bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐475: Dynamic Conformal Sub‐Arc Vs 3D Planning for SBRT Lung Treatment
Authors: Baydush A , Wiant D , Terrell J , Pursley J , Yount C , Maurer J , Sintay B
Published: July 2013

Purpose: To introduce and compare dynamic conformal sub‐arc (DCSA) planning to static 3D planning for SBRT lung treatments. Methods: Discussions about standard dynamic conformal arcs revealed little control to adjust the dose cloud. This was especially relevant in areas such as a tumor in between the chest wall and lung. DCSA was devised to overcome this lack of adjustability. DCSA reduces an arc field into multiple conformal sub‐arcs, which can then be adjusted using field weights to alter dose contribution and shift the dose cloud. In this initial study, we investigated two DCSAs, where each sub‐arc covered 45°: 8‐field 360° DCSA and 6‐field 270° DCSA. The 360° plans used the fit and shield tool in Eclipse for any field that entered through the spine. The 270° partial arc plans were positioned to have no entry dose into the spine or contralateral lung. These DCSA plans were compared to static 3D SBRT plans for isodose and RTOG dosimetric measurements. Results: Results show minimal differences with the largest differences being a slight increase in volume of the 50% isodose coverage. 360° DCSA plans showed similar MU versus 3D, while 270° DCSA plans showed reduced MU. All organs‐of‐interest doses were significantly below RTOG criterion. The 3D plans incorporated 3 couch kicks, while the DCSA plans had none. Treatment time for the 3D plans was approximately 20–30 minutes because of the couch kicks and repeat imaging, while the DCSA plans can be treated in under 4 minutes at 600 MU/min and beam automation on a Varian Truebeam linac. Isodose comparisons are presented, as well. Conclusion: Full arc DCSA and partial arc DCSA offer comparable treatment plans to 3D planning but can be delivered much more efficiently. This reduces patient treatment time, alleviates the need for any second CBCT localization, removes couch kicks, and may reduce overall MU.
SU‐E‐T‐477: Study of MLC Apertures and Their Significance for Volumetric Arc Therapy in the Management of Non‐Small Cell Lung Cancer Using Stereotactic Body Radiotherapy
Authors: Spaans J , Mackowiak L , Malhotra H
Published: July 2013

Purpose: Electronic disequilibrium concerns limit the minimum field sizes to 3.5 × 3.5 cm2 for NSCLC patients per various RTOG protocols which is easily achievable using 3DCRT. However, the inverse planning nature of VMAT does not provide any control on the field‐size or MLC shapes and may often violate it. This study aims to analyze the difference between VMAT and 3DCRT planning techniques by evaluating aperture size & hence conformity with RTOG protocols. Methods: 25 patients with NSCLC [5 tumors per lobe] who were treated using 3DCRT were retrospectively inverse planned for VMAT. Dosimetrically, VMAT plans were almost identical with that of clinical 3DCRT plans. DICOM plans were imported into MATLAB to determine the actual MLC area of the active leaves per control point and its relative weight in the overall dose delivery of that VMAT arc. The effective‐weight of all the MLC areas which were less than 3.5 × 3.5 cm2 was summated (VMAT3.5fs). We also studied the conditions where the total MLC area did satisfy the minimum requirement of 3.5 × 3.5 cm2 but where any two opposite MLC leaf positions were less than 3.5 cm apart (VMAT3.5MLCwt). These evaluations are contained in VMAT3.5MLCwt which gives the weighted sum of all small MLC segments in the entire VMAT arc. A zero value of both ensures that the VMAT arc satisfies the RTOG requirement of minimal jaws size in its entirety. Results: Respective values of VMAT3.5fs & VMAT3.5MLCwt were found to be 0.88 ± 0.23 & 0.9 ± 0.21 for 63 VMAT arcs. Only 2 patients had a value of 0 for both indices implying compliance to RTOG field size limits. Conclusion: Proposed indices provide an objective method to evaluate the suitability of VMAT plans in satisfying RTOG constraints for NSCLC treatment using SBRT.
SU‐E‐T‐479: Dosimetric Evaluation of 3 Dimensional Conformal Radiotherapy (3DCRT), Dynamic Conformal Arc Therapy (DCAT) & Volumetric Arc Therapy (VMAT) in the Management of Non‐Small Cell Lung Cancer (NSCLC) Using Stereotactic Body Radiotherapy (SBRT)
Authors: Rauschenbach B , Mackowiak L , Malhotra H
Published: July 2013

