Index of content:
Volume 36, Issue 9, September 2009
The Chief Information Technology Officer in a Radiation Oncology department should be a medical physicist36(2009); http://dx.doi.org/10.1118/1.3168969View Description Hide Description
- RADIATION THERAPY PHYSICS
36(2009); http://dx.doi.org/10.1118/1.3176936View Description Hide DescriptionPurpose:
AAPM TG-56 recommends the use of a specific dosimetric dataset for each brachytherapysource model. In this study, a full dosimetric dataset for indigenously developedsource models, namely, the CSA1 and CSA2, in accordance with the AAPM TG-43U1 formalism is presented. The study includes calculation of dose-to-kerma ratio in water around these sources including stainless steel encapsulated sources such as RTR, 3M, and selectron/LDR .Methods:
The Monte Carlo–based EGSnrcMP code system is employed for modeling the sources in vacuum and in water. Calculations of air-kerma strength, for the investigated sources and collision kerma in water along the transverse axis of the RTR source are based on the FLURZnrc code. Simulations of water-kerma and dose in water for the CSA1, CSA2, RTR, 3M, and selectron/LDR sources are carried out using the DOSRZnrc code. In DOSRZnrc calculations, water-kerma and dose are scored in a cylindrical water phantom having dimensions of .Results:
The calculated dose-rate constants for the CSA1 and CSA2 sources are 0.945(1) and 1.023(1) cGy/(h U), respectively. The calculated value of per unit source activity, for the CSA1 and CSA2 sources is . The EGSnrcMP-based collision kerma rates for the RTR source along the transverse axis (0.25–10 cm) agree with the corresponding GEANT4-based published values within 0.5%. Anisotropy profiles of the CSA1 and CSA2 sources are significantly different from those of other sources. For the selectron/LDR single pellet spherical source (modeled as a cylindrical pellet with dimensions similar to the seed selectron), the values of at 1 and 1.25 mm from the capsule are 1.023(1) and 1.029(1), respectively. The value of at 1 mm from the CSA1, CSA2, RTR, and 3M source capsules (all sources have an external radius of 1.5 mm) is 1.017(1) and this ratio is applicable to axial positions to . This is in contrast to a published GEANT4-based Monte Carlodosimetric study on RTR and 3M sources wherein the authors have assumed that collision kerma is approximately equal to absorbed dose at 1 mm from the source capsules. Collision kerma is approximately equal to absorbed dose for distances from source capsules as opposed to reported in published studies. A detailed electron transport is necessary up to 2 mm from source capsules.Conclusions:
The Monte Carlo–calculated dose-rate data for the CSA1 and CSA2 sources can be used as input data for treatment planning or to verify the calculations by radiotherapytreatment planning system.
Comparisons of treatment optimization directly incorporating random patient setup uncertainty with a margin-based approach36(2009); http://dx.doi.org/10.1118/1.3176940View Description Hide Description
The purpose of this study is to incorporate the dosimetric effect of random patient positioning uncertainties directly into a commercial treatment planning system’s IMRT plan optimization algorithm through probabilistic treatment planning (PTP) and compare coverage of this method with margin-based planning. In this work, PTP eliminates explicit margins and optimizes directly on the estimated integral treatmentdose to determine optimal patient dose in the presence of setup uncertainties. Twenty-eight prostate patient plans adhering to the RTOG-0126 criteria are optimized using both margin-based and PTP methods. Only random errors are considered. For margin-based plans, the planning target volume is created by expanding the clinical target volume (CTV) by 2.1 mm to accommodate the simulated 3 mm random setup uncertainty. Random setup uncertainties are incorporated into IMRTdose evaluation by convolving each beam’s incident fluence with a Gaussian prior to dose calculation. PTP optimization uses the convolved fluence to estimate dose to ensure CTV coverage during plan optimization. PTP-based plans are compared to margin-based plans with equal CTV coverage in the presence of setup errors based on dose-volume metrics. The sensitivity of the optimized plans to patient-specific setup uncertainty variations is assessed by evaluating dose metrics for dose distributions corresponding to halving and doubling of the random setup uncertainty used in the optimization. Margin-based and PTP-based plans show similar target coverage. A physician review shows that PTP is preferred for 21 patients, margin-based plans are preferred in 2 patients, no preference is expressed for 1 patient, and both autogenerated plans are rejected for 4 patients. For the PTP-based plans, the average CTV receiving the prescription dose decreases by 0.5%, while the mean dose to the CTV increases by 0.7%. The CTV tumor control probability (TCP) is the same for both methods with the exception of one case in which PTP gave a slightly higher TCP. For critical structures that do not meet the optimization criteria, PTP shows a decrease in the volume receiving the maximum specified dose. PTP reduces local normal tissue volumes receiving the maximum dose on average by 48%. PTP results in lower mean dose to all critical structures for all plans. PTP results in a 2.5% increase in the probability of uncomplicated control , along with a 1.9% reduction in rectum normal tissue complication probability (NTCP), and a 0.7% reduction in bladder NTCP. PTP-based plans show improved conformality as compared with margin-based plans with an average PTP-based dosimetric margin at 7100 cGy of 0.65 cm compared with the margin-based 0.90 cm and a PTP-based dosimetric margin at 3960 cGy of 1.60 cm compared with the margin-based 1.90 cm. PTP-based plans show similar sensitivity to variations of the uncertainty during treatment from the uncertainty used in planning as compared to margin-based plans. For equal target coverage, when compared to margin-based plans, PTP results in equal or lower doses to normal structures. PTP results in more conformal plans than margin-based plans and shows similar sensitivity to variations in uncertainty.
