and dose‐volume goals were separated by at least 1.4 sigmas (standard deviation). The average for the percentage breast volume receiving 60 Gy, a major concern because of the large boost PTVs, was (29.7±7.3)%, well below the required value of 40%. Conclusion: The dose‐volume criteria required for the new multi‐center protocol of radiotherapy for multiple ipsilateral breast cancers can be met with no major difficulties with the standard planning techniques.
SU‐E‐T‐465: A Patient Pebble Bed Pool for TBI
Authors: Koren S , Bernstein K , Chen S , Passarelli D , No D , Tai C , Kalach N , Furhang E
Published: July 2013

Purpose: To introduce a patient pebble bed pool that will enable a homogeneous patient dose distribution for TBI. Methods: We demonstrate the technique feasibility using a Rando phantom immersed in several hundred golf balls. We override the high density golf balls (HU ∼800) to a water equivalent. A uniform dose distribution using a two lateral TBI treatment beams with an extended SSD was calculated using Eclipse (Varian inc.). A future system will have an optimized pebble radius size, while the smaller the radius the more homogeneous the surrounding media is. Larger pebble diameter will be used around the head in order to allow the patient to breath. Results: The body dose was found significantly more homogeneous using the pebble bed pool. The maximum dose was calculated about 11.5% higher without using the pool. For a quarter of the body volume the dose is about 2.6 times lower when the pool is used, while the other three quarters are co mparable. A homogeneous neck axial cross‐section dose profile is shown, overco ming an about 20% heterogeneity dose difference; between a pool to a none pool delivery.This technique may also be suitable for proton and heavy ion beams therapy by allowing the usage of different beam angels while using the same co mpensator. Co nclusion: We have demonstrated the feasibility of pebble bed pool usage for TBI cases where homogeneous dose distribution is needed. Future research includes pebble size optimization and investigation of the applicability of this technique for different treatment modalities co nducted with a Monte Carlo technique in order to overco me the large amount of inhomogeneities.
SU‐E‐T‐469: Dosimetric Impact of Whole Breast Treatment Planning with Tissue Heterogeneity Correction
Authors: Nishioka S , Kim L , Goyal S , Khan A , Haffty B , Yue N
Published: July 2013

Purpose: Dose to normal organs, such as lung and heart, are more and more considered in the design of whole breast radiation treatments. In this study, the dosimetric impacts of heterogeneity correction on doses of such cases are investigated. Methods: Six clinical whole breast + SCV treatment cases were used in the study. These cases were first planned using static 6 MV photon beams with multileaf collimators and wedges where necessary. The plans were calculated using AAA without heterogeneity correction and the plans met the dose constraints set for the breast and critical organs. The plans were then recalculated with heterogeneity correction using the same MUs. We compared doses for breast, ipsilateral lung and heart. Results: With tissue heterogeneity correction, the breast mean and maximum doses increased, on average, by 0.8 % from 98.9 % and 0.5 % from 110.2 % of the prescribed doses, respectively. The breast volume receiving 95 % of the prescribed dose (V95%) increased by 1.5 % from 83.6 %. The ipsilateral lung mean and maximum doses increased by 2.3 % from 25.3 % and −0.2 % from 100.6 % of the prescribed dose, respectively. The ipsilateral lung V40% and V20% increased by 1.1 % from 25.1 % and 5.4 % from 29.2 %, respectively. The mean and maximum doses for heart increased by 0.1 % from 6.9 % and 0.8 % from 87.1 % of the prescribed dose, respectively, and the heart V20% increased by 0.2 % from 1.4 %. Conclusion: Calculated doses to breast and critical structures with tissue heterogeneity correction tend to be a little higher than those without the correction Dose constraints derived from experience without the correction may need to be adjusted accordingly if heterogeneity correction is used.
SU‐E‐T‐473: Preplanning of Stereotactic Radiosurgery Using the Brainlab IPlan Treatment Planning System
Authors: Ahn K , Ozturk N , Smith B , Aydogan B
Published: July 2013