Purpose: Initial clinical results of NSCLC usually delivered using 3DCRT & using extreme hypo‐fractionated radiotherapy (SBRT) have been very encouraging. Dynamic‐conformal‐arc therapy or volumetric‐arc therapy could also be potentially used to deliver the tumorcidal dosage. This study aims to evaluate 3D conformal radiation therapy techniques (3DCRT) vs DCAT & VMAT planning methods using dosimetric indices and doses to critical structures as per various Radiation Therapy Oncology Group (RTOG) protocols 0236, 0813, and 0915 for treating Non‐Small Cell Lung Cancer (NSCLC) patients using SBRT for tumors based on their geographical location within thorax. Methods: 25 patients (5 in each lobe) previously treated with 3DCRT were retrospectively re‐planned with DCAT & VMAT methods. All treatment plans were designed to ensure 95% of the PTV volume gets 100% of the prescription dose. Same dosimetric indices & dose to critical structures were used to evaluate respective plan qualities. Results: The conformity index of 3DCRT, DCAT & VMAT was 1.2 ±0.1, 1.2±.1, 1.0±.0, respectively. The respective volume (cc) of PTV exceeding 105% of the prescription were 3.3 ±2.5, 3.1±.2.8, 0.2±.0.3. The ratio of 50% isodose volume over PTV volume was found to be 4.1 ±0.9, 1±0.5, 3.6±.0.3, respectively for 3DCRT, DCAT & VMAT. The respective V20 values for 3DCRT, DCAT & VMAT were found to 4.0 ±2.6, 4.2±3.1, 3.8±.2.9. Conclusion: Our analysis show that almost all DCAT & VMAT plans provided better conformity with the PTV and generally better sparing of tumor critical structures & satisfied respective RTOG constraints. DCAT & VMAT are possible alternative to 3DCRT for the treatment of NSCLC with SBRT regardless of the location of the tumor within in the thorax. The use of VMAT for NSCLC cases has the potential to shorten treatment planning times as well as treatment times experienced by the patient while satisfying planning constraints.
SU‐E‐T‐480: Can PTV Density Be Overridden for SBRT Lung Planning?
Authors: Fu  , Perera H , Rudoler S , Yu Y
Published: July 2013

Purpose: To demonstrate a PTV density overridden method that may reduce planned MU while still delivering sufficient dose to the tumor for SBRT lung planning. Methods: The tumor motion inside the planning target volume (PTV) during treatment changes the density distribution inside the PTV. The worst case scenario occurs when CTV fills the PTV at the same moment, so the PTV density becomes the same as the CTV density. Our hypothesis is that the MU calculated with appropriate PTV density should be sufficient for delivering the prescription dose to the tumor. To find support for this, the plan MU with the PTV density assigned as 0.8g/cm3 were recalculated using Pinnacle planning software for twelve patients. Three of those patients had four CTV structures copied and manually placed at the border of the PTV. The new CTV density was overridden to 1g/cm3 and the density of the PTV, excluding new CTV, was overridden to 0.2 g/cm3. The plans were recalculated using the MU from the PTV density overridden plan. Each new plan with the CTV coverage and dose distribution was compared to the plan with the original density. Results: MU was reduced by more than 10% for three patients whose CTV sizes were smaller than 1.2cc. For the remaining patients, MU was reduced by an average of 3%. Without considering density redistribution due to tumor relocation, the new CTVs of those three patients only had 58%, 74%, and 47% of their volumes receiving the prescription dose. However, the coverage of the CTVs increased to 95% in average if density redistribution was applied. The prescription isodose line changed with the CTV location. Conclusion: This work finds support for a significant reduction in MU for very small CTV lung SBRT plan, while still delivering sufficient dose to the tumor.
SU‐E‐T‐483: Factors Contributing to the Increased Proton RBE at the Distal Edge of Bragg Peak
Authors: Cheng C‐W , Takashina M , Suga M , Das IJ , Moskvin V
Published: July 2013

Purpose: RBE in proton beam is variable and it is expected that the values are different at Bragg peak. This is investigated using Monte Carlo simulation. The objective is to determine the physical quantities contributing to the spatial variation of RBE in proton beam. Methods: The Monte Carlo codes PHITS and FLUKA are used in this study. A mono‐energetic unmodulated proton beam is incident on a water phantom. The average proton energy along the proton beam path, the proton flux, Φ(#protons/cm2/incident proton) and the linear energy transfer, LET (keV/μm) integrated over the energy range 0‐Ep,max are computed at selected depths from d=0 to 1cm beyond the Bragg peak for incident proton energies 50–250 MeV. The quantity LET*Φ /ρ which is a direct measure of energy absorbed/unit mass at each point of the proton beam path is also determined. Results: The proton energies at the BP are 4, 4, 12, 19 and 25 MeV for incident proton energies of 50, 100, 150, 200 and 250 MeV respectively. As the proton energy decreases with depth, the LET increases, while the proton flux decreases due to the broadening of the proton spectrum with depth. Thus the product, LET*Φ attains a maximum somewhere on the distal edge of the BP (or the SOBP). Lower energy protons have a higher LET*F at the distal edge than that higher energy beams. Conclusion: RBE is related to the relative dose (energy absorbed/unit mass) to a reference radiation for the same biological endpoint. The variation of LET*Φ indirectly shows that the increase in RBE at the distal edge is due to the two competing factors in play, the LET and the proton flux as a function of depth. Strictly speaking, secondary electrons contribute to the2 LET and flux, and will affect the RBE, to a small extent.
SU‐E‐T‐485: Comparison of the Oncogenic Potential for Radiation‐Associated, Second Malignant Neoplasms for Several Prostate Radiotherapy Modalities as a Function of Relative OAR & PTV Volumes
Authors: Price MJ , Fontenot JD , Lian J , Nichols G , Burkhardt K , Erwin R , VanderWalde N , Dooley J , Tracton G , Chang S
Published: July 2013

