Index of content:
Volume 33, Issue 7, July 2006
- FIFTY‐SECOND ANNUAL MEETING OF THE CANADIAN ORGANIZATION OF MEDICAL PHYSICISTS AND THE CANADIAN COLLEGE OF PHYSICISTS IN MEDICINE
- J.R. Cunningham Young Investigators Symposium
33(2006); http://dx.doi.org/10.1118/1.2244666View Description Hide Description
We assessed the performance of the induction motor of a rotating‐anode x‐ray tube in the magnetic fringe field of a clinical MRI scanner. A standard rotating‐anode x‐ray tube insert was placed into the fringe field of a 1.5 T unshielded research MRI scanner. The induction motor in the x‐ray tube was aligned so that the magnetic field lines were in the plane of its stator core. The induction motor of the x‐ray tube was placed in magnetic fields ranging from 0 to 500 G. The magnetic fields were measured with a Model 4048 Gauss meter. The rotation speed of the anode was measured using a strobe light. The power consumed by the motor during operation in the fringe field was measured separately using a PLM‐1 power meter. The anode rotation speed dropped from 3437 +/− 6.8 rpm to 2744 +/− 5.3 rpm when the magnetic fringe field was increased from 0 to 400 G. The average power consumed by the motor increased from 70.5 +/− 0.4 W to 78.2 +/− 0.1 W when the fringe field was increased from 0 to 500 G. This work indicates the feasibility of safely operating an x‐ray tube induction motor in the fringe field of an MRI scanner. Power consumption did not significantly increase and anode rotation speed did not fall below 3000 rpm until a fringe field exceeding 300 G was applied.
Sci‐Fri AM General‐02: Planning and automatic delivery of energy modulated electron therapy (EMET): a novel technique33(2006); http://dx.doi.org/10.1118/1.2244667View Description Hide Description
Energy modulated electron therapy (EMET) with conventional clinical accelerators has lagged behind photon intensity modulated therapy (IMRT) despite the potential of the former modality to achieve highly conformal dose distributions tailored to superficial targets which are inherently difficult to plan with IMRT. The main reason for this lag is the lack of an automated collimating device that allows sequential beam shaping of a series of variable field openings. In this study, we present a novel technique to deliver EMET using a prototype of a “few leaf electron collimator” (FLEC) that consists of four motor‐driven leaves fit in a standard clinical applicator. An inverse treatment planning procedure was developed using a Monte Carlo‐based optimization algorithm that makes use of patient‐specific dose kernels. With this procedure optimal plans of three clinical sites were compared to 3D‐conformal radiation therapy and IMRT. Output measurements using radiochromic film and ionization chambers were obtained to validate the Monte Carlo calculations. Further comparisons of 2D dose distributions were also performed using gamma‐index analysis. The FLEC was CAD designed and constructed after optimizing its physical configurations based on Monte Carlo studies.Treatment planningMonte Carlo studies also showed that EMET, delivered using FLEC, is able to provide conformal dose to the superficially located targets, reduce dose to organs at risk, and lower whole body dose without significant perturbation of the dose homogeneity in the target. Comparisons between Monte Carlo calculations and measurements of complex delivery maps show an overall agreement within 3%.
Sci‐Fri AM General‐03: Variation in the relative volumes and ionization response of four cylindrical ion chambers using micro‐computed tomography33(2006); http://dx.doi.org/10.1118/1.2244668View Description Hide Description
High accuracy in absolute dose measurements is required in radiation therapy and dosimetric protocols require the use of air ionization chambers. These protocols involve chamber specific calibration factors, which depend fundamentally upon the chamber volume. In this work, we investigate the ability of micro‐computed tomography (micro‐CT) to determine differences in the chamber volume of four nominally identical Exradin A1SL cylindrical ion chambers. A GE Locus micro‐CT imaging system (General Electric Healthcare, London, Ontario) was used to acquire 20 micron resolution images of the four ion chambers. The ionization response of each chamber in a 10×10cm2 10 MV x‐ray reference field was also measured, and the readings were corrected for polarity and ion recombination effects. GE MicroView software (Version 2.1.1) and its semi‐automatic pixel volume measurement option was used to determine the sensitive air volume of the ion chambers. Both the ionization response of the chambers and the volumes were normalized to the chamber with the largest values. The relative differences in response and volume for each chamber agreed within 1%. Micro‐CT is a promising tool for the accurate determination of chamber volume, and potentially for the independent determination of chamber calibration factors.
