Volume 39, Issue 7, July 2012
Index of content:
Radiation-induced lung injury (RILI) is the primary dose-limiting toxicity for radiation therapy of the lung, and although the effects of radiation dose on RILI development have been well characterized, the influence of chronic obstructive pulmonary disease (COPD) on the development of RILI and other outcomes is not well understood. The purpose of this small pilot study was to evaluate the relationship between hyperpolarized3He magnetic resonance imaging(MRI) measurements of COPD with RILI and 12-month survival in lungcancer patients undergoing radical radiotherapy and to evaluate the feasibility of pulmonary functional MRI as an image guidance/planning tool for radiation therapy.Methods:
Fifteen non-small cell and small cell lungcancer patients underwent pulmonary function tests, x-ray computed tomography(CT), and hyperpolarized3He MRI prior to radical radiation therapy (≥60 Gy). Conventional thoracic 1H and hyperpolarized 3He MRI were acquired to generate ventilation defect percent and the apparent diffusion coefficient for the ipsilateral and contralateral lungs independently. CT was acquired postradiation therapy and qualitatively evaluated for radiological evidence of RILI and 12-month survival was reported.Results:
Hyperpolarized3He MRI measurements of COPD classified 10/15 subjects with contralateral lung COPD (CLC), and five subjects without COPD [contralateral lung normal (CLN)]. Of the 10 subjects with CLC, only four had a previous clinical diagnosis of COPD. CTimages were acquired postradiation therapy for 13 subjects, and for eight (62%) of these there was qualitative evidence of RILI, including 5/9 CLC and 3/4 CLN subjects. The one-year survival was 2/10 for CLC and 3/5 for CLN subjects.Conclusions:
In this small pilot study, we report the use of3He MRI to stratify lungcancer patients based on MRI evidence of COPD and showed that comorbid COPD was present in the majority of lungcancer subjects stratified for radiation therapy.Lungcancer patients with imaging evidence of COPD did not have an increased incidence of RILI compared to patients without COPD. However, preliminary data presented here indicated that one-year survival in COPD subjects was lower than expected based on previously published survival rates, which may have implications for radiation therapy in lungcancer patients with comorbid COPD.
- FIFTY‐FOURTH ANNUAL MEETING OF THE CANADIAN ORGANIZATION OF MEDICAL PHYSICISTS AND THE CANADIAN COLLEGE OF PHYSICISTS IN MEDICINE
- Scientific Session: Radiation Therapy Tx Planning & Dosimetry
Sci—Thur AM: Planning ‐ 01: Experimental and Monte Carlo verification of Acuros XB calculations near low and high density heterogeneities39(2012); http://dx.doi.org/10.1118/1.4740086View Description Hide Description
The purpose of this study was to examine the accuracy of AcurosXB and AAA algorithms near low and high density heterogeneities of different densities using EBT2 film, MOSFET detector “MOSkin” and Monte Carlo calculations using BEAMnrc/DOSXYZnrc. Three different interfaces were used that included a solid water phantom with 2×2×30cm3 rectangular air gap, rectangular steel insert, and a slab of water embedded between two slabs of lung material. 6MV photon beam with field size of 10×10cm2 was used for the first two geometries and a 3×3cm2‐field was used for the third. Percentage Depth Doses were measured and calculated at the beam central axis. Calculation voxel of 0.1×0.1×0.1cm3 was used by all three algorithms. For all configurations, AcurosXB and AAA agreed to within ±1.3% with MC before the inhomogeneity. The PDD measurements using MOSkin and EBT2 in water, apart from 0.2cm layer near heterogeneity, agreed with the MC within ±2.2%. Within 0.1cm before the water‐air interface AcurosXB and AAA overestimated the dose by 4.7% and 1.6%, respectively. Whereas, in the 0.1cm beyond the air‐water interface, AcurosXB and AAA overestimated the dose by 2.4% and 16.2% respectively. In the 0.1cm before the water‐steel interface, AcurosXB overestimated the dose by 4.7% and AAA underestimated the dose by 9.5%; beyond the steel‐water interface AcurosXB and AAA overestimated the dose by 3.6% and 7.7% respectively. For the lung phantom configuration, AcurosXB and AAA were in agreement with MC within 2% throughout the phantom. These results demonstrate improved performance of AcurosXB as compared to AAA in considered conditions.