36(2009); http://dx.doi.org/10.1118/1.3176951View Description Hide DescriptionPurpose:
A helical tomotherapy accelerator presents a dosimetric challenge because, to this day, there is no internationally accepted protocol for the determination of the absolute dose. Because of this reality, we investigated the different alternatives for characterizing and measuring the absolute dose of such an accelerator. We tested several dosimetric techniques with various metrological traceabilities as well as using a number of phantoms in static and helical modes.Methods:
Firstly, the relationship between the reading of ionization chambers and the absorbed dose is dependent on the beam quality value of the photon beam. For high energy photons, the beam quality is specified by the tissue phantom ratio and it is therefore necessary to know the to calculate the dose delivered by a given accelerator. This parameter is obtained through the ratio of the absorbed dose at 20 and depths in water and was measured in the particular conditions of the tomotherapy accelerator. Afterward, measurements were performed using the ionization chamber (model A1SL) delivered as a reference instrument by the vendor. This chamber is traceable in absorbed dose to water in a Co-60 beam to a watercalorimeter of the American metrology institute (NIST). Similarly, in Switzerland, each radiotherapy department is directly traceable to the Swiss metrology institute (METAS) in absorbed dose to water based on a watercalorimeter. For our research, this traceability was obtained by using an ionization chamber traceable to METAS (model NE 2611A), which is the secondary standard of our institute. Furthermore, in order to have another fully independent measurement method, we determined the dose using alanine dosimeters provided by and traceable to the British metrology institute (NPL); they are calibrated in absorbed dose to water using a graphite calorimeter. And finally, we wanted to take into account the type of chamber routinely used in clinical practice and therefore measured the dose using a Farmer-type instrument (model NE 2571) as well.Results:
We found the tomotherapy value to be around 0.629, which is close to a conventional linear accelerator value. During static irradiation, the secondary standard and the alanine dosimeters were compatible within 0.5%. The A1SL relative deviation to the secondary standard was 1.2% and the NE2571 relative deviation to the secondary standard was . The measurement in dynamic helical mode found the different dosimeters compatible within 1.4% and the alanine dosimeters and the secondary standard were even found under 0.2%.Conclusions:
We found that the different methods are all within uncertainties as well as globally coherent, and the specific limitations of the various dosimeters are discussed in order to help the medical physicist design an independent reference system. We demonstrated that, taking into account the particular reference conditions, one can use an ionization chamber calibrated for conventional linear accelerators to assert the absolute dose delivered by a tomotherapy accelerator.
Considering marker visibility during leaf sequencing for segmental intensity-modulated radiation therapy36(2009); http://dx.doi.org/10.1118/1.3177313View Description Hide DescriptionPurpose:
Segmental intensity-modulated radiation therapy(IMRT)delivers a sequence of segments to obtain a desired intensity distribution. Many leaf sequencing algorithms for segmental IMRT have been developed with the aim of reducing delivered monitor units (MUs) and (or) number of segments and, consequently, to reduce the total treatmentdelivery time. With the development of real-time detection technology, it is useful to develop leaf sequencing algorithms that consider the detecting probability of markers implanted into or near the target volume.Methods:
In this study, the authors defined the concept of marker visibility to denote the marker’s detecting probability and proposed a new leaf sequencing algorithm based on the Kamath algorithm. The new algorithm first uses the Kamath algorithm to generate an initial leaf sequence and then performs a series of column transformations to obtain a new leaf sequence that is optimal in terms of MU efficiency and marker visibility. The authors evaluated the performance of the new algorithm with six artificial fields that had randomly generated intensity matrices and 15 clinical fields that had intensity matrices from the IMRT plans for three prostate cancer patients.Results:
Compared to the Kamath algorithm, the new algorithm does not increase the total delivered intensity but increases the marker visibility. For the artificial fields, the marker visibility increased from 66.67% to 91.67% for small radiation fields, from 39.29% to 42.86% for medium size fields, and from 31.48% to 37.04% for large fields. For the clinical fields, the marker visibility increased 9%–20% for four fields, 20%–30% for three fields, 30%–40% for two fields, and more than 40% for one field. However, the marker visibility did not change for 4 out of 15 fields.Conclusions:
The authors developed a new leaf sequencing algorithm for optimal MU efficiency and marker visibility and also rigorously proved its optimality.
