DVH Comparison Research Articles

SU-E-T-385: Accelerated Beam Delivery with MLC Gaps in IMRT Fields.

By default, the 4-mm wide leaf-pairs of the Elekta Beam Modulator MLC not used for field shaping are moved to under the primary collimator to minimize the leakage dose. The trade-off is the increased beam delivery time in the case of IMRT. This study examines the dosimetric impact and improved delivery efficiency by allowing the unused leaf-pair as 2mm gap to stay within the PTV aperture during beam-on. The capability of the treatment planning system (Pinnacle) in modeling the central and off-axis dose through 2mm MLC gaps, respectively, was investigated. EBT film measurement performed in a solid water phantom was compared with the computed dose. For two representative IMRT cases, the dosimetric features of treatment plans generated with and without MLC gaps in the field were compared. Similarly, the delivery times for 13 IMRT cases with and without MLC gaps were compared. The peak dose of the leaf gap computed with Pinnacle was dependent on dose grid resolution. The difference between a 0.5mm and 2mm dose grid was about 16.5% for 6MV and 15MV beam respectively. Using a 0.5mm dose grid, the maximum difference between measured and calculated dose was < 1% for a central axis gap, and < 3% for an off-axis gap. The maximum dose through a 2mm×4mm leaf gap was estimated < 10cGy for a typical IMRT plan. DVH comparison showed an increase of target dose up to 2% for the IMRT plans with gap, with minimal dose change to all critical structures. For the 13 IMRT cases, the beam delivery times were reduced by 27+/-9.7%, from 8.7+/-4.5 minutes to 6.5+/-4.0 minutes. The beam delivery is accelerated by utilizing minimal gaps in the IMRT fields. The increased dose to the target is acceptable and is minimal to all critical structures.

An experimental feasibility study on the use of scattering foil free beams for modulated electron radiotherapy

The potential benefit of using scattering foil free beams for delivery of modulated electron radiotherapy is investigated in this work. Removal of the scattering foil from the beamline showed a measured bremsstrahlung tail dose reduction just beyond Rp by a factor of 12.2, 6.9, 7.4, 7.4 and 8.3 for 6, 9, 12, 16 and 20 MeV beams respectively for 2 × 2 cm2 fields defined on-axis when compared to the clinical beamline. Monte Carlo simulations were matched to measured data through careful tuning of source parameters and the modification of certain accelerator components beyond the manufacturer's specifications. An accelerator model based on the clinical beamline and one with the scattering foil removed were imported into a Monte Carlo-based treatment planning system (McGill Monte Carlo Treatment Planning). A treatment planning study was conducted on a test phantom consisting of a PTV and two distal organs at risk (OAR) by comparing a plan using the clinical beamline to a plan using a scattering foil free beamline. A DVH comparison revealed that for quasi-identical target coverage, the volume of each OAR receiving a given dose was reduced, thus reducing the dose deposited in healthy tissue.

Reirradiation of paraaortic lymph node metastasis by brachytherapy with hyaluronate injection via paravertebral approach: With DVH comparison to IMRT

PURPOSE/INTRODUCTION: To safely irradiate retroperitoneal targets as paraaortic lymph node by separating abdominal at-risk organs from the target during irradiation, we created a percutaneous paravertebral approach of high-dose-rate brachytherapy with hyaluronate gel injection (HGI). We report a case treated with this technique. We encountered a patient with symptomatic regrowth of paraaortic lymph node metastasis from prostatic cancer. He had previously received 58.4Gy of radiotherapy to the same region 12 months prior. Brachytherapy needles and a HGI needle were deployed via the paravertebral approach under local anesthesia at our outpatient clinic. A single dose of 22.5Gy (equivalent to 60.94Gy in 2Gy per fraction schedule calculated at α/β=10) was delivered to the target, with preservation of the surrounding small intestine by HGI with D(2cc) (minimum dose to the most irradiated volume of 2mL) of 5.05Gy. Therapeutic ratio was 3.64 times higher for this brachytherapy plan compared with an intensity-modulated radiation therapy plan. At followup at 1 year after brachytherapy, the symptoms had disappeared, tumor size had reduced with no fluorodeoxyglucose accumulation, and prostate-specific antigen level had decreased. We consider that high-dose-rate brachytherapy with the HGI procedure offers effective treatment even in this type of reirradiation situation.

