Comparison of cell casted and 3D-printed plastic scintillators for dosimetry applications.

Currently, the most used methods of plastic scintillator (PS) manufacturing are cell casting and bulk polymerisation, extrusion, injection molding, whereas digital light processing (DLP) 3D printing technique has been recently introduced. For our research, we measured blue-emitting EJ-200, EJ-208, green-emitting EJ-260, EJ-262 cell cast and two types of blue-emitting DLP-printed PSs. The light output of the samples, with the same dimension of 10 mm × 10 mm × 10 mm, was compared. The light output of the samples, relative to the reference EJ-200 cell-cast scintillator, equals about 40-49 and 70-73% for two types of 3D-printed, and two green-emitting cell-casted PSs, respectively. Performance of the investigated scintillators is sufficient to use them in a plastic scintillation dosemeter operating in high fluence gamma radiation fields.

Effect of Magnetic Field Strength on Plastic Scintillation Detector Response.

PURPOSE: To characterize the response of plastic scintillation detectors (PSDs) to high-energy photon radiation as a function of magnetic field strength. MATERIALS AND METHODS: PSDs were placed inside a plastic phantom held at the center point between 2 magnets and irradiated using a 6-MV photon beam from a linear accelerator. The magnetic field was varied from 0 T to 1.5 T by 0.3-T increments. The light emission and stem-effect-corrected response as a function of magnetic field strength were obtained for both a commercial PSD (Exradin W1, Standard Imaging) and an in-house hyperspectral PSD. Spectral signatures were obtained for the in-house PSD, and light emission from a bare fiber was also measured. RESULTS: Light emission increased as magnetic field strength increased for all detectors tested. The tested PSDs exhibited an increase in light intensity of 10% to 20%, mostly owing to the increase in Cerenkov light produced within and transmitted along the optical fiber. When corrected for stem effects, the increase in PSD response went down to 2.4% for both detectors. This most likely represents the change in the inherent dose deposition within the phantom. CONCLUSION: PSDs with a suitable stem-effect removal approach were less dependent on magnetic field strength and had better water equivalence than did ion chambers tested in previous studies. PSDs therefore show great promise for use in both quality assurance and in-vivo dosimetry applications in a magnetic field environment.

3D prompt gamma imaging for proton beam range verification.

We tested the ability of a single Compton camera (CC) to produce 3-dimensional (3D) images of prompt gammas (PGs) emitted during the irradiation of a tissue-equivalent plastic phantom with proton pencil beams for clinical doses delivered at clinical dose rates. PG measurements were made with a small prototype CC placed at three different locations along the proton beam path. We evaluated the ability of the CC to produce images at each location for two clinical scenarios: (1) the delivery of a single 2 Gy pencil beam from a hypo-fractionated treatment (~9 × 108 protons), and (2) a single pencil beam from a standard treatment (~1 × 108 protons). Additionally, the data measured at each location were combined to simulate measurements with a larger scale, clinical CC and its ability to image shifts in the Bragg peak (BP) range for both clinical scenarios. With our prototype CC, the location of the distal end of the BP could be seen with the CC placed up to 4 cm proximal or distal to the BP distal falloff. Using the data from the simulated full scale clinical CC, 3D images of the PG emission were produced with the delivery of as few as 1 × 108 protons, and shifts in the proton beam range as small as 2 mm could be detected for delivery of a 2 Gy spot. From these results we conclude that 3D PG imaging for proton range verification under clinical beam delivery conditions is possible with a single CC.

Feasibility Studies of a New Event Selection Method to Improve Spatial Resolution of Compton Imaging for Medical Applications.

A Compton imaging method for medical applications that includes new energy determination and data filtering techniques has been tested using several point sources with known emission lines. Using a prototype Compton camera, a distance-of-closest approach technique has been employed to determine the initial energy of the incoming γs and to ensure the reconstructed source position is within an acceptable distance from the known γ source location. Further analysis is done by implementing a Compton line filtering technique, keeping only those interactions whose deposited energy in the first interaction matches the theoretical energy deposition predicted by the Compton equation. Using this new event filtering method, we see improvements in the full width at half maximum in the lateral profiles of γ point sources of up to 70% over standard Compton imaging methods, as well as achievable spatial resolutions in the reconstructed images of better than 2 mm. In addition, this new Compton imaging method was able to reconstruct an extended source of γ rays emitted during irradiation of a water tank with a clinical proton radiotherapy beam.

