Volume 33, Issue 7, July 2006
Index of content:
- FIFTY‐SECOND ANNUAL MEETING OF THE CANADIAN ORGANIZATION OF MEDICAL PHYSICISTS AND THE CANADIAN COLLEGE OF PHYSICISTS IN MEDICINE
Scientific Session: Radiation Therapy Physics — Tx Planning & Verification
33(2006); http://dx.doi.org/10.1118/1.2244615View Description Hide Description
The purpose of the study was to investigate absorbed dose in the build up region for oblique intensity modulated 6MV beams. For locally advanced breast carcinoma using multi‐field IMRT it is imperative to know with good precision the dose at the surface. This study intended to determine the effect of IMRT on the surface dose compared with open beams, to benchmark our Monte Carlo algorithm (BEAMnrc) in the build up region for clinical implementation and to characterize the dose calculation accuracy of our treatment planning system (Eclipse ™ 7.3) at the surface and depths up to dmax.
Measurements were performed in solid water using a parallel plate ionization chamber for a 10 × 10 cm2 open field, 1 cm slit and a typical breast IMRT treatment field, both delivered with sliding window technique at 00°, 350° and 700° angles of incidence. Compared with the open field the 1cm gap sliding window decreased the surface dose up to 10% for normal incidence and up to 5% for 70° incidence. The measured depth‐dose curves along the central axis are in excellent agreement within 2% of the Monte Carlo simulations. The TPS using the pencil beam algorithm overestimates the dose at the surface by a factor of 3 at normal incidence for both open and IMRT fields. The TPS does not take into account the increase in the surface dose due to oblique incidence. Monte Carlo can be used to calculate with accuracy the surface dose for a clinical multi‐ field IMRT plan.
Sci‐Thur PM Therapy‐02: Accounting for the off‐axis detector response for improved a‐Si EPID dosimetry33(2006); http://dx.doi.org/10.1118/1.2244616View Description Hide Description
In portal imaging applications using an a‐Si electronic portal imaging device(EPID), a calibration procedure must be performed before clinical images may be acquired. This calibration procedure incorporates a dark‐field image acquisition and a flood‐field image acquisition. Every clinical image is corrected by subtraction of the dark field image and then division by the flood‐field image. The flood‐field image exhibits two main features, the pixel‐to‐pixel sensitivity (present due to inherent limitations in the manufacturing process), and the beam profile shape as measured by the detector (a function of off‐axis distance, ie. energy spectra and incident fluence). While it is necessary to correct for the pixel‐to‐pixel sensitivity, the beam profile shape as measured by the detector is also removed by this process, which is undesirable for dosimetric applications. This work develops an approach which allows for the pixel‐to‐pixel sensitivity correction to be made, while still preserving the inherent response of the detector to the incident beam.
The approach consists of directly measuring the beam profile shape independently from the pixel‐to‐pixel sensitivity. This profile information may then be reintroduced into the clinical image. The beam profile measurement is achieved by building a small “point” detector using materials that are analogous to the clinical EPID. The point detector is scanned in two dimensions across the open radiation flood‐field at the same source‐to‐detector distance used for clinical flood‐field acquisition. Clinical images are then multiplied by this acquired 2D profile (normalized to central axis) to reintroduce the beam profile shape..
Sci‐Thur PM Therapy‐03: Automated Monte Carlo Simulation and Validation of an External Beam Radiotherapy Treatment Plan33(2006); http://dx.doi.org/10.1118/1.2244617View Description Hide Description
We present the implementation of an integrated EGSnrc‐based Monte Carlo(MC) system as a tool for the verification of Varian's Eclipse clinical external beam radiotherapytreatment planning system (TPS). Once a TPS plan is generated an automated and fully integrated MC simulation is performed based on the information contained in the DICOM RT Plan, RT Sset, RT Dose and CTimage set. MC simulation parameters are determined from information extracted from the RT Plan. The CTimage set is then used to generate a virtual phantom in a manner analogous to that of CTcreate (the relevant EGSnrc program), but with the added advantage of incorporating the structures in RT Sset for volume definition within the phantom. The MC simulation is broken into stages that progress from the target in the head of the accelerator, through the various stages of collimation, and into the phantom using both BEAMnrc and DOSXYZnrc on a dedicated cluster. Using RT Dose, we are then able to compare absolute dose differences between the approaches quantitatively in 3D using a χ metric. To illustrate the process, we compare dose distributions for an unblocked four field pelvic treatment generated both using Eclipse and the MC system. Significant differences (> 4%) between the MC and TPS dose matrixes are observed in the vicinity of the bony anatomy.