An approach for online evaluations of dose consequences caused by small rotational setup errors in intracranial stereotactic radiation therapy

The purpose of this work is to investigate the impact of small rotational errors on the magnitudes and distributions of spatial dose variations for intracranial stereotactic radiotherapy (SRT) treatment setups, and to assess the feasibility of using the original dose map overlaid with rotated contours (ODMORC) method as a fast, online evaluation tool to estimate dose changes (using DVHs) to clinical target volumes (CTVs) and organs-at-risks (OARs) caused by small rotational setup errors. Fifteen intracranial SRT cases treated with either three-dimensional conformal radiation therapy (3DCRT) or intensity-modulated radiation therapy (IMRT) techniques were chosen for the study. Selected cases have a variety of anatomical dimensions and pathologies. Angles of ±3° and ±5° in all directions were selected to simulate the rotational errors. Dose variations in different regions of the brain, CTVs, and OARs were evaluated to illustrate the various spatial effects of dose differences before and after rotations. DVHs accounting for rotations that were recomputed by the treatment planning system (TPS) and those generated by the ODMORC method were compared. A framework of a fast algorithm for multicontour rotation implemented by ODMORC is introduced as well. The average values of relative dose variations between original dose and recomputed dose accounting for rotations were greater than 4.0% and 10.0% in absolute mean and in standard deviation, respectively, at the skull and adjacent regions for all cases. They were less than 1.0% and 2.5% in absolute mean and in standard deviation, respectively, for dose points 3 mm away from the skull. The results indicated that spatial dose to any part of the brain organs or tumors separated from the skull or head surface would be relatively stable before and after rotations. Statistical data of CTVs and OARs indicate the lens and cochleas have the large dose variations before and after rotations, whereas the remaining ROIs have insignificant dose differences. DVH comparisons suggest that the ODMORC method is able to estimate the DVH of CTVs fairly accurately (within 1.5% of relative dose differences for evaluation volumes). The results also show that most of the OARs including the brain stem, spinal cord, chiasm, hippocampuses, optic nerves, and retinas, which were relatively distal from the skull and surface, had good agreement (within 2.0% of relative dose differences for 0.1 cc of the volumes ) between the ODMORC method and the recomputation, whereas OARs more proximate to the bone-tissue interface or surface, such as the lenses and cochlea, had larger dose variations (greater than 5.0%) for some cases due to the incapability of the ODMORC to account for scatter contribution variations proximate to interfaces and intrinsic dose calculation uncertainties for ROIs with small volumes. The ODMORC method can be implemented as an online evaluation system for rotation-induced dose changes of CTVs and most OARs and for other related dose consequence analyses.

Visualization of a variety of possible dosimetric outcomes in radiation therapy using dose-volume histogram bands

PurposeDose-volume histograms (DVH) are the most common tool used in the appraisal of the quality of a clinical treatment plan. However, when delivery uncertainties are present, the DVH may not always accurately describe the dose distribution actually delivered to the patient. We present a method, based on DVH formalism, to visualize the variability in the expected dosimetric outcome of a treatment plan. MethodsFor a case of chordoma of the cervical spine, we compared 2 intensity modulated proton therapy plans. Treatment plan A was optimized based on dosimetric objectives alone (ie, desired target coverage, normal tissue tolerance). Plan B was created employing a published probabilistic optimization method that considered the uncertainties in patient setup and proton range in tissue. Dose distributions and DVH for both plans were calculated for the nominal delivery scenario, as well as for scenarios representing deviations from the nominal setup, and a systematic error in the estimate of range in tissue. The histograms from various scenarios were combined to create DVH bands to illustrate possible deviations from the nominal plan for the expected magnitude of setup and range errors. ResultsIn the nominal scenario, the DVH from plan A showed superior dose coverage, higher dose homogeneity within the target, and improved sparing of the adjacent critical structure. However, when the dose distributions and DVH from plans A and B were recalculated for different error scenarios (eg, proton range underestimation by 3 mm), the plan quality, reflected by DVH, deteriorated significantly for plan A, while plan B was only minimally affected. In the DVH-band representation, plan A produced wider bands, reflecting its higher vulnerability to delivery errors, and uncertainty in the dosimetric outcome. ConclusionsThe results illustrate that comparison of DVH for the nominal scenario alone does not provide any information about the relative sensitivity of dosimetric outcome to delivery uncertainties. Thus, such comparison may be misleading and may result in the selection of an inferior plan for delivery to a patient. A better-informed decision can be made if additional information about possible dosimetric variability is presented; for example, in the form of DVH bands.