Sci‐Fri AM General‐04: High Contrast Imaging Using Orthogonal Bremsstrahlung Beams: An Experimental Study of Radiation Quality33(2006); http://dx.doi.org/10.1118/1.2244669View Description Hide Description
Since portal images are created by megavoltage, forward‐directed bremsstrahlung beams, their image quality is inferior to that of images produced by kilovoltage beams. In this study, characteristics of orthogonal bremsstrahlung photons produced by megavoltage electron pencil beams were studied and suitability of their use for improved radiotherapy imaging was evaluated. A 10 MeV electron beam emerging through the research port of a Varian Clinac‐18 linac was made to strike targets of carbon,aluminum and copper. PDD and attenuation measurements of both the forward and orthogonal beams were performed, and experimental results were compared with Monte Carlo‐calculated findings. Images of contrast objects were acquired with Agfa 400 diagnostic films and their contrast levels were analyzed. Photon yield and effective energy are lower for orthogonal beams than for forward beams, and the differences are more pronounced for targets of lower atomic number. The effective energy of a spectrum produced by carbon dropped by a factor of 10 from 1535 keV in the forward direction to 151 keV in the orthogonal direction, while for aluminum the effective energy dropped by 77% to 425 keV, and for copper by 37% to 1107 keV. The imagecontrast of films exposed with orthogonal beams was qualitatively determined to be superior to that obtained using the forward megavoltage beams. Using their relatively low effective energy, orthogonal bremsstrahlung beams produced by megavoltage electrons striking low atomic number targets yield images with a higher contrast than do forward bremsstrahlung beams.
Sci‐Fri AM General‐05: Measurement of daily interfraction prostate motion with tomotherapy MVCT images33(2006); http://dx.doi.org/10.1118/1.2244670View Description Hide Description
It is well known that the interfraction motion of the prostate gland can have a significant impact on the efficacy of radiotherapy in the treatment of prostate cancer. As we move towards more complex treatment protocols involving dose escalation and adaptive therapy, accurate daily prostate localization will increase in importance. In this work, the daily interfraction prostate motion has been measured for research patients receiving prostate treatment on the Cross Cancer Institute's TomoTherapy Hi*Art unit. An automated rigid registration algorithm designed to overlay the rigid bony anatomy was used to align the daily on‐line MVCT images with their respective planning CTimages. The position of the prostate in the MVCT images was achieved using a prostate localization algorithm based on mutual information registration that only requires prostate delineation on the planning CTimage. A multi‐start optimization approach was employed to reduce registration uncertainty and eliminate any gross miss‐registrations. The technique was validated with kVCT and MVCT images of two patients with localization seeds implanted in their prostates. Accurate convergence could only be achieved when assuming translations only, thus neglecting any prostate rotation. The maximum interfraction motion observed in the anterior‐posterior, superior‐inferior, and lateral directions was 5.7mm, 5.2mm, and 2.0mm, respectively. Mean and standard deviation values were −1.1mm, 0.4mm, and 0.3mm, and 1.7mm, 2.4mm, and 0.7mm, respectively.
Sci‐Fri AM General‐06: The Moving Slanted‐Edge Method: A Novel Technique to Measure the Temporal Response of a Fluoroscopic System33(2006); http://dx.doi.org/10.1118/1.2244671View Description Hide Description
Fluoroscopic procedures are generally acknowledged to lead to large patient doses. It is important to have a method of measuringdetector performance. Applications of cascaded‐systems theory to measuredetector performance of fluoroscopic systems have produced unexpectedly high results due to the reduction in measured noise caused by temporal averaging between frames as a result of system lag. Methods have been proposed to remove this affect of lag, which require the system temporal resolution through the system temporal modulation transfer function(MTF). No method currently exists to easily and accurately measure the system temporal MTF of a fluoroscopic system. We have developed a novel technique, the “moving slanted‐edge method”, to make measurements of system temporal MTF from the image data of a fluoroscopic system.