Sci—Thur AM: Planning ‐ 02: Validation of XiO's eMC module using Gafchromic EBT3 films and triple channel dosimetry39(2012); http://dx.doi.org/10.1118/1.4740087View Description Hide Description
The aim of this study is to validate the electron Monte Carlo module implemented in XiO, a treatment planning system commercialized by Elekta CMS inc. Two types of phantoms were investigated: homogeneous water phantoms with irregular surfaces and phantoms containing slab and 3D heterogeneities. The phantoms were CT scanned, and dose to water calculations were performed in the eMC module using 2 ×2 × 2 mm2 voxels and a mean relative statistical uncertainty of 0.5%. Concurrently, Gafchromic EBT3 film measurements were performed in the same phantoms. To obtain reliable absolute dose readings from the films, a new method using triple channel dosimetry in the Film QA Pro software was developed. The accuracy of the proposed method was determined empirically and an uncertainty of ±1.5% was found over the range [75, 800] cGy. Dose comparisons between film and simulations were done using an in‐house MATLAB program. XiO's eMC module provides accurate dose distributions in the presence of surface irregularities and slab heterogeneities for 12 MeV beams. In the presence of 3D heterogeneities, the percent dose difference comparisons highlighted the need to perform 3D gamma comparisons. In conclusion, the electron Monte Carlo module offered in the XiO treatment planning system is promising and could greatly improve the accuracy of clinical dose calculations. The validation of the software is ongoing, notably concerning more complex phantom geometries. Small field calculations, oblique incidences and cutout factors will also be investigated.
Sci—Thur AM: Planning ‐ 03: Extensive patient specific QA for field junction regions for craniospinal irradiation with Jagged‐Junction IMRT approach without beam edge matching for field junctions39(2012); http://dx.doi.org/10.1118/1.4740088View Description Hide Description
Purpose: Jagged‐Junction IMRT was developed for craniospinal irradiation. An extensive QA was performed for the field junction regions. Methods and Materials: The Jagged‐Junction IMRT plan employed three field sets, each with unique isocentres (Iso1,2,3). Fields from adjacent sets were overlapped and the dose was smoothly integrated inside the overlapped junction. The delivered dose in the junction regions were verified with film and ion chamber measurements on phantoms. An anthropomorphic‐wax phantom was created for verifying the cranio‐spinal junction. For measuring at the spinal‐spinal junction, a solid water phantom was used. The influence of beam mismatching due to setup and mechanical inaccuracy was investigated by shifting all the fields from Iso1 and Iso3 superiorly and inferiorly by 3 mm and at the same time keeping all the fields from Iso2 without any shift. Results: The patient‐averaged difference between the measured dose with ion chamber and planned dose in the cranio‐spinal junction is 0.34 % ± 0.40% and in the spinal‐spinal junction this difference is 0.03% ± 0.71%. The dose profile comparison shows that measured and planned dose profiles match well to each other over a junction region. The patient‐averaged dose difference discrepancy between the film measurement and the planned is 1.1% ± 1.3% at the cranio‐spinal junction and −0.14% ± 1.8% at the spinal junction. Conclusions: Jagged‐Junction IMRT planning provided smooth dose coverage to the target in the field junction region. The junction dose for the Jagged‐Junction IMRT plan is not sensitive to the setup error during the treatment.
Sci—Thur AM: Planning ‐ 04: Evaluation of the fluence complexity, solution quality, and run efficiency produced by five fluence parameterizations implemented in PARETO multiobjective radiotherapy treatment planning software39(2012); http://dx.doi.org/10.1118/1.4740089View Description Hide Description
Purpose: PARETO (Pareto‐Aware Radiotherapy Evolutionary Treatment Optimization) is a novel multiobjective treatment planning system that performs beam orientation and fluence optimization simultaneously using an advanced evolutionary algorithm. In order to reduce the number of parameters involved in this enormous search space, we present several methods for modeling the beam fluence. The parameterizations are compared using innovative tools that evaluate fluence complexity, solution quality, and run efficiency. Methods: A PARETO run is performed using the basic weight (BW), linear gradient (LG), cosine transform (CT), beam group (BG), and isodose‐projection (IP) methods for applying fluence modulation over the projection of the Planning Target Volume in the beam's‐eye‐view plane. The solutions of each run are non‐dominated with respect to other trial solutions encountered during the run. However, to compare the solution quality of independent runs, each run competes against every other run in a round robin fashion. Score is assigned based on the fraction of solutions that survive when a tournament selection operator is applied to the solutions of the two competitors. To compare fluence complexity, a modulation index, fractal dimension, and image gradient entropy are calculated for the fluence maps of each optimal plan. Results: We have found that the LG method results in superior solution quality for a spine phantom, lung patient, and cauda equina patient. The BG method produces solutions with the highest degree of fluence complexity. Most methods result in comparable run times. Conclusion: The LG method produces superior solution quality using a moderate degree of fluence modulation.