Purpose: To compare plan quality for a comprehensive set of OAR & PTV geometries considering relative risk estimates for the induction of secondary bladder and rectal cancers due to prostate radiotherapy using several modern delivery techniques. Methods: IMRT / 3D‐conformal, volumetric‐arc (VMAT), and Tomotherapy (TT) plans were generated using PlanUNC, Pinnacle3 ver. 9.2, and TomoTherapy treatment planning systems, respectively, for 12 prostate cases using, RTOG 0815‐derived planning goals. Selected cases were representative of relative (i.e. small (Svol), medium (Mvol), large (Lvol)) PTV, rectal & bladder structure volumes encountered clinically. PlanUNC was used to calculate the elevated relative‐risk (ERR) of secondary cancer induction in normal tissues, the bladder and rectal ERR, PTV conformity index (CI), dose heterogeneity index (DHI), and bladder and rectal V5, V15, V35, V50 (Bvx / Rvx) for all cases. All metrics were averaged for each relative tissue / target volume, and normalized to corresponding 3D‐conformal cases for comparison. Results: Considering relative rectal volumes, all metrics fell within the standard error for each modality excluding bladder ERR (TT 13%ave, IMRT −9% ave & VMAT −18% ave) and BV15 (VMAT −24% ave) for Lvol rectum cases. Considering relative bladder volumes, bladder ERR (TT 4%, IMRT −10%, VMAT −18%) for Lvol and (VMAT −20%) for Mvol; rectal ERR (TT −40%) for Mvol were observed to be significant in addition to bladder ERR (TT 13%) for Svol PTV cases. Generally, all modalities performed better when compared to 3D conformal delivery excluding nominal (<5%) increases in normal tissue dose and increased bladder ERR for several TT cases. Conclusion: We compared three prostate EBRT modalities considering OAR ERR as well as traditional plan quality metrics for a class of cases defined by relative structure size. Preliminary results indicate comparable improvements in PTV conformity & OAR dosing for each modality but differences in OAR ERR.
SU‐E‐T‐486: Evaluation of the Fisher‐Kolmogorov Glioma Growth Model for Radiotherapy Target Delineation
Authors: Unkelbach J , Dittmann F , Menze B , Konukoglu E , Shih H
Published: July 2013

Purpose: Gliomas infiltrate the adjacent brain parenchyma far beyond the tumor mass visible on current imaging modalities. In current clinical practice, an isotropic margin is applied to account for infiltrative disease. However, histopathology suggests that glioma growth is anisotropic: (1) the ventricles and the falx represent barriers for migrating tumor cells; (2) gray matter is infiltrated less than white matter; and (3) glioma cells appear to preferentially spread along white matter fibers. Developing objective and quantitative approaches to account for these growth patterns may improve and automate target delineation for gliomas. Methods: We study the Fisher‐Kolmogorov glioma growth model, which represents a partial differential equation for the tumor cell density and replicates the observed growth characteristics. Via brain segmentation into anatomical barriers, cerebrospinal fluid, and white and gray matter, the model equations are solved on the patient‐specific anatomy. Preferential spread along white matter fibers is incorporated through DTI imaging. The radiotherapy target can be defined as an isoline of the derived tumor cell density. We performed a retrospective study involving 10 glioblastoma patients to (1) identify the anatomical situations for which the model suggests target volumes that differ from the manually drawn targets, and (2) fully understand the capabilities and limitations of the model for target delineation. Results: It was found that tumors located close to the corpus callosum potentially benefited the most from model‐based target delineation because it consistently predicted the contralateral tumor extension across the corpus callosum in addition to easily modeling falx and ventricles as boundaries. Furthermore, for tumors located close to major sulci (Sylvian fissure), modeling reduced gray matter infiltration can potentially reduce the amount of brain tissue targeted for irradiation. Conclusion: The tumor growth model represents a promising tool to objectively create target volumes for radiotherapy of gliomas by consistently account ing for known growth patterns.
SU‐E‐T‐487: A Method to Vectorize the Dose Distribution, the Dose Volume Histogram
Authors: Mayo C , Zankowski C , Herman M , Miller R , Olivier K , Suominen J
Published: July 2013

Purpose: A programmatic method to incorporate spatial information into a dose volume histogram (DVH) by adding vector data on the location of pixels in the dose array relative to structures in the plan to construct a vectorized dose distribution (VDD) is defined and demonstrated. With this data the DVH can be sub grouped according to a wide array of vector constraint sets, defining the spatial relationship of pixels to one or several structures to construct a vectorized DVH (VDVH) to reveal vector relationships of dose regions to structures. Methods: Set theory models for construction of the VDD and VDVH are described and a dose‐vector‐histogram (DVctH) is introduced as a means of specifying the position, with respect to other structures, of focal dose features such as hot spots, distributed dose regions and conformality. Practical detail on a programmatic approach to implement the methods is provided. A set of tests utilizing phantom, prostate and SBRT lung image sets were carried out to demonstrate ability of VDVH and DVctH to reveal clinically relevant spatial detail in dose distributions. Results: The VDVH and DVctH enabled decomposing DVH curves to reveal the relative location of pixels contributing to dose points on the curve. The metrics enabled specificity in defining the location and magnitude of dose features relevant to treatment plan evaluation. Conclusion: The VDD, VDVH and DVctH differ from other methods described in the literature as a Result of using vector based constraints for each pixel, rather than focusing only on distance by construction of a set of shells around or within a structure and then subgrouping pixels in the overlap region. The method is an effective means to combine spatial information with DVH metrics and provides a practical means of specifying the location of dose features with respect other structures in the treatment plan. Varian Medical Systems
SU‐E‐T‐488: Secondary Malignancy Risks in Lung Cancer Radiotherapy with Helical Tomotherapy, Intensity Modulated Radiotherapy, and Conformal Radiotherapy
Authors: Papanikolaou N , Komisopoulos G , Rodriguez S , Stathakis S , Esquivel C , Nikiforidis G , Sakellaropoulos G , Mavroidis P
Published: July 2013