Innovations in Radiotherapy Technology.

Many low- and middle-income countries, together with remote and low socioeconomic populations within high-income countries, lack the resources and services to deal with cancer. The challenges in upgrading or introducing the necessary services are enormous, from screening and diagnosis to radiotherapy planning/treatment and quality assurance. There are severe shortages not only in equipment, but also in the capacity to train, recruit and retain staff as well as in their ongoing professional development via effective international peer-review and collaboration. Here we describe some examples of emerging technology innovations based on real-time software and cloud-based capabilities that have the potential to redress some of these areas. These include: (i) automatic treatment planning to reduce physics staffing shortages, (ii) real-time image-guided adaptive radiotherapy technologies, (iii) fixed-beam radiotherapy treatment units that use patient (rather than gantry) rotation to reduce infrastructure costs and staff-to-patient ratios, (iv) cloud-based infrastructure programmes to facilitate international collaboration and quality assurance and (v) high dose rate mobile cobalt brachytherapy techniques for intraoperative radiotherapy.

In vivo dosimetry: trends and prospects for brachytherapy.

The error types during brachytherapy (BT) treatments and their occurrence rates are not well known. The limited knowledge is partly attributed to the lack of independent verification systems of the treatment progression in the clinical workflow routine. Within the field of in vivo dosimetry (IVD), it is established that real-time IVD can provide efficient error detection and treatment verification. However, it is also recognized that widespread implementations are hampered by the lack of available high-accuracy IVD systems that are straightforward for the clinical staff to use. This article highlights the capabilities of the state-of-the-art IVD technology in the context of error detection and quality assurance (QA) and discusses related prospects of the latest developments within the field. The article emphasizes the main challenges responsible for the limited practice of IVD and provides descriptions on how they can be overcome. Finally, the article suggests a framework for collaborations between BT clinics that implemented IVD on a routine basis and postulates that such collaborations could improve BT QA measures and the knowledge about BT error types and their occurrence rates.

Determination of the quenching correction factors for plastic scintillation detectors in therapeutic high-energy proton beams.

Plastic scintillation detectors (PSDs) have many advantages over other detectors in small field dosimetry due to their high spatial resolution, excellent water equivalence and instantaneous readout. However, in proton beams, the PSDs undergo a quenching effect which makes the signal level reduced significantly when the detector is close to the Bragg peak where the linear energy transfer (LET) for protons is very high. This study measures the quenching correction factor (QCF) for a PSD in clinical passive-scattering proton beams and investigates the feasibility of using PSDs in depth-dose measurements in proton beams. A polystyrene-based PSD (BCF-12, ϕ0.5 mm × 4 mm) was used to measure the depth-dose curves in a water phantom for monoenergetic unmodulated proton beams of nominal energies 100, 180 and 250 MeV. A Markus plane-parallel ion chamber was also used to get the dose distributions for the same proton beams. From these results, the QCF as a function of depth was derived for these proton beams. Next, the LET depth distributions for these proton beams were calculated by using the MCNPX Monte Carlo code, based on the experimentally validated nozzle models for these passive-scattering proton beams. Then the relationship between the QCF and the proton LET could be derived as an empirical formula. Finally, the obtained empirical formula was applied to the PSD measurements to get the corrected depth-dose curves and they were compared to the ion chamber measurements. A linear relationship between the QCF and LET, i.e. Birks' formula, was obtained for the proton beams studied. The result is in agreement with the literature. The PSD measurements after the quenching corrections agree with ion chamber measurements within 5%. PSDs are good dosimeters for proton beam measurement if the quenching effect is corrected appropriately.

Verification of proton range, position, and intensity in IMPT with a 3D liquid scintillator detector system.