We translate a tungsten sheet across the detector with constant velocity, vo , and calculate a spatiotemporal coordinate for each pixel based on distance and time from passage of the edge. The resulting data is used to calculate a vo ‐dependent MTF. The system (spatial) MTF and vo ‐dependent MTF are mapped onto the temporal axis using the edge velocity. The system temporal MTF is calculated by dividing the vo ‐dependent MTF by the (spatial) MTF.
This method was validated on a bench‐top fluoroscopic system consisting of a three‐year old Dunlee image intensifier coupled to a CCDcamera. The system temporal MTF was calculated using the moving slanted‐edge method (vo =45 cm/s) and the optical decay curve of the image intensifier. Excellent agreement was found between the two methods.
Sci‐Fri AM General‐07: Dual Isotope SPECT to Track Transplanted Cells in Canine Myocardium Using Molecular Imaging33(2006); http://dx.doi.org/10.1118/1.2244672View Description Hide Description
Introduction: Non‐invasive cell tracking techniques that provide repetitive and functional assessments of transplanted cells will be an asset in cell‐based therapies. Specifically, reporter gene (RG) expression may be used to signal certain molecular events as they occur in vivo.
Objective: To compare the in vivo kinetics of a radiolabeled reporter probe (RP) specifically targeted by RG expression to a non‐specific radiolabel (NSL) in canine bone marrow cells (BMC). Cells were co‐labeled in vitro, transplanted into normal canine myocardium, and tracked using dual‐isotope SPECT.
Methods: Following canine bone marrow harvest (n=1), BMCs were isolated, transfected with RG, and grown in culture for 4 weeks. Transfected cells (5.4×106) were then incubated with RP and NSL followed by direct injection into the left canine ventricle. The recipient of transplanted cells was also the donor. Forty 1‐hr SPECTimages were acquired to obtain washout kinetics of RP and NSL. Time activity curves (TAC) were generated from SPECT region of interest analysis. After sacrifice, ex vivo measurements of RP and NSL activity remaining in the myocardium were made with a high purity germanium gamma‐ray well counter.
Results: Decay‐corrected TACs demonstrated faster washout kinetics for RP compared to NSL with biological half‐lives of 19 and 39 hours, respectively. This suggests that RP loss is greater than that attributed to cell death alone and demonstrates differences in label stability. Ex vivotissue analysis confirmed these results.
Conclusion: Multispectral SPECT shows potential for monitoring RG stability and expression to follow cell viability and function non‐invasively in large animals.
33(2006); http://dx.doi.org/10.1118/1.2244673View Description Hide Description
Endovascular therapy describes a class of minimally invasive treatments for certain vascular diseases, including the placement of stents to correct arterial stenoses, and the administration of intra‐arterial thrombolysis for ischemic stroke. Our objective is to develop a real‐time system to guide interventional endovascular procedures with MR imaging. Our approach in developing this system is to mimic, where appropriate and possible, the functionality of current clinical x‐ray fluoroscopy systems. Our endovascular MR system is comprised of two major components: a data acquisition module and an image reconstruction module. Data acquisition was performed using a 3‐tesla MR scanner (Signa VH/i; General Electric Healthcare; Waukesha, WI). Image reconstruction was performed on a separate dedicated workstation (2.2‐GHz Athlon processor‐equipped workstation running Windows XP). We achieved catheter visualization by filling catheters with MR contrast and imaging using a multi‐cycle projection dephaser to suppress the background signal. We used image‐processing filters available in the VTK library to isolate the catheter in the background‐suppressed images. The output of this image‐filteringpipeline was then used as a mask for an anatomical roadmap image. The image fusion process described herein has successfully been used to combine background‐suppressed in vivoimages of a catheter with anatomical roadmap images. The fusion process removed most of the noise and residual background signal from the background‐suppressed image, leaving the catheter with improved conspicuity. Our endovascular MR system functions in a similar manner to x‐ray fluoroscopic systems and has applications in the treatment of stroke and other vascular diseases.
