Rapid portal imaging with a high-efficiency, large field-of-view detector

The design, construction, and performance evaluation of an electronic portal imaging device (EPID) are described. The EPID has the same imaging geometry as the current mirror-based systems except for the x-ray detection stage, where a two-dimensional (2D) array of 1 cm thick CsI(Tl) detector elements are utilized. The approximately 18% x-ray quantum efficiency of the scintillation detector and its 30 x 40 cm2 field-of-view at the isocenter are greater than other area-imaging EPIDs. The imaging issues addressed are theoretical and measured signal-to-noise ratio, linearity of the imaging chain, influence of frame-summing on image quality and image calibration. Portal images of test objects and a humanoid phantom are used to measure the performance of the system. An image quality similar to the current devices is achieved but with a lower dose. With approximately 1 cGy dose delivered by a 6 MV beam, a 2 mm diam structure of 1.3% contrast and an 18 mm diam object of 0.125% contrast can be resolved without using image-enhancement methods. A spatial resolution of about 2 mm at the isocenter is demonstrated. The capability of the system to perform fast sequential imaging, synchronized with the radiation pulses, makes it suitable for patient motion studies and verification of intensity-modulated beams as well as its application in cone-beam megavoltage computed tomography.     

Tomotherapeutic portal imaging for radiation treatment verification

In their tomotherapy concept Mackie and co-workers proposed not only a new technique for IMRT but also an appropriate and satisfactory method of treatment verification. This method allows both monitoring of the portal dose distribution and imaging of the patient anatomy during treatment by means of online CT. This would enable the detection of inaccuracies in dose delivery and patient set-up errors. In this paper results are presented showing that a single electronic portal imaging device (EPID) could deliver all data necessary to establish such a complete verification system for tomotherapy and even other IMRT techniques. Consequently it has to be shown that it is able to record both the low-intensity photon fluences encountered in tomographic imaging and the intense photon transmission of each treatment field. The detector under investigation is a video-based EPID, the BIS 710 (manufactured by Wellh?fer Dosimetrie, Schwarzenbruck, Germany). To examine the suitability of the BIS for CT at 6 MV beam quality, different phantoms were scanned and reconstructed. The agreement between a diamond detector and BIS responses is quantitative. Tomographic reconstruction of a complete set of these transmission profiles resulted in images which resolve 3 cm large objects having a (theoretical) contrast to water of less than 9%. Three millimetre objects with a 100% contrast are clearly visible. The BIS signal was shown to measure photon fluence distributions. The reconstructed images possess a spatial and contrast resolution sufficient for accurate imaging of the patient anatomy, needed for treatment verification in many clinical cases.     

Electronic portal imaging with on-line correction of setup error in thoracic irradiation: clinical evaluation

PURPOSE: To analyze setup errors and the feasibility of their on-line correction using electronic portal imaging in the irradiation of lung tumors. METHODS AND MATERIALS: Sixteen patients with lung cancer were irradiated through opposed anteroposterior fields. Localization images of anteroposterior fields were recorded with an electronic portal imaging device (EPID). Using an in-house developed algorithm for on-line comparison of portal images setup errors were measured and a correction of table position was performed with a remote couch control prior to treatment. In addition, residual errors were measured on the EPID verification image. Global and individual mean and standard deviation of setup errors were calculated and compared. The feasibility of the procedure was assessed measuring intra- and interobserver variability, influence of organ movement, reproducibility of error measurement, the extra time fraction needed for measuring and adjusting and the fraction of dose needed for imaging. RESULTS: In two setups the procedure could not be finished normally due to problems inherent to the procedure. The reproducibility, intraobserver variability, and influence of organ movements were each described by a distribution with a mean value less than or equal to 1 mm and a standard deviation (SD) of less than 1.5 mm. The interobserver variability showed to be a little bit larger (mean: 0.3 mm, SD: 1.7 mm). The mean time to perform the irradiation of the anteroposterior field was 4 +/- 1 min. The mean time for the measurement and correction procedure approximated 2.5 min. The mean extra time fraction was 65 +/- 24% (1 SD) with more than half of this coming from the error measurement. The dose needed for generation of EPID images was 5.9 +/- 1.4% of total treatment dose. The mean and SD of setup errors were, respectively, 0.1 and 4.5 mm for longitudinal and -2.0 and 5.7 mm for transversal errors. Of 196 measured translational errors 120 (61%) exceeded the adjustment criteria. For individual patients systematic and random setup errors can be as high as, respectively, 15.8 and 7.5 mm. Mean residual error and SD were for longitudinal direction 0.08 and 1.2 mm and for transversal direction -0.9 and 1.0 mm (pooled data). For individuals, the mean residual errors were smaller than 1 mm, with a typical SD per patient of less than 2 mm. CONCLUSION: Setup errors in thoracic radiation therapy are clinically important. On-line correction can be performed accurately with an objective measurement tool, although this prolongs the irradiation procedure for one field with 65%.     