Sci—Thur AM: Planning ‐ 05: Lung SBRT: Dosimetric accuracy of the Analytical Anisotropic Algorithm (AAA) for 6MV FFF RapidArc planning39(2012); http://dx.doi.org/10.1118/1.4740090View Description Hide Description
Purpose: Stereotactic Body Radiation Therapy(SBRT) requires the delivery of a high biologically effective dose in only a few fractions. These large doses per fraction can necessitate long treatment times. The Varian Truebeam is capable of RapidArc delivery and also has the optional Flattening Filter Free (FFF) modes which greatly increase the dose rate. We have commissioned the 6MV FFF beam (1400 MU/min) for RapidArc lungSBRT, and verified heterogeneous dose calculations with Monte Carlo (MC).Methods: The standard commissioning data was acquired for Varian's Analytical Anisotropic Algorithm (AAA) beam model. Measurements were acquired with the IBA Blue Phantom, using the CC13 and CC01 ion chambers and PTW diode. MLC‐defined fields were also acquired for model verification. The Dosimetric Leaf Gap (DLG) was measured and then optimized using RapidArc lungSBRT plans, matching Eclipse with ion chamber measurements. Heterogeneous dose calculations were independently verified using MC.Results: There were some discrepancies regarding leaf transmission and penumbra, but the AAA model was generally well within 2% and 2 mm. A nominal DLG value of 1.6 mm was chosen. A representative lungSBRT case utilizing FFF RapidArc was calculated with MC. For the high dose region, 99% matched Eclipse within 3% and 3 mm. The mean dose difference of the PTV was 0.7%. Conclusions: Although we have observed some minor infield discrepancies between the AAA and Monte Carlo calculations in heterogeneous media, the Eclipse AAA is reasonably accurate for complex FFF, RapidArc, SBRTlung planning.
Sci—Thur AM: Planning ‐ 06: Planning target volume margin suitability in lung stereotactic body radiation therapy: A preliminary evaluation using cone‐beam computed tomography39(2012); http://dx.doi.org/10.1118/1.4740091View Description Hide Description
Stereotactic body radiation therapy(SBRT) requires precise delivery of radiation to the target; intra‐ and inter‐fraction lungtumour motion may adversely impact local tumourcontrol. The purpose of this study was to retrospectively evaluate the impact of planning target volume (PTV) margin size on the coverage of the internal target volume (ITV) as localized in pre‐ and post‐treatment cone‐beam computed tomography(CBCT)images. Data from two patients undergoing SBRT were evaluated. For planning, free‐breathing and 4DCT scans were performed, and used to contour the ITV. A 5mm margin was added to create the PTV. During treatment, 14 CBCTs were collected pre‐ and post‐beam delivery. A data set comprising the average 4DCT intensities where available and treatment planningCT intensities for voxels that were beyond the field of view of the 4DCT was constructed. Registration of the combined planning image to each CBCT was performed using a deformable image registration algorithm. The transformations aligning the combined planning image with the CBCTs were applied to the planning ITV to obtain the treatment ITVs. For each CBCT, the fraction of treatment ITV within the PTV was determined using Boolean logic. This was repeated for various PTV margins ranging from 0 to 10 mm at 1mm intervals. The 3 and 5 mm PTV margins covered 95.1 ± 5.9% and 99.0 ± 2.0% of the ITV, respectively. Analysis of additional patients will be performed to confirm these preliminary results, which reinforce the use of a 5mm PTV margin for lungSBRT.