PURPOSE: Intensity-modulated proton therapy (IMPT) using spot scanned proton beams relies on the delivery of a large number of beamlets to shape the dose distribution in a highly conformal manner. The authors have developed a 3D system based on liquid scintillator to measure the spatial location, intensity, and depth of penetration (energy) of the proton beamlets in near real-time. METHODS: The detector system consists of a 20 × 20 × 20 cc liquid scintillator (LS) material in a light tight enclosure connected to a CCD camera. This camera has a field of view of 25.7 by 19.3 cm and a pixel size of 0.4 mm. While the LS is irradiated, the camera continuously acquires images of the light distribution produced inside the LS. Irradiations were made with proton pencil beams produced with a spot-scanning nozzle. Pencil beams with nominal ranges in water between 9.5 and 17.6 cm were scanned to irradiate an area of 10 × 10 cm square on the surface of the LS phantom. Image frames were acquired at 50 ms per frame. RESULTS: The signal to noise ratio of a typical Bragg peak was about 170. Proton range measured from the light distribution produced in the LS was accurate to within 0.3 mm on average. The largest deviation seen between the nominal and measured range was 0.6 mm. Lateral position of the measured pencil beam was accurate to within 0.4 mm on average. The largest deviation seen between the nominal and measured lateral position was 0.8 mm; however, the accuracy of this measurement could be improved by correcting light scattering artifacts. Intensity of single proton spots were measured with precision ranging from 3 % for the smallest spot intensity (0.005 MU) to 0.5 % for the largest spot (0.04 MU). CONCLUSIONS: Our LS detector system has been shown to be capable of fast, submillimeter spatial localization of proton spots delivered in a 3D volume. This system could be used for beam range, intensity and position verification in IMPT.

Sci-Thur PM: YIS - 10: A new optically encoded single-fiber plastic scintillation detector for multi-point radiation dosimetry.

PURPOSE: To develop a new multi-point plastic scintillation detector (mPSD) that allows for simultaneous dose measurements at multiple points and uses a single optical guide. MATERIALS AND METHODS: Two different prototypes were built. A two-point mPSD was built and light discrimination was based on the use of multiple color filters at the outputs of a network of optical fiber splitters. Light intensity was measured by an EMCCD camera. For the three-point mPSD, the light discrimination setup was replaced by a low-noise spectrometer. Depth-dose and profiles measurements were obtained on a 6 MV photon beam with the mPSDs inside a water phantom. An ion chamber was also used for comparison purpose. Finally, the three-point mPSD was tested under an Ir-192 high-dose-rate (HDR) brachytherapy dose delivery and compared to the treatment planning system. RESULTS: A good agreement was found between the measured and expected dose for both mPSDs. The average relative differences to the ion chamber measurement for the two-point mPSD were of (2.4 ± 1.6)% and (1.3 ± 0.8)%. For the three-point mPSD, these differences were of (2.3±1.1)%, (1.6±0.4)% and (0.32±0.19)%. The latter mPSD was shown very versatile, being able to measure dose from HDR brachytherapy with an average accuracy of (2.3±1.0)% per catheter. CONCLUSIONS: The practical feasibility of mPSDs using a single optical guide has been demonstrated under irradiation from a 6 MV photon beam and an Ir-192 HDR brachytherapy source. Their application for pre-treatment quality assurance and in vivo dosimetry will be various.

Sci-Sat AM: Brachy - 07: Plastic scintillation detector validation for kV dosimetry.

PURPOSE: To characterize the plastic scintillation detectors (PSDs) response in the diagnostic energy range. A fast and adaptable method for real-time dosimetry in superficial x-ray therapy and interventional radiology is proposed. METHOD: A PSD (1 mm diameter and 10 mm long) is coupled to a 5 m long optical fiber. Scintillation photons are guided to a polychromatic photodiode which provides an electrical current proportional to the input light signal. If the incident energy spectrum is known, the dose measured in the PSD's polystyrene sensitive volume can be converted to score dose in any other media such as air, water or soft tissues using the large cavity theory (LCT). A software simulating x-ray tube spectra and filtration has been benchmarked and is used for analysis. The method is confirmed by Monte Carlo simulations. RESULTS: PSDs cannot be assumed energy independent with low-energy photons as a factor 2 has been observed in the energy response between 80 kVp and 150 kVp. When the dose is converted to the desired medium, the PSD's energy dependence is compensated and a 2.1% standard deviation was observed upon the studied energy ranges, which is inside the measurement and calculation uncertainties. Percent depth dose (PDD) measurements are in good agreement with Monte Carlo simulations and results can be improved if the proposed method is applied to compensate beam hardening. CONCLUSION: PSDs present great potential for real-time dose measurements with radiologic photon energy.