SU‐E‐T‐690: Fast Monte Carlo Treatment Planning for Prostate Brachytherapy: A Comparison with VariSeed

Purpose: Treatment planning for prostate Brachytherapy with seeds uses mainly the TG43 algorithm for dose calculations. It doesnˈt take into account the interseed attenuations (known as the interseed effect) due to its complexity. A fast Monte Carlo dose calculation engine, taking into account the interseed effect, has been developed, using the MCNP5 code. Methods: Three MC techniques were used to calculate the dose distributions of the prostate implants: MCOSST, FMCT and MCOSSIT. The two last techniques take into account the interseed effect. IsoSeed (Bebig) seeds were chosen for this work. A 3D comparison has been performed to validate the new MC techniques for 8 patients. Dose distributions are compared on each ultrasound imaging slide. Moreover, a DVH comparison is also performed for some organs. Results: The interseed effect was investigated by comparing MCOSST with FMCT. It leads to the appearance of cold spots in the planar dose distribution which varies from 4% to 6%. It depends on number, position and composition of seeds. Moreover, the interseed effect becomes more important as the number of implanted seeds increases. We can especially notice it at the end of the strand and also behind set of seeds implanted each 5 mm in the same plane. DVH comparison between VariSeed and MC techniques was also performed. VariSeed overestimates the average dose deposited in the prostate by 3% up to 5%. MCOSSIT can be considered being a more accurate and faster (2–3 sec) calculation method in a near future as it taks into account the interseed effect and can also be applied on line Conclusions: MCOSSIT is a new calculation technique of dose distribution for prostate Brachytherapy that can be used for all types of seeds. All these advantages make from MCOSSIT very accurate and fast system for online permanent brachytherapy of the prostate.This work is submited by UCL University to get a patent.

SU-E-T-391: Is 1D Anisotropy Function Recommended by TG-43U1 Protocol a Good Candidate for Clinical Brachytherapy Dosimetry?

Purpose: The effect of the anisotropy on dose distributions for real permanent prostate implants with 125I seeds has been evaluated and compared with Monte Carlo (MC) calculations. Methods: soSeed (Bebig) was used. Dose distribution of the prostate implants was calculated using MCNP5 code and compared to TG43 calculation. A DVH comparison between 2D and 1D approximations was performed for prostate, urethra and rectum for 20 patients. Results: A DVHs comparison between the use of 1D & 2D anisotropy functions are presented for 20 prostates implants. For prostate, Dmean, D90 and D98 are underestimated by 1D approximation by about 3%, 1.5% and 1% respectively. It leads also to inconsistent results for Dmin and D100. 1D approximation overestimates the dose for about of 15% in planes situated at the two ends of implant containing no implanted seeds while it underestimates dose about 5% in the other planes. This yields a compensating effect in the resulting DVH's. These over dosage and under dosage must be attributed to an averaging effect between 2D and 1D anisotropy function. It can be noticed that the effect of 1D approximation on D98 and Dmin of prostate varies from patient to another. This is due to the many factors, for example, prostate volume, number of seeds and seeds positions relative to prostate…etc. D90 is always underestimated by 1D approximation where for 70% of patients D90 exceeds 185 Gy. For the urethra, 1D approximation underestimates the Dmax in which 60% of patients, their maximum dose for the urethra exceeds the limits. Finally, 1D approximation underestimates V100 about 50% les than 2D approximation Conclusions: The use of the 2D anisotropy function is preferred to the 1D anisotropy function as this approximation yields more consistent results for the 20 patients studied. The observed overestimation and the variability among the patients can influence dose—outcome.

SU‐E‐T‐827: The Impact of Artefact Contouring on the Optimization of Head and Neck Cancers Using IMRT

Purpose: IMRT planning of head and neck (H&amp;N) cancers routinely employs artefact contouring in the oral cavity due to dental restorations (DR). The differentiation between teeth and soft tissue in the region immediately surrounding these DR however is uncertain. A study was carried out on patients with varying number of DR/artefacts to determine the impact on optimization. Methods: Five H&amp;N patients who had been planned using IMRT were selected with varying PTV locations, degrees of DR and artefacts. These patients were originally planned with the artefacts surrounding the DR contoured and the density of these converted to water/soft tissue (HU=0). The DR were given a maximum CT number assigned for our CT scanner (HU∼2700). These patients were then re‐optimized (using identical constraints as in the original plan) without any compensation for artefacts. Optimizations were carried out in both scenarios using the Eclipse™ planning system Pencil Beam (PBC)8.6.15 and then carried out using the AAA algorithm with/without artefacts contouring as well. Results: In the cases where the PTV didnˈt overlap the teeth/DR, DVH as well as axial slice comparisons showed insignificant differences between contouring the artefacts or not. In those cases where the PTV did overlap the DR, a difference was seen between the two optimizations; with and without artefacts converted to water/soft tissue. For large PTV volumes, this may not translate into a significant effect based on DVH comparisons since the volume of DR artefact is relatively small compared to the PTV volume. However, for smaller PTV volumes, the impact could be quite significant. This effect was seen using both algorithms: PBC and AAA. Conclusion: For H&amp;N cancers with PTVs within the oral cavity, artefact contouring does have an impact on the optimization of IMRT plans and the proper contouring of DR artefacts can minimize the impact.