33(2006); http://dx.doi.org/10.1118/1.2244674View Description Hide Description
In the treatment of kidney stones, knowing stone composition has been established as an important aid to the understanding of stone formation and in preventing recurrences, particularly the composition of the initial “core” of the stone. Traditionally, stone composition has come from laboratory techniques such as infrared spectroscopy and x‐ray diffraction. These methods require taking multiple samples of excised stone fragments and powdering them — losing structural information in the process. Furthermore, since stone composition is rarely homogenous, samples must be drawn from enough materially distinct regions of the stone or some components may not be identified. Low‐angle coherent scatter (CS) at diagnostic energies provides tissue‐composition information that can distinguish between different biological materials. Previously, our lab has developed a system to capture these CS patterns and to match them to known patterns of pure component materials. A drawback of the system is that it uses a polyenergetic x‐ray source. Since the CS angular distribution is energy‐dependent, an angular blurring of the measured cross section occurs, impairing the system's ability to accurately identify composition. To overcome this drawback, the spectral width of the x‐rays must be reduced. One way to achieve this uses two filters to create two CS patterns, which are then subtracted. If the filters are chosen with similar atomic numbers, the subtracted scatter pattern will be formed primarily by x‐rays in a narrow energy range. In this article we present an application of this “balanced filter technique” that will improve the scatter pattern uniqueness of kidney stone components.
33(2006); http://dx.doi.org/10.1118/1.2244675View Description Hide Description
Purpose: To identify the chemical and or physical cause of a threshold dose in the low dose response of FXG gel dosimeters.
Methods:Gels were prepared by mixing stock solutions of ferrous‐ammonium‐sulphate and different chemical supplies of xylenol‐orange (XO) together with sulfuric acid into a solution of gelatin and distilled water. Concentrations of ferrous‐ammonium‐sulphate, XO and sulfuric acid were varied to identify one of these chemicals as a source of impurities potentially causing the threshold dose effect. Samples in 1 or 10cm acrylic cuvettes were irradiated to doses between 0–5Gy. Optical transmissions were measured through the samples and referenced to water at 589 and 543nm, respectively, using an absorption spectrophotometer. This procedure was repeated for gelatin‐free FX solutions and with gel containing varying concentrations of ferric‐ammonium‐sulphate.
Results: The threshold dose decreased to 0.20±0.05 Gy using reagent grade XO. There was no correlation between threshold dose and varying concentrations of ferrous and sulphuric acid although decreasing the XO concentration from 0.05 to 0.025mM lowered the threshold dose by a factor of two. Gelatin‐free FX solutions showed a threshold dose. Using a ferric concentration of 7.5uM decreased the threshold dose to 0.04±0.03 Gy. The threshold dose effect was not present at 543nm.
Conclusions: Chemical impurities are not responsible for the threshold dose. The “apparent” threshold dose is caused by the production of different Fe(III)i:XOj species (each with their own respective spectral absorptivity). These species are formed at different dose levels and sensitivity to detecting these species requires optimization of the readout wavelength.
- Poster Session
33(2006); http://dx.doi.org/10.1118/1.2244628View Description Hide Description
The standard technique for electron therapy has several inherent problems. First, the cutouts in electron cones for field shaping are labour intensive to produce, cumbersome to use and reduce the clearance between the treatment machine and the patient. Secondly, it is difficult to modulate treatment depth and match multiple fields with electron beams due to electron multiple scattering. We propose the use of photonMLC for electron field shaping to improve treatment efficiency and convenience. Unfortunately, using the photonMLC to shape an electron beam significantly increases the penumbra of the treatment field, which can be reduced using a subsequent photonIMRT boost field. Electron beams shaped with a Varian Millennium MLC were recently commissioned in Pinnacle V7.6 (Philips). Using forward or inverse intensity‐modulated radiotherapy(IMRT) planning techniques, MLC segments for a step‐and‐shoot IMRT field were determined for improving electron beam penumbra, depth modulation and electron‐photon field. The technique has been applied to a few clinical cases, such as whole CNS irradiation and cancer of parotid. Inverse planned IMRT will be used to optimize photon‐electron‐combined irradiation for cancer of the parotid. Generated plans are verified using film and ion chamber measurements. Using boost IMRT fields, we showed a similar 50%–95% penumbra for the MLC shaped electron field compared to the penumbra with an electron applicator (100cm SSD). In conclusion, combined photonIMRT and electron beam improves dose uniformity, beam junction, and depth modulation.