Sci—Thur AM: Planning ‐ 07: A fast and accurate source model for energy and intensity modulated electron beams39(2012); http://dx.doi.org/10.1118/1.4740092View Description Hide Description
The purpose of this study is to develop a highly accurate and fast method for calculating electron beamdose distributions in Modulated Electron Radiation Therapy (MERT). An algorithm has been developed for creating phase‐space files at the exit of a linear accelerator for any arbitrary intensity and energy electron beam without the need of full Monte Carlo simulations. The model assigns each particle to one of the 3 following sources: primary, secondary collimator and electron collimatorscatter. The primary component is derived by fast MC transport in air. The scatter components are derived by the use of MC pre‐calculated leaf kernels. Each kernel includes the fluence distribution, energy distribution and scatter probability of generating an electron from a leaf. The original position is sampled from tunable Gaussian or uniform distributions. The direction is estimated by geometrical means. According to the projection of the direction a particle is rejected if it is expected to suffer a leaf‐hit. A leaf‐hit counter is used to calculate the output of scatter particles based on the pre‐calculated scatter probabilities. To account for multiple coulomb scattering in air a MC‐corrected version of the Fermi‐Eyges scattering theory was implemented. Depth and profile dose distributions were derived for the largest and smallest square field sizes, as well as for irregular and off‐axis fields. The model agreed with full MCdose distributions within 3 % in all cases. Output at the depth of maximum dose exhibited discrepancies less than 2.6 % in all cases. The model was 16–22 times faster in generating a phase‐space file than a full MC simulation with the BEAMnrc code.
Sci—Thur AM: Planning ‐ 08: Validation of a commercial Monte Carlo code used for stereotactic radiosurgery and stereotactic body radiation therapy39(2012); http://dx.doi.org/10.1118/1.4740093View Description Hide Description
Our project consisted of validating the BrainLab iPlan Monte Carlo algorithm, used in conjunction with the stereotactic radiosurgery(SRS) mode of the Varian Novalis TX linear accelerator, for clinical use. Our approach was to “benchmark” the iPlan algorithm by comparing dose distributions with those obtained using a BEAMnrc model of the Novalis SRS mode. The BEAMnrc model was obtained by modifying an existing accelerator model to include the SRS flattening filter and source characteristics of the Novalis TX, and by reprogramming a component module to model the high definition 120‐leaf multi‐leaf collimator. The free parameters of interleaf air gap and leaf density were adjusted by matching to interleaf leakage profiles measured with EBT2 film. The BEAMnrc model was used to perform comparisons of depth dose curves and planar distributions for fields in homogeneous and heterogeneous slab phantoms between both MC codes and film. The source parameters of electron beam energy, size and angular spread were determined to be 6.6 MeV, 0.7 mm and 0.8 mm (cross and in‐plane), and 1.27°, respectively. Comparisons between iPlan and EGSnrc MC codes show agreement within 2% for PDD curves, and a high pass rate (>98%) on gamma analysis (3%/3mm) for planar distributions, when the scored quantity is dose to medium. Discrepancies between both MC codes and film measurements were seen near bone inhomogeneities, where the film trend agrees somewhat with iPlan MC reporting dose‐to‐water. Further work is being performed to understand these differences and how film is used to measure dose near bone.
39(2012); http://dx.doi.org/10.1118/1.4740094View Description Hide Description
In a previous study, the variogram fractal dimension (FD) method was found to be very accurate at identifying planned head and neck IMRT fields that are overly‐modulated. In the current study, the authors used MATLAB® to develop FracMod, a graphical user interface (GUI) and variogram FD analysis tool to assess modulation complexity of dynamic IMRT fields designed for treatments of the prostate alone and prostate plus pelvic nodes. A set of 5 prostate plans (25 fields) and 5 prostate plus pelvic node plans (35 fields) were used to choose FD cut‐points that ensure no false positives (100% specificity) in distinguishing between moderate field modulation (typical modulation used clinically at the authors' institution) and high modulation. Field modulation was controlled by adjusting fluence smoothing parameters in the Eclipse™ treatment planning system. The area under the curve (AUC) from receiver operating characteristic (ROC) analysis was used to quantitatively compare the ability of FD and the number of monitor units (MUs) for distinguishing between the moderate and high modulation fields. The variogram FD method gave AUCs of 0.96 (almost perfect classification) and 1.00 (perfect classification) for the prostate alone and the prostate plus pelvic node fields, respectively. The variogram FD method is an accurate metric; performing better than the number of MUs at identifying high modulation IMRT fields planned for the treatment of prostatic carcinoma. Hence, FracMod will enable RadiotherapyPhysicists to easily and accurately quantify the degree of modulation of IMRT fields and adjust overly‐modulated fields at the treatment planning stage.