SU‐E‐T‐411: Validation of Plan Dose Perturbation Software for Use in Patient Specific IMRT Quality Assurance

Purpose: Patient‐specific IMRT quality assurance typically compares measured and calculated dose distributions in‐phantom. The clinical importance of disagreement between measured and calculated dose is often difficult to interpret. Recently, software has been developed (3DVH Sun Nuclear, Florida USA) that calculates the “delivered” dose distributions in patients by perturbing the calculated dose using errors detected in planar dose measurements. The aim of this work was to validate the accuracy of 3DVH versus commercial treatment planning software (Varian Eclipse 8.9). Methods: 17 DMLC prostate IMRT treatment plans were modified by Matlab script by adding randomly distributed positional errors of 1 or 3 mm in the planned MLC positions. These modified plans were delivered to a 2D diode array (MapCheck2, Sun Nuclear). Measured doses were compared to the dose planes from the original or recalculated MLC modified plans and 3DVH calculated a “delivered” dose in the patient. DVH comparisons were made based on mean dose and D99 for the PTV, and mean and maximum dose for the bladder and rectal for each patient and MLC error. Results: Paired Data showed good agreement for planning system and 3DVH dose volume histograms for both targets and organs at risk for both modified and unmodified plans, regardless of MLC error induced. Structure mean dose values as measured for PTV, Bladder and Rectum agreed to within 1% and dose maximum data showing a discrepancy of 2%. Conclusions: 3DVH is a potentially useful tool to calculate delivered dose distributions during patient specific IMRT quality assurance. The 3DVH data allows the user to analyze the difference between planned and delivered dose to given structures. By introducing random delivery errors, we showed that the 3DVH software was able to detect small fluence changes. This demonstrates the accuracy of 3DVH software against a commercial treatment planning system.

Dynamic tomotherapy delivery

Several dynamic techniques are introduced to speed up TomoTherapy delivery and improve longitudinal conformity. These techniques include dynamic jaws, dynamic couch, and their combinations. In general, dynamic jaws techniques allow jaws to move during a treatment. On the one hand, the jaws open wide to increase efficiency and thus reduce beam-on time. On the other hand, the jaws can close and follow the target border when sharp penumbra is required near the superior and inferior borders of tumor sites, which results in improved longitudinal dose conformity. The main purpose of the dynamic couch technique is to move the couch as fast as possible at variable speed to reduce beam-on time. Delivering most conformal dose as fast as possible requires a combination of dynamic jaws and dynamic couch techniques (DJDC). Motions of the jaws and couch are determined from the longitudinal fluence profile, which is calculated from an optimized leaf sinogram of small jaw width regular delivery or running start and stop delivery (RSS). We focused on RSS and DJDC in this study and also discussed other delivery techniques. Several conceptual cases are simulated to compare different delivery techniques. The results show that beam-on time can be reduced by about 60% compared to regular delivery with a 2.5 cm jaw width (REG 2.5 cm) for these cases and arbitrary longitudinal fluence profiles can be delivered. Two clinical cases, a prostate and a head-and-neck case, with different delivery techniques are calculated. The results show that plan quality yielded by DJDC with a maximum 5.0 cm jaw width is overall comparable to or better than that of the existing REG 2.5 cm. The DVH comparisons show better critical structure avoidance with the dynamic techniques. At the same time, beam-on time is reduced by about one half compared to REG 2.5 cm. Dynamic delivery techniques provide users more tools to speed up delivery and/or improve plan quality.