Purpose: The aim of the study is to examine the risk of inducing secondary malignancies in association with the clinical effectiveness of treatment plans in terms of tumor control and normal tissue complication probabilities. Methods: Four patients having been treated for lung cancer were examined. For each of them, three treatment plans were developed based on the Helical Tomotherapy (HT), MLC‐based Intensity Modulated Radiation Therapy (IMRT), and 3D‐Conformal Radiation Therapy (CRT) radiation modalities. The different plans were evaluated using the complication‐free tumor control probability (P+), the overall probability of injury (PI), the overall probability of control/benefit (PB), and the biologically effective uniform dose (BEUD). The risks for secondary malignancies in the spinal cord, heart and contra‐lateral lung were calculated for the three radiation modalities. Results: Regarding the overall evaluation of the different radiation modalities based on the P+ index, the average values of the HT, IMRT and CRT are 67.3%, 61.2% and 68.2%, respectively. The corresponding average values of PB are 75.6%, 70.5% and 71.0%, whereas the average values of PI are 8.3%, 9.3% and 2.8%, respectively. Regarding the risk for secondary cancer, for spinal cord the CRT modality produces the lowest average risk (1.4%) compared with the HT (2.1%) and IMRT (1.6%) radiation modalities. In heart, the lowest average risk is produced by IMRT (3.8%) compared with the HT (5.1%) and CRT (4.7%), whereas the three radiation modalities show almost equivalent results regarding the contra‐lateral lung (0.8% for HT, 0.9% for IMRT and 0.9% for CRT, respectively). Conclusion: The plans that appear to be more effective for the treatment are prone to show the highest risk of secondary malignancy. Radiobiologically‐based treatment planning taking into account the risk of secondary cancer can be established as an effective clinical tool for treatment optimization.
SU‐E‐T‐501: A Sensitivity Study of Tissue Characterization for Brachytherapy Monte Carlo Dose Calculation
Authors: Bedwani S , Carrier J , Bouchard H
Published: July 2013

Purpose: To establish the reliability of electron density (ED) mapping and tissue segmentation techniques using CT images for brachytherapy, considering stochastic and systematic Hounsfield unit (HU) variations. Methods: Most common artifacts are simulated within a Monte Carlo theoretical framework. A set of CT data is first generated with the EGSnrc suite followed by their reconstruction performed with an iterative algorithm. A statistical analysis of HU values from reconstructed images is performed to determine uncertainties and systematic effects introduced by the iterative algorithm and the presence of artifacts. Maps of ED and tissue indexes are retrieved from an HU‐ED curve calibrated at 120 kVp using experimental measurements and ICRU data. The mean energy absorption coefficient for each tissue is computed from ICRU data for an Iridium‐192 source in order to evaluate the impact on dose calculations. A probabilistic approach is used to compute the uncertainty on absorbed dose generating random distributions of HU and determining the probabilistic effects on the extracted ED and absorption coefficients. Results: For an uncertainty of +/− 20 HU, absorption coefficient uncertainties raise up to 3% when HU values are near the fat‐muscle intersection and up to 9% near the muscle‐spongiosa intersection. Uncertainties on ED are found to be less than 1% for HU above 0 and up to 3% for fat. A systematic effect of 50 HU caused by typical artifacts yields absorption coefficient errors up to 30% for HU values near the muscle‐spongiosa region, and errors in ED up to 8% for fat. Conclusion: Results show that the main source of dose calculation uncertainty is caused by the sensitivity of the tissue segmentation technique. This study suggests that improvements in such techniques are yet to be achieved.
SU‐E‐T‐502: In Search of the Optimum Ion for Radiotherapy
Authors: Guan F , Titt U , Bangert M , Mohan R
Published: July 2013

Purpose: To investigate the physical and biological properties and the clinical potential of several different ions through computer simulations. Methods: Using the Geant4 Monte Carlo toolkit, we calculated spatial dose and LET distributions in a water phantom for monoenergetic beams for four candidate ions with the same range (28 cm in water): 207 MeV/u protons, 244 MeV/u 3He ions, 206 MeV/u 4He ions, and 400 MeV/u 12C ions. An ideal broad beam of 6 cm in diameter was used in the simulations. The scoring voxel volume was set to 1 cubic millimeter. We also recorded the dose contributions from several major nuclear fragments. Furthermore, data for a 5‐cm RBE‐weighted dose SOBP for 12C ions with a range of 30 cm were analyzed. Results: Simulation results showed advantages and disadvantages of each ion. For instance, a 12C ion beam produces the sharpest and narrowest Bragg peak, and the smallest penumbra, but it has the highest entrance to peak dose ratio, and highest and longest nuclear fragmentation tail. Furthermore, the lateral dose halo effect of 12C is substantially greater than other ions. For the same range in water, 3He and 4He ions show very similar features. They have smaller lateral penumbrae than protons, and much smaller fragmentation tails compared with 12C ions. Although protons have the largest lateral penumbra and widest Bragg peak, they do not have an observable fragmentation tail. For a pristine beam of 400 MeV/u 12C ions, the fragmentation tail dose is 15% of the peak dose. In the case of 5‐cm SOBP with 30‐cm range of 12C ions, the fragmentation tail dose can be as high as 30% of the maximum dose. Conclusion: Each type of ion has special advantages and disadvantages. Further research is needed to consider additional factors such as energy dependence, cost effectiveness and RBE. NCI grant P01CA021239
SU‐E‐T‐510: Mitigation Strategies to Reduce the Relative Extrafocal Dose (RED)
Authors: Constantin M , Hsu H , Mansfield S , Svatos M
Published: July 2013