Sci‐Thur PM Therapy‐04: Comparison of treatment planning system and Monte Carlo calculated dose distributions in an extreme water‐lung interface phantom33(2006); http://dx.doi.org/10.1118/1.2244618View Description Hide Description
The study compares the performance of the analytical anisotropic algorithm (AAA), a new superposition‐convolution algorithm recently implemented in Eclipse™ Integrated Treatment Planning System (TPS), to that of the pencil beam convolution (PBC) algorithm for a variety of clinical beam configurations at nominal beam energies of 6 MV and 18 MV, respectively in an extreme water‐lung interface phantom using Monte Carlo(MC) calculated dose distributions as benchmarks. Dose profiles at specific depths, 2‐D dose difference (TPS — MC) maps and histograms as well as first order statistics were used to quantify the accuracy of both photon dose calculation algorithms. For each open beam configuration, the AAA algorithm yielded smaller means, standard deviations and confidence limits. On average, the standard deviation of the dose differences was reduced by half with the AAA dose calculation model. Confidence limits for the PBC algorithm were ⩾ 3%, ranging from 3.0% for the 6 MV, 10×10 cm2 beam to 11.2% for the 18 MV, 4×4 cm2 beam while for the AAA algorithm they were ⩽ 4% for all beam configurations. The worst agreement of AAA algorithm with MC results was observed for the highest energy and smallest field combination (18 MV, 4×4 cm2). The standard deviation of the differences (2.4%) and confidence limit (4.0%) were however right between the standard deviations and confidence limits provided by the current clinical PBC model of the 6MV 4×4 and 10×10 cm2 beams.
Sci‐Thur PM Therapy‐05: Evaluation of male pelvic phantom for megavoltage cone‐beam computed tomography33(2006); http://dx.doi.org/10.1118/1.2244619View Description Hide Description
Computed tomography (CT)‐based image guidance has the feature of including the tumour as well as normal tissue in the imagelocalization process during radiation treatment. We propose to use a new anthropomorphic male pelvic phantom (CIRS Inc.) to test the feasibility of using a linear‐accelerator‐based megavoltage (MV) cone‐beam CT system for imaging the prostate. The objective of this work is to verify the tissue‐equivalency of the phantom to determine whether it is representative of prostate cancer patients. We evaluated the phantom in two ways. (1) The linear attenuation coefficient was determined by measuring the photon transmission through uniform samples of the phantom materials (e.g., prostate, muscle, bladder, etc.) using (2) The phantom material CT numbers (kVCT) were compared to CT number data acquired from randomly selected planning CT scans of prostate cancer patients at our institution. The measured linear attenuation coefficients agreed to ∼ 1% of the manufacturer's specifications, while the reproducibility of the measurement was ∼ 1%. The kVCT numbers in the phantom were also in good agreement with the manufacturer's specifications and the patient data. However, the bony structures are comprised of solid average bone equivalent material (cortical and trabecular bone), which caused artifacts within some regions of the kVCT images. However, MVCT imaging is expected to suppress these artifacts. These findings indicate that the phantom is representative of prostate cancer patients and will be a valuable tool in investigating the feasibility of MV cone‐beam CT.
This work was partially supported by Siemens Medical Solution USA, Inc.
Sci‐Thur PM Therapy‐06: Helical Tomotherapy for Adaptive Radiotherapy of Bladder Cancer: Treatment Planning Considerations33(2006); http://dx.doi.org/10.1118/1.2244620View Description Hide Description
We have developed a method that leverages the image‐guidedradiotherapy capabilities of a tomotherapy unit to adapt daily treatments to changes in bladder shape and volume. Each patient receives three treatment‐planningCT scans: empty bladder; partially full bladder; and full bladder. For each scan, the radiation oncologist contours the initial and boost target volumes (PTV1 and PTV2) and organs at risk, and an initial treatment plan is generated to deliver 40 Gy in 20 fractions to PTV1. A second treatment plan is designed to deliver 20 Gy in 10 fractions to PTV2 (a total of six treatment plans per patient). Patients are asked to void immediately before each treatment. An MVCT of the pelvis is performed before each fraction to verify patient position and to assess the bladder volume. If the treating therapist concludes that the patient's bladder extends to within 10 mm of the PTV contour, then the corresponding treatment plan for a larger bladder volume is downloaded and the MVCT is repeated. Approximately 20% of fractions required a treatment change based on perceived bladder volume. Three of four patients treated to‐date have required the use of a larger volume treatment plan at least once, even though they are always instructed to empty their bladders just prior to treatment. Our early experience is that relying on patient compliance for treating “empty bladder” is insufficient to ensure proper target coverage, and that generous internal margins are required to ensure target coverage in the absence of adaptive IGRT capability.
