Phantom System Research Articles

CT-guided procedures: evaluation of a phantom system to teach accurate needle placement

To evaluate the use of a phantom system to help teach the basic techniques of accurate CT-guided needle placement, thereby avoiding the risks associated with teaching on patients. Gelatine phantoms with five, 1.9 cm embedded spherical wooden targets were constructed. Four trainee operators performed 15 simulated biopsy procedures on the targets (series one) and repeated identical procedures 2 weeks later (series two). Statistical analysis of accuracy of needle placement and subject confidence were performed. Significant sequential improvement in axial plane angular error was noted with the average error decreasing by 0.33 degrees after every five procedures performed (95% CI: -0.58 to -0.08, p=0.01). Operator confidence indicated significant improvement both within each series and from series one to series two (95% CI: 0.08 to 1.17, p=0.025 and 95% CI: 0.00 to 0.58, p=0.05) respectively. However, variability in operator performance made statistically significant improvement in other variables unproven. Despite the study comprising a relatively small number of participants and procedures, it clearly demonstrated the effectiveness of teaching operators to perform CT-guided procedures using a phantom system. Needle placement accuracy significantly improved, with a reduction in axial angular error, and improved operator confidence without the risks associated with training on patients. Three of the operators in this study had never performed a CT-guided procedure previously, and their proficiency, after a relatively short but intense period of training, was impressive. The use of phantoms should be considered routinely for basic training of CT-guided needle placement.

A motion phantom study on helical tomotherapy: the dosimetric impacts of delivery technique and motion

Helical tomotherapy (HT) can potentially be used for lung cancer treatment including stereotactic radiosurgery because of its advanced image guidance and its ability to deliver highly conformal dose distributions. However, previous theoretical and simulation studies reported that the effect of respiratory motion on statically planned tomotherapy treatments may cause substantial differences between the calculated and actual delivered radiation isodose distribution, particularly when the treatment is hypofractionated. In order to determine the dosimetric effects of motion upon actual HT treatment delivery, phantom film dosimetry measurements were performed under static and moving conditions using a clinical HT treatment unit. The motion phantom system was constructed using a programmable motor, a base, a moving platform and a life size lung heterogeneity phantom with wood inserts representing lung tissue with a 3.0 cm diameter spherical tumour density equivalent insert. In order to determine the effects of different motion and tomotherapy delivery parameters, treatment plans were created using jaw sizes of 1.04 cm and 2.47 cm, with incremental gantry rotation periods between the minimum allowed (10 s) and the maximum allowed (60 s). The couch speed varied from 0.009 cm s−1 to 0.049 cm s−1, and delivered to a phantom under static and dynamic conditions with peak-to-peak motion amplitudes of 1.2 cm and 2 cm and periods of 3 and 5 s to simulate human respiratory motion of lung tumours. A cylindrical clinical target volume (CTV) was contoured to tightly enclose the tumour insert. 2.0 Gy was prescribed to 95% of the CTV. Two-dimensional dose was measured by a Kodak EDR2 film. Dynamic phantom doses were then quantitatively compared to static phantom doses in terms of axial dose profiles, cumulative dose volume histograms (DVH), percentage of CTV receiving the prescription dose and the minimum dose received by 95% of the CTV. The larger motion amplitude resulted in more under-dosing at the ends of the CTV in the axis of motion, and this effect was greater for the smaller jaw size plans. Due to the size of the penumbra, the 2.47 cm jaw plans provide adequate coverage for smaller amplitudes of motion, ±0.6 cm in our experiment, without adding any additional margin in the axis of motion to the treatment volume. The periodic heterogeneous patterns described by previous studies were not observed from the single fraction of the phantom measurement. Besides the jaw sizes, CTV dose coverage is not significantly dependent on machine and phantom motion periods. The lack of adverse synchronization patterns from both results validate that HT is a safe technique for treating moving target and hypofractionation.