SU-GG-T-474: Feasibility Study of MVCT Imaging Guided Adaptive Proton Therapy for Head and Neck Cancers

Purpose: Extending MVCT's potential for daily patient position and anatomy verification, we investigated the feasibility of using MVCT to assess the daily delivered dose for head and neck patients treated with proton beams. Method and Materials: A Rando head phantom was scanned on a Philips Brilliance Big Bore CT scanner to obtain the planning kVCT images. The corresponding MVCT images were acquired on a helical Tomotherapy unit. To establish CT-electron density curves, a Gammex RMI 465 electron density phantom was scanned on both kVCT and MVCT scanners. A typical 4-field plan was created for the phantom and dose distribution was calculated using pencil beam algorithm from the XIO treatment planning system. After MVCT and kVCT images manually registered, kVCT-based treatment plan with structure contours and beam configuration including beam shaping devices was mapped to MVCT images for dose re-computation. Dose map, isodose curves, dose profiles, gamma maps, and dose-volume histograms were used for dose comparison. Results: Although the noise level in MVCT images is higher than that with the comparabl kVCT the dose distribution computed on MVCT image is comparable to the dose distribution computed on the kVCT images. The dose comparison showed an agreement of higher than 97% between the two plans given the criteria of 3% dose difference and 3 mm distance to agreement. The biggest dose difference between two plans occurred around the periphery of the target. There was negligible difference in the DVH comparison. Conclusions: MVCT images acquired for daily setup verification in image-guided proton therapy are feasible for assessment of daily delivered proton dose distributions with accuracy comparable to that of kVCT-based dose calculation. The original treatment plan based on kVCT can be transferred to the daily MVCT image set to evaluate the actual dose distribution for adaptive proton therapy.

SU-GG-T-138: Dosimetric Benefits of Non-Coplanar Arcs in VMAT Delivery for Intracranial Targets

Purpose: VMAT is an arc‐based IMRT technique delivered using conventional linear accelerator. The current application of VMAT has been focused on axial coplanar delivery. However, for certain treatment sites, especially some intracranial targets, non‐coplanar arcs may bring extra dosimetric benefits. This work was carried out to study the non‐coplanar VMAT delivery in terms of both plan quality and delivery efficiency. Materials and Methods: Five GBM cases were selected for this study. For each case, VMAT plans using one axial coplanar arc and multiple non‐coplanar arcs were generated using either Pinnacle3 SmartArc® or a home‐grown arc sequencer. DVH comparisons were made between the two sets of plans. VMAT deliveries using Elekta's PreciseBeam® VMAT control system were performed for each case to compare the delivery efficiency. Results: Both coplanar and non‐coplanar VMAT plans have very similar target dose coverage. For all five cases, the average V95, defined as the target volume receiving at least 95% of the prescribed dose, remained unchanged at 99.8%. As to the critical structure sparing, however, the non‐coplanar plans show significant improvement. For example, on average, the maximum and mean doses to brainstem were reduced from 3277 and 1016 cGy to 2524 and 668 cGy in the non‐coplanar plans. These correspond to 23% and 34% reduction. The average maximum dose to chiasm was also reduced from 876 cGy to 346 cGy in the non‐coplanar plan, a 61% reduction. The average treatment delivery time increased from 1.9 to 4.0 minutes for the non‐coplanar plans. Conclusions: Comparing to the axial coplanar delivery, the VMAT plans using non‐coplanar arcs can significantly improve the critical structure sparing while maintaining the target dose coverage and uniformity. However, these dosimetric benefits come at the cost of a prolonged treatment delivery time. This work was sponsored in part through a grant from Elekta.

SU-GG-T-21: Dosimetry Benefits of Online Adaptive Replanning for Whole Breast Irradiation

Purpose: Interfraction breast shape change (deformation) can be substantial (e.g., large or pendulous breasts in supine position), which, in turn, can degrade dose uniformity in breast. This change cannot be fully corrected for by patient repositioning. We have previously developed an online adaptive replanning technique to address interfraction variations. This work aims to demonstrate that this online replanning scheme can effectively account for the daily variation including setup errors and anatomic changes in whole breast irradiation (WBI). Method and Materials: Daily CT scans collected during IGRT for representative breast cancer patients treated in supine position using a CT-on-Rails (CTVision, Siemens) were analyzed. For each patient, a WBI IMRT plan with tangential 6MV beams was generated based on the planning CT using a planning system (Prowess Inc.). An adaptive plan was generated based on each daily CT using the system (RealART, Prowess Inc) with the online replanning algorithms implemented. For comparison, the plan (repositioning plan) with the patient repositioned based on the registration between the daily CT and the planning CT was reconstructed by copying the original plan over to the daily CT with shifts considered. A total of 20 adaptive plans and 20 repositioning plans were generated and compared. Results: DVH comparison shows that the adaptive plans, which are similar to the original plans, are generally superior than the repositioning plan in term of target coverage and/or critical structure sparing. Repositioning plans suffer from reduced target coverage. For example, values of V45Gy (breast volume covered by 45 Gy) and V50Gy are reduced by 10% and 6% respectively, for repositioning plans, while no changes in these values for adaptive plans. Conclusion: Interfractional variations in whole breast irradiation cause dosimetry degradation which cannot be accounted for by patient repositioning, but can be corrected by the online adaptive replanning.