Purpose: Several studies [1,2] indicate that IMRT may lead to peripheral (extrafocal) dose outside the intended treatment‐field. The purpose of this study is to: (a) obtain an accurate assessment of the relationship between the relative extrafocal dose (RED) and beam energy, modulation factor, treatment modality, and Y‐jaw settings in connection with the high‐definition HD120‐leaf MLC from Varian; and (b) identify the most efficient mitigation strategies to reduce RED when significant. Methods: IMRT fields with varying modulation were simulated by using an accurate representation of the TrueBeam linac equipped with HD120‐MLC in Geant4 (v9.4.p02). Dose to water phantom calculations were performed relative to a 10×10cm2 field. RED distributions were normalized to dmax for the 6MV, 6FFF, 10MV, 10FFF and 15MV beams. Inline profiles indicated the spatial location of the RED maximum, while simulations with and without the flattening filter revealed its origin. The effect of reducing the Y‐jaw position was quantified for different incident energies. Results: RED is mainly due to photons scattered by the flattening filter, therefore it is not a concern (∼0.7%) when the filter is removed (i.e. 6FFF and 10FFF). The RED maximum is located 4cm outside the outermost boundary leaf (i.e. 15cm off‐axis for HD120‐MLC). Its magnitude can be reduced below 1% by decreasing the Y‐jaw setting, lowering the photon beam energy, lowering the modulation factor of the plan or by using FFF beams, if available. At 15MV, the RED magnitude was reduced from 2.6% to 0.9% by setting the Y‐jaw limit to 8cm. Conclusion: Methodologies to effectively lower the relative extrafocal dose were identified for various photon beams on a Varian linac with HD120‐MLC, including using FFF modes, choosing lower‐energy beams, limiting the Y‐field size and lowering the modulation.[1]E.E. Klein et al., Med.Phys.33 (2006), 2525‐1531[2]D.S. Sharma et al., B.J.R.79 (2006), 331–335
SU‐E‐T‐511: A Dosimetric Comparison Between Superposition Algorithm and Monte Carlo Simulation for SBRT
Authors: Luo W , Meacham A , Molloy J
Published: July 2013

Purpose: To verify the superposition dose calculation algorithm for SBRT plans using the Monte Carlo simulation and investigate the related issues. Methods: The SBRT plans with 6 MV were originally created on CMS Xio treatment planning system with superposition algorithm. The same patient CT, beam geometry and MUs were used in the Monte Carlo simulation (MC) on MCSIM. MCSIM is an EGS4‐based MC dose calculation system for photon and electron beams. The MC plans were compared with the Xio plans to verify Xio superposition algorithm for SBRT. The electron disequilibrium was particularly investigated by comparing the DVHs for a 2‐mm thick peel of the GTV. The 18‐MV plans and one‐and two‐beam plans were also created for the dosimetric comparison. Results: Five SBRT lung plans created on CMS Xio and delivered on Varian 21 EX linac were included in this study. The tumor GTV ranged from 1.4 cc to 11 cc and the dose ranged from 1950 cGy to 5400 cGy. The comparisons were made in terms of DVHs, mean doses, minimal doses, and maximal doses for GTV. The results showed the mean dose values of the real patient plans agreed with MC to within 3%. However, the differences between Xio and MC were increased to up to 10% for 1‐beam and 2‐beam plans, and the differences were more pronounced for the 18‐MV plans than the 6‐MV plans. Conclusions: The Xio superposition algorithm may not adequately account for electron disequilibrium for one or two beam plans; however, it can perform accurate dose calculation for SBRT with large number of beams. Compared to 18‐MV beams, 6‐MV is preferable as the dose calculation for the 6‐MV plans was more accurate based on the Monte Carlo simulation.
SU‐E‐T‐512: Monte Carlo Dose Verification of Pencil Beam Scanning Proton Therapy
Authors: Giantsoudi D , Clasie B , Grassberger C , Dowdell S , Depauw N , Madden T , Kooy H , Paganetti H
Published: July 2013

Purpose: To verify a clinical pencil (PB) beam dose calculation algorithm for scanned beam intensity modulated proton therapy (IMPT), using TOPAS (TOol for PArticle Simulation), a GEANT4 based Monte Carlo (MC) simulation system. Methods: Seven patients, previously treated with IMPT for various treatment sites and prescriptions, were selected from our patient database. Proton fluence maps of the treated plans were exported for each field from our clinical treatment planning system (ASTROID) and imported to TOPAS along with the patient and beam geometry. The absolute dose distribution of each individual beam was calculated and compared to the PB algorithm‐based calculation from ASTROID. Results: The differences observed in mean and median target doses were less than ±1% for all cases, while D02 and D98 (surrogates for maximum and minimum dose values respectively) differed by less than ±3% for the majority of beams. Differences in the mean dose for the organs at risk (OARs) ranged from −8.9% to 3.7%, with reference to MC calculation, with an average over all the OARs of −0.1%, indicating no systematic over‐or under‐estimation of the dose by the PB algorithm. 3D gamma analysis (2%/2mm) for the PB to MC dose comparison resulted in an average 95.2% (±5.0) of the target volume having an absolute gamma value equal or less than 1 and 99.2% (±1.2%) equal or less than 2. For the healthy tissue receiving at least 5% of the target mean dose, the corresponding percentages were 99.6% (±0.3%) and 99.9% (±0.1%). Conclusion: We have clinically implemented MC for IMPT plan recalculation. Our PB calculation algorithm for IMPT was found to be in overall good agreement with MC calculations. Clinically significant deviations in OAR mean dose can be attributed to lung tissue or bone anatomy in the beam path.
SU‐E‐T‐513: Evaluation Output Dose of Electron Beams Based On Monte Carlo
Authors: Liu D , Chi Z , Gao C , Han C
Published: July 2013