Measurements of dose discrepancies due to inhomogeneities and radiographic contrast in balloon catheter brachytherapy36(2009); http://dx.doi.org/10.1118/1.3183497View Description Hide Description
Recently, a device called MammoSite®, consisting of a balloon and a catheter, was developed to perform partial-breast irradiation using a high-dose-rate (HDR) brachytherapy unit with ease and reproducibility. However, the actual dose to the skin does not agree well with the calculated dose by the treatment planning system because of the difference between the calculation condition and the real treatment condition (i.e., homogeneous water and full scatter condition vs contrast solution and lack of full scatter condition). In this study, the authors experimentally estimated dose discrepancies due to contrast and lack of full scatter in breast HDR brachytherapy with MammoSite®. Using metal-oxide-semiconductor field-effect transistor detectors and a breast phantom, the dose discrepancies between the calculation and the treatment conditions were measured according to contrast concentration (10% and 20% volume ratios), balloon size (35 and ), and source to detector distance ranging from 25 to 50 mm. The source was an Ir-192 isotope from Nucletron HDR unit. The dose discrepancies from the calculation condition due to both contrast and lack of full scatter combined ranged from about to in the studied cases (error bound is in two sided confidence interval of 80% based on Student’s distribution). In all cases, the effect of lack of full scatter was dominant to that of contrast and significant dose discrepancies existed between the calculation and the real treatment conditions, indicating that the actual skindose is less than that which is currently calculated.
Megavoltage planar and cone-beam imaging with low- targets: Dependence of image quality improvement on beam energy and patient separation36(2009); http://dx.doi.org/10.1118/1.3183499View Description Hide DescriptionPurpose:
The purpose of this study is to investigate the improvement of megavoltage planar and cone-beam CT(CBCT)image quality with the use of low atomic number external targets in the linear accelerator.Methods:
In this investigation, two experimental megavoltage imagingbeams were generated by using either 3.5 orelectrons incident on aluminum targets installed above the level of the carousel in a linear accelerator (2100EX, Varian Medical, Inc., Palo Alto, CA). Images were acquired using an amorphous silicon detector panel. Contrast-to-noise ratio(CNR) in planar and CBCTimages was measured as a function of dose and a comparison was made between the imagingbeams and the standard therapy beam. Phantoms of variable diameter were used to examine the loss of contrast due to beam hardening. Porcine imaging was conducted to examine qualitatively the advantages of the low- target approach in CBCT.Results:
In CBCTimagingCNR increases by factors as high as 2.4 and 4.3 for the 7.0 andbeams, respectively, compared to images acquired with . Similar factors of improvement are observed in planar imaging. For the imagingbeams,beam hardening causes a significant loss of the contrast advantage with increasing phantom diameter; however, for the beam and a phantom diameter of , a contrast advantage remains, with increases of contrast by factors of 1.5 and 3.4 over for bone and lung inhale regions, respectively. The spatial resolution is improved slightly in CBCTimages for the imagingbeams.CBCTimages of a porcine cranium demonstrate qualitatively the advantages of the low- target approach, showing greater contrast between tissues and improved visibility of fine detail.Conclusions:
The use of low- external targets in the linear accelerator improves megavoltage planar and CBCTimage quality significantly. CNR may be increased by a factor of 4 or greater. Improvement of the spatial resolution is also apparent.