SU-GG-T-119: 124 Patients Treatment Plans Evaluation Experience between Tomotherapy Helical, Eclipse Rapid-Arc and Static IMRT

Purpose: Advances in radiotherapy external beam equipments has evolved to such a degree, it is difficult to compare quality, delivery efficiency, and accuracy of each treatment scheme, without considering the skill of the operators. With multiple systems in a new radiation oncology department, the authors did exhaustive trials on each of the treatment methods on every eligible IMRT patient for comparison. Method and Material: The physics team of two persons with over 50 years combined treatment planning experience, evaluated 124 IMRT treatment plans between Tomotherapy helical system, Varian Eclipse 3D-conformal-sliding-window static IMRT, Rapid Arc, single and multiple arcs. Plans are based on identical CTV, PTV, Organ of Interest constraints. Multiple trials were done to determine the best plan which is normalized identically for DVH comparisons. The plans were compared for: efficiency/ease of planning including operator interaction time, computation time; Plan quality including conformality, homogeneity and organ protection; dose delivery accuracy including absolute point dose, and 2D relative dose matching using MATRIXX 2D-ion-chamber array; delivery efficiency including setup plus treatment time. Results: There was no single “winner” when all parameters were taken into account. Tomotherapy planning took less interactive-time, required knowledge and skill in choosing beam-width, pitch, constraints and other parameters. Batch beam-let calculation saves time. Eclipse planning required more interactive time, allowed progressive improvements by trial and error up to a limit. Tomotherapy is straight-forward and less likely to cause treatment error. Varian system allowed flexibility. Rapid arc took less time to deliver, CBCT was not as straight-forward. Both system were capable to achieve clinical tolerance of 5% and 3 mm. Conclusion: This multivariable comparison gave a realistic view. Final choice depended on clinical preference, region of interest, ease of delivery, error-free treatments and end results expected. Each system had it's own merits for a given clinical condition.

A New Hierarchical Symmetric Optical Flow-based Image Registration Method and its Clinical Evaluation

To determine clinical applicability for radiation treatment planning of a new symmetric optical flow image registration method that allows for better handling of larger displacements and deformations. The classical variational optical flow method and the resultant field of the velocities is based on a gradient fields extracted from a template image. We have extended this model through an addition of an extra virtual image incorporated into the model as a computational construct half-way along displacements. Thus a gradient field of a scene is incorporated into the algorithm. The computation is built upon the multi-resolution approach based on coarse-to-fine computation with spline pyramids. We explored the sensitivity of the calculation to gradient evaluation which is critical for this approach. Three methods were used: (1) finite difference method, (2) B-spline differentiation and (3) paired/matched multi-point filters. Registered images are obtained with the backward reconstruction. The software program was implemented with C-programming language. CT scans of thorax used for treatment planning of lung cancer patients were employed for this study. The accuracy of the method was estimated at 1.5 pixel. The worst results were obtained with finite difference method of gradient estimation. The multi-point spatial and temporal gradient filters and B-spline differentiation are qualitatively more robust, however they are also more time consuming methods. The registration was used to propagate contours along 3D-CT volumes and model the thoracic tumor motion. This segmentation results differed from human delineated contours up to few pixels. The majority of the generated contours were gauged acceptable using human visual judgment criterion. The structures' DVH comparison confirmed their congruity. However, a human validation is still required after the registration. The tumor motion modeling is less sensitive to minor local discrepancies. The evaluated modified optical flow image registration shows promise in handling larger deformations of anatomical volumes as those due to respiration for lung cancer patients. This new modification enhances a classical multi-resolution technique that is naturally incorporated in our fusion scheme. This is a unimodal non-rigid registration that relies on the image intensity conservation. It can be applied to 2D, 3D and 4D images.