Purpose: The purpose of this study is to investigate the influence factor of output dose for electron beams used in radiotherapy by Monte Carlo technique. Methods: MCTP code was used to calculate a group of output dose from Varian Clinac 23EX. According to the contributions, the calculated output dose was divided into four parts which can help to identify the physical reasons for the variation of output dose, dose from direct particles which was arisen from the machine head directly, dose from indirect particles which was arisen from the collimation devices, including jaws, scrapers and shielding. Results: 1. Appling same cones and same jsw settings but different field inserts, considering the 6 Mev beams, due to direct particles as well as indirect particles the output dose increases from the smallest to the largest field insert. 2. Different cones with same jaw settings and field size, for 6MeV electron beams, the use of a wider cone resulted in a higher output dose, which is mainly because of the direct particles. 3. Same field size with different cones and jaw settings, considering the 18 MeV beams, the wider cone exhibits a reduced output dose, which is attributed to indirect particles, and jaw contributes more than others. 4. Open cone, for the 6 MeV beams the wider cones that also has wider jaw settings showed an increased contribution to the output dose majorly from the indirect particles compared with the smaller cones. Similar trends were seen from the 9 MeV beams, although they were diminished. A reverse Result was indicated for the 18 MeV beams. Conclusion: The variation of output dose depends in a complicated manner on the characteristics of the beams and the modifications that the various cone‐insert combinations introduce to these characteristics.
SU‐E‐T‐601: Optimal Beam Selection for Intensity Modulated Radiation Therapy: High Intensity Flattening‐Filter Free Versus Conventional Flattened Beams
Authors: Duan J , Shen S , Yuan Y , Wu X , Cardan R , Popple R , Brezovich I
Published: July 2013

Purpose: To investigate the effect of target size and magnitude of intensity modulation on the required number of monitor units (MU) in IMRT delivered by high intensity flattening filter free (HIFFF) beam. To provide guidelines for the selection of beams that optimizes delivery time. Methods: A 29cm×1cm×5cm target in the center of a 20cm×40cm rectangular phantom was divided into 29 small 1cm×1cm×5cm subtargets along the MLC leaf motion direction. Various sizes of new targets consisting of 5 to 29 subtargets were generated. By prescribing two different doses to adjacent subtargets, the modulation index (MI), defined as the ratio of the nominal dose of the low dose subtargets to that of the high dose subtargets, was varied from 0.2 to 1.0. IMRT plans consisting of simple AP and PA beams were optimized for various combinations of target sizes and MI with 10X HIFFF, 6X HIFFF and 6X flattened beams, respectively. MLC leaf sequences and MUs for delivering 2Gy/fraction were computed using the maximum MU rates of 2400, 1400 and 600 MU/min, respectively. Results: For any combination of target size and MI the 10X HIFFF beam required the highest number of MUs while 6X flattened beam required the lowest. For HIFFF beams, the MU increased with the target size in linear quadratic fashion while decreased approximately linearly with MI. For all tested conditions, 10X HIFFF beam required the shortest delivery time, while 6X beam required 1.4 to 3.7 times as long. Conclusion: Despite its requiring the most MUs among the three beams investigated, the 10X HIFFF beam takes the shortest time to deliver an IMRT plan, regardless of target size and MI. 10X HIFFF beam is the choice of beam for optimal delivery time when increased leaf leakage dose is not a concern.
U‐E‐T‐620: Radiobiological Implication of Margin for Target Expansion in Head and Neck IMRT with Daily IGRT
Authors: Srivastava S , Das I , Cheng C
Published: July 2013

Purpose: To account for organ motion and set up uncertainties a margin is added to the clinical target volume (CTV) to form the Planning Target Volume (PTV). There exists significant institutional variability of margins employed between CTV to PTV. The margin used has significant implication for extra tissue (PTV‐CTV) dose and directly related with normal tissue complication. This study quantifies the setup uncertainties to optimize the PTV margin and its radiobiological implication for target expansion in H&N cancer. Methods: Nine Nine H&N patients treated with IMRT and daily IGRT with on‐board‐imaging (OBI) were chosen for this study. Using Eclipse treatment planning system (TPS), treatment plans were generated for different margins ranging from 0 to 10 mm subject to the same optimization criterion for PTV and OAR using 6 MV photon beam. The DVH, extra tissue volume and total MU were recorded for all these IMRT plans. NTCP was calculated using the Lyman Kutcher Burman model from DVH data. The daily setup errors from OBI for the entire treatment were also analyzed. Results: Analysis of the 9 patients setup with over 800 data points showed that 98% of the points are within ±5mm using daily IGRT. With increasing margin, the PTV volume increases linearly. There is a 4‐fold relative increase in extra tissue volume for margin increase from 0–10 mm With increasing PTV margin the NTCP also increases linearly for the parotid glands. Similar patterns were noticed for all other OARs. Conclusion: With OBI the setup uncertainty could be easily achieved within ±5 mm for 98% of the H&N treatments. Increase in PTV margin increases NTCP. It is concluded that PTV margin should be limited to ±5mm to reduce extra volume irradiation and also reduces NTCP of OARs.
SU‐E‐T‐622: Feasibility of Partial Volumetric Modulated Arc Therapy for Boost Treatment of Breast Cancer
Authors: Yuan Y , Duan J , Klepczyk L , Meredith R , Wu X , Cardan R , Brezovich I , Shen S
Published: July 2013