Sci—Thur AM: Planning ‐ 10: Improved dosimetric accuracy for patient specific quality assurance using a dual‐detector measurement method for cyberknife output factors39(2012); http://dx.doi.org/10.1118/1.4740095View Description Hide Description
The measurement of output factors for small fields is challenging and can lead to large dose errors in patient treatments if corrections for detector size and scatter from high‐Z material are not applied. Due to its high spatial resolution and near tissue equivalence, GAFCHROMIC® film potentially provides a correction free measure of output factors but it can be challenging to obtain high quality dosimetric results using this film. We propose minimizing errors in the clinical determination of small field output factors by employing diode measurements with Monte‐Carlo generated corrections for small fields ≤10 mm diameter and using small volume ion chambers for apertures >10 mm diameter with independent validation using radiochromic film. We performed patient specific quality assurance (QA) measurements for 9 patients using GAFCHROMIC® film and an A16 small volume ion chamber in a head‐shaped phantom, employing this hybrid dual detector method for relative output factor measurements within the Multiplan treatment planning system. Our results suggest that consistent output factors can be determined using this method with experimental verification using GAFCHROMIC® film dosimetry. For the patient specific QA using film, we achieve good dosimetric agreement (<2σ) of the measured and calculated average dose for pixels within the 80% isodose line. For patient specific QA using the micro‐ion chamber, we get good agreement (<3%) for cone sizes greater than 5 mm. The differences observed for the 5 mm cone plans are consistent with a 1 mm radial setup uncertainty for patient positioning using the Cyberknife system.
Sci—Thur AM: Planning ‐ 11: The impact of distributed calculation framework settings on plan calculation time39(2012); http://dx.doi.org/10.1118/1.4740096View Description Hide Description
Some treatment planning system can divide a treatment plan calculation into multiple threads and allow both local and network computing resources to perform the calculation concurrently, which significantly reduces the calculation time for a calculation‐demanding planning such as Volumetric Modulated Arc Therapy (VMAT) or electron Monte Carlo (eMC). This study tested in Eclipse (Varian, V10.0.39) the impact of Distributed Calculation Framework (DCF, V10.0.0.757) settings on calculation time in a planning environment that consists of 20 workstations with 8 core processors and 16GB RAMs installed on most of them. It is found that for an arc plan increasing the control point field parallelization factor reduces the total calculation time at beginning but lengthens the total calculation time after a certain level as a result of data sending time increase. Further increasing the factor may cause a serious net work traffic or even failure of a calculation. For an eMC plan the calculation time decreases monotonously with the increase of Monte carlo field parallelization factor, and the data sending time is insignificant compared to the calculation time. Increasing the local servant numbers reduces the data sending time but raises the calculation time for arc and eMC plans. The calculation time increment is more and more significant with the increase of local servants. The optimal DCF setting for a facility depends on the total number of calculation workstations available, the hardware configuration of the workstations, and the data transfer rate of the network. No conflict of interest exists in the study.
Sci—Thur AM: Planning ‐ 12: Comparative study of SBRT lung dose calculation using Eclipse and Monte Carlo39(2012); http://dx.doi.org/10.1118/1.4740097View Description Hide Description
Stereotactic Body Radiation Therapy(SBRT) is an option for early stage non‐small cell lungcancertreatment. In SBRTtreatment, high biological effective dose is delivered to the patient within a small number of fractions. High level of confidence in accuracy is required in the entire treatment procedure, from patient setup, tumour delineation, treatment simulation and planning, to the final dose delivery. SBRTlungtreatment utilizes small fields that are incident on large tissue inhomogeneities within the patient. It is difficult for commercially available treatment planning systems (TPS) to model the lack of charged particle equilibrium and the dose near tissue‐lung interfaces accurately. The Monte Carlo (MC) technique calculates the dose distribution from the first principles thereby providing a feasible tool for verifying the dose distribution computed from TPS. In this study, we compared the SBRTdose distribution between Eclipse 8.9 and BEAMnrc/DOSXYZnrc for both conformal and RapidArc plans. Calculation results for five clinical SBRT conformal lung plans were compared. Eclipse and MC results for each plan showed good agreement in dose received by organs at risk. MC simulation predicted uniformly hotter or similar PTV coverage for three cases with tumor either small or attached to the chest wall. When tumor is inside lung and at relatively medium to larger size for SBRT,MC predicted lower PTV coverage. The variation in dose coverage may depend on the tumour size and its position within the lung.Dose comparison for RapidArc plans shows similar dependence.