36(2009); http://dx.doi.org/10.1118/1.3183820View Description Hide Description
In this work, the authors have evaluated ten different ionization chambers for the relative dosimetry of kilovoltage x-ray beams in the energy range of 50–280 kVp. Percentage depth doses in water and relative detector response (in Solid Water and in air) were measured for each of the x-ray beams studied using a number of chambers. Measured depth dose data were compared with Monte Carlo calculated depth doses using the EGSnrc Monte Carlo package and the BEAMnrc user code. The accuracy of the phase space files generated by BEAMnrc was verified by calculating the half-value layer and comparing with the measured half-value layer of each x-ray beam. The results indicate that the Advanced Markus, Markus, NACP, and Roos parallel plate ionization chambers were suitable for the measurement of depth dose data in this beam quality range with an uncertainty of less than 3%, including in the regions close to the watersurface. While the relative detector response of the Farmer and scanning thimble chambers exhibited a better energy response, they were not suitable for depth dose measurements in the first 5 mm below the watersurface with differences of up to 12% in the surfacedose measurement for the 50 kVp x-ray beam. These differences were due to dose artifacts generated by the chamber size and the dose gradient. However, at depths greater than 5 mm, the Farmer and thimble scanning chambers gave uncertainties of less than 3% for the depth dose measurements for all beam energies. The PTW PinPoint 31006 chamber was found to give varying dose differences of up to 8% depending on the x-ray beam energy; this was attributed to the steel central electrode. The authors recommend that one of the parallel plate ionization chambers investigated be used to determine depth dose data for kilovoltage x-ray beams in the energy range studied and give correct dose information close to the surface and at depth in the water phantom.
36(2009); http://dx.doi.org/10.1118/1.3184695View Description Hide DescriptionPurpose:
This article presents an analytical dose calculation method for high-dose-ratebrachytherapy, taking into account the effects of inhomogeneities and reduced photon backscatter near the skin. The adequacy of the Task Group 43 (TG-43) two-dimensional formalism for treatment planning is also assessed.Methods:
The proposed method uses material composition and density data derived from computed tomography images. The primary and scatterdose distributions for each dwell position are calculated first as if the patient is an infinite water phantom. This is done using either TG-43 or a database of Monte Carlo (MC)dose distributions. The latter can be used to account for the effects of shielding in water. Subsequently, corrections for photon attenuation, scatter, and spectral variations along medium- or low- inhomogeneities are made according to the radiological paths determined by ray tracing. The scatterdose is then scaled by a correction factor that depends on the distances between the point of interest, the body contour, and the source position. Dose calculations are done for phantoms with tissue and lead inserts, as well as patient plans for head-and-neck, esophagus, and MammoSite balloon breast brachytherapy treatments. Gamma indices are evaluated using a dose-difference criterion of 3% and a distance-to-agreement criterion of 2 mm. PTRAN_CTMC calculations are used as the reference dose distributions.Results:
For the phantom with tissue and lead inserts, the percentages of the voxels of interest passing the gamma criteria are 100% for the analytical calculation and 91% for TG-43. For the breast patient plan, TG-43 overestimates the target volume receiving the prescribed dose by 4% and the dose to the hottest of the skin by 9%, whereas the analytical and MC results agree within 0.4%. are 100% and 48% for the analytical and TG-43 calculations, respectively. For the head-and-neck and esophagus patient plans, are for both calculation methods.Conclusions:
A correction-based dose calculation method has been validated for HDRbrachytherapy. Its high calculation efficiency makes it feasible for use in treatment planning. Because tissue inhomogeneity effects are small and primary dose predominates in the near-source region, TG-43 is adequate for target dose estimation provided shielding and contrast solution are not used.
From the limits of the classical model of sensitometric curves to a realistic model based on the percolation theory for GafChromic™ EBT films36(2009); http://dx.doi.org/10.1118/1.3187226View Description Hide DescriptionPurpose:
Modern radiotherapy uses complex treatments that necessitate more complex quality assurance procedures. As a continuous medium, GafChromic EBT films offer suitable features for such verification. However, its sensitometric curve is not fully understood in terms of classical theoretical models. In fact, measured optical densities and those predicted by the classical models differ significantly. This difference increases systematically with wider dose ranges. Thus, achieving the accuracy required for intensity-modulated radiotherapy(IMRT) by classical methods is not possible, plecluding their use. As a result, experimental parametrizations, such as polynomial fits, are replacing phenomenological expressions in modern investigations. This article focuses on identifying new theoretical ways to describe sensitometric curves and on evaluating the quality of fit for experimental data based on four proposed models.Methods:
A whole mathematical formalism starting with a geometrical version of the classical theory is used to develop new expressions for the sensitometric curves. General results from the percolation theory are also used. A flat-bed-scanner-based method was chosen for the film analysis. Different tests were performed, such as consistency of the numeric results for the proposed model and double examination using data from independent researchers.Results:
Results show that the percolation-theory-based model provides the best theoretical explanation for the sensitometric behavior of GafChromic films. The different sizes of active centers or monomer crystals of the film are the basis of this model, allowing acquisition of information about the internal structure of the films. Values for the mean size of the active centers were obtained in accordance with technical specifications. In this model, the dynamics of the interaction between the active centers of GafChromic film and radiation is also characterized by means of its interaction cross-section value.Conclusions:
The percolation model fulfills the accuracy requirements for quality-control procedures when large ranges of doses are used and offers a physical explanation for the film response.