2514: Biological Lung Phantom for Image-Guided Proton Therapy Validation

To evaluate the feasibility of a biological lung phantom to validate image-guided proton beam therapy. Human lung tissue is a complex, dynamic system consisting of lung parenchyma, blood vessels, and airways, each having different electron densities. Furthermore, this system deforms and moves in a variable manner during normal respiration. In contrast, conventional radiological phantoms (e.g., the Rando® phantom) assume a static, uniform-density lung field. A more realistic phantom system that takes regional density variations and ventilation-induced motion and deformation into account is needed to verify the ability to deliver proton treatment plans to well-defined penetration depths. To this end, swine lung preserved to retain vital mechanical properties was obtained and compared to human lung for suitability as a phantom. The phantom was ventilated and imaged on a GE Lightspeed CT scanner at various lung filling states. Volume rendering of the CT image data using BioImage (SCIRun BioPSE software) was performed to visualize and determine coordinates of airway bifurcations. To demonstrate the residual range variation in proton track length in a dynamic, heterogeneous lung tissue system, a virtual lung tissue model based on lung histology and matched to measured swine lung phantom CT density at different points in the ventilatory cycle was generated. A 100 MeV proton pencil beam traversing the 10 cm thick tissue model was then simulated with the proton beam terminating in a water beamstop to calculate residual range. Preserved swine lung was found to be equivalent to human lung in terms of radiological and physical properties, lobar structure, airway architecture, volume and overall mass, and has been determined to be histologically comparable to human lung by a board-certified pathologist. Analysis of volume rendered CT images yielded a conservative minimum of 31 reproducible anatomic landmarks (airway bifurcations) evenly distributed throughout the lung. Residual ranges varying from 3 and 76 mm in the post-model water beamstop were calculated (mean of 39 mm, standard deviation of 30 mm), dependent on the breathing phase of the lung phantom. The preserved swine lung lasted approximately 8 weeks before degrading, which may be extended through improved preservation techniques. Simulation results suggest that respiratory motion not only compromises the therapeutic dose delivery to a moving target, but can result in significant excess dose delivery to critical structures downstream of the target volume (e.g., spinal cord, esophagus) through variations in residual range. Advantages of the dynamically ventilated preserved swine lung phantom for proton depth-dose studies with a proton range telescope include realistic geometry, motion, deformation, and the ability to implant radio-opaque clips and tumor surrogates. Disadvantages include compromises in true lung deformation due to external ventilation instead of physiological respiration, as well as the inevitable degradation of the system. We believe that this dynamic biological lung phantom will be useful as a verification tool for image-guided proton therapy, in that it will allow for the planning, scanning, and delivery of proton dose to a reproducible, complex, spatially and temporally variable system.

TU‐FF‐A1‐05: A Motion Phantom Study On Helical Tomotherapy: The Dosimetric Impacts of Delivery Technique and Motion

Purpose: To determine the optimal delivery technique for treating moving targets with Helical Tomotherapy (HT) we measured the dosimetric effects of various motion and Tomotherapy plan parameters using a lung motion phantom. Method and Materials: A motion phantom system was constructed using a programmable motor driving a moving platform. Tomotherapy plans were delivered to a phantom which mimicked lung heterogeneity by using wood inserts with the density of lung tissue and a 3.3cm spherical tissue density material as tumor. A cylindrical planning target volume (PTV) was used with a length and diameter of 3.3cm including the sphere. Treatment plans were created using jaw sizes of 1.04 and 2.47cm with incremental gantry rotation times ranging from (10s) to (60s). Treatments based on these plans were delivered to the phantom with motion periods of 3 and 5 seconds, and amplitudes of +/−6 and 10mm. All plans were normalized to 2.0 Gy fractional doses to 95% of the PTV and Kodak EDR‐2 film was used for 2‐D dose measurements. Axial dose profiles and cumulative dose volume histograms (DVH) of plans for moving and static phantom conditions were compared. Results: Target edge under‐dosing is less with a jaw size of 2.47cm than 1.04cm in this experimental system. Shorter motion periods (3s) and greater gantry rotation (>5 times motion period) resulted in better PTV coverage referenced to static phantoms. Greater than 90% of the PTV received the prescription dose even when the phantom moves +/−10mm. When the gantry rotation time to motion period ratio approached 3 PTV coverage was compromised. Conclusions: PTV expansions smaller than motion excursion may be possible for lung tumor treatment by HT. Larger jaw width provides better target coverage because of the wider inferior/superior penumbra. Increasing the ratio of the gantry rotation time to the motion period improves target edge coverage.