SU-E-T-116: The Water Equivalence of Organic Liquid Scintillators for Proton Dosimetry.

PURPOSE: Organic liquid scintillators are currently under investigation for use in proton dosimetry. The purpose of this work is to evaluate the water equivalence of these materials as a preliminary step to identify scintillators that are well-suited to this purpose. METHODS: Stopping powers were calculated for 0.001-1000 MeV protons in water, polystyrene, and two organic liquid scintillators: BC-531 and OptiPhase 'Hi-Safe' 3 at 0%, 25%, and 50% concentrations of water. Angular scatter was quantified by theta0, a characteristic multiple Coulomb scattering angle analogous to the standard deviation of a Gaussian distribution of proton angles relative to the incident beam axis. Theta0 was calculated as a function of depth over the range of 200 MeV protons in these materials. RESULTS: Collisional stopping power in BC-531 ranged from +44% to +1% ofthat in water. It remained within 6% from 2-600 MeV. OptiPhase ranged from +24% to -2%, with smaller deviations at increased water concentrations. At all concentrations, OptiPhase showed smaller deviations than polystyrene and BC-531 and remained within 1% of water from 2-600 MeV.Theta0 was very similar for all materials, with deviations from water of 5 milliradians or less over the majority of the proton range. BC-531 showed deviations of 10 milliradians or more in the last few millimeters of the range. OptiPhase showed smaller deviations than BC-531 or polystyrene, and these deviations decreased with increasing water concentration. CONCLUSIONS: OptiPhase was found to be more water equivalent than BC-531 or polystyrene in stopping power and angular scatter, and increased water concentration improved both quantities. Large deviations in stopping power were only found below 2 MeV for any material, where proton range is less than 0.1 millimeter. The deviations from water found in angular scatter were less significant, and probably too small to affect measurement.

WE-G-BRB-05: On the Importance of Fluorescence Within the Stem Effect of Scintillation Detectors.

PURPOSE: To quantify the nature and composition of the light produced in optical fibers under different irradiation conditions and evaluate its impact on dosimetry. METHODS: Irradiation of a bare PMMA optical fiber (Mitsubishi ESKA Premier) was performed using a superficial therapy unit, an Ir-192 HDR brachytherapy source, a Co-60 external-beam unit as well as photon and electron beams from a linear accelerator. Spectra of the radiation-induced visible light in the fiber were acquired and signals were compared as a function of depth and irradiation type. Irradiation of a 75 kVp beam from the superficial therapy unit was used to isolate the fluorescence spectrum. Isolation of the Cerenkov spectrum component was obtained from irradiation of a 15 MeV electron beam at a 45 degree angle. Relative composition in fluorescence and Cerenkov of the stem effect light has been determined for all irradiations. RESULTS: The total stem effect spectra can be represented by a linear superposition of the fluorescence and Cerenkov spectra. The fluorescence contribution was shown to strongly differ between the superficial therapy unit (99%±1%), the Ir-192 HDR source (25%±3%) and higher energy irradiations (3%±2%). Variations within each energy regime (kV, HDR brachytherapy and MV) were small at 3% or lower. These were observed for irradiations at angle or when the fiber was near the surface. This study suggests it is better to calibrate the stem effect of a scintillation detector using the same irradiation modality. CONCLUSIONS: Stem effect light was shown to be composed of fluorescence and Cerenkov light in different proportions depending on the geometry of the experimental setup, nature of the irradiation, and irradiation energy. Calibrating detectors separately for fluorescence and Cerenkov may lead to better performance of the stem effect removal technique.