Purpose: Frame‐based stereotactic radiosurgery (SRS) requires fixation of an invasive head ring to ensure accurate targeting. Minimizing waiting time with head ring on is important for patient comfort and satisfaction. We report a practical preplanning solution for the Brainlab iPlan treatment planning system (TPS) that reduces waiting time by expediting the planning process on treatment day. Methods: A water‐filled anthropomorphic head phantom was used to acquire a surrogate CT image set for preplanning and fused with patient MRI image set which is usually available before the treatment day. Once an acceptable preplan was obtained, it was saved as a plan template and the phantom image set was removed from the Brainlab database to prevent any confusion and mix‐up. On treatment day, patient CT was fused with MRI, customized beam settings from the preplan template were applied and optimized. Considerable reduction in treatment planning time was demonstrated by bench testing with both a conformal beam and a circular arc case. Results: Preplans were generated in 20–25 minutes to satisfy <130% homogeneity, 100% coverage and <2.0 conformity index with the phantom CT. After applying the plan template and fine‐tuning the pre‐configured beam settings, treatment plans of comparable evaluation indices were obtained in 5 min and 1 min for the conformal and arc cases, respectively. This technique was successfully applied to our last 16 patient treatments. Conclusion: The SRS preplanning technique implemented in this study for the Brainlab iPlan TPS offers an opportunity to thoroughly explore possible beam configurations, optimize planning parameters, resolve gantry angle clearance issues, and communicate and address challenges with physicians before treatment day. Preplanning has proven useful and time‐saving in our clinic particularly for multiple‐isocenter and dosimetrically challenging cases.
SU‐E‐T‐476: 4‐Dimensional Dosimetric Comparison of Stereotactic Body Radiation Therapy for Lung Tumor With Versus Without Tumor Tracking Technique
Authors: Ding C , Hrycushko B , Solberg T
Published: July 2013

Purpose: Recently a new treatment device capable of gimbal‐based tumor tracking (Vero, BrainLAB AG, Feldkirchen, Germany) was installed at our institution. The purpose of this study is to compare dose‐volume and radiobiological parameters associated with treatment efficacy and normal tissue toxicity for lung SBRT using a conventional linac with and without tumor tracking. Methods: A digital torso phantom having the capability to simulate respiratory and cardiac motion was used for this work. Spherical targets having a diameter of 20mm were centrally located within the right lung. 12 non‐coplanar, non‐opposing conformal beams were designed for linac planning with and without tumor tracking to deliver 60Gy in 3 fractions, prescribed to cover 95% of the target. 4D dose calculations were performed using the Monte Carlo algorithm in iPlan™ using a voxel tracking technique for the moving target and surrounding tissue. Diaphragm motion was varied from 10–30 mm in the superior‐inferior direction to simulate different motion amplitudes while limiting chest wall motion to 10mm in the anterior‐posterior direction. Dose‐volume analysis was performed using parameters previously shown to correlate with normal tissue toxicity for lung SBRT. Results: All indices used to assess normal tissue toxicity were reduced for the tumor tracking model compared with non‐tracking delivery. Mean lung dose, equivalent ipsilateral lung dose delivered in 2Gy fractions, and V13 were reduced by 17%, 21%, and 22% and 27%, 30%, and 36% for small and large diaphragm motion, respectively. The maximum heart dose was reduced by 7% for small diaphragm motion and by 9% for larger diaphragm motion. Conclusion: This work validates the ability of tumor tracking techniques to increase the dose to moving targets while reducing dose to surrounding normal tissues. It is expected that SBRT dose escalation using tumor tracking techniques will improve local control while reducing rates of normal tissue toxicity.
SU‐E‐T‐478: Prescription to 50–75% Isodose Line May Be Optimum for Linac Radiosurgery of Cranial Lesions
Authors: Zhao B , Jin J , Wen N , Chetty I , Ryu S
Published: July 2013