36(2009); http://dx.doi.org/10.1118/1.3187229View Description Hide Description
Purpose: The purpose of this study is to describe the University of Texas M. D. Anderson proton therapysystem (PTC-H) including the accelerator, beam transport, and treatment delivery systems, the functionality and clinical parameters for passive scattering and pencil beam scanning treatment modes, and the results of acceptance tests.
Methods: The PTC-H has a synchrotron and four treatment rooms. An overall control system manages the treatment, physics, and service modes of operation. An independent safety system ensures the safety of patients, staff, and equipment. Three treatment rooms have isocentric gantries and one room has two fixed horizontal beamlines, which include a large-field treatment nozzle, used primarily for prostate treatments, and a small-field treatment nozzle for ocular treatments. Two gantry treatment rooms and the fixed-beam treatment room have passive scattering nozzles. The third gantry has a pencil beam scanning nozzle for the delivery of intensity modulated protontreatments (IMPT) and single field uniform dose (SFUD) treatments. The PTC-H also has an experimental room with a fixed horizontal beamline and a passive scattering nozzle. The equipment described above was provided by Hitachi, Ltd. Treatment planning is performed using the Eclipse system from Varian MedicalSystems and data management is handled by the MOSAIQ system from IMPAC MedicalSystems, Inc. The large-field passive scattering nozzles use double scattering systems in which the first scatterers are physically integrated with the range modulation wheels. The proton beam is gated on the rotating range modulation wheels at gating angles designed to produce spread-out-Bragg peaks ranging in size from . Field sizes of up to can be achieved with the double scattering system. The IMPT delivery technique is discrete spot scanning, which has a maximum field size of . Depth scanning is achieved by changing the energy extracted from the synchrotron (energy can be changed pulse to pulse). The PTC-H is fully integrated with DICOM-RT ION interfaces for imaging,treatment planning,data management, and treatment control functions.
Results: The proton therapysystem passed all acceptance tests for both passive scattering and pencil beam scanning. Treatments with passive scattering began in May 2006 and treatments with the scanning system began in May 2008. The PTC-H was the first commercial system to demonstrate capabilities for IMPT treatments and the first in the United States to treat using SFUD techniques. The facility has been in clinical operation since May 2006 with up-time of approximately 98%.
Conclusions: As with most projects for which a considerable amount of new technology is developed and which have duration spanning several years, at project completion it was determined that several upgrades would improve the overall system performance. Some possible upgrades are discussed. Overall, the system has been very robust, accurate, reproducible, and reliable. The authors found the pencil beam scanning system to be particularly satisfactory; prostate treatments can be delivered on the scanning nozzle in less time than is required on the passive scattering nozzle.
36(2009); http://dx.doi.org/10.1118/1.3187785View Description Hide Description
Portal dosimetricimages acquired for IMRT pretreatment verification show dose errors of up to 15% near the detector edges as compared to dose predictions calculated by a treatment planning system for these off-axis regions. A method is proposed to account for these off-axis effects by precisely correcting the off-axis output factors, which calibrate the imager for absolute dose. Using this method, agreement between the predicted and the measured doses improves by up to 15% for fields near the detector edges, resulting in passing rate improvements of as much as 60% for gamma evaluation of , 3% within the collimator jaws.