患者呼吸模擬システムを用いた呼吸同期照射法に対する線量評価

Respiratory-gated (RG) radiotherapy is useful for minimizing the irradiated volume of normal tissues resulting from the shifting of internal structures caused by respiratory movement. In this technique, although improvement in the dose distribution of the target can be expected, the actual absorbed dose distribution is not clearly determined. Therefore, it is important to clarify the absorbed dose at the tumor and at the evaluation points according to the patient's respiration. We have developed a phantom system that simulates patient respiration (TNK Co., Ltd.), to evaluate the absorbed dose and ensure precise RG radiotherapy. Actual patient respiratory signals were obtained using a respiratory synchronization and gating system (AZ-733V, Anzai Medical). The acquired data were then transferred to a phantom system driven by a ball screw to simulate the shifting of internal structures caused by respiratory movement. We measured the absorbed dose using a micro-ionization chamber dosimeter and the dose distribution using the film method for RG irradiation at expiratory phase by using Linac (PRIMUS, Toshiba Medical Systems Corp.) X-rays. When the distance of phantom movement was set to the average patient respiratory movement distance of 1.5 cm, we first compared absorbed dose with RG irradiation with a gating signal of 50% or less, and without RG irradiation. The absorbed dose at the iso-center was improved by 6.0% and 4.4% at a field size of 4x4 cm2, and by 1.3% and 0.7% at a field size of 5x5 cm2 with an X-ray energy of 6 MV and 10 MV, respectively. There was, however, no dose change at a field size of 10x10 cm2 and 15x15 cm2. When the gating signal was reduced to 25% and 10%, absorbed dose was also improved. With regard to the flatness of the dose profile, no changes in dose distribution were observed in the lateral direction, e.g., beam flatness was within 1.4% and 1.6% at field sizes of 5x5 cm2 and 10x10 cm2, respectively, with an X-ray energy of 6 MV. In the cranial-caudal direction, the dose profile was relatively large even if a gating signal of 50% was applied, i.e., 8.1% and 10.4% at field sizes of 5x5 cm2 and 10x10 cm2, respectively. Beam flatness without RG was much worse, i.e., 37.8% and 38.2%, at field sizes of 5x5 cm2 and 10x10 cm2, respectively. In both cases, the dose was insufficient in the expiratory direction. Although RG radiotherapy is quite useful, the margins in the inspiratory and expiratory phases should be considered based on the level of gating signal and field size in order to formulate appropriate radiotherapy planning in terms of the shifting of internal structures. To ensure accurate radiotherapy, the characteristics of the RG irradiation technique and the radiotherapy equipment must be clearly understood when this technique is to be employed in clinical practice.

患者呼吸模擬ファントムシステムを用いた呼吸同期照射法に対する照射位置の評価

Respiratory-gated (RG) radiotherapy is useful for minimizing the irradiated volume of normal tissues resulting from the shifting of internal structures caused by respiratory movement. The present study was conducted to evaluate the treatment field in RG radiotherapy using a phantom system simulating patient respiration. A phantom system consisting of a 3-cm ball-shaped dummy tumor and film placed in a cork lung phantom was used (THK Co., Ltd.). RG radiotherapy was employed in the expiratory phase. The phantom movement distance was set to 2 cm, and the gating signals from a respiratory-gating system (AZ-733V, Anzai Medical) were varied. The settings used for irradiation were an X-ray energy of 6 MV (PRIMUS, Toshiba Medical Systems), treatment field of 5 cm x 7 cm, and X-ray dose of 100 MU. Images were acquired using an electric portal-imaging device (EPID, OPTIVUE 500), and the X-ray dose distribution was measured by the film method. In images acquired using the EPID, the tumor margins became less clear when the gating signals were increased, and the ITVs were determined to be 3.6 cm, 3.7 cm, 4.2 cm, and 5.1 cm at gating rates of 10%, 25%, 50%, and no gate, respectively. With regard to the X-ray dose distribution measured by the film method, the dose profile in the cephalocaudal direction was shifted toward the expiratory phase, and the degree of shift became greater when the gating signals were increased. In addition, the optimal treatment fields in the cephalocaudal direction were determined to be 5.2 cm, 5.2 cm, 5.6 cm, and 7.0 cm at gating rates of 10%, 25%, 50%, and no gating, respectively. Although RG radiotherapy is useful for improving the accuracy of radiotherapy, the characteristics of the RG radiotherapy technique and the radiotherapy system must be clearly understood when this method is to be employed in clinical practice. Image-guided radiotherapy (IGRT) is now assuming a central role in radiotherapy, and properly identifying internal margins is an important issue for ensuring optimal treatment. The results of this study confirmed that it is necessary to ensure the optimal treatment field in radiotherapy of the trunk and that it is essential to confirm tumor position on the basis of image evaluation.