Purpose: The percentage‐isodose‐line to prescribe (RxIDL) in linac‐based SRS varies among institutions. An optimal RxIDL may lead to a treatment plan with the highest therapeutic ratio. Gradient index (GI), defined as the ratio of total volume receiving 50% of prescription dose and target volume receiving prescription dose, is an important parameter to characterize dose falloff. For radiosurgery prescribed at 16–20Gy, GI corresponds to the normal brain volume receiving 8–10Gy (V8–10Gy), which has been found to correlate to brain necrosis. Thus, GI can be considered as a clinically relevant criteria correlating to therapeutic ratio. This study aims to explore the optimal RxIDL using the GI criteria. Methods: 30 patients with brain tumors were retrospectively studied using iPlan system (BrainLAB, Feldkirchen, Germany). MLC‐based dynamic conformal arc technique was used for planning. For each patient, 6–8 plans with different RxIDLs were generated with different MLC margins. All plans were normalized so that 95% of target volume receives prescription dose. RxIDL was calculated as prescription dose divided by maximum dose. GI was calculated for each plan and the plan with minimum GI was considered optimal. Results: GI decreases (6.3 to 2.4) as target volumes increases (0.15cm3 to 50.09cm3), and the optimal RxIDL shifts to higher percentile. Median optimal RxIDL is 52.7±8.8% for targets <5cm3 and 71.8±3.5% for those >5cm3. Average actually treated RxIDL is 83.6±3.1%. The lower optimal RxIDL results in smaller V8–10Gy and higher mean dose to target volume. About 22cm3 reduction in V8–10Gy is observed in the case that has the largest tumor volume. Conclusion: The optimal RxIDL appears to be between 50% and 75% which is lower than the treated RxIDL. Larger targets tend to have higher optimal RxIDLs. By choosing an optimal RxIDL, we could reduce the volume of irradiated normal brain while delivering higher dose to target volume.
SU‐E‐T‐482: Novel Planning Technique for Treating Large Volume Arteriovenous Malformations Using SRS
Authors: Desai P , Caroprese B , McKellar H
Published: July 2013

Purpose: An Arteriovenous malformation (AVM) is an abnormal connection between an artery and a vein bypassing a capillary. AVMs if left untreated can bleed and cause other neurological complications including death. Stereotactic radiosurgery is used as an alternative to conventional surgery to obliterate the AVM nidus (the central core of the AVM). SRS is generally restricted to small volume (less than 5cc) AVMs. Typically single fraction SRS doses of 1600–2000cGy are delivered to the contoured volume of the AVM while ensuring sharp gradients at the interface of AVM and normal brain. We discuss a novel strategy of dosing large volume AVMs (5–20cc) using a differential dose scheme that delivers 1600cGy at the rim and 2000cGy at the nidus of the AVM. Methods: Accuray CyberKnife (G‐3), Multiplan V3.5 treatment planning system. Patient CT, MRI and other imaging studies are sent to the Multiplan planning system. The neurosurgeons and the radiation oncologist at our site contour the AVM and the relevant anatomy. The contoured AVM volume is contracted by 2.5mm, and a rim is created by subtracting the previously contracted structure from the AVM. A treatment plan is generated using iterative planning,using custom planning structures. The treatment plan delivers 2000cGy to the core of the AVM, and 1600cGy at the rim, while keeping the dose gradient at the AVM normal brain interface high, enabling us to spare surrounding brain tissue. Results: Post treatment follow‐up imaging done at 3, 6 and 12 months shows a gradual diminishing and obliteration of the nidus. Conclusion: We conclude that this scheme of differential dosing enables us to treat larger volume AVMs effectively and safely.
SU‐E‐T‐484: A Closed Parameterization of Microscopic Damage as a Function of Energy in Dose Deposition by Charged Particles
Authors: Van den Heuvel F
Published: July 2013

Purpose: To present the derivation and application of a closed formalism for the impact of charged particle radiation on damage induced in DNA. The formalism is valid for all types of charged particles and due to its closed nature is suited to provide fast conversion of dose to DNA‐damage. As photon treatments can be reduced to energy deposition by electrons it is also possible to calculate damage for classical radiation treatments. Methods: A simple geometrical model allows to parameterize the impact of charged particles in terms of single and double strand breaks. The parameters of the model contain: double strand threshold (i.e. the maximal distance two single lesions can have to be categorized as a double strand break), the diameter of the DNA‐strand, and a scaling factor describing distance between ionizing collisions in terms of particle energy. The model is validated using microdosimetric Monte Carlo calculations for electrons and protons. Finally, example applications are constructed calculating Relative Biological Effectratios for proton therapy compared to a therapeutic 6MV beam. Results: For both protons and electrons the model fits the Monte Carlo calculations almost perfectly as assessed by a χ2‐test. The energy scaling factor seems to be the only difference between the modalities of protons and electrons. The RBE‐factor for a mono‐energetic proton beam varies depending on the depth and is of the order of 1.1 rising to almost 2 at the Bragg peak. Conclusions: We have shown that it is possible to provide a generalized expression parameterizing the DNA‐damage induced by different modalities, indicating that the change in biological effect is mainly governed by geometrical considerations rather than a biological impact.
SU‐E‐T‐490: A Geant4 Implementation of the Generalized Boltzmann Fokker‐Plank Method
Authors: Dixon D , Prinja A
Published: July 2013