36(2009); http://dx.doi.org/10.1118/1.3190156View Description Hide DescriptionPurpose:
The graphical processing unit (GPU) on modern graphics cards offers the possibility of accelerating arithmetically intensive tasks. By splitting the work into a large number of independent jobs, order-of-magnitude speedups are reported. In this article, the possible speedup of PLATO’s ray tracing algorithm for dose calculations using a GPU is investigated.Methods:
A GPU version of the ray tracing algorithm was implemented using NVIDIA’s CUDA, which extends the standard C language with functionality to program graphics cards. The developed algorithm was compared based on the accuracy and speed to a multithreaded version of the PLATO ray tracing algorithm. This comparison was performed for three test geometries, a phantom and two radiotherapy planning CT datasets (a pelvic and a head-and-neck case). For each geometry, four different source positions were evaluated. In addition to this, for the head-and-neck case also a vertex field was evaluated.Results:
The GPU algorithm was proven to be more accurate than the PLATO algorithm by elimination of the look-up table for indices that introduces discretization errors in the reference algorithm. Speedups for ray tracing were found to be in the range of 2.1–10.1, relative to the multithreaded PLATO algorithm running four threads. For dose calculations the speedup measured was in the range of 1.5–6.2. For the speedup of both the ray tracing and the dose calculation, a strong dependency on the tested geometry was found. This dependency is related to the fraction of air within the patient’s bounding box resulting in idle threads.Conclusions:
With the use of a GPU, ray tracing for dose calculations can be performed accurately in considerably less time. Ray tracing was accelerated, on average, with a factor of 6 for the evaluated cases. Dose calculation for a single beam can typically be carried out in for clinically realistic datasets. These findings can be used in conventional planning to enable (nearly) real-time dose calculations. Also the importance for treatment optimization techniques is evident.
Verification and source-position error analysis of film reconstruction techniques used in the brachytherapy planning systems36(2009); http://dx.doi.org/10.1118/1.3191665View Description Hide Description
A method was presented that employs standard linac QA tools to verify the accuracy of film reconstruction algorithms used in the brachytherapy planning system. Verification of reconstruction techniques is important as suggested in the ESTRO booklet 8: “The institution should verify the full process of any reconstruction technique employed clinically.” Error modeling was also performed to analyze seed-position errors. The “isocentric beam checker” device was used in this work. It has a two-dimensional array of steel balls embedded on its surface. The checker was placed on the simulator couch with its center ball coincident with the simulator isocenter, and one axis of its cross marks parallel to the axis of gantry rotation. The gantry of the simulator was rotated to make the checker behave like a three-dimensional array of balls. Three algorithms used in the ABACUStreatment planning system: orthogonal film, 2-films-with-variable-angle, and 3-films-with-variable-angle were tested. After exposing and digitizing the films, the position of each steel ball on the checker was reconstructed and compared to its true position, which can be accurately calculated. The results showed that the error is dependent on the object-isocenter distance, but not the magnification of the object. The averaged errors were less than within the tolerance level defined by Roué et al. [“The EQUAL-ESTRO audit on geometric reconstruction techniques in brachytherapy,” Radiother. Oncol.78, 78–83 (2006)]. However, according to the error modeling, the theoretical error would be greater than if the objects were located more than away from the isocenter with a 0.5° reading error of the gantry and collimator angles. Thus, in addition to carefully performing the QA of the gantry and collimator angle indicators, it is suggested that the patient, together with the applicators or seeds inside, should be placed close to the isocenter as much as possible. This method could be used to test the reconstruction techniques of any planning system, and the most suitable one can be chosen for clinical use.
36(2009); http://dx.doi.org/10.1118/1.3193677View Description Hide Description
Purpose: The purpose of this study is to establish the in vivo verification of protonbeam path by using proton-activated positron emission distributions.
Methods: A total of 50 PET/CT imaging studies were performed on ten prostate cancer patients immediately after daily proton therapy treatment through a single lateral portal. The PET/CT and planning CT were registered by matching the pelvic bones, and the beam path of delivered protons was defined in vivo by the positron emission distribution seen only within the pelvic bones, referred to as the PET-definedbeam path. Because of the patient position correction at each fraction, the marker-definedbeam path, determined by the centroid of implanted markers seen in the post-treatment (post-Tx) CT, is used for the planned beam path. The angular variation and discordance between the PET- and marker-defined paths were derived to investigate the intrafraction prostate motion. For studies with large discordance, the relative location between the centroid and pelvic bones seen in the post-Tx CT was examined. The PET/CT studies are categorized for distinguishing the prostate motion that occurred before or after beam delivery. The post-PET CT was acquired after PETimaging to investigate prostate motion due to physiological changes during the extended PET acquisition.