- J.R. Cunningham Young Investigator Symposium
Sci—Thur PM: YIS — 01: Inverse treatment planning for modulated electrons and mixed photon and electron radiotherapy39(2012); http://dx.doi.org/10.1118/1.4740098View Description Hide Description
Modulated electron radiotherapy (MERT) takes advantage of the low distal dose of electrons to reduce dose to healthy tissue. The dosimetric advantage of MERT is clear when compared against single‐field electron irradiation where MERT demonstrates superior target homogeneity and sparing; however the dosimetric advantage is unclear when comparing MERT with photon intensity‐modulated radiotherapy(IMRT) where MERT techniques struggle to match the IMRT target homogeneity but with less total energy delivered to healthy tissues. In an effort to improve dosimetric benefits of MERT, this study investigated an inverse planning technique for the creation of hybrid MERT‐IMRT mixed beam radiotherapy (MBRT) plans. The optimization process decouples the photon and electron beamlets for combined modality optimization. The input to the optimization algorithm was a series of patient‐specific 3D dose distributions for the corresponding electron and photon beamlets, while the output was a list of weights that satisfied the optimization constraints. A photonIMRT Eclipse (Varian, Palo Alto, CA) plan and a MERT plan were created for a patient‐specific sarcoma irradiation. The MERT plan was competitive in its ability to reduce dose to organs at risk and total‐body dose; however, the plan suffered from poorer target conformity compared with the IMRT plan. The MBRT plan was created by adding two photon fields, divided into beamlets, to the electron beamlets of the MERT plan for reoptimization. The MBRT plan improved MERT target coverage with only minimal cost to healthy tissuedose. The MBRT plan provided clear dosimetric advantages over the IMRT and MERT plan.
Sci—Thur PM: YIS — 02: A validated approach for clinical linacs to accurately determine the photon spectra and the incident electron energy39(2012); http://dx.doi.org/10.1118/1.4740099View Description Hide Description
In clinical photon beams, independent determination of the photon spectra and the incident electron energy is useful for beam (re)commissioning and for detector response modelling. In this study, an approach is developed for that purpose, and validated on a research linac whose photon spectra and electron beams are directly and independently known. In this approach, an optimized combination of transmission curves is measured using multiple attenuators and detectors to maximize energy differentiation. For validation, transmission measurements are made for 8 beams from 10–30 MV, with bremsstrahlung targets from Be to Pb. A protocol is established to account for many influence quantities including linac drifts (2%), polarity (6%), ion recombination (0.2%), leakage (0.3%), room scatter (0.8%), non‐ideal attenuation (1.5%), attenuator mass thickness (4%), and photonuclear effect (5.6%). The experimental accuracy on the smallest signals is 0.4%. EGSnrc is upgraded to model photonuclear attenuation (without tracking secondary particles), and then used to model the full experiment. For direct transmission comparisons, the agreement is 2%. This allows for an estimate of 0.5% on the upper limit of photon cross section uncertainties, which is much better than the current estimate of 1–2%. The unfolded spectra agree with the benchmark ones within 4.5%. The incident electron energy is accurate within 5%, with 95% confidence. The overall improvement over the commonly used methods is a factor of 3. This transmission study is the first to independently determine the incident electron energy, and to recognize the significant role of the photonuclear effect at higher energies.
39(2012); http://dx.doi.org/10.1118/1.4740100View Description Hide Description
Purpose: To evaluate the treatment plan qualities of 4D‐VMAT, gated‐VMAT and 3D‐VMAT in the treatment of non‐small cell lungcancer(NSCLC) in stereotactic body radiation therapy(SBRT).Methods: 4D‐VMAT is a motion compensation strategy that aims to exploit relative target and OAR motion to increase OAR sparing over 3D‐VMAT without the long treatment times associated with gated‐VMAT. The 4D‐VMAT algorithm incorporates the entire patient respiratory cycle and 4D‐CT in the optimization process. Resulting treatment plans synchronize the delivery of each MLC aperture to a specific phase of the target motion. Using software developed in Matlab™, SBRTtreatment plans for 4D‐VMAT, gated‐VMAT and 3D‐VMAT were generated on 3 patients with NSCLC.Tumour motion ranged from 1.4–3.4 cm. The fractionation scheme was 48Gy in 4 fractions with the GTV receiving 100% of the prescribed dose. For gated‐VMAT, the treatment window constrained residual tumour motion to 3 mm or less corresponding to duty cycles of 40–60%. In 3D‐VMAT, the ITV was generated by merging the GTV from all phases. A b‐spline transformation model was used to register the 4D‐CT images and DVHs were calculated from total dose accumulated on the max expiration phase. Results and Conclusion: For the majority of OARs, gated‐VMAT provided the greatest radiation sparing but significantly extended treatment times (25–35 gantry interruptions/arc). For 3D‐VMAT, only 2 patients had clinically acceptable plans that met all the strict dose limits. OAR sparing in 4D‐VMAT was comparable to gated‐VMAT but with significantly improved delivery efficiency.