Purpose: A geant4 implementation of the Generalized Boltzmann Fokker‐Plank (GBFP) method, an advanced, computationally efficient Monte Carlo method for charged particle transport, is studied. Methods: The GBFP method is a systematic approach for extending the mean free path (MFP) of particles while exactly preserving moments of the true differential cross section (DCS) to some order, $N$, and approximating higher order moments by the exact moments of the approximate DCS. The approximate DCS preserves important physical quantities like mean scattering cosine, stopping power, and energy straggling. The GBFP method is a single event method, so transport mechanics are restored and step size and boundary crossing limitations inherent to condensed history methods are a non‐issue. A geant4 application with GBFP physics was developed for testing speed and accuracy of the GBFP method dose calculations. Results: Initial testing revealed that speed‐ups for the discrete GBFP model are consistent with previous work on the GBFP method. Computed dose profiles using the discrete GBFP method are in agreement with benchmarks within statistical uncertainty (in most cases < 1$\%) for sufficiently sampled regions. This indicates that the geant4 implementation was successful. A framework for further testing is now established. Conclusion: The GBFP method provides systematic control of calculation speed and accuracy for electron transport calculations. The GBFP method is single event Monte Carlo so pre‐existing production code transport algorithms are all that are needed for the implementation. Therefore, no major changes are necessary for incorporation of the GBFP method into production code systems.
SU‐E‐T‐492: High Precision Cross Sections and Physics Models for Proton Interactions Below 20 MeV in Geant4
Authors: Enger SA , Giusti V , Arce P
Published: July 2013

Purpose: Geant4 is a Monte Carlo (MC) toolkit with an increasing use in the field of medical physics. Electromagnetic processes are well validated, but for hadronic interactions of charged particles at energies of the order of a few MeV, particularly for low Z materials, there are no physics models able to reproduce the experimental data. With the default version of Geant4, the cross sections and production of secondary particles for proton‐nucleus interactions at an energy below 20 MeV are not satisfactory. In this study, a new physics package to simulate particles below 20 MeV has been added to Geant4. Methods: A new Geant4 package has been developed to simulate proton interactions below 20 MeV using the data on reaction cross sections and production of secondary particles contained in evaluated nuclear databases. The new package has been tested making use of the TENDL‐2009 and TENDL‐2012 databases to simulate the 18O(p,x) interactions for energies ranging from 4 MeV up to 20 MeV. For this particular reaction, we have also included in the database the accurate data that can be found in the IAEA medical database. Results: The obtained energy spectra of the secondary particles produced by the 18O(p,n) and 18O(p,gamma) reactions were compared with those calculated with the standard Geant4 physics, QGSP_BIC_HP, and also with suitable MCNPX calculations. The neutron and gamma production obtained with the new Geant4 package and TENDL/IAEA‐libraries are significant from 4 MeV, while with the standard Geant4 physics there is no production of neutrons for energy below 8 MeV, while above 8 MeV the gamma and neutron production shows a wrong yield. Conclusion: A new high precision physics package with corresponding cross section libraries have been successfully added to the Geant4 MC toolkit for particle energies below 20 MeV, improving the accuracy and production rates of different secondaries.
SU‐E‐T‐495: Monte Carlo Simulations of Relative DNA Damage From KV CBCT Radiation
Authors: Kirkby C , Ghasroddashti E , Poirier Y , Tambasco M , Stewart R
Published: July 2013