Sci‐Thur PM Therapy‐07: Improving IMRT plans delivery for head and neck cases using aperture‐based MLC segments33(2006); http://dx.doi.org/10.1118/1.2244621View Description Hide Description
Purpose: To investigate the possibility of performing IMRT in head and neck treatment sites with less segments and monitor units (MU).
Materials and methods: Six pharyngeal cases (n = 6) were analysed and four cases (n = 4), in the sinonasal region. For each one, an IMRT plan was first realized using a commercial software (Pinnacle3 — IMFAST segmentation algorithm —). Then, an‐in‐house inverse planning system, called Ballista, based on predetermined segments, was used to realize comparable plans. Its segments are generated with the subtraction of the projection of the OARs with the PTV (planning target volume).
Results: For the pharyngeal Ballista plans, the average volume of the PTV that received at least 100% of the prescribed dose (V100) was 85.0±4.5% for the first prescription (PTV1) and the V100 for the second prescription (PTV2 — simultaneous integrated boost —) was 78.5±10.9%. With Pinnacle3, the V100 value was 86.6±4.8% and 81.5±12.4% respectively for PTV1 and PTV2 (see figure 2a and 2b). On average, Ballista plans have required 932±124 MUs and 52±10 segments compared to 1238±230 MU and 117±7 segments for Pinnacle3. For the sinonasal Ballista plans, the average V100 obtained was 80.0±3.1%. With Pinnacle3, the V100 gave 75.7±2.7%. Ballista plans have required an average of 406±54 MUs and 22±1 segments compared to 697±133 MUs and 99±14 segments for beamlet‐based IMRT.
Conclusion: In step‐and‐shoot head and neck IMRT, an anatomy‐based MLC optimization system can achieve similar dosimetric plans comparable to traditional beamlet‐based IMRT with less number of segments and MU.
Sci‐Thur PM Therapy‐08: Intra and Inter Observer Variability and Systematic Error in Prostate Delineation33(2006); http://dx.doi.org/10.1118/1.2244622View Description Hide Description
Individual sub‐processes of external beam radiation therapy can affect patient outcome. Specifically geometric uncertainties play a major role in tumor control and normal tissue complications. It is known that tumor definition varies among different observers but importantly, at least to our knowledge, few studies have reported their findings against a true gold standard, most relying on the population average as the reference value. This work uses the Visible Human Project® data for a set of cross‐indexed CT and anatomical photographic digital images of a human male cadaver. We use the anatomical images to define a gold standard for the prostate as defined by a group of experts. Six radiation oncologists then repeatedly (20 times over several weeks) defined the prostate alone on the corresponding CTimages allowing us to quantify inter and intra observer random and importantly to measure systematic error.
Individual and population means were tested for systematic error against the gold standard. We found that the observers routinely over estimated the prostate volume, and that our intra and inter observer variability is not significantly different than that reported in the literature. Significantly we report a systematic error in target definition, with the physicians failing to include all of the posterior prostate volume near the rectum. In contrast the observers rarely missed true prostate volume in the left, right or anterior quadrants, but we observe a systematic error in that normal tissue anterior to the prostate was routinely included as target tissue.
33(2006); http://dx.doi.org/10.1118/1.2244623View Description Hide Description
Magnetic Resonance Imaging is the imaging modality of choice for target volume delineation required by radiotherapy planning (RTP) due to its superior soft‐tissue contrast. However, MRimages are affected by distortions that alter the imagespatial accuracy which is crucial for RTP. In the present work, we investigate a complete 3D phantom‐based protocol that determines and corrects both geometrical and object‐induced distortions. A 3D oil‐filled phantom consisting of parallel plastic grid sheets equally distributed inside the phantom was scanned on a 3T MR unit and CT (spatially accurate reference dataset). To determine the geometrical distortions we developed a novel automatic procedure. This allows us to accurately identify and register the MR and CT control points and to determine iteratively the 3D distortion field. To correct the sample‐induced effects we applied a standard method as per Chang and Fitzpatrick . For the entire field of view the control points were located accurately within one pixel resolution. We found that the total distortion in the transverse plane is approximately 7 mm and the z‐distortion is around 4 mm for a 15×15×15 cm3 volume. The MRimages are corrected by applying a spatial and a pixel intensity 3D interpolation procedure, the residual distortion after applying these transformations being under one pixel resolution. We investigated a procedure that relies on a new system distortion correction technique and a standard sample‐induced correction method to successfully rectify the MRimages required by Treatment Planning.