Sci—Thur PM: YIS — 04: Forcing lateral electron disequilibrium to spare lung tissue: A novel technique for SBRT of small lung tumours39(2012); http://dx.doi.org/10.1118/1.4740101View Description Hide Description
Stereotactic body radiation therapy(SBRT), a technique that uses tightly conformed Megavoltage(MV) x‐ray fields, improves local control of lungcancer. However, small MV x‐ray fields can cause lateral electron disequilibrium(LED), which reduces the dose within lung. These effects are difficult to predict and are presently a cause of alarm for the radiotherapy community. Previously, we developed The Relative Depth Dose Factor(RDDF), which is an indicator of the extent of LED (RDDF < 1). We propose a positive application of LED for lung sparing in SBRT:LED can be exploited to irradiate a small tumor while greatly reducing the dose in surrounding lungtissue. The Monte Carlo code, DOSXYZnrc, was employed to calculate dose within a cylindrical lung phantom. The phantom's diameter and height were set to 25 cm, and consisted of water and lung (density = 0.25g/cm3) shells surrounding a small water tumor (volume = 0.8 cm3). Two 180° 6MV arcs were focused onto the tumor with field sizes of 1×1cm2(RDDF∼0.5) and 3×3cm2(RDDF∼1). Analyzing dose results, the 1×1cm2 arc reduced dose within lung and water tissues by 70% and 80% compared to the 3×3cm2 arc. Although, central tumordose was also reduced by 15% using the 1×1cm2 arc, these reductions can be offset by escalating the prescription dose appropriately. Using the RDDF as a guideline, it's possible to design a SBRT treatment plan that reduces lungdose while maintaining relatively high tumordose levels. Clinical application requires an accurate dose algorithm and may lower SBRTdose‐induced toxicity levels in patients.
Sci—Thur PM: YIS — 05: Tomographic dosimetry using scintillating fibers and its application to 2D and 3D dosimetry39(2012); http://dx.doi.org/10.1118/1.4740102View Description Hide Description
Purpose: To present the proof of concept and the experimental validation of tomographic dosimetry (tomodosimetry), where a tomographic acquisition of the incident deposited dose is performed using long scintillating fibers. Method: 2D tomodosimetry: 50 long scintillating fibers were aligned on a 20cm diameter disk inside a 30cm diameter masonite phantom. 3D tomodosimetry: 128 long scintillating fibers of various orientation were simulated on the surface of two cylindrical regions of radius 7.5 and 3.75cm inside a 20cm diameter, 20cm long cylindrical phantom. In both case, the dose projections were acquire each 5 degrees over a 180 degrees (2D) or 360 degrees (3D) rotation of the device, and the dose in each scintillating fiber plane was reconstructed using a total variation minimization reconstruction iterative algorithm at a resolution of 1×1mm2. The 3D dose was obtained by interpolating between in each cylindrical plane in the 3D prototype. Results: 3%/3mm gamma tests conducted in the isocentre plane for both configurations achieved a success rate of more than 99% of the dose pixels in the region over 50% of the maximum dose. Absolute dose differences in the high dose low gradient region of each scintillating fiber plane were on average below 1% for the 2D configuration and below 1.3% for the 3D configuration. Conclusions: This work illustrates the potential and capacity of scintillating fiber based 2D and 3D tomodosimeters. The presented methodology allows for millimeter resolution dosimetry in a whole 2D plane or 3D volumes in real‐time using only a limited number of detectors.