33(2006); http://dx.doi.org/10.1118/1.2244629View Description Hide Description
We have compared four computational methods for quantifying the effect of set‐up error and uncertainty on delivered doses to targets and organs at risk in the IMRT treatment of head and neck cancer. These four methods were direct simulation, simple convolution plus two modified convolution approaches: the corrected convolution and the truncated convolution proposed by our group. Discrepancies of up to 20% in the Equivalent Uniform Dose (EUD) between direct simulation and simple convolution were estimated for the relatively superficial parotid gland at a systematic set‐up error of 6mm standard deviation and a random uncertainty of 2mm standard deviation. Truncated convolution agreed with direct simulation to within 6% for all situations studied. However, of the four methods, only direct simulation can quantify the range of outcomes (EUD) associated with a finite number of courses and fractions. Our results are particularly relevant to the design of dose escalation studies in head and neck cancer.
33(2006); http://dx.doi.org/10.1118/1.2244630View Description Hide Description
In modern radiotherapytreatment planning, the information in diagnostic CTimages is used for two purposes: to delineate tumour and surrounding critical structures and to provide an electron density map of the patient that is used to calculate the dose distribution resulting from exposure to a certain beam arrangement. In pelvic cancer patients with hip prostheses, the metal implants produce artifacts in the diagnostic CTimages such that both the location of the tumour and accurate electron densities are either difficult or impossible to obtain. We have used megavoltage CT (MVCT) images for treatment planning in an attempt to quantify the impact of metal artifacts and overcome the problems they introduce. This has been done in three different sets of experiments. The first was a calibration of the megavoltage CT number‐to‐electron density curve using a CIRS phantom. This also allowed for measurements of the impact of metal artifacts on apparent relative electron density in both kVCT and MVCT images. The second was the comparison of treatment plans generated for patients with metal implants using both diagnostic and megavoltage CT studies. This allowed for quantitative measurements of the calculated dosimetric effect of metal artifacts. The final set of experiments compared MVCT and kVCT treatment plans of a water tank containing a stainless steel 316L rod. Dose measurements were taken at various points and compared to the doses calculated using both MVCT and kVCT studies.
Po‐Thur Eve General‐04: Squeezing a balloon: the co‐dependences of calculated photon beam characteristics on model parameters33(2006); http://dx.doi.org/10.1118/1.2244631View Description Hide Description
Modeling photon beams in modern treatment planning systems is an iterative procedure as the model parameters generally influence the behaviour of more than one feature of the computed beam characteristics. In this work we explore the co‐dependence of the calculation of specified regions of beam profiles on the adjustable source parameters in Pinnacle®. The regions chosen are those used by Venselaar in recommending tolerances to be applied during the modeling process. A reference model, at 15MV, was modified in a controlled fashion so as to primarily influence one region of either the depth dose or cross beam profile at 10cm depth in a 20×20cm2 field. The effect on the five other profile regions was examined thus illuminating the degree of co‐dependence of the different regions on the model parameters. The regions of interest were: build‐up, descending depth dose, horns, tail, high and low penumbra. Our study suggests a sequence which could enhance efficiency in modeling measured photon beam data.