Evaluation of Absorbed Dose in Respiratory Gated Radiotherapy Using a Phantom System that Simulates Patient Respiration

Purpose/Objective: Respiratory gated (RG) radiotherapy is useful for minimizing the irradiated volume of normal tissues resulting from the shifting of internal structures due to respiratory movement. In this technique, although improvement in the dose distribution of the target can be expected, the actual absorbed dose distribution is not clearly determined. Therefore, it is important to clarify the absorbed dose at the tumor and at the evaluation points according to the patient′s respiration. We have developed a phantom system that simulates patient respiration (TNK Co., Ltd.) to evaluate the absorbed dose and ensure precise RG radiotherapy.

MRI-based treatment planning for radiotherapy: Dosimetric verification for prostate IMRT

Magnetic resonance (MR) and computed tomography (CT) image fusion with CT-based dose calculation is the gold standard for prostate treatment planning. MR and CT fusion with CT-based dose calculation has become a routine procedure for intensity-modulated radiation therapy (IMRT) treatment planning at Fox Chase Cancer Center. The use of MRI alone for treatment planning (or MRI simulation) will remove any errors associated with image fusion. Furthermore, it will reduce treatment cost by avoiding redundant CT scans and save patient, staff, and machine time. The purpose of this study is to investigate the dosimetric accuracy of MRI-based treatment planning for prostate IMRT. A total of 30 IMRT plans for 15 patients were generated using both MRI and CT data. The MRI distortion was corrected using gradient distortion correction (GDC) software provided by the vendor (Philips Medical System, Cleveland, OH). The same internal contours were used for the paired plans. The external contours were drawn separately between CT-based and MR imaging-based plans to evaluate the effect of any residual distortions on dosimetric accuracy. The same energy, beam angles, dose constrains, and optimization parameters were used for dose calculations for each paired plans using a treatment optimization system. The resulting plans were compared in terms of isodose distributions and dose-volume histograms (DVHs). Hybrid phantom plans were generated for both the CT-based plans and the MR-based plans using the same leaf sequences and associated monitor units (MU). The physical phantom was then irradiated using the same leaf sequences to verify the dosimetry accuracy of the treatment plans. Our results show that dose distributions between CT-based and MRI-based plans were equally acceptable based on our clinical criteria. The absolute dose agreement for the planning target volume was within 2% between CT-based and MR-based plans and 3% between measured dose and dose predicted by the planning system in the physical phantom. Magnetic resonance imaging is a useful tool for radiotherapy simulation. Compared with CT-based treatment planning, MR imaging-based treatment planning meets the accuracy for dose calculation and provides consistent treatment plans for prostate IMRT. Because MR imaging-based digitally reconstructed radiographs do not provide adequate bony structure information, a technique is suggested for producing a wire-frame image that is intended to replace the traditional digitally reconstructed radiographs that are made from CT information.

Phantom field withO(N)symmetry in an exponential potential

In this paper, we study the phase space of phantom model with O(\emph{N}) symmetry in exponential potential. Different from the model without O(\emph{N}) symmetry, the introduction of the symmetry leads to a lower bound $w>-3$ on the equation of state for the existence of stable phantom dominated attractor phase. The reconstruction relation between the potential of O(\textit{N}) phantom system and red shift has been derived.

Migration versus apparent migration: importance of errors due to positioning variation in plain radiographic follow-up of aortic stent-grafts.