MR image-guided portal verification for brain treatment field

PURPOSE: To investigate a method for the generation of digitally reconstructed radiographs directly from MR images (DRR-MRI) to guide a computerized portal verification procedure. METHODS AND MATERIALS: Several major steps were developed to perform an MR image-guided portal verification procedure. Initially, a wavelet-based multiresolution adaptive thresholding method was used to segment the skin slice-by-slice in MR brain axial images. Some selected anatomical structures, such as target volume and critical organs, were then manually identified and were reassigned to relatively higher intensities. Interslice information was interpolated with a directional method to achieve comparable display resolution in three dimensions. Next, a ray-tracing method was used to generate a DRR-MRI image at the planned treatment position, and the ray tracing was simply performed on summation of voxels along the ray. The skin and its relative positions were also projected to the DRR-MRI and were used to guide the search of similar features in the portal image. A Canny edge detector was used to enhance the brain contour in both portal and simulation images. The skin in the brain portal image was then extracted using a knowledge-based searching technique. Finally, a Chamfer matching technique was used to correlate features between DRR-MRI and portal image. RESULTS: The MR image-guided portal verification method was evaluated using a brain phantom case and a clinical patient case. Both DRR-CT and DRR-MRI were generated using CT and MR phantom images with the same beam orientation and then compared. The matching result indicated that the maximum deviation of internal structures was less than 1 mm. The segmented results for brain MR slice images indicated that a wavelet-based image segmentation technique provided a reasonable estimation for the brain skin. For the clinical patient case with a given portal field, the MR image-guided verification method provided an excellent match between features in both DRR-MRI and portal image. Moreover, target volume could be accurately visualized in the DRR-MRI and mapped over to the corresponding portal image for treatment verification. The accuracy of DRR-MRI was also examined by comparing it to the corresponding simulation image. The matching results indicated that the maximum deviation of anatomical features was less than 2.5 mm. CONCLUSION: A method for MR image-guided portal verification of brain treatment field was developed. Although the radiographic appearance in the DRR-MRI is different from that in the portal image, DRR-MRI provides essential anatomical features (landmarks and target volume) as well as their relative locations to be used as references for computerized portal verification.     

Accurate portal dose measurement with a fluoroscopic electronic portal imaging device (EPID) for open and wedged beams and dynamic multileaf collimation

Measuring portal dose with an electronic portal imaging device (EPID) in external beam radiotherapy can be used to perform routine dosimetric quality control checks on linear accelerators and to verify treatments (in vivo dosimetry). An accurate method to measure portal dose images (PDIs) with a commercially available fluoroscopic EPID has been developed. The method accounts for (i) the optical 'cross talk' within the EPID structure, (ii) the spatially nonuniform EPID response and (iii) the nonlinearity of the EPID response. The method is based on a deconvolution algorithm. Measurement of the required input data is straightforward. The observed nonlinearity of the EPID response was largely due to the somewhat outdated EPID electronics. Nonlinearity corrections for more modern systems are expected to be smaller. The accuracy of the method was assessed by comparing PDIs measured with the EPID with PDIs measured with a scanning ionization chamber in a miniphantom, located at the same position as the fluorescent screen. For irradiations in open, wedged and intensity modulated 25 MV photon beams (produced with dynamic multileaf collimation) EPID and ionization chamber measurements agreed to within 1% (1 SD).     