WE-G-BRB-04: BEST IN PHYSICS (THERAPY) - A Novel Multi-Point Plastic Scintillation Detector for in Vivo Dosimetry and Quality Assurance in Radiation Therapy.

PURPOSE: To develop a novel multi-point plastic scintillation detector (mPSD) capable of accurately measuring dose at multiple positions simultaneously with the use of a single optical guide. METHODS: We built a new generation of plastic scintillation detectors composed of multiple scintillating elements along a same optical transmission line. Three different scintillating fibers were optically coupled to a single collecting optical fiber. A primary challenge for this new type of detector is that the output signal is a superposition of multiple scintillation spectra and contaminating elements. Acquisition with a spectrometry setup allows for the implementation of a new hyperspectral approach that accounts for each light-emitting component separately, and allows spectral unmixing. The mPSD and an ion chamber were irradiated in a water phantom with a 6 MV photon beam. Profiles and depth-dose curves were measured and compared between detectors. This detector and the corresponding calibration approach were also applied to Ir- 192 HDR brachytherapy. RESULTS: Doses measured with the mPSD were in good agreement with the ion chamber measurements for external beam irradiations. Average relative differences of (2.3±1.1)%, (1.6±0.4)% and (0.32±0.19)% were observed for each scintillating element. The mPSD measurements tended to be at least as accurate as published measurements from single-point PSDs. For the Ir-192 HDR brachytherapy application, the average difference between the treatment planning system and the measurements were (4.6±1.0)% per dwell-position and (2.1±1.0)% per catheter. The accuracy of each scintillating element was shown to depend on light attenuation and on the similarity of its scintillation spectrum in comparison to the other light emitters. CONCLUSIONS: The feasibility and accuracy of mPSDs using a single transmission line was demonstrated. In addition to well-documented advantages of single-point PSDs, the multi-point capability of this single-fiber detector makes mPSDs a very promising new technique for quality assurance and on-line in vivo dosimetry.

SU-E-J-79: Using the Distance of Closest Approach to Improve Compton Camera Image Quality.

PURPOSE: The prompt gamma radiation emitted from tissue during proton therapy offers a means of beam range verification. Compton camera (CC) imaging systems offer high efficiency and 3D imaging capability. However, Doppler broadening and the inaccuracies in the measurement of the scattering positions and energies reduce the image resolution. The purpose of our study is to determine if removing events with a distance-of-closest-approach greater than a threshold value will improve the resolution of images reconstructed using the stochastic origins ensemble (SOE) algorithm. METHODS: We first simulated a 3-stage CC detecting gammas from a 0.511 MeV point source. We then used SOE to reconstruct images from the point source and from a) all gammas, b) gammas with DCA < 3 mm, and c) gammas with DCA < 1 mm. We measured the point-spread-function for the point source for a), b), and c). Next, we simulated a 3-stage CC detecting prompt gammas emitted from tissue during proton therapy. We reconstructed the gammas using SOE and compared 2D images of all gammas, gammas with DCA < 3 mm, and gammas with DCA < 1 mm. RESULTS: The FWHM of the PSF of the 0.511 MeV point source was reduced by 50% when DCA was required to be < 5 mm, and it was reduced by 65% when DCA was required to be < 3 mm. 2D images of a proton beam are of visibly higher quality as the DCA requirement is lowered. CONCLUSIONS: The DCA for MC events can be used to identify the events with significant resolution loss due to the detector effects. Removing these events before running the reconstruction algorithm results in higher quality images. We discuss methods to predict the DCA based on the measured scatter data, so that a similar technique can be applied to data from real detectors. This work was supported by the National Institutes of Health through award number R21CA137362 from the National Cancer Institute.

Measurement and calculation of characteristic prompt gamma ray spectra emitted during proton irradiation.

