Differences In Dose Distributions Research Articles

Sci-Sat AM(2): Brachy-05: Dosimetry effects of the TG-43 approximations for two iodine seeds in LDR brachytherapy.

This work consists of studying the interseed and tissue composition effects for two model iodine seeds: the IBt Interseed-125 and the 6711 model seed. Three seeds were modeled with the MCNP MC code in a water sphere to evaluate the interseed effect. The dose calculated at different distances from the centre was compared to the dose summed when the seeds were simulated separately. The tissue composition effect was studied calculating the radial dose function for different tissues. Before carrying out post-implant studies, the absolute dose calculated by MC was compared to experiment results: with LiF TLDs in an acrylic breast phantom and with an EBT Gafchromic film placed in a water tank. Afterwards, the TG-43 approximation effects were studied for a prostate and breast post-implant. The interseed effect study shows that this effect is more important for model 6711 (15%) than for IBt (10%) due to the silver rod in 6711. For both seed models the variations of the radial dose function as a function of the tissue composition are quasi similar. The absolute dose comparisons between MC calculations and experiments give good agreement (inferior to 3% in general). For the prostate and breast post-implant studies, a 10% difference between MC calculations and the TG-43 is found for both models of seeds. This study shows that the differences in dose distributions between TG43 and MC are quite similar for the two models of seeds and are about 10% for the studied post-implant treatments.

SU‐GG‐T‐523: Benchmarking of An MMLC for Intensity‐Modulated Proton Radiotherapy, with Emphasis On Secondary Neutron Dose

Purpose. We discuss the results of a feasibility study for multileaf collimator driven intensity modulated proton radiotherapy. The main focus is directed towards secondary neutron dose when using a miniature multileaf collimator. We also present a dosimetric comparison with conformal brass apertures. Method and Materials. Mechanical properties of an Integra Radionics MMLC were tested. The dosimetry of the MMLC in a fixed intra‐cranial proton beamline was assessed by comparing depth and lateral dose profiles, as well as the conformity of 2D dose distributions. Neutron dose to the patient is a consequence of protons impinging on beam modifying devices, such as scatterers and collimators. Neutron dose was measured for both brass apertures and MMLC shaping at different distances from the aperture edge for varying proton energies, airgaps and field sizes. Activation of the MMLC leaves was quantified. Results. Mechanical reproducibility and centering of the MMLC leaves are excellent. As expected we observe differences in dose distributions shaped by brass aperture and MMLC with regards to lateral fall‐off, conformity and the field size dependence of the Bragg peak‐to‐entrance ratio. These differences are clinically negligible. Neutron dose depends strongly on particle energy, airgap, distance from isocenter and collimator material. It is increased for the tungsten MMLC, but the required increased airgap reduces the difference between MMLC and brass aperture. Because of the design of our intra‐cranial fixed beamline the neutron dose is a factor of 10 to 20 smaller than in our passive‐scattered IBA gantry system. Conclusion. Extensive measurements proved that secondary neutrons do not prevent the implementation of intensity modulated proton therapy in our fixed beamline. Our results will furthermore be used as basis for the development of an MMLC dedicated for proton therapy.

SU-GG-T-402: Model to Account for Organ Deformation During Respiration Using the Power Law Dependence of Biological Outcome On Volume (mass)