SU‐GG‐T‐443: Effect of Inhomogeneity On Dose Distribution for Gamma Knife(R) Perfexion(TM) Treatment

Purpose: To investigate the dose perturbation in Gamma Knife® Perfexion™ Stereotactic Radiosurgery due to inhomogeneity. Method and Materials: We applied Monte Carlo (MC) simulation for Gamma Knife® Perfexion™. Approximation parameters for geometries and materials were used to reconstruct 4mm, 8mm and 16mm collimators in Gamma Knife® Perfexion™ unit. A total of 192 Cobalt‐60 sources with the same activity were simulated. Phase space files stored all particles passing through collimator and reaching a sphere surface of surrounding the isocenter. Patient's geometries were rebuilt from CT data. The doses were calculated using MC simulation with and without inhomogeneity correction. Dose perturbation effect was derived from the comparison of DVH between homogeneous and inhomogeneous geometries. Results: Three patients were investigated. For each patient, at least 2 sites were selected: one in brain and another one close to air cavity. A total of 14 Monte Carlo simulations were performed for 16mm collimator with and without inhomogeneity correction. The statistical error for isocenter dose is around 0.2%. Monte Carlo results show that inhomogeneity effects lowered the dose, on average, by 4.6% in brain site. For sites close to the air cavity, the inhomogeneity effects are mixed. The inhomogeneity effect ranged from 2.7% to −1.3%. Less attenuation from air cavity increases the dose to the site. However, the high density area such as spine results more attenuation and it eventually reduces the dose. Conclusion: Monte Carlo shows that the inhomogeneity effect could cause lower delivered doses from Gamma Knife® Perfexion™ in brain sites and higher or lower doses in those sites close to air cavity. More investigations are necessary to determine accurate dose for treatment sites close to air cavity for Gamma Knife® Perfexion™.

SU‐GG‐T‐529: Pre‐Clinical Evaluation of a 4D Tracking DmMLC‐Based Delivery to Lung Tumor

Purpose: The aim of this study is to provide a pre‐clinical evaluation of four‐dimensional tracking radiation therapy to lung tumor using a prototype tracking system. The evaluation was based on films dosimetric analysis, time delay measurements and treatment planning DVH analysis by using two types of dynamic phantoms. Method and Materials: The key component of the 4DTRT system was a prototype of TrackBeam. It consists of an image processing tools and a first‐of‐its‐kind dual‐layer micro MLC. DmMLC has two layers of orthogonal leaves which provide advantages in speed and conformality when forming beam aperture for tracking. The TrackBeam was mounted to a Varian Linac and connected to a workstation which process the online MV fluence and controls each leaf's motion. A Quasar dynamic phantom was used for radiographic film irradiation with 4DTRT and also 3DCRT. The phantom has a Gafchromic film insert and a gold marker in the insert. It can move in Sinusoid mode as well as real patient respiratory cycle. Another tissue‐equivalent thorax dynamic phantom was used for DVH analysis after a phantom‐based 3DCRT planning and a 4DTRT planning developed respectively. Results: The synchronization of marker motion and the DmMLC leaf motion was achieved within less than 0.05 seconds. The films analysis indicated that total 29.91% over the tolerance of 5% and 5.09% of over the tolerance of 5% when the 3DCRT and 4DTRT films compared to a static film. The DVH comparisons indicate 4DTRT reduces significant dose to the Ring from 75% (80% volume) to 60% (50% volume). 4DTRT also reduces considerable amount dose to the total lung from 33% (30% volume) to 22% (22%). Conclusion: The 4D tracking using MLC provides a feasible solution delivering conformal dose to lung tumor and spare the surrounding tissue. Conflict of Interest: The work was partially supported by Initia‐RT medical device.

SU-GG-T-198: Measurements and Evaluation of Doses Outside the Treatment Volume From Photon External Beam Radiation Therapy