In this paper, we present results of initial measurements and calculations of prompt gamma ray spectra (produced by proton-nucleus interactions) emitted from tissue equivalent phantoms during irradiations with proton beams. Measurements of prompt gamma ray spectra were made using a high-purity germanium detector shielded either with lead (passive shielding), or a Compton suppression system (active shielding). Calculations of the spectra were performed using a model of both the passive and active shielding experimental setups developed using the Geant4 Monte Carlo toolkit. From the measured spectra it was shown that it is possible to distinguish the characteristic emission lines from the major elemental constituent atoms (C, O, Ca) in the irradiated phantoms during delivery of proton doses similar to those delivered during patient treatment. Also, the Monte Carlo spectra were found to be in very good agreement with the measured spectra providing an initial validation of our model for use in further studies of prompt gamma ray emission during proton therapy.

Experimental validation of a Monte Carlo proton therapy nozzle model incorporating magnetically steered protons.

The purpose of this study is to validate the accuracy of a Monte Carlo calculation model of a proton magnetic beam scanning delivery nozzle developed using the Geant4 toolkit. The Monte Carlo model was used to produce depth dose and lateral profiles, which were compared to data measured in the clinical scanning treatment nozzle at several energies. Comparisons were also made between measured and simulated off-axis profiles to test the accuracy of the model's magnetic steering. Comparison of the 80% distal dose fall-off values for the measured and simulated depth dose profiles agreed to within 1 mm for the beam energies evaluated. Agreement of the full width at half maximum values for the measured and simulated lateral fluence profiles was within 1.3 mm for all energies. The position of measured and simulated spot positions for the magnetically steered beams agreed to within 0.7 mm of each other. Based on these results, we found that the Geant4 Monte Carlo model of the beam scanning nozzle has the ability to accurately predict depth dose profiles, lateral profiles perpendicular to the beam axis and magnetic steering of a proton beam during beam scanning proton therapy.

Prompt gamma-ray emission from biological tissues during proton irradiation: a preliminary study.

In this paper, we present the results of a preliminary study of secondary 'prompt' gamma-ray emission produced by proton-nuclear interactions within tissue during proton radiotherapy. Monte Carlo simulations were performed for mono-energetic proton beams, ranging from 2.5 MeV to 250 MeV, irradiating elemental and tissue targets. Calculations of the emission spectra from different biological tissues and their elemental components were made. Also, prompt gamma rays emitted during delivery of a clinical proton spread-out Bragg peak (SOBP) in a homogeneous water phantom and a water phantom containing heterogeneous tissue inserts were calculated to study the correlation between prompt gamma-ray production and proton dose delivery. The results show that the prompt gamma-ray spectra differ significantly for each type of tissue studied. The relative intensity of the characteristic gamma rays emitted from a given tissue was shown to be proportional to the concentration of each element in that tissue. A strong correlation was found between the delivered SOBP dose distribution and the characteristic prompt gamma-ray production. Based on these results, we discuss the potential use of prompt gamma-ray emission as a method to verify the accuracy and efficacy of doses delivered with proton radiotherapy.

Sci-Thurs PM: Delivery-02: Improving image quality produced by CCD cameras exposed to stray radiation from a medical linac.

PURPOSE: Charge coupled devices (CCDs) are increasingly used in radiation therapy. CCDs are ideal for applications such as two-dimensional dosimetry of scintillator sheets or to read arrays of miniature scintillation detectors. However, CCDs are sensitive to stray radiation. Radiation-induced noise strongly alters images and limits their quantitative analysis. We have characterized radiation-induced noise and developed filtration algorithms to restore image quality. METHOD AND MATERIALS: Two models of CCD cameras were used for measurements in linac environments. Images were acquired with and without radiation. The structure of the transient noise was characterized. Then, four methods of noise filtration were compared: median filtering of a time series of images, uniform median filtering of single images, an adaptive filter with switching mechanism and a modified version of the adaptive filter. RESULTS: The intensity distribution of noisy pixels was similar in both cameras. However, the spatial distribution of the noise was different: the average noise cluster size was 1.2±0.6 and 3.2±2.7 pixels for each of the two cameras. The median of a time series of image resulted in the best filtration and minimal image distortion. For applications where time series is impractical, adaptive filtration must be used to reduce image distortion. CONCLUSION: We have characterized the transient noise produced in CCDs by scattered radiation from a linac and have developed an efficient filtration scheme to remove this noise and restore image quality. Use of our filtration scheme allows detailed quantitative analysis of an image even when subjected to scattered radiation.