Purpose: To investigate extending a model of biological outcome across the respiratory cycle to encompass the differences of dose distribution and tissue deformation at different respiratory phases during treatment delivery. Method and Materials: This problem is addressed using the equivalent uniform dose (EUD), calculated w.r.t. mass and volume in each voxel at different phases. An organ of interest is modeled as existing in two phases such that phase 1 has two and phase 2 has four voxels. The volume of each calculation voxel is taken to be a constant and total volume of the organ to change from one phase to the other due to deformation. Mass of each voxel changes within and in between the phases, whereas the total mass of the organ which is essentially the mass of cells within the organ, remains unchanged. The EUD and normal tissue complication probability (NTCP) in Lyman‐Kutcher‐Burman (LKB) model is calculated based on power law w.r.t mass and volume of voxels. Results: As the mass and dose of the voxels differ over time, meff, the effective mass irradiated to a reference dose (dref) at each phase differs and hence Meff, the total effective mass also varies. Dose‐dependent differences were observed in EUD calculated w.r.t. mass and volume. For dose variations analogous to that of a target, EUD calculated with volume and mass shows differences throughout the range of the volume‐dependence parameter, a (=1/n). Conclusion: This work potentially takes dose distribution in all treated respiratory phases by considering fixed calculation voxels instead of a single phase or an ITV concept. EUD for a deforming organ is dependent on variations in the dose distribution across different respiratory phases. Dose distributions will be analyzed from treatment plans in phantom and patients using the proposed model in subsequent work.

Comparison of dose contribution to normal pelvic tissues among conventional, conformal and intensity-modulated radiotherapy techniques in prostate cancer

High-energy external radiotherapy has become one of the most common treatment in localized prostate cancer. We compared the difference of dose distribution, mainly at the 5–30 Gy dose level, in the irradiated pelvic volume among three modalities of radiotherapy for patients with prostate cancer: conventional, conformal and intensity-modulated radiotherapy (IMRT). We selected six patients with prostate cancer treated by conformal radiotherapy at the doses of 46 Gy to PTVN (prostate and seminal vesicles), and 70 Gy to PTV-T (prostate). The conventional technique": an 8-field arrangement was used; the conformal technique 4 fields with a boost through 6 fields. For IMRT, a five-beam arrangement was used. Dose-volume histograms (DVH) were analyzed and compared among the three techniques. The IMRT technique significantly increased the pelvic volume covered by the isodose surfaces below 15 Gy as compared with the conventional and conformal techniques. The mean absolute increase for the pelvic volume included between 5–30 Gy for the IMRT technique, was about 2 900 ml as compared with the conventional technique. However, IMRT significantly reduced the irradiated volume of the rectum in the dose range of 5 to 40 Gy, also significantly reduced the irradiated volume of bladder and femoral heads, and obtained a similar or improved isodose distribution in the PTVs. In addition, the use of IMRT slightly increased the relative dose delivered to the body volume outside the pelvis, as estimated by the use of specific software. A long-term follow-up will be needed to evaluate potential late treatment complications related to the use of IMRT and the low or moderate irradiation dose level obtained in the pelvis and in the whole body.

Target motion tracking with a scanned particle beam

Treatment of moving targets with scanned particle beams results in local over- and under-dosage due to interplay of beam and target motion. To mitigate the impact of respiratory motion, a motion tracking system has been developed and integrated in the therapy control system at Gesellschaft für Schwerionenforschung. The system adapts pencil beam positions as well as the beam energy according to target motion to irradiate the planned position. Motion compensation performance of the tracking system was assessed by measurements with radiographic films and a 3D array of 24 ionization chambers. Measurements were performed for stationary detectors and moving detectors using the tracking system. Film measurements showed comparable homogeneity inside the target area. Relative differences of 3D dose distributions within the target volume were 1 +/- 2% with a maximum of 4%. Dose gradients and dose to surrounding areas were in good agreement. The motion tracking system successfully preserved dose distributions delivered to moving targets and maintained target conformity.