To demonstrate the influence of radiographic positioning on the assessment of stent-graft migration using plain radiographs following endovascular abdominal aortic aneurysm repair. Equations were derived to correct for artifactual stent-graft migration introduced by geometric distortion due to variations in positioning between radiographs acquired at different times. A phantom system was used to validate the equations. Errors in stent position increase with (1) the distance of the aortic stent-graft from the midline and (2) differences in radiographic centering points in the craniocaudal direction; other variables have little effect. For typical stent positions, errors are small if the centering changes by <8 cm. Consistent radiographic positioning to within 4 cm on successive imaging studies limits errors to 1.5 mm. Even if artifactual migration is large, the true migration can be reliably calculated to within 2 mm. Artifactual migration due to variation in radiographic centering is not usually clinically significant if care is taken to center radiographs consistently. Radiographs in which artifactual migration may be important are readily identified, and mathematical correction is straightforward.

Migration Versus Apparent Migration: Importance of Errors Due to Positioning Variation in Plain Radiographic Follow-up of Aortic Stent-Grafts

PURPOSE: To demonstrate the influence of radiographic positioning on the assessment of stent-graft migration using plain radiographs following endovascular abdominal aortic aneurysm repair. METHODS: Equations were derived to correct for artifactual stent-graft migration introduced by geometric distortion due to variations in positioning between radiographs acquired at different times. A phantom system was used to validate the equations. RESULTS: Errors in stent position increase with (1) the distance of the aortic stent-graft from the midline and (2) differences in radiographic centering points in the craniocaudal direction; other variables have little effect. For typical stent positions, errors are small if the centering changes by <8 cm. Consistent radiographic positioning to within 4 cm on successive imaging studies limits errors to 1.5 mm. Even if artifactual migration is large, the true migration can be reliably calculated to within 2 mm. CONCLUSIONS: Artifactual migration due to variation in radiographic centering is not usually clinically significant if care is taken to center radiographs consistently. Radiographs in which artifactual migration may be important are readily identified, and mathematical correction is straightforward.

Bi-exponential proton transverse relaxation rate (R 2) image analysis using RF field intensity-weighted spin density projection: potential for R 2 measurement of iron-loaded liver

A bi-exponential proton transverse relaxation rate (R 2) image analysis technique has been developed that enables the discrimination of dual compartment transverse relaxation behavior in systems with rapid transverse relaxation enhancement. The technique is particularly well suited to single spin-echo imaging studies where a limited number of images are available for analysis. The bi-exponential R 2 image analysis is facilitated by estimation of the initial proton spin density signal within the region of interest weighted by the RF field intensities. The RF field intensity-weighted spin density map is computed by solving a boundary value problem presented by a high spin density, long T 2 material encompassing the region for analysis. The accuracy of the bi-exponential R 2 image analysis technique is demonstrated on a simulated dual compartment manganese chloride phantom system with relaxation rates and relative population densities between the two compartments similar to the bi-exponential transverse relaxation behavior expected of iron loaded liver. Results from analysis of the phantoms illustrate the potential of bi-exponential R 2 image analysis with RF field intensity-weighted spin density projection for quantifying transverse relaxation enhancement as it occurs in liver iron overload.

A Critical Study of the Mechanical and Electrical Properties of the PHANToM Haptic Interface and Improvements for Highperformance Control

This paper presents a critical study of the mechanical and electrical properties of the PHANToM haptic interface and improvements to overcome its limitations for applications requiring high-performance control. Target applications share the common requirements of low-noise/granularity/latency measurements, an accurate system model, high bandwidth, the need for an open architecture, and the ability to operate for long periods without interruption while exerting significant forces. To satisfy these requirements, the kinematics, dynamics, high-frequency dynamic response, and velocity estimation of the PHANToM system are studied. Furthermore, this paper presents the details of how the unknown subsystems of the stock PHANToM can be replaced with known, high-performance systems and how additional measurement electronics can be interfaced to compensate for some of the PHANToM's shortcomings. With these modifications, it is possible to increase the maximum achievable virtual wall stiffness by 35%, active viscous damping by 120%, and teleoperation loop gain by 50% over the original system. With the modified system, it is also possible to maintain higher forces for longer periods without causing motor overheating.

Modification of reference array attachment for image guided neurosurgery: direct cranial fixation.