Po‐Thur Eve General‐05: Novel Intermediate Energy Photon Treatment of Small Lesions: A Monte Carlo Simulation and Treatment Planning Study33(2006); http://dx.doi.org/10.1118/1.2244632View Description Hide Description
Stereotactic radiosurgery affords great conformality for small tumour volumes. Our study proposes that the radiological penumbra for an intermediate energy photon beam (IEP, 0.2 – 1 MeV) is greatly reduced compared to a megavoltage beam. From Monte Carlo simulation, an 800kV beam of field size less than 2×2 cm2 was generated from electrons impinging upon a 0.5mm tungsten target. This beam generated a radiological penumbral width (80%–20%) of less than 300μm for small field sizes at depth=5cm in water. A virtual IEP treatment unit (PDD's and profiles generated from Monte Carlo) was created in a Pinnacle treatment planning system (v6.2). An 11 beam non‐coplanar arrangement was used to cover a target volume situated in the middle of a phantom head and at 1mm from a critical structure. Dose volume histograms generated for both the 800kV and a standard 6MV beam showed that the volume of critical structure receiving 10% of the prescription dose was 27% versus 41%. The maximum dose received by the target was 110% (800kV) and 127% (6MV). The 800kV and 6MV beams were dosed to 92% and 78% isodose lines respectively for comparable target coverage. The reduction of radiological penumbra is linked to reduced photon scattering (using small field sizes) and the reduced secondary electron range (using IEP). An 800 kV beam shows superiority over a standard 6MV beam resulting in greater homogeneity and conformality to the target and better sparing of a critical structure in close contact with the target.
33(2006); http://dx.doi.org/10.1118/1.2244633View Description Hide Description
The Medical Physics departments of the Tom Baker Cancer Center (TBCC) and the Cross Cancer Institute (CCI) independently performed preliminary evaluation of the new Analytical Anisotropic Algorithm (AAA) implemented in Varian's Eclispe (v. 6.0) treatment planning system (TPS). The TPS was pre‐commissioned with “Golden Beam Data” from the vendor. We measured central and off‐axis profiles in several beam configurations including: open square, rectangular and asymmetric (half‐blocked) beams; wedged square and half‐blocked beams; square fields at three SSDs; open and wedged oblique beams; irregular field defined by MLC and cerrobend blocks. All measurements were performed on Varian 2100EX linear accelerators. Measurements were made to assess the dose in heterogeneous media at both the CCI (CIRS Thorax IMRT phantom) and at the TBCC (TLDs in a Rando phantom). Profiles were evaluated in the buildup, penumbra, inner and outer beam regions as per AAPM Task Group 53.
Measured and calculated profiles agreement was very good in all regions except for the inner beam region at the CCI, attributed a difference in interpolation schemes at the two institutions and the large volume ion chamber used for measurements. The AAA penumbra was also found to be steeper than measured penumbra since AAA was pre‐commissioned using diode measurements. Total scatter factors for most measurements differed by less than 2% from the calculated ones except for the hard wedges where differences up to 4% were found. Anthropomorphic phantoms measurements differed from AAA by as much as 5.6%.
Funding provided by Varian.
Po‐Thur Eve General‐07: Dosimetry of Small Lung Lesions with EGSnrc Monte Carlo and Treatment Planning Systems33(2006); http://dx.doi.org/10.1118/1.2244634View Description Hide Description
Early stage lungcancer, presenting as a small solitary pulmonary mass, is treated with radiation when concurrent disease precludes a surgical option. These small lesions are usually surrounded by less dense normal lung, which affects the ability to deliver a homogenous dose to the target volume. In low‐density tissue, such as lung, there is increased transmission of photons. In addition, lateral scatter of electrons out of the beam can lead to increased penumbral width. The magnitude of these effects is known to be dependent on beam energy. Some of the commonly used commercial treatment planning systems have had limited success in predicting accurately dose distributions under these highly inhomogeneous conditions.
We present a quantitative comparison between Monte Carlo simulation and commercial planning systems for a select range of clinically relevant target geometries and beam parameters. Small water equivalent cylindrical lungtumors of diameter 3 and 5 cm were incorporated within a CT dataset at different locations. A Parallel Opposed Pair (POP) field arrangement with 6MV or 15MV photons and variable field‐edge margins were considered. These plans were calculated using BEAMnrc Monte Carlo code and on two planning systems; ADAC Pinnacle III Version 7.4 and MDS Nordion Theraplan Plus v3.8.
The analysis of dose profiles and DVH's show considerable and unique differences between Monte Carlo and the results from each TPS within the tumor and at the junction between tumor and lung. For both planning systems, the severity of these errors, increases with photon energy, and decreases with field size.