Accelerated partial breast irradiation

Breast-conserving surgery followed by whole-breast radiotherapy (WBRT) has become the standard treatment for the majority of patients with early breast cancer. Whereas the indications for systemic adjuvant treatment have continuously expanded, there is a tendency to restrict postoperative radiotherapy to accelerated partial breast irradiation (APBI) instead of WBRT. The different techniques of APBI are described and their respective advantages or potential drawbacks outlined. Moreover, the results described in the literature are briefly reviewed as a basis for the consensus statements and recommendations of the German Society of Radiation Oncology, the German Society of Senology, and the Working Group for Gynecological Oncology of the German Cancer Society. The methods mainly used for APBI are: interstitial radiotherapy with multicatheter technique, intraoperative radiotherapy (IORT) using either electrons produced by linear accelerators or 50 kV x-rays (Intrabeam), the balloon-catheter technique (MammoSite), or 3D conformal external beam radiotherapy. These techniques have marked differences in dose distribution and homogeneity. The published range of local recurrence rates varies between 0% to 37%, the median follow-up from 8 to 72 months. To date, follow-up times mostly do not yet permit a definite judgment concerning the long-term effectiveness and side effects of APBI. The relevant societies in Germany support randomized clinical studies comparing APBI with WBRT in a well-defined subset of low-risk patients. However, the authors expressly discourage the routine use of APBI outside clinical trials. Until definite results show that APBI neither impairs therapeutic outcome nor cosmetic results, WBRT remains the gold standard in the treatment of early breast cancer.

Tomotherapy for prostate adenocarcinoma: A report on acute toxicity

Background and purpose To analyze the impact of Tomotherapy (TOMO) intensity modulated radiotherapy (IMRT) on acute gastrointestinal (GI) and genitourinary (GU) toxicity in prostate cancer. Materials and methods The records of 55 consecutively treated TOMO patients were reviewed. Additionally a well-matched group of 43 patients treated with LINAC-based step and shoot IMRT (LINAC) was identified. Acute toxicity was scored according to Radiation Therapy Oncology Group acute toxicity criterion. Results The grade 2–3 acute GU toxicity rates for the TOMO vs. LINAC groups were 51% vs. 28% ( p = 0.001). Acute grade 2 GI toxicity was 25% vs. 40% ( p = 0.024), with no grade 3 GI toxicity in either group. In univariate analysis, androgen deprivation, prostate volume, pre-treatment urinary toxicity, and prostate dose homogeneity correlated with acute GI and GU toxicity. With multivariate analysis use of Tomotherapy, median bladder dose and bladder dose homogeneity remained significantly correlated with GU toxicity. Conclusions Acute GI toxicity for prostate cancer is improved with Tomotherapy at a cost of increased acute GU toxicity possibly due to differences in bladder and prostate dose distribution.

TU‐D‐AUD‐08: Monte Carlo Kernel‐Based Convolution‐Superposition Dose Calculation for Intensity‐Modulated Arc Therapy (IMAT)

Purpose: Dose calculations for intensity‐modulated arc therapy (IMAT) have traditionally been performed by approximating continuous arcs with multiple static beams. Not only that the calculated dose may not truthfully represent the delivered dose, but also the large number of beams used to approximate an arc requires very long calculation time. We propose to use our homegrown Monte Carlo (MC)/superposition dose calculation algorithm to compute doses for rotational delivery. Method and Materials: We first generate static IMRT plans with Pinnacle3 P3 IMRT module utilizing 36 equi‐spaced beams. The resulting intensity maps are inputted to the continuous intensity map optimization (CIMO) IMAT leaf sequencer. CIMO outputs deliverable arcs, each represented by aperture shapes spaced at 10‐degree intervals. The apertures are then interpolated at 1‐degree intervals. Dose calculations for both 36‐field plan and 351‐field plan (which closely represents the actual delivery) are performed using our homegrown MC‐based dose calculator. The time needed for calculating dose for the two plans are compared and the differences in dose distributions are also examined. Results and Conclusion: Four IMAT plans, a prostate, a head and neck, an orbit, and a brain, were selected for the study. Using history‐based sampling, the dose computation time only increased slightly for the 351‐beam plans. Each case took approximately 30 minutes to compute on a single CPU running Mac OSX. No significant differences in plan quality between the 36‐beam and the 351‐beam plans were observed. However, finger‐like artifacts at the lower isodose levels became evident when using 36 beams to approximate an arc. The dose calculation time using our MC‐based dose calculator is largely independent of the number of beam portals. For the cases studied, using 36 beams to approximate an arc delivery is sufficient, but more accurate calculation with finer angular spacing is achievable without a significant increase in computation time.