Sci‐Thur PM Therapy‐10: Is there an optimal starting gantry angle for equiangular‐spaced beam arrangements in IMRT?33(2006); http://dx.doi.org/10.1118/1.2244624View Description Hide Description
Equiangular‐spaced beam arrangements are the standard for intensity‐modulated radiotherapy(IMRT). However, since the choice of starting gantry angle is arbitrary, the question arises of whether there exist optimal starting gantry angles which will produce clinically significant improvements. To investigate the effect of beam orientation, the starting gantry angles in a 5‐beam equiangular‐spaced arrangement were incremented by 5°. Fifteen IMRT plans were generated per patient using 6 MV photon beams with static MLC delivery for 5 prostate, 5 lung, and 5 head‐and‐neck cancer patients. The plans were compared based on Planning Target Volume (PTV) and Organ‐at‐Risk (OAR) dose‐volume histograms, PTV mean dose, OAR maximum dose, total number of segments, and total number of monitor units. A fundamental geometric PTV conformity analysis was also conducted for each increment in gantry angle. To achieve this, divergent ray tracing was performed from the beam source coordinates through the PTV Beam's‐Eye‐View to produce a 3D cone beam matrix. The geometric conformity index (ratio of beam overlap volume to PTV volume) was then calculated. Clinically insignificant variations in PTV mean dose were found for all treatment sites as the starting gantry angles were incremented. This observation was further supported by the small variation in geometric conformity index with gantry increment. The variations in rectal V 75 Gy, spinal cord DVH, and parotid V 30 Gy were clinically significant for the prostate, lung, and head‐and‐neck patients, respectively. It is evident that the choice of starting gantry angle in IMRT equiangular‐spaced beam arrangements must be chosen with care.
33(2006); http://dx.doi.org/10.1118/1.2244625View Description Hide Description
We examine reconstruction of volumetric objects from megavoltage rotating and stationary beam projections. Quality of the reconstruction depends on the transverse and depth resolutions of the projections. In the rotating beam geometry, as the gantry angle varies, the depth resolution improves and thus more detailed anatomical structure of the object along the central axis of the radiation beam can be reconstructed, while the transverse resolution decreases. In the stationary beam geometry, where the source and detector remain fixed but the location of the object along the beam central ray is varied, the depth resolution is poor while the transverse resolution can be very high. We use the algebraic reconstruction technique (ART) and exact weighting factors to reconstruct 3‐D objects from 2‐D projections. When projections are noise‐free as those generated by a computer, the object is accurately reconstructed as long as the number of iterations is sufficiently high. For real world projections, in which noise can be significant, the reconstruction quality can be poor. Because both the transverse and depth resolutions are important in clinical applications, our work points to potential advantages of combining rotating‐ and stationary‐beam projections.
Sci‐Thur PM Therapy‐12: Use of patient‐specific planning target volumes in conformal radiotherapy of lung cancer: simple estimates of potential dosimetric benefit in tumor motion management33(2006); http://dx.doi.org/10.1118/1.2244627View Description Hide Description
Purpose: To estimate the volume of lung receiving at least 20 Gy (V20) for three tumor‐motion management strategies: population‐based uniform 15 mm margin, free‐breathing individualized margins, and reduced margins for gating with a 0.33 duty cycle.
Methods: We have used a simple formula to combine set‐up error with target motion and to individualize planning target volumes (PTV) for four non‐small cell lungcancer patients. Previous measurements of set‐up error and fluoroscopy data synchronized to a spirometer were used as inputs for our calculations. Tumor motion was characterized in terms of the standard deviation of superior‐inferior (sup‐inf) displacements in each fluoroscopyimage. Single‐phase treatment plans were generated such that at least 97% of the PTV was enclosed by the 60 Gy isodose, spinal cord dose was kept below 45 Gy, and V20 was minimized.
Results: The table shows the PTV margins in the sup‐inf direction and the corresponding V20 values, as read from the dose‐volume histograms.
Conclusion: Reductions in lung V20 and spinal cord dose are possible by use of patient‐specific margins. For more mobile tumors, gating may provide additional gains. Since tumor motion is predominantly in the superior‐inferior direction, uniform margins are not optimal.
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.