Purpose: There has been increased concern about the amount of dose delivered outside the treatment volume from advanced radiation therapy techniques. It has been suggested that these treatments, such as IMRT, IGRT, and SBRT, present a potential impact on the induction of second malignancies due to a larger scattered and leakage radiation. Children have much higher risks to develop a radiation-induced cancer from radiotherapy. In this study, doses outside the treatment volume calculated from the Eclipse planning system were evaluated and compared with measurements from MOSFET and TL dosimeters. An anthropomorphic phantom and a 3-D polymer gel dosimeter were employed for measurements. Method and Materials: The “ATOM” anthropomorphic phantom was CT- scanned into the Eclipse-Helios system. An IMRT prostate plan was designed for the ATOM phantom. Both MOSFET detectors and TLD were calibrated based on ion chamber dosimetry. The MOSFET and TLD were precisely placed in positions corresponding to various internal organs, allowing point-dose measurements and comparison. BANG® polymer gel, prepared in a cylindrical container with a volume similar to that of a 10-year child, was placed next to the phantom head for 3D dose distribution measurement in a brain IMRT. The DVH in the gel cylinder, analyzed with an optical CT scanner, was compared with that from the planning system. Results: Our results show that the agreement between the MOSFET measurements and the calculated results are within 5% for points within the target. At medium-dose regions (2–50%), discrepancies are within 15%. At low dose regions (<2%) the measured results from MOSFET and TLD agree well, and can be 2–3 times larger than the planning results. DVH comparison between gels and the treatment planning will be presented. Conclusion: Anthropomorphic phantom with MOSFET and TL detectors as well as polymer gels can provide evaluation of doses outside the treatment volume.

TH-D-AUD B-01: Comparison of Single-Arc and Multiple-Arc Approaches for Delivering of Volumetric Modulated Arc Therapy

Purpose:Linear accelerator vendors have recently introduced deliverycontrol systems with the ability to deliver Volumetric Modulated Arc Therapy (VMAT). These systems can deliver VMAT using either a single‐arc or a multi‐arc approach. The goal of this study is to compare single‐arc and multi‐arc delivery techniques in terms of both plan quality and delivery efficiency. Method and Materials: Two prostate cases and three head‐and‐neck cases were included in this study. For each case, single‐arc and three‐arc VMAT plans were generated using our home‐grown arc sequencing algorithm that converts optimized fluence maps into deliverable arcs. Each VMAT plan was evaluated using Pinnacle's superposition dose calculation, and DVH comparisons were made between the two sets of VMAT plans. VMAT deliveries using Elekta's PreciseBeam Infinity™ control system were performed for each patient to compare the delivery efficiency. Results: For the two prostate cases, the single‐arc and three‐arc VMAT plans resulted in similar target dose coverage and organ at risk (OAR) sparing. For the three head‐and‐neck cases, the three‐arc plans provided improved target dose coverage as compared with the single‐arc plans with the average standard deviation in the target dose reduced from 7.06 to 5.82 cGy. The target volume covered by 95% of the prescribed dose also increased from 97.5% to 98.5% for the three‐arc VMAT plans. For single‐arc VMAT delivery, the average delivery time was approximately 3.2 minutes for prostate cases and 5 minutes for head‐&‐neck cases. While for three‐arc VMAT delivery, these values increased to 4.5 and 5.6 minutes. Conclusion: For simple cases such as prostate, single arc delivery provides comparable plan quality and improved delivery efficiency. However, for more complex cases, multi‐arc VMAT plans are preferable due to the ability to achieve improved dose conformity while maintaining acceptable treatment times. This work was sponsored in part through a grant from Elekta.

SU‐GG‐T‐137: Patient Based IMRT QA Using Fluence Map Measurements

Purpose: Traditionally IMRT QA involves measuring dose maps and comparing to the planning system using a gamma analysis. This methodology has been successful, but removes the link between the measured dose maps and DVHs relating to the patient used to define a clinically acceptable plan. A system containing a feedback loop for the treatment planning system and evaluating the dose distribution based on measured dose maps has been developed for H&amp;N IMRT. Method and Materials: After an IMRT plan has been created, dose maps measured by film at Dmax and 0° Gantry are measured for each IMRT beam. The dose maps are converted to fluence maps by deconvolving the dose map with a Gaussian point spread function approximating phantom scatter present in the measurement geometry. The derived fluence maps are then re‐introduced into the Philips Pinnacle treatment planning system and the dose re‐calculated to provide a dose distribution based on the patient anatomy and the deliverable fluence from each beam. Once re‐calculated, DVH comparisons and dose statistics can be used to assess the deliverable plan. Results: The methodology is currently in the testing phase for H&amp;N IMRT plans. The feedback system has provided a comparison of the planned and deliverable dose distributions for 5 IMRT plans so far. The methodology has shown some differences in doses delivered to the nodal regions, and small differences to organs‐at‐risk such as the spinal cord and contra‐lateral parotid. To date, no clinically significant differences in dose have been found by the system providing confidence in our H&amp;N IMRT programme. Conclusion: The introduction of an IMRT QA system based on delivered fluence maps to establish the delivered dose distribution to the patient has been developed. The system allows the IMRT QA to be assessed based on clinical tolerances to the individual patient.