Purpose: Daily imaging protocols using kilovoltage cone beam CT (kV CBCT) have the potential to accumulate non‐trivial doses (up to several percent of the prescription dose) to the imaged volume which could have implications for planning decisions or the induction of secondary cancers. When CBCT dose is added to therapeutic dose, it may be necessary to account for differences in the relative biological effectiveness (RBE). We conducted a Monte Carlo study to investigate the relative number and complexity of DNA double strand breaks (DSBs) that Result from kV CBCT radiation compared to Co‐60 as a reference for general external beam radiation. Methods: A method was developed for coupling the radiation transport code (PENELOPE) with a Monte Carlo Damage Simulation (MCDS) code that predicts relative numbers and complexity of DSBs. The PENELOPE simulations generated electron spectra relevant for the MCDS input consisting of (i) electrons generated outside of a cell nucleus that drift in and (ii) electrons created within the nuclear volume, both without the continued slowing within the nucleus itself that is simulated implicitly by the MCDS code. Various configurations were considered (surface/depth, bowtie filter thickness, kV energy, and degrees of oxygenation). Results: The RBE for DSB induction in the kV CBCT sources ranged from 1.14 to 1.19 with increased complexity compared to that of Co‐60 for the majority of conditions; however, values as high as 1.45 were calculated in extreme circumstances. The RBE increased for anoxic conditions, at the phantom surface, for lower energies and for less filtration. Conclusions:Our results suggest kV CBCT sources produce more DNA damage than therapeutic radiation for a given absorbed dose and the relative difference can depend on several factors. This has potential implications for calculating the probability for inducing secondary cancers through the use of image guidance radiation.
SU‐E‐T‐503: Exploiting the Rotational Symmetry of Tomotherapy to Reduce Dose Perturbations From MRI‐Guided Radiotherapy: A Monte Carlo Investigation
Authors: Yang YM , Geurts M , Smilowitz JB , Sterpin E , Bednarz B
Published: July 2013

Purpose: There is growing excitement over new treatment modalities that integrate magnetic resonance (MR) image guidance with radiotherapy delivery systems. The therapeutic gain from reducing tumor position uncertainty using intra‐fraction MR imaging during radiotherapy will only be achieved by adequately accounting for impact of the magnetic field on dose distributions during treatment delivery. We propose that integrating MR‐image guidance with a more rotationally symmetric modality such as helical Tomotherapy delivery might reduce or even eliminate dose perturbations due to magnetic fields. The purpose of this study was to investigate the magnetic field effect on clinical Tomotherapy dose distributions in two disparate patient geometries using Monte Carlo methods. Methods: The Monte Carlo code Geant4 was used for all simulations performed in this study. To simulate magnetic resonance imaging guided radiation therapy (MRIgRT), we investigated dose distributions from Tomotherapy patient plans subjected to homogeneous transverse magnetic fields of varying field strengths. For this study, perturbations in the dose distributions from a prostate and lung case are presented. Results: Results show that perturbations in the overall dose profile due the magnetic field can increase the dose heterogeneity to the target volume when a transverse magnetic field was applied to the patient geometry. The perturbations were dependent on the heterogeneity of the patient geometry, symmetry of the primary beam fluence, and magnetic field strength. The heterogeneous lung geometry demonstrated a significant impact to the tumor volume due to the magnetic field, whereas relatively homogeneous prostate geometry showed minimal dose perturbations. Conclusion: This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality.
SU‐E‐T‐507: Dosimetric Validation of An IntrabeamTM GATE Model, Based On Monte Carlo GEANT4 Toolkit, for IORT Applications
Authors: Bouzid D , Visvikis D , Dupre P , Bert J , Pradier O , Nicolas B
Published: July 2013

Purpose: Intra‐operative Radiation Therapy (IORT) with Intrabeam™ is delivered using a miniaturized radiation device with applicators generating the prescribed dose directed to the tumor bed. However, the dose delivered to the patient is not well known. In this context, a Monte Carlo (MC) calculation can accurately assess the dose delivered to the patient by simulating the particle transport. The aim of this study is to create and validate an Intrabeam model on the GATE platform, a MC code based on Geant4. Methods: A model of the Intrabeam™ was created on GATE taking into account dimensions and materials of each element of the system as well as physical processes. This model was extended to include simulations based on patient CT images including the presence of applicators within the same simulation framework allowing their inclusion in the dose calculation. For breast cancer simple simulations in a water phantom were performed including the bare probe and the spherical applicators, using soft X‐Ray PTW ionization chambers. For spinal metastases, the RANDO phantom was used to simulate a patient and realistic treatment conditions allowing dose delivery verifications using thermo‐luminescent dosimeters (TLD). Finally a breast cancer patient CT acquisition, carried out during the operation with the applicator in place, was also used in the validation of the Intrabeam™ GATE model and associated MC based dosimetry using TLDs. Results: Depth dose curve and isotropy were compared showing a good agreement in water. For breast application, the comparison between TLD measurements and GATE simulations on patient and phantom CT images showed a good agreement with less than 1% of deviation (one billion simulated particles). Conclusion: The Monte Carlo GATE platform can simulate accurately relative dose distributions delivered by the Intrabeam system. The proposed model has been validated and will be used for personalized 3D dose prediction in IORT.
SU‐E‐T‐516: Limitations and Clinical Implications of the TG‐43 Formalism for High‐Dose‐Rate Skin Brachytherapy
Authors: Granero D , Perez‐Calatayud J , Vijande J , Ballester F , Candela‐Juan C , Rivard M
Published: July 2013