We have modified the cranial fixation technique of the reference array used for the Stealth (Medtronics Inc., Minneapolis, MN) image guided neurosurgical workstation to avoid rigid immobilization and to accommodate patients undergoing awake procedures. The modification allows attachment of a reference array directly to the skull prior to registration, avoiding the requirement for rigid cranial fixation. The accuracy of fiducial registration for the modified reference array was compared to the conventional reference array using a phantom system yielding similar registration results and target accuracy. The successful application of the modified system to two operative cases is described.

A novel quantitative dual-isotope method for simultaneous ventilation and perfusion lung SPET.

A quantitative dual-isotope single-photon emission tomography (SPET) technique for the assessment of lung ventilation (V) and perfusion (Q) using, respectively, technetium-99m labelled Technegas (140 keV) and indium-113m labelled macro-aggregated albumin (392 keV), is presented, validated and clinically tested in a healthy volunteer. In order to assess V, Q and V/Q distributions in quantitative terms, algorithms which correct for down scattering, photon scattering and attenuation, as well as an organ outline algorithm, were implemented. Scatter and down-scatter correction were made in the spatial domain by pixel-wise image subtraction of projection-dependent global scattering factors obtained from the energy domain. The attenuation correction was based on an iterative projection/back-projection method. All studies were made on a three-headed SPET system (Trionix) with medium-energy parallel-hole collimators. The set of input data for quantification was based on SPET acquisition of emission data in four separate energy windows, the associated cumulative energy spectra and transmission data. The attenuation correction routine as well as the edge detection algorithm utilized data from (99m)Tc transmission tomography. Attenuation data for (113m)In were obtained by linear scaling of the (99m)Tc attenuation maps. The correction algorithms were experimentally validated with a stack phantom system and applied on a healthy volunteer. The mean difference between the corrected SPET data of the dense stack lung phantom and those obtained from the corresponding scatter- and attenuation-"free" version was only 1.9% for (99m)Tc and 0.9% for (113m)In. The estimated fractional V/Q distribution in the 3-D lung phantom volume had its peak at V/Q=1, with a width (FWHM) of 0.31 due to noise, particularly in the (113m)In images, and to partial volume effects. For a healthy volunteer, the corresponding values were 0.9 and 0.35, respectively. This method allows accurate assessment of radionuclide distribution on a regional basis. For basic lung physiology and clinical practice, the method allows assessment of the global frequency functions of the V, Q and V/Q distributions.

Tagged MRIによる脳動脈瘤モデルの血流動態解析

The purpose of this study was to investigate the possibility of visualization of hemodynamics in an intracranial aneurysm replica by using tagged MR imaging and particle image velocimetry (PIV) software. Our phantom system included a pulsatile flow pump and a replica of an anterior communicating artery aneurysm (Shelley Medical Imaging Technologies). Tagged MR imaging was carried out using a 1.5 T superconducting MR system and using ECG gated fast spoiled gradient echo sequence combined with spatial modulation of magnetization technique. Tag stripes were placed at 3mm intervals in a grid pattern in the aneurysm model. Analysis of tagged MR image data with PIV software revealed the velocity and direction of flowing fluid at each location in the aneurysm model every nine milliseconds. Tagged MR image velocimetry combined with tagged MR imaging and PIV analysis would be a promising and noninvasive technique for revealing hemodynamics in intracranial aneurysms.

Phantom Harmonic Gradient Estimators for Nonpreemptive Priority Queueing Systems

This paper presents a new gradient estimator for the steady-state expected sojourn (system) time in a nonpreemptive priority queueing system. The estimator uses the concept of a phantom system, together with the basic ideas in harmonic gradient estimation, to develop a single simulation run estimator, termed the phantom harmonic gradient (PHG) estimator. The estimator is shown to be strongly consistent and strongly consistent in the average sense as the sample size grows. An upper bound for the variance of the PHG estimator is presented. This bound is used to show that under mild conditions, the variance of the PHG estimator tends to zero as both the number of phantom systems and the sample size approach infinity. A variance-reduction technique that simultaneously uses both common and antithetic random numbers is presented. Computational results on several nonpreemptive queueing systems illustrate the effectiveness of the method and show that common and antithetic random numbers can be used simultaneously to reduce the variance of the phantom harmonic gradient estimator.