Dosimetric characteristics of a liquid-filled electronic portal imaging device

PURPOSE: To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled ionization chambers, for transmission dose measurements during patient treatment. METHODS AND MATERIALS: Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type ionization chamber in air in a miniphantom and in an extended water phantom. RESULTS: The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the ionization chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the ionization chambers is small. The sensitivity increases about 0.5% for all ionization chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the ionization chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID ionization chambers were between those of an ionization chamber in a miniphantom and in a water phantom. CONCLUSION: The matrix ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.     

An observer study for direct comparison of clinical efficacy of electronic to film portal images

PURPOSE: To directly compare clinical efficacy of electronic to film portal images. METHODS AND MATERIALS: An observer study was designed to compare clinical efficacy of electronic to film portal images acquired using a liquid matrix ion-chamber electronic portal imaging device and a conventional metal screen/film system. Both images were acquired simultaneously for each treatment port and the electronic portal images were printed on gray-level thermal paper. Four radiation oncologists served as observers and evaluated a total of 44 sets of images for four different treatment sites: lung, pelvis, brain, and head/neck. Each set of images included a simulation image, a double-exposure portal film, and video paper prints of electronic portal images. Eight to nine anatomical landmarks were selected from each treatment site. Each observer was asked to rate each landmark in terms of its clinical visibility and to rate the ease of making the pertinent verification decision in the corresponding electronic and film portal images with the aid of the simulation image. RESULTS: Ratings for the visibility of landmarks and for the verification decision of treatment ports were similar for electronic and film images for most landmarks. However, vertebral bodies and several landmarks in the pelvis such as the acetabulum and public symphysis were more visible in the portal film images than in the electronic portal images. CONCLUSION: The visibility of landmarks in electronic portal images is comparable to that in film portal images. Verification of treatment ports based only on electronic portal images acquired using an electronic portal imaging device is generally achievable.     

Clinical use of on-line portal imaging for daily patient treatment verification

PURPOSE: To determine the ease of use by clinical staff and reliability of an electronic portal imaging system and evaluate the potential to utilize on-line imaging to assess accuracy of daily patient treatment positioning in radiation therapy. METHODS AND MATERIALS: A computer controlled fluorescent screen-mirror imaging system was used to acquire on-line portal images. A physician panel assessed on-line image quality relative to standard portal film. Clinical use of the imager was implemented through a protocol where images were obtained during the first six monitor units of external beam. The images were visually compared to a reference portal and patient setup was adjusted for errors exceeding 5 mm. Subsequent off-line analysis was utilized to give insight into the magnitude of clinical setup error in the visually accepted images. RESULTS: Physician evaluation of on-line image quality with an initial 211 images found that 70% were comparable or superior to standard film portal images. Eighty percent of treatment fields fit completely within the on-line imaging area. Eight percent of on-line images were rejected due to poor image quality. Twelve percent of the daily treatment setups imaged required adjustment overall, but specific field types predictably required more frequent adjustment (pelvic and mantle fields). Off-line analysis of accepted images demonstrates that 18% of the final images had setup errors exceeding 5 mm. CONCLUSION: On-line imaging facilitated daily portal alignment and verification. Ease of use, almost instantaneous viewing and consistent ability to identify and locate anatomical landmarks imply the potential for on-line imaging to replace film based approaches. Retrospective analysis of daily images reveals that visual assessment of setup is not sufficient for eliminating localization errors. Further improvement is required with respect to detecting localization error and fully encompassing larger field sizes.     