33(2006); http://dx.doi.org/10.1118/1.2244635View Description Hide Description
Helical tomotherapy represents the state of the art in intensity modulated radiation therapy(IMRT) and image guided adaptive radiotherapy (IGAR). This work is aimed at carrying out Monte Carlo (MC)dose calculations of tomotherapy deliveries to real phantom and virtual phantom/patient CT data. All MC calculations are performed with the EGSnrc‐based MC simulation codes, BEAMnrc and DOSXYZnrc. Various MC parameters and variance reduction techniques were investigated and optimized. Single projection simulations are carried out to get percent depth dose (PDD) and beam profiles at various depths and lateral field dimensions. Simulations are compared with measured results taken with an A1SL ionization chamber in a water tank. A complex delivery was simulated with the 64 leaf binary multileaf collimator(MLC) modulating at fixed radiation angle for a solid water phantom. Further, a rotational treatment plan to a cheese phantom CT data set was also simulated. Simulations are performed by taking the sinogram (leaf opening vs. time) of the treatment plan and decomposing it into different projections, each of which is segmented further into several opening configurations with different MLC settings and weights, which corresponded to leaf opening time. Then the projection is simulated with the result of sum of all of the opening configurations, and the overall rotational treatment is simulated with the result of sum of all of the projection simulations. Measured and simulated profiles and PDDs are found agree with each other well. Preliminary simulations of full treatment plans are also presented.
Po‐Thur Eve General‐09: Dynamic evaluation for the treatment outcome incorporating prostate organ motion33(2006); http://dx.doi.org/10.1118/1.2244636View Description Hide Description
This paper is to calculate the cumulative dose considering internal organ motion and patient repositioning of prostate patient using the one fractional treatment plan method for a specific phantom and patients. In the multi‐fractional radiation treatment, it is desirable to consider the internal organ motion from fraction to fraction. The PTV positional variations can be measured and expressed as the fractional equivalent uniform dose (FEUD) deviation by randomly picking up the FEUD using Monte Carlo method in every fraction. The average EUD deviations for PTV with hundreds of simulations are 3.3% for phantom and 1.8% for 5 prostate patients. The changes of rectal volume and position are also simulated by shifting the geometric center of the rectum and changing the rectal radius to model the rectal deformation using Pinnacle3 planning system and FEUD for rectum are estimated. The prostate margin, together with the inter‐fraction variations in rectal position and shape, affects the delivered dose to the rectum. The effect of the dose fluctuations is larger for the normal tissues than the target, and the change of rectal position would cause more complication compared to the rectal volume change. The method provides a simple way to estimate the tumor control probability and normal tissue complication probability dynamically with considering internal organ motion and repositioning errors throughout the whole treatment course. It is possible to test a treatment plan against a range of possible patterns of organ motion with this dynamic evaluation for the treatment outcome.
Po‐Thur Eve General‐10: Dose gradient analyses in the prostate organ motion: treatment plan evaluation independent of DVH33(2006); http://dx.doi.org/10.1118/1.2244637View Description Hide Description
The aim of this study is to investigate internal organ motion using the profiles of the PTV dose gradients independent of DVH. In this study, the four‐beam box (4BB) and 5‐beam IMRT of three prostate patients were analyzed. Patients with small (17.3 cm3), medium (51.6 cm3) and large (87.1 cm3) prostate volumes were selected from Grand River Regional Cancer Center (GRRCC). A Pinnacle3 planning system was used to determine the PTV dose profiles crossing the isocenter in the L‐R, A‐P and S‐I directions. The convolution of the “static” dose and the Gaussian distribution were used to describe the discrepancy between the prescribed and delivered dose affected by patient repositioning and internal organ motion. The “shoulder” shape of the PTV profile determines the dose delivered to the tumor and critical organs when internal organ motion was considered. A “line equivalent uniform dose” (LEUD) was used to evaluate the shoulder dose deviation. The profile shapes influence the organ at risk (OAR) dose distribution more significantly for 4BB in comparison with IMRT especially in the posterior direction. The LEUD deviation in the range from 7.7% to 2.8% for S‐I and A‐P directions, respectively, was observed for the standard deviation of 1. The analysis of the PTV profile shape is a powerful tool for further improvement of IMRT treatment plan optimization. The results can be included as a parameter in the objective function used in the IMRT optimization.