The properties of the ultramicrocylindrical ionization chamber for small field used in stereotactic radiosurgery

Accurate dosimetry of small-field photon beams tends to be difficult to perform due to the presence of lateral electronic disequilibrium and steep dose gradients. In stereotactic radiosurgery (SRS), small fields of 6-30 mm in diameter are used. Generally thermoluminescence dosimetry chips, Farmer, Thimble ion chamber, and film dosimetry are not adequate to measure dose in SRS beams. These techniques generally do not provide the required precision due to their energy dependence and/or poor resolution. It is necessary to construct a small, accurate detector with high spatial resolution for the small fields used in SRS. The ultramicrocylindrical ionization chamber (UCIC) with a gold wall of 2.2 mm in diameter and 4.0 mm in length has dual sensitive volumes of air (8.0 mm3) and borosilicate (2.6 mm3) cavity. Reproducibility, linearity, and radiation damage with respect to absorbed dose, beam profile of small beam, and independence of dose rate of the UCIC are tested by the dose measurements in high energy photon (5, 15 MV) and electron (9 MeV) beams. The UCIC with a unique supporting system in the polystyrene phantom is demonstrated to be a suitable detector for the dose measurements in a small beam size.

Reconstruction and navigation system for intraoperative brachytherapy using the flab technique for colorectal tumor bed irradiation

Purpose: To present the development of a new navigation and reconstruction system based on an electromagnetic free-hand tracker and on CT imaging for treatment planning of intraoperative high-dose-rate brachytherapy (IORT-HDRB) in the sacral region. Our aim is to improve accuracy and to enable individualized treatment planning and dose documentation to be performed for IORT-HDRB using a flab technique. Methods and Materials: The material consists of an electromagnetic 3D tracker system, a PC workstation with Microsoft Windows NT 4.0 operating system, and a recognition program for continuous speech. In addition, we designed an external reference system constructed of titanium and Perspex, which is positioned in the pelvis, and a special digitizer pen for reconstruction of the flab geometry. The flab design incorporates a series of silicon 10-mm-diameter spherical pellets. Measurements were made with a pelvic phantom in order to study the accuracy of the system. The reconstruction results are stored and can be exported via network or floppy to our different treatment planning systems. Results: Our results for the reconstruction of a flab with six catheters and a total of 100 spherical pellets give mean errors in the range (2.5 ± 0.6) mm to (3.5 ± 0.8) mm depending on the positions of the pelvic phantom and transmitter relative to the operation table. These errors are calculated by comparing the reconstruction results of our system with those using a CT-based reconstruction of the flab geometry. For the accuracy of the navigation system for the pelvic phantom, we obtained mean errors in the range (2.2 ± 0.7) mm to (3.1 ± 1.0) mm. Conclusions: The new system we have developed enables navigation and reconstruction within the surgical environment with a clinically acceptable level of accuracy. It offers the possibility of individualized treatment planning and effective documentation of the 3D dose distribution in IORT-HDRB using a flab technique.

A practical technique for verification of three-dimensional conformal dose distributions in stereotactic radiosurgery.

The trend toward conformal techniques in stereotactic radiosurgery necessitates an accurate and practical method for verification of irregular three-dimensional dose distributions. This work presents the design and evaluation of a phantom system facilitating the measurement of conformal dose distributions using one or more arrays of up to 20 radiographic films separated by 3.2 mm-thick tissue-equivalent spacers. Using Electron Gamma Shower version 4 (EGS4) Monte Carlo simulation, we show that for 6 MV radiosurgical photon beams this arrangement preserves tissue-equivalence to within 1%. The phantom provides 0.25 mm in-plane spatial resolution and multiple sets of films may be used to resample the dose volume in orthogonal planes. Dedicated software has been developed to automate the process of ordering and orienting of scanned film images, conversion of scanned pixel value to dose, resampling of one or more sets of film images and subsequent export of images in DICOM format for coregistration of planned and measured dose volumes. Calculated and measured isodose surfaces for a simple, circular-beam treatment agree to within 1.5 mm throughout the dose volume. For conformal radiosurgical applications, the measured and planned dose distributions agree to within the uncertainty of the manufacture of irregularly shaped collimators. The sensitivity of this technique to minor spatial inaccuracies in beam shaping is also demonstrated.