X-ray quantum limited portal imaging using amorphous silicon flat-panel arrays

We have measured the linearity, spatial resolution (MTF), noise (NPS), and signal-to-noise characteristics (DQE) of an electronic portal imaging device (EPID) based on an amorphous silicon flat-panel array. The array has a 128 x 128-pixel matrix and each pixel is 0.75 x 0.75 mm2 in dimension so the array covers an area of 96 x 96 mm2. The array acts like a large area light sensor and records the optical signals generated in a metal plate/phosphor screen x-ray detector when the detector is irradiated by a megavoltage x-ray beam. In addition, approximately 0.5% of the total signal is generated by nonoptical processes. The noise measurements show that the device is quantum noise limited with the noise power generated by the x-ray quanta being up to 100 times greater than the noise added by the external readout electronics and flat-panel light sensor itself. However, the flat-panel light sensor does reduce the spatial resolution (compared to a perfect optical sensor with infinitesimal pixel size) because of its moderate pixel size and because optical spread can occur in the transparent glues used to attach the phosphor screen to the flat-panel light sensor. The response of the sensor is very linear and does not suffer from the glare phenomenon associated with TV camera-based EPIDs--characteristics which suggest that the amorphous silicon EPID will be well suited to transit dosimetry. Nevertheless, some limitations need to be overcome before these devices can be used clinically. These include developing larger flat-panel light sensors, the elimination of "noisy" pixels with high dark signal, and improvements in the uniform sensitivity of the sensors. This last requirement is only needed for transit dosimetry applications where it would greatly simplify calibration of the device. In addition, an image acquisition scheme must be developed to eliminate artifacts created by the pulsed x-ray beam generated by linear accelerators. Despite these limitations, our studies suggest that the amorphous silicon EPIDs are very well suited to portal imaging.     

Combining various projection access schemes with the algebraic reconstruction technique for low-contrast detection in computed tomography

We separately applied three different projection access orders (the multilevel scheme (MLS), the sequence access scheme (SAS) and the random permutation scheme (RPS)) to the algebraic reconstruction technique (ART) in an attempt to improve low-contrast object detection in computed tomography (CT). We used both simulated (from various numbers of projections of low-contrast and contrast detail phantoms) and actual data (from megavoltage portal CT) during our study. When coupled with ART, SAS and RPS led to poor low-contrast detection and required multiple iterations for convergence. In contrast, one-iteration MLS yielded the most uniform reconstructions with the highest contrast-to-noise ratios. Therefore, MLS-ART provides the highest dose efficiency of all current reconstruction techniques for imaging low-contrast objects from a small number of projections.     

Frame slippage verification in stereotactic radiosurgery

PURPOSE: To develop a method for detecting frame slippage in stereotactic radiosurgery by interactively matching in three dimensions Digitally Reconstructed Radiographs (DRRs) to portal images. METHODS AND MATERIALS: DRRs are superimposed over orthogonal edge-detected portal image pairs obtained prior to treatment. By interactively manipulating the CT data in three dimensions (rotations and translations) new DRRs are generated and overlaid with the orthogonal portal images. This method of matching is able to account for ambiguities due to rotations and translations outside of the imaging plane. The matching procedure is performed with anatomical structures, and is used in tandem with a fiducial marker array attached to the stereotactic frame. The method is evaluated using portal images simulated from patient CT data and then tested using a radiographic head phantom. RESULTS: For simulation tests a mean radial alignment error of 0.82 mm was obtained with the 3D matching method compared to a mean error of 3.52 mm when using conventional matching techniques. For the head phantom tests the mean alignment displacement error for each of the stereotactic coordinates was found to be delta(x) = 0.95 mm, delta(y) = 1.06 mm, delta(z) = 0.99 mm, with a mean error radial of 1.94 mm (SD = 0.61 mm). CONCLUSION: Results indicate that the accuracy of the system is appropriate for stereotactic radiosurgery, and is therefore an effective tool for verification of frame slippage.     

Clinical applications of composite and realtime megavoltage imaging

The versatility of electronic portal imaging devices (EPIDs) is best demonstrated by their ability to perform novel megavoltage imaging protocols, which are still pertinent to good radiotherapy practice. This paper examines two such techniques: composite and realtime imaging. Our EPID can be programmed to acquire and manipulate images very easily, allowing images from segmented treatment protocols to be mixed and displayed, giving a composite image of the effective treatment result. Its use for verifying the efficacy of spinal shielding using a segmented, offset collimator technique is described. By acquiring images very quickly