Results: The less than 2° of angular variation indicates that the patient roll was minimal within the immobilization device. Thirty of the 50 studies with small discordance, referred as good cases, show a consistent alignment between the field edges and the positron emission distributions from the entrance to the distal edge. For those good cases, average displacements are 0.6 and 1.3 mm along the anterior-posterior and superior-inferior directions, respectively, with 1.6 mm standard deviations in both directions. For the remaining 20 studies demonstrating a large discordance (more than 6 mm in either or ), 13 studies, referred as motion-after-Tx cases, also show large misalignment between the field edge and the positron emission distribution in lipomatous tissues around the prostate. These motion-after-Tx cases correspond to patients with large changes in volume of rectal gas between the post-Tx and the post-PET CTs. The standard deviations for and are 5.0 and 3.0 mm, respectively, for these motion-after-Tx cases. The final seven studies, referred to as position-error cases, which had a large discordance but no misalignment, were found to have deviations of 4.6 and 3.6 mm in and , respectively. The position-error cases correspond to a large discrepancy on the relative location between the centroid and pelvic bones seen in post-Tx CT and recorded x-ray radiographs.
Conclusions: Systematic analyses of proton-activated positron emission distributions provide patient-specific information on prostate motion () and patient position variability () during daily protonbeam delivery. The less than 2 mm of displacement variations in the good cases indicates that population-based values of and used in margin algorithms for treatment planning at the authors’ institution are valid for the majority of cases. However, a small fraction of PET/CT studies (approximately 14%) with displacement variations may require different margins. Such data are useful in establishing patient-specific planning target volume margins.
Monte Carlo model for a prototype CT-compatible, anatomically adaptive, shielded intracavitary brachytherapy applicator for the treatment of cervical cancer36(2009); http://dx.doi.org/10.1118/1.3193682View Description Hide DescriptionPurpose:
Current, clinically applicable intracavitary brachytherapy applicators that utilize shielded ovoids contain a pair of tungsten-alloy shields which serve to reduce dose delivered to the rectum and bladder during source afterloading. After applicator insertion, these fixed shields are not necessarily positioned to provide optimal shielding of these critical structures due to variations in patient anatomies. The authors present a dosimetric evaluation of a novel prototype intracavitary brachytherapy ovoid [anatomically adaptive applicator], featuring a single shield whose position can be adjusted with two degrees of freedom: Rotation about and translation along the long axis of the ovoid.Methods:
The dosimetry of the device for a HDR was characterized using radiochromic film measurements for various shield orientations. A MCNPX Monte Carlo model was developed of the prototype ovoid and integrated with a previously validated model of a v2 mHDR source (Nucletron Co.). The model was validated for three distinct shield orientations using film measurements.Results:
For the most complex case, 91% of the absolute simulated and measured dose points agreed within 2% or and 96% agreed within 10% or .Conclusions:
Validation of the Monte Carlo model facilitates future investigations into any dosimetric advantages the use of the may have over the current state of art with respect to optimization and customization of dose delivery as a function of patient anatomical geometries.
36(2009); http://dx.doi.org/10.1118/1.3196182View Description Hide Description
Purpose: The purpose of this work is to characterize the x-ray volume imager (XVI), the cone-beam computed tomography(CBCT) unit mounted on the Elekta Synergy linac, with F1 bowtie filter and to calculate the three-dimensional dose delivered to patients using volumetric acquisition.
Methods: The XVI is modeled in detail using a new Monte Carlo(MC) code, BEAMPP, under development at the National Research Council Canada. In this investigation, a new component module is developed to accurately model the unit’s bowtie filter used in conjunction with the available beam collimators at the clinical energy of . The modeling is compared against percentage depth dose (PDD) and profile measurements. Kilovoltage radiation beams’ phase space files are also analyzed. The authors also describe a method for the absolute dosecalibration of the MCmodel of the CBCT unit when used in a clinical volumetric acquisition mode. Finally, they calculate three-dimensional patient dose from CBCT image acquisition in three clinical cases of interest: Pelvis, lung, and head and neck.
Results: The agreement between measurement and MC is shown to be very good: Within for the PDD and within inside the radiation field for all the collimators with the F1 bowtie filter. A full account of the absolute calibration method is given and dose calculation is validated against ion chamber measurements in different locations of a plastic phantom. Calculations and experiments agree within or better in both at the center and the periphery of the phantom, with worst agreement of 4.5% at the surface of the phantom and for one specific combination of collimator and filter. Patient dose from CBCT scan reveals that dose to tissue is between 2 and for a pelvis or a lung full acquisition. For H&N dose to tissue is , with the unit presets used in this work. Dose to bony structures can be two to three times higher than dose to tissue.