Collapsed cone and analytical anisotropic algorithm dose calculations compared to VMC++ Monte Carlo simulations in clinical cases

The purpose of this work was to study and quantify the differences in dose distributions computed with some of the newest dose calculation algorithms available in commercial planning systems. The study was done for clinical cases where large density inhomogeneities were present. Three dose algorithms were used: a pencil beam like algorithm, the anisotropic analytic algorithm (AAA), a convolution superposition algorithm, collapsed cone convolution (CCC) and a Monte Carlo program, voxel Monte Carlo (VMC++). The dose calculation algorithms were compared under static field irradiations at 6 MV and 15 MV using multileaf collimators and hard wedges where necessary. Five clinical cases were studied: three lung and two breast cases. We found that the CCC algorithm performed overall better than AAA compared to VMC++, but AAA remains an attractive option for routine use in the clinic due to its short computation times. Dose differences between the different algorithms for the median value of the planning target volume (PTV) were typically 0.4% (range: 0.0–1.4%) in the lung and −1.3% (range: −2.1–−0.6%) in the breast for the few cases we analysed. As expected, PTV coverage and dose homogeneity turned out to be more critical in the lung than in the breast cases with respect to the accuracy of the dose calculation. This was observed in the dose volume histograms obtained from the Monte Carlo simulations.

SU-FF-T-128: Compatibility of IMRT Plans for the Matched Beam Linear Accelerators

Purpose: To investigate the compatibility of IMRT plans for closely matched beam linacs in terms of patient‐specific quality assurance results. We are interested to know if there is a systematic passing rate difference between IMRT QA results when planned and delivered on the same machine and when planned and delivered on different machines. Method and Materials: An Elekta Precise (L5) and Elekta Synergy (L6) linacs with closely matched 6 MV beam data were commissioned in Pinnacle3 (version 7.6c) for planning of IMRT. A total of 56 fields were planned on L5 and 40 fields were planned on L6. Planar dose distributions of all the fields were measured with a MapCHECK 2D diode array. Comparison between the planned and measured dose distributions were done using both the 3%/3mm and 2%/2mm criteria in the absolute dose comparison mode. Results: When planned and delivered on L5, the average passing rates were 98.3±1.4 % with 3mm/3% criteria and 93.2 ± 3.1 % with 2mm/2% criteria. The same plan delivered on L6 had the average passing rates of 97.0 ± 2.5 % with 3mm/3% criteria and 88.6 ± 5.0 % with 2mm/2% criteria. For the plans planned and delivered on L6, the average passing rates were 96.2 ± 3.5 % with 3mm/3% criteria and 88.5 ± 6.2 % with 2mm/2% criteria. When the same plans were delivered on L5, the average passing rates were 97.9 ± 1.8 % with 3mm/3% criteria and 92.1 ±4.0 % with 2mm/2% criteria. The average passing rates were about 98.1% on L5 and 96.6% on L6 with 3%/3mm criteria regardless of which beam data were used for the planning. Conclusion: No systematic differences were observed in the currently adopted QA criteria. More planning studies are needed to investigate the differences in dose distribution and DVH for typical clinical cases.

Collapsed cone convolution and analytical anisotropic algorithm dose calculations compared to VMC++ Monte Carlo simulations in clinical cases