Purpose: To evaluate the possible dosimetric advantage of volumetric modulated arc therapy (VMAT) over conventional 3D conformal therapy for left breast tumor bed boost treatment. Methods: Ten patients were retrospectively planned with VMAT technology for treatment of left breast after lumpectomy. 6 patients were simulated under deep inspiration breath hold, 4 patients were free breath. In each plan, a partial ARC of 200° to 220° was used. All VMAT plans were normalized to deliver 100% of the 16Gy in 8 fractions prescription dose to 95% of PTV volume. The plans were evaluated by calculation of heart maximum dose (Dmax_heart), percentage of lung volume receiving 10% of prescribed dose (V10_lung), percentage of ipsilateral breast receiving 50% of prescribed dose (V50_breast), maximum dose (Dmax), and the location of hot spot. These dosimetric parameters were compared with those for clinical plans using conventional 3D conformal technique. Results: For these ten patients (mean breast volume 1198 cc, mean boost PTV 161 cc), the mean dosimetric parameters for ARC plans/3D plans were 8.8%/28.5% of prescription dose for Dmax_heart, 5.4%/17.7% of total lung volume for V10_lung, 35.8%/40.0% of ipsilateral breast volume for V50_breast, 111.2%/111.6% of prescription dose for Dmax. Hot spots were located inside PTV in all 10 cases for ARC plans, but only in 6 out of 10 cases for 3D conformal plans. Conclusion: Initial results suggest that partial arc VMAT plans have dosimetric advantages for breast boost treatment. The plan quality was superior to conventional 3D conformal plans. In addition, with a template of standardized ARC beam arrangement and dose constraints for optimization, treatment planning and delivery for breast tumor bed boost can be more efficient.
SU‐E‐T‐624: Population Based Respiratory Analysis of Partial Breast IMRT and Impact of Breast Shape and Size
Authors: Quirk S , Conroy L , Smith W
Published: July 2013

Purpose: IMRT and partial breast irradiation (PBI) are used to improve cosmetic outcomes in early stage breast cancer patients; however, homogeneity and dose coverage are degraded by respiratory motion during delivery. We investigate the impact of respiratory motion on PBI IMRT using population respiratory data. Methods: The fluences from five different PBI IMRT plans were convolved with a population probability density function (PDF) to simulate dose delivery during respiration. Plan quality and coverage of the CTVsu (CTV plus 5 mm set‐up margin, trimmed back from skin and chestwall) were evaluated using DVH, hotspot, cold spot, and uniformity index. Results: All plans showed blurring (loss of coverage) in the shoulder region of the DVH curves for CTVsu and increased hotspot to the ipsilateral breast. For the patient with the largest breast volume and the smallest DEV:breast volume ratio, the CTVsu is surrounded by tissue. In this patient, the CTVsu coverage is slightly degraded and the hot spot becomes slightly hotter. The patients with a target volume near skin, lung or severe contour changes experience coverage loss near inhomogeneities and increased hot spots. This occurs for trimmed DEVs, although flash was added appropriately to all plans and coverage to the PTV (not only DEV) is adequate in the original plan. In patients with small breasts the CTVsu was noticeably hotter under respiratory conditions than static with the hot spot (2cc) in the CTVsu increasing by 7% and the 95% coverage increasing by 2%. Conclusions: The impact of respiratory motion on PBI is nontrivial and mainly patient dependent. Breast and target shape are the major determining factors in degradation of plan quality with respiration.
SU‐E‐T‐625: Dosimetric Dependence On Patient Size Reduction Between Prostate IMRT a1 nd VMAT2
Authors: Chow J , Jiang R
Published: July 2013

Purpose: This study compared the dosimetric impact between prostate intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) when patient weight loss occurs, which resulted in a reduction of external contour in the course of treatment. Methods: Six prostate patients were planned by the 7‐beam IMRT and VMAT technique using the same set of target and critical organ dose‐volume criteria and prescription dose per fraction (79 Gy per 39 fractions). Based on the original plan, doses in IMRT and VMAT plans were recalculated with the external contour of the patient reduced by 0.5–2 cm anteriorly and laterally to mimic the patient size reduction. Dose coverage and dose‐volume points of the targets and critical organs (rectum, bladder and femoral heads) were compared between IMRT and VMAT. Results: It is found in IMRT plans that increases of the D99% (target volume having 99% of the prescription dose) in PTV and CTV were 4.0 +/− 0.1% per cm of the reduced depth, which were higher than those in VMAT plans (2.7 +/−0.24% per cm). For increases of the D30% in rectum and bladder, 4.0 +/− 0.2% per cm and 3.5 +/− 0.5% per cm were found in IMRT plans, which were higher than those of VMAT (2.2 +/− 0.2% per cm and 2.0 +/−0.6% per cm), respectively. The increase of the D5% in right femoral head for IMRT (3.7 +/− 0.1% per cm) was also found higher than VMAT (3.3 +/− 0.1% per cm). Conclusion: For increases of dose‐volume criteria of critical organs in IMRT plans, it is concluded that VMAT would be preferred to IMRT in prostate radiotherapy, when a patient has potentials to have weight loss in the treatment.
SU‐E‐T‐627: An Optimal Treatment Planning Scheme for Breast IMRT
Authors: Yao W
Published: July 2013