Determination of prospective displacement-based gate threshold for respiratory-gated radiation delivery from retrospective phase-based gate threshold selected at 4D CT simulation.

Four-dimensional (4D) computed tomography (CT) imaging has found increasing importance in the localization of tumor and surrounding normal structures throughout the respiratory cycle. Based on such tumor motion information, it is possible to identify the appropriate phase interval for respiratory gated treatment planning and delivery. Such a gating phase interval is determined retrospectively based on tumor motion from internal tumor displacement. However, respiratory-gated treatment is delivered prospectively based on motion determined predominantly from an external monitor. Therefore, the simulation gate threshold determined from the retrospective phase interval selected for gating at 4D CT simulation may not correspond to the delivery gate threshold that is determined from the prospective external monitor displacement at treatment delivery. The purpose of the present work is to establish a relationship between the thresholds for respiratory gating determined at CT simulation and treatment delivery, respectively. One hundred fifty external respiratory motion traces, from 90 patients, with and without audio-visual biofeedback, are analyzed. Two respiratory phase intervals, 40%-60% and 30%-70%, are chosen for respiratory gating from the 4D CT-derived tumor motion trajectory. From residual tumor displacements within each such gating phase interval, a simulation gate threshold is defined based on (a) the average and (b) the maximum respiratory displacement within the phase interval. The duty cycle for prospective gated delivery is estimated from the proportion of external monitor displacement data points within both the selected phase interval and the simulation gate threshold. The delivery gate threshold is then determined iteratively to match the above determined duty cycle. The magnitude of the difference between such gate thresholds determined at simulation and treatment delivery is quantified in each case. Phantom motion tests yielded coincidence of simulation and delivery gate thresholds to within 0.3%. For patient data analysis, differences between simulation and delivery gate thresholds are reported as a fraction of the total respiratory motion range. For the smaller phase interval, the differences between simulation and delivery gate thresholds are 8 +/- 11% and 14 +/- 21% with and without audio-visual biofeedback, respectively, when the simulation gate threshold is determined based on the mean respiratory displacement within the 40%-60% gating phase interval. For the longer phase interval, corresponding differences are 4 +/- 7% and 8 +/- 15% with and without audiovisual biofeedback, respectively. Alternatively, when the simulation gate threshold is determined based on the maximum average respiratory displacement within the gating phase interval, greater differences between simulation and delivery gate thresholds are observed. A relationship between retrospective simulation gate threshold and prospective delivery gate threshold for respiratory gating is established and validated for regular and nonregular respiratory motion. Using this relationship, the delivery gate threshold can be reliably estimated at the time of 4D CT simulation, thereby improving the accuracy and efficiency of respiratory-gated radiation delivery.

Radioimmunoguided imaging of prostate cancer foci with histopathological correlation.

PURPOSE: We have previously presented a technique that fuses ProstaScint and pelvic CT images for the purpose of designing brachytherapy that targets areas at high risk for treatment failure. We now correlate areas of increased intensity seen on ProstaScint-CT fusion images to biopsy results in a series of 7 patients to evaluate the accuracy of this technique in localizing intraprostatic disease. METHODS AND MATERIALS: The 7 patients included in this study were evaluated between June 1998 and March 29, 1999 at Metrohealth Medical Center and University Hospitals of Cleveland in Cleveland, Ohio. ProstaScint and CT scans of each patient were obtained before transperineal biopsy and seed implantation. Each patient's prostate gland was biopsied at 12 separate sites determined independently of Prostascint-CT scan results. RESULTS: When correlated with biopsy results, our method yielded an overall accuracy of 80%: with a sensitivity of 79%, a specificity of 80%, a positive predictive value of 68%, and a negative predictive value of 88%. CONCLUSION: The image fusion of the pelvic CT scan and ProstaScint scan helped identify foci of adenocarcinoma within the prostate that correlated well with biopsy results. These data may be useful to escalate doses in regions containing tumor by either high-dose rate or low-dose rate brachytherapy, as well as by external beam techniques such as intensity modulated radiotherapy (IMRT).