The purpose of this work was to study and quantify the differences in dose distributions computed with some of the newest dose calculation algorithms available in commercial planning systems. The study was done for clinical cases originally calculated with pencil beam convolution (PBC) where large density inhomogeneities were present. Three other dose algorithms were used: a pencil beam like algorithm, the anisotropic analytic algorithm (AAA), a convolution superposition algorithm, collapsed cone convolution (CCC), and a Monte Carlo program, voxel Monte Carlo (VMC++). The dose calculation algorithms were compared under static field irradiations at 6 MV and 15 MV using multileaf collimators and hard wedges where necessary. Five clinical cases were studied: three lung and two breast cases. We found that, in terms of accuracy, the CCC algorithm performed better overall than AAA compared to VMC++, but AAA remains an attractive option for routine use in the clinic due to its short computation times. Dose differences between the different algorithms and VMC++ for the median value of the planning target volume (PTV) were typically 0.4% (range: 0.0 to 1.4%) in the lung and −1.3% (range: −2.1 to −0.6%) in the breast for the few cases we analysed. As expected, PTV coverage and dose homogeneity turned out to be more critical in the lung than in the breast cases with respect to the accuracy of the dose calculation. This was observed in the dose volume histograms obtained from the Monte Carlo simulations.

Optimization of extracranial stereotactic radiation therapy of small lung lesions using accurate dose calculation algorithms.

BackgroundThe aim of this study was to compare and to validate different dose calculation algorithms for the use in radiation therapy of small lung lesions and to optimize the treatment planning using accurate dose calculation algorithms.MethodsA 9-field conformal treatment plan was generated on an inhomogeneous phantom with lung mimics and a soft tissue equivalent insert, mimicking a lung tumor. The dose distribution was calculated with the Pencil Beam and Collapsed Cone algorithms implemented in Masterplan (Nucletron) and the Monte Carlo system XVMC and validated using Gafchromic EBT films. Differences in dose distribution were evaluated. The plans were then optimized by adding segments to the outer shell of the target in order to increase the dose near the interface to the lung.ResultsThe Pencil Beam algorithm overestimated the dose by up to 15% compared to the measurements. Collapsed Cone and Monte Carlo predicted the dose more accurately with a maximum difference of -8% and -3% respectively compared to the film. Plan optimization by adding small segments to the peripheral parts of the target, creating a 2-step fluence modulation, allowed to increase target coverage and homogeneity as compared to the uncorrected 9 field plan.ConclusionThe use of forward 2-step fluence modulation in radiotherapy of small lung lesions allows the improvement of tumor coverage and dose homogeneity as compared to non-modulated treatment plans and may thus help to increase the local tumor control probability. While the Collapsed Cone algorithm is closer to measurements than the Pencil Beam algorithm, both algorithms are limited at tissue/lung interfaces, leaving Monte-Carlo the most accurate algorithm for dose prediction.

Pencil beam approach for correcting the energy dependence artifact in film dosimetry for IMRT verification

The higher sensitivity to low-energy scattered photons of radiographic film compared to water can lead to significant dosimetric error when the beam quality varies significantly within a field. Correcting for this artifact will provide greater accuracy for intensity modulated radiation therapy (IMRT) verification dosimetry. A procedure is developed for correction of the film energy-dependent response by creating a pencil beam kernel within our treatment planning system to model the film response specifically. Film kernels are obtained from EGSnrc Monte Carlo simulations of the dose distribution from a 1 mm diameter narrow beam in a model of the film placed at six depths from 1.5 to 40 cm in polystyrene and solid water phantoms. Kernels for different area phantoms (50 x 50 cm2 and 25 x 25 cm2 polystyrene and 30 x 30 cm2 solid water) are produced. The Monte Carlo calculated kernel is experimentally verified with film, ion chamber and thermoluminescent dosimetry (TLD) measurements in polystyrene irradiated by a narrow beam. The kernel is then used in convolution calculations to, predict the film response in open and IMRT fields. A 6 MV photon beam and Kodak XV2 film in a polystyrene phantom are selected to test the method as they are often used in practice and can result in large energy-dependent artifacts. The difference in dose distributions calculated with the film kernel and the water kernel is subtracted from film measurements to obtain a practically film artifact free IMRT dose distribution for the Kodak XV2 film. For the points with dose exceeding 5 cGy (11% of the peak dose) in a large modulated field and a film measurement inside a large polystyrene phantom at depth of 10 cm, the correction reduces the fraction of pixels for which the film dose deviates from dose to water by more than 5% of the mean film dose from 44% to 6%.