Purpose: To ensure dose coverage at shallow depth and to reduce breathing effect during adjuvant radiotherapy for breast cancer patients, by using optimal treatment planning. Methods: When treating the breast or chest wall with tangential fields, low energy beams should be used as much as possible to ensure dose coverage to the target volume at shallow depth close to the skin, provided that the dose variation within the target volume is clinically acceptable. When using a hybrid plan, namely a plan that consists of open beams and IMRT segments, the weightings of open beams should be set as high as possible to minimize breathing effect in the skin region during IMRT delivery. Based on these two considerations, a treatment planning scheme is proposed to assign optimal weightings to low and high energy open beams as well as to IMRT segments. Delivery of the IMRT segments in the hybrid plans was verified by using a MapCheck device attached to a motion simulation table. Results: Compared with 3DCRT plans and non‐optimal hybrid plans, i.e. plans with non‐optimized weightings, the optimal hybrid plans demonstrated better dose uniformity, conformity and coverage of the target, especially at shallow depth close to the skin. As to the verification of the delivery of the IMRT segments in the hybrid plans, when breathing was shallow, the pass rate of gamma‐test (3%/3mm and 10% threshold) was greater than 95% for both optimal and non‐optimal plans; however, when breathing was deep, the pass rate was about 95% for optimal plans and 91% for non‐optimal plans. Conclusion: Our optimal hybrid planning can effectively improve the quality of treatment plans in terms of improving dose uniformity (including adequate dose coverage at shallow depth close to the skin and avoidance of cold/hot spots at beam junction) and minimizing breathing effect.
SU‐E‐T‐629: Initial Results of VMAT Re‐Planning for On‐Line Adaptive Radiotherapy
Authors: Jiang L , Kearney V , Zhong Z , Yordy J , Chen S , Nedzi L , Solberg T , Mao W
Published: July 2013

Purpose: The volumetric‐modulated arc therapy (VMAT) has been widely used due to its fast delivery and good dose confirmation to target. Adaptive radiation therapy becomes essential to address the anatomic changes during a course of radiotherapy. A fast algorithm to re‐plan treatment has been investigated to adapt the changes of targets and OARs. Methods: All studies were based on data of head/neck cancer patients treated by VMAT in our institution. These patients had second CT scans, contours, and treatment plans in the middle of the treatment course. CT image datasets, ROI structures, and arc beam apertures were exported from a commercial treatment planning system (Pinnacle v9.0, Philips Radiation Oncology Systems, Madison, WI). The beam apertures were adjusted based on volume change of targets and OARs volume contoured based on second CT scans. Every beam segment was loaded to Eclipse treatment plan system (Varian Medical Systems, Palo Alto, CA) to calculate dose distribution of individual beam separately. The original planning dosimetric restrictions were recalculated as individual cost functions and their weighted summation formed an overall cost function. A Limited‐memory Broyden‐Fletcher‐Goldfarb‐Shanno (L‐BFGS) algorithm was applied to optimize the weighting factor of every segment by minimizing the overall cost function. Results: Data of three patients were used. Three plans with 2 to 4 arcs (178 ∼ 256 beams) were re‐planned. Although 15 to 17 cost functions were involved, high quality of VMAT plans were obtained within a short time. Conclusions: This initial research demonstrates the feasibility of on‐line VMAT re‐planning for adaptive radiotherapy. Though this initial work focuses on optimization of arc beams weight, it can be intergraded with on‐board imaging and GPU calculating technique for online adaptive radiotherapy system which include automatic reconstruction of on‐board images, deformable image registration, dose evaluation, apertures adjustment, dose calculation, re‐optimization, and MLC sequencing. This research is supported by CPRIT Individual Investigator Award RP110329.
SU‐E‐T‐641: Incorporation of Non‐Uniform Segment Sampling Into Fluence‐Map Based VMAT Planning
Authors: Kim H , Li R , Xing L
Published: July 2013

Purpose: VMAT has been widely used due to enhanced delivery efficiency with high angular coverage. However, general VMAT optimized by single aperture at the control points cannot meet sufficient intensity modulation in some directions. To improve the plan quality over the limit, this work proposes to use the non‐uniform segments sampling for VMAT planning by fluence‐map based optimization approach. Methods: Reweighted L1‐minimization can optimize the fluence‐map for VMAT as it successfully eliminates unnecessary information by further simplifying the fluence‐map. To increase the connectivity of adjacent fluence‐maps for VMAT planning, this study considers the similarity of neighboring fluence‐maps by combining the simple penalized term with the reweighted L1‐minmization. Importantly, instead of taking one segment uniformly, the non‐uniform segments sampling that takes additional segment at certain directions is introduced to enhance the plan quality for VMAT plans. To choose the appropriate directions giving great benefits in plan quality from the additional segment, we sequentially quantified the respective cost at a specific field by adding extra segment to the field. The extra segments were assigned to 6 to 8 field directions with the lowest costs, which were redistributed into adjacent fields and linked by linear interpolation for VMAT delivery. Prostate patient data was employed to evaluate and compare the uniform/non‐uniform segment sampled VMAT in plan quality and treatment time. Results: Introducing the additional term into optimization effectively enhanced the similarity of adjacent fluence‐maps, which were used to form uniform/non‐uniform segments sampled VMAT plans. Significantly, non‐uniform sampling VMAT formed by assigning 8 segments to the 8 chosen fields has better dose conformity to the target (0.8245 to 0.8409) than uniform sampled VMAT plan at the small expense of the estimated treatment time (63s to 65s). Conclusion: Fluence‐map based VMAT planning incorporated by non‐uniform segments sampling can achieve efficient VMAT plan with enhanced plan quality.