Purpose: In high‐dose‐rate (HDR) skin brachytherapy, sources are located outside, in contact with, or implanted at some depth below the skin surface. Nowadays, most TPS use the TG‐43 formalism that is based on sources within an infinite water medium, without accounting for the scatter defect due to surrounding air. The purpose of the present study is to evaluate the limitations of the TG‐43 formalism in HDR skin brachytherapy. Methods: This study examined two HDR sources (Ir‐192 and Co‐60) together with a hypothetical Yb‐169 source. Dose distributions were obtained using Monte Carlo methods (i.e., GEANT4). The following configurations were investigated: 1)A typical clinical case consisting of a 5×5 cm2 superficial mould, using an Ir‐192 HDR source, embedded within an infinite water medium (i.e., TG‐43 conditions), 2)The same mould positioned over the skin in a semi‐infinite water medium, and 3)A single source, instead of a mould, with the same configuration as in 1), but with 0, 1, or 2 cm of bolus. Results: Dose rate distribution differences between cases 1) and 2) ranged from 1.5% to 3% at the skin surface. At the typical prescription depth of 0.5 cm, differences were 2.5% to 3%. For case 3) using a Ir‐192 source without bolus, dose rate differences were within 2% compared with TG‐43 data for depths < 0.5 cm. When 1 cm or 2 cm of bolus was added, the differences became negligible. For Co‐60 with no bolus, the differences approached 15% near the surface. From a clinical perspective, these differences were negligible considering the dose gradients at the typical prescription depth (0.5 cm). Conclusion:Although dose differences exist amongst the different scatter configurations, they are not clinically significant.
SU‐E‐T‐597: Online Adaptive Replanning for Sequential Boost After Whole Breast Irradiation
Authors: Chen X , Qiao Q , Currey A , Nascimento J , Wilson J , Li X
Published: July 2013

Purpose: For whole breast irradiation (WBI) followed by sequential boost of the lumpectomy cavity (LC), the LC shape and volume can change significantly after WBI. Consequently, the boost plan, normally generated together with WBI plan before the start of treatment, may not be optimal. Here we propose the use of online adaptive replanning at the time of boost to account for the LC change. Methods: Daily diagnostic‐quality CT sets acquired during IGRT using an in‐room CT (CTVision, Siemens) for 19 breast cancer patients treated with WBI with sequential boost in prone position were used. Contours of LC, treated breast, ipsilateral lung, and heart were generated by deformably registering the planning CT with the fraction CT acquired on the first boost fraction using an auto‐segmentation tool (ABAS, Elekta) with manual editing, if necessary. Three plans were generated based on the daily CT: (1) repositioning plan by applying the original boost plan with the shift, (2) adaptive plan by quickly modifying the original plan using a tool (RealArt, Prowess), and (3) reoptimization plan by a fully‐blown optimization. Results: Significant changes were observed in the LC volume and shape. The LC volume on the first day of boost changes in the range of 30% and 130% from that on planning CT. The plan quality of the adaptive plans and the re‐optimization plans were comparable. Compared to the repositioning plans, the adaptive plans generally lead to improvement in target coverage and normal tissue sparing, with an average increase in LC V95 of 2.3% and decrease in breast tissue V50 of 3.0%. Conclusion: Significant changes in LC shape and volume at the time of boost from the original plan for WBI with sequential boost can be addressed by the online replanning at the first boost fraction.