Monte Carlo dose voxel kernel calculations of beta-emitting and Auger-emitting radionuclides for internal dosimetry: A comparison between EGSnrcMP and EGS4

Dose-point kernels (DPKs) can be widely applied to therapeutic nuclear medicine to obtain more accurate absorbed dose assessments in internal dosimetry assuming a spherical geometry. Recently, EGSnrc-the latest in the family of EGS Monte Carlo codes--has been tested for isotropic monoenergetic electrons and Y-90 beta spectrum in spherical geometry. The availability of SPECT images allows one to take into account heterogeneities in activity distribution within tumors, and to perform dose calculations using voxel dosimetry based on Monte Carlo simulations in a Cartesian geometry. The purpose of this study is to evaluate the differences of dose distributions scored in Cartesian voxels also known as Dose Voxel Kernels (DVKs) for five beta-emitting (131I, 89Sr, 153Sm, 186Re, and 90Y) and Auger-emitting (111In) radionuclides, when their computation is made using these two Monte Carlo codes from the same family to check if the new physics in EGSnrc simulation system produces DVK very different from those calculated with EGS4. We have calculated the DVKs for point and voxel sources in Cartesian scoring grids of different spatial resolutions. Our results for the point source, scored in the finer spatial resolution, show a poor agreement between EGSnrc and EGS4 (up to about 20%) for voxels closer to the origin, and a better agreement (below 5%) for longer distances for all radionuclides. For the voxel source, where doses were scored in the coarser spatial resolution, dose deposition in the central voxel is in good agreement for all the radionuclides; while surrounding voxels exhibit a slightly worse agreement.

The effects of intrafraction motion on dose homogeneity in a breast phantom with physical wedges, enhanced dynamic wedges, and ssIMRT

This study attempts to compare how breathing motion affects intact-breast cancer patients between three different treatment techniques and to determine the degree of improvement on dose homogeneity when implementing gating therapy. A breast phantom and respiratory simulator were designed to simulate respiratory motion to a first-order approximation. Film was used as a dosimeter, and static dosimetry data were used as a control for comparison. Three velocities of the breast phantom were studied, and gating therapy was introduced for each data set. Dose area histograms (DAHs) were calculated for a breast and a "lung" planning target area (PTA), and Normalized Agreement Test (NAT) indices were calculated in reference to the static case. Deviations from the static case were highest if the collimator speed was of the same magnitude as the speed of the target. In general, gating therapy improved dose uniformity to the breast PTA by up to 14% and reduced dose to the "lung" PTA by up to 24%. With step-and-shoot intensity-modulated radiation therapy (ssIMRT), gating the beam may compromise dose coverage of the breast PTA if the timing interval of the gate is too large. Gating the beam decreased NAT indices by 9 for physical wedges, by 16 for enhanced dynamic wedges, and by 6 for ssIMRT. Both the phantom and respiratory simulator are adequate for showing differences in dose distributions for all three treatment modalities. Gating therapy improves dose homogeneity to the PTAs and decreases the dose delivered to areas below the posterior border of the beams.

SU‐FF‐J‐123: The Effect of Respiratory Motion On Two Breast Radiotherapy Techniques: A Phantom Study