Conclusions: The XVI CBCT unit has been fully modeled including the F1 bowtie filter. Absolute dose distribution from the unit has been successfully validated. Full MC patient dose calculation has shown that the three-dimensional dose distribution from CBCT is complex. Patient dose from CBCT exposure cannot be completely accounted for by using a numerical factor as an estimate of the dose at the center of the body. Furthermore, additional dose to bone should be taken into account when adopting any IGRT strategy and weighed vs the unquestionable benefits of the technique in order to optimize treatment. Full three-dimensional dose calculation is recommended if patient dose from CBCT is to be integrated in any adaptive planning strategy.
36(2009); http://dx.doi.org/10.1118/1.3197062View Description Hide DescriptionPurpose:
A commonly used beam quality index for high-energy photon beams is the tissue phantom ratio for a square field of and SDD of . On some specialized radiotherapy treatment equipment such a reference collimator setting is not achievable. Likewise a flat beam profile, not explicitly required in dosimetry protocols, but certainly influences the measurement of , is not always produced. In this work, a method was developed in order to determine at any field size, especially for small and nonflattened beams.Methods:
An analytical relationship was derived between for arbitrary field sizes and [the ] as quality index. The proposed model equation was fitted to the measured and published data in order to achieve three general fit parameters. The procedure was then tested with published data from TomoTherapy and CyperKnife treatment devices.Results:
For standard flattened photon fields, the uncertainty in measured at any field size using the parameters derived from this study is better than 1%. In flattening-filter free beams, the proposed procedure results in a reliable for any field size setting.Conclusions:
A method is introduced and successfully tested in order to measure the beam quality under nonstandard conditions. It can be used, e.g., to get energy dependent correction factors as tabulated in dosimetry codes of practice even if standard conditions are not adjustable.
36(2009); http://dx.doi.org/10.1118/1.3183498View Description Hide DescriptionPurpose:
Several authors studied the problem of geometrical matching of fields produced by medicallinear accelerators. However, a general solution has yet to be published. Currently available solutions are based on parallelism arguments. This study provides a general solution, considering not only parallelism but also field sizes.Methods:
A fixed field with arbitrary field size, gantry, collimator, and couch angle is considered, and another field with a fixed gantry angle is matched to it. A single reference system attached to the treatment couch is used, and two approaches are followed. In the first approach, fixed field sizes are assumed and parallelism of the adjacent field-side planes is imposed. In the second approach, fixed isocenter positions are considered and both parallelism and coincidence between field-side planes are required.Results:
When fixed field sizes are assumed, rotation angles are obtained; however, the isocenters may need to be shifted to make side planes coincident and therefore achieve a proper match. When fixed isocenter positions are considered, solutions for all parameters, including the field size, are obtained and an exact geometrical match is achieved.Conclusions:
General expressions to the field-matching problem are found for the two approaches followed, fixed field sizes, and fixed isocenter positions. These results can be applied to any treatment technique and can easily be implemented in modern treatment planning systems.
36(2009); http://dx.doi.org/10.1118/1.3196236View Description Hide DescriptionPurpose:
Clinically safe and effective treatment of intrafractionally moving targets with scanned ion beams requires dedicated delivery techniques such as beam tracking. Apart from treatment delivery, also appropriate methods for validation of the actual tumorirradiation are highly desirable. In this contribution the feasibility of four-dimensionally (space and time) resolved, motion-compensated in-beam positron emission tomography (4DibPET) was addressed in experimental studies with scanned carbonion beams.Methods:
A polymethyl methracrylate block sinusoidally moving left-right in beam’s eye view was used as target. Radiological depth changes were introduced by placing a stationary ramp-shaped absorber proximal of the moving target. Treatment delivery was compensated for motion by beam tracking. Time-resolved, motion-correlated in-beam PET data acquisition was performed during beam delivery with tracking the moving target and prolonged after beam delivery first with the activated target still in motion and, finally, with the target at rest. Motion-compensated 4DibPET imaging was implemented and the results were compared to a stationary reference irradiation of the same treatment field. Data were used to determine feasibility of 4DibPET but also to evaluate offline in comparison to in-beam PET acquisition.Results:
4D in-beam as well as offline PETimaging was found to be feasible and offers the possibility to verify the correct functioning of beam tracking. Motion compensation of the imaged-activity distribution allows recovery of the volumetric extension of the delivered field for direct comparison with the reference stationary condition. Observed differences in terms of lateral field extension and penumbra in the direction of motion were typically less than 1 mm for both imaging strategies in comparison to the corresponding reference distributions. However, in-beam imaging retained a better spatial correlation of the measured activity with the delivered dose.Conclusions:
4DibPET is a feasible and promising method to validate treatment delivery of scanned ion beams to moving targets. Further investigations will focus on more complex geometries and treatment planning studies with clinical data.