39(2012); http://dx.doi.org/10.1118/1.4740103View Description Hide Description
Compressed Sensing MRSI (CS‐MRSI) offers the ability to accelerate MRSI sequences while suffering minimal artifacts compared to conventional fast MRSI techniques. CS‐MRSI exploits the inherent sparsity of MRSIimages and incoherent artifacts of pseudo‐random sub‐Nyquist sampling of k‐space combined with non‐linear reconstruction to produces MRSIimages. CS‐MRSI can be used as an acceleration tool to decrease the scan time while maintaining acceptable spatial definition or to enable the acquisition of higher resolution scans while minimizing the associated time penalty. In this work we adopt the compressed sensing technique to accelerate a clinically relevant 2‐D point resolved spectroscopy sequence. However, the process of weighing the cost and benefit of applying such a fast imaging technique is complicated due to the unique non‐linear nature of the reconstruction process and has largely relied on qualitative assessments. Moreover, pseudo‐random sub‐Nyquist sampling of k‐space can have unwanted effects on the modulation transfer function. In this work we set out to quantify the loss in image quality associated with CS‐MRSI. We used simulations of a phantom based method to investigate the MTF behaviour of CS‐MRSI with regard to different k‐space sampling patterns. As expected, the k‐space sampling patterns tested were found to have a direct effect on the MTFs. Moreover, limiting the deviation of the resulting k‐space sampling pattern from the prescribed probability distribution function had a positive effect on the MTF overall. Not only was low‐resolution response improved, but we also noticed an improvement of ∼ 26% in resolution at 0.1 MTF.
Sci—Thur PM: YIS — 07: Monte Carlo simulations to obtain several parameters required for electron beam dosimetry39(2012); http://dx.doi.org/10.1118/1.4740104View Description Hide Description
When current dosimetry protocols were written, electron beam data were limited and had uncertainties that were unacceptable for reference dosimetry.. Protocols for high‐energy reference dosimetry are currently being updated leading to considerable interest in accurate electron beam data. To this end, Monte Carlo simulations using the EGSnrc user‐code egs_chamber are performed to extract relevant data for reference beam dosimetry. Calculations of the absorbed dose to water and the absorbed dose to the gas in realistic ion chamber models are performed as a function of depth in water for cobalt‐60 and high‐energy electron beams between 4 and 22 MeV. These calculations are used to extract several of the parameters required for electron beamdosimetry — the beam quality specifier, R50, beam quality conversion factors, kQ and kR50, the electron quality conversion factor, k′R50, the photon‐electron conversion factor, kecal, and ion chamber perturbation factors, PQ. The method used has the advantage that many important parameters can be extracted as a function of depth instead of determination at only the reference depth as has typically been done. Results obtained here are in good agreement with measured and other calculated results. The photon‐electron conversion factors obtained for a Farmer‐type NE2571 and plane‐parallel PTW Roos, IBA NACP‐02 and Exradin A11 chambers are 0.903, 0.896, 0.894 and 0.906, respectively. These typically differ by less than 0.7% from the contentious TG‐51 values but have much smaller systematic uncertainties. These results are valuable for reference dosimetry of high‐energy electron beams.
39(2012); http://dx.doi.org/10.1118/1.4740105View Description Hide Description
Current generation electronic portal imaging devices(EPID) contain a 1.0 mm copper conversion plate to increase detection efficiency of a therapeutic megavoltage spectrum. When using these EPIDs for low‐Z target imaging, the conversion plate largely attenuates the large populations of diagnostic energy photons, thereby decreasing the benefits of low‐Z target imaging. In this work we measure directly the effect the variation in thickness of a copper conversion plate has on image quality in planar and cone beam computed tomographyimaging. Monte Carlo modeling was used to quantify changes to the diagnostic spectrum and detector response for low‐Z target beams generated with 2.35 and 7.00 MeV electrons incident on a carbon target. Planar contrast‐to‐noise ratio (CNR) measurements were made as a function of copper thickness. Cone beam computed tomography(CBCT)imageCNR measurements were made as a function of dose both with and without the copper plate present in the EPID. The presence of copper in the EPID decreased the diagnostic photon population by up to 20% and suppressed the peak detector response at 60 kV by a factor of 6.4. Planar CNR was increased by a factor ranging from 1.4 to 4.0 with no copper present compared to 1.0 mm thickness. Increases in CBCTimageCNR ranged from a factor of 1.3 to 2.1 with the copper plate removed. As a result of this we suggest that the copper conversion plate be removed from the EPID when used for low‐Z target planar or CBCTimaging.