Purpose: Many centers are looking beyond conventional wedged tangential fields for the treatment of breast cancer by using intensity modulation techniques for improvements in the dose distribution, reduced late tissue toxicity, and improved cosmesis. The improvement in dose uniformity may be mitigated in the presence of respiratory motion. We have compared a forward planned intensity modulated technique with a conventional wedged tangent approach for irradiation of the whole breast. The purpose of this study was to compare these techniques under both static and dynamic conditions to simulate the effect of respiratory motion. Method and Materials: We planned these two techniques (conventional wedged tangents and forward planned intensity modulated) on a stationary beeswax breast phantom containing a cork lung insert, using a CT acquired 3D dataset. Treatments were performed with the phantom stationary, and mounted on a moving platform having a sinusoidal waveform. The resulting dose distributions were measured using radiographic film in the anterior‐posterior (AP) and medial‐lateral (ML) directions. The amplitude of motion and angle of tilt of the breast, representing the relative contributions of the cranio‐caudal (CC) and AP components of breathing motion, were varied. Results: The dose distributions for the conventional and forward planned techniques were not significantly affected by motion in both the CC and ML directions, for 1cm and 2cm amplitude, and tilt angles of 15, 30 or 45 degrees. This can be explained as the majority of dose is delivered through open fields, with only a small modulation component using MLCs for forward planning. Conclusion: This study demonstrates negligible differences in dose distribution with conventional and simple forward‐planned IMRT techniques. The dosimetric benefits of forward planned 3D compensated breast radiotherapy are not adversely affected by respiratory motion.

SU-FF-T-207: Efficient IMRT Delivery - Fast Generation of Integer-Valued Beam Profiles

Purpose:The delivery time for an intensity-modulated radiotherapy plan using the step-and-shoot method may be impractical for complex beam profiles that require a large number of segments. We propose a fast algorithm that simplifies beam profiles to integer-valued intensities, which results in dramatic reduction of leaf-segments, while maintaining the “goodness” of the original-optimized-plan. Method-and-Materials:An optimization model was designed to simplify a beam profile to integer-valued intensities. The user specifies the permitted intensity level values, the maximum number of intensity levels, and the acceptable percentage of total under-/over-dosage. The model minimizes the absolute difference between each beamlet intensity and a weighted average of the intensities of the beamlet's nearest neighbors(and itself) in the resulting plan. The method is tested on several cases of head-and-neck and prostate cases, each with seven beams. In the original-optimized-plan, the number of intensity levels in each beam ranges from 97 to 153. Results: For all beams, an optimal integer solution was obtained within 15seconds. This held true even after the total intensity delivered by the simplified-beam-profile was constrained to be either the floor or ceiling of the total intensity of the original beam profile(the tightest possible). The simplified-beam-profiles were permitted to use up to ten distinct integer values(between 1–20). There is virtually no difference in conformity, homogeneity and dose distribution in PTV(all within 0.02%), as well as in all OARs. The total intensities deposited between the original-optimized-plan versus the resulting simplified plans are within 0.45%. Running through Siochi's leaf-sequencer, the resulting segments reduce from 89–151 to 14–39 for all simplified plans. Conclusions: This work indicates the potential of a quick algorithm for simplifying complex intensity profiles while maintaining good plan quality. The resulting beam complexity reduction improves deliverability of the leaf sequence of each beam. The algorithm takes only seconds to complete, thus making it realistic for clinical implementation.

SU-FF-T-226: Evaluation of Radiation Therapy Treatment Plans Through Conventional IMRT and Inversely Planned Intensity Modulated ARC Therapy (IMAT), A Clinical Comparison

Purpose: The goals of this study were to evaluate the difference in target dose uniformity and dose distribution through mean DVH and OAR analysis for a given PTV. Dosimetric analysis were also extended to compare sparing of normal tissue and critical structures between two treatment planning techniques. Methods and Material: The ADAC Pinnacle inverse planning system was used to generate conventional IMRT inverse plans for a few head & neck patients. These plans were then compared with treatment plans generated using the IMAT software package manufactured by 3D line USA Medical System Corp. Using axial CT for the same patients, targets, and numerous critical structures were delineated and “best” plans were developed in each environment. In IMAT planning, continuous and dynamically shaped arcs were planned through aperture optimization and leaf sequencing. The dose delivery was accomplished through a specially designed MICRO DMLC with 3 mm leaf width and double focus focalization. Results: For IMRT plans, 5 or 7 co‐planar arcs were used and treatment plans were generally optimized using a predefined set of dose objectives, penalties, min and max dose and percentage of volume required. The IMAT plans were designed with typically two co‐planar arcs of 180 degrees around the patient with similar dosimetry objective parameters. The percentage coverage for V95 in targets 1&2 were 94.9 ± 2.8 and 95.1 ± 2.1 for IMRT and 98.4 ± 2.6 & 98.1 ± 2.8 for IMAT respectively. Sparing of critical structures was generally better in IMAT, a detail of which will be presented at the meeting. Conclusions: In the IMAT technique with direct aperture optimization,dose distribution within the target was found to be better by 8%. Superior critical structure sparing was achieved as compared with similar plan using IMRT technique. The IMAT technique may be considered as an alternative to tomotherapy.

SU‐FF‐T‐176: Dose Verification in Moving Targets During Helical Tomotherapy Beam Delivery

Purpose: To compare dose distributions from stationary and moving targets subject to simulated intra‐fraction motion during treatment deliveries on a helical tomotherapy unit. Method and Materials: We have constructed an in house platform that moves in the superior‐inferior direction in a controlled manner. Using a stationary CT scan of a thorax phantom, a fictitious target volume was defined and a helical tomotherapy plan was generated. The phantom was placed on top of the moving platform and set in motion at a frequency of 16.7 cycles/min and 20 mm peak‐to‐trough excursion. An IMRT plan was delivered while the phantom was in motion and when it was still. Ion chamber readings were recorded at the center of the target. Different phases were artificially introduced by starting the platform motion at different times before the start of irradiation. In each delivery a film was placed in the coronal plane inside the phantom to register 2D dose. Results: Profiles along the direction of motion from the co‐registered images of moving and stationary deliveries have shown dose differences as large as 70% in the penumbra region. Gamma function maps for different dose and distance to agreement criteria revealed the regions at which the gamma criteria fails and was shown to be a strong function of initial motion phase. Ion chamber point measurements recorded insignificant differences between still and moving deliveries. Conclusions: We have shown that there is a significant difference in 2D dose distributions in the penumbra region both in the inferior‐superior and transverse directions between stationary and moving deliveries that simulate breathing motion. Our results indicate that the differences are highly dependent on the breathing motion initial phase. The phantom motion had little effect on the delivered dose at the central target region.

SU-FF-T-198: Dosimetry of Small Lung Lesions with EGSnrc Monte Carlo and Treatment Planning Systems

Background: The ability to deliver a homogenous dose distribution to small lung lesions is usually affected by the surrounding less dense normal lung. In low‐density tissue, there is increased transmission of photons relative to that in tissue. The lateral scatter of electrons out of the beam can lead to loss of field flatness and increased penumbral width. The magnitude of these effects is known to be very significant at high energies. Some commonly used radiotherapy treatment planning systems have had limited success in predicting accurately the dose distribution under these inhomogeneous conditions. Purpose: The purpose of this work is to study the magnitude of these effects using the EGSnrc/BEAMnrc Monte Carlo code on a typical patient CT dataset and to assess the limitations in treatment planning of such lungtumours with three Treatment Planning Systems (TPS); ADAC Pinnacle III v7.4, Varian Eclipse IMX and MDS Nordion Theraplan Plus v3.8. Methods: Small lungtumours (GTV) of diameter (3cm and 5cm) were simulated as water equivalent volumes and graphically inserted into the CT dataset at three different locations (Anterior, Central and Posterior) in the right lung. Using 6MV and 15MV beams, modelled for a Varian 21EX with the BEAMnrc Monte Carlo dose distributions were simulated using a Parallel Opposed Pairs (POP) technique. The field sizes were varied such that the distance from GTV to field edge was 1.0cm, 1.5cm and 2.0cm. Results and Discussion: Compared to the Monte Carlo calculated dose distribution, considerable and unique differences were observed in predicted dose by the TPS around the periphery of tumour and in the lung. These differences can be attributed to changes in the electron transport in lung, which are not adequately taken into account. The severity of these uncertainties, increases with photon energy, and decreases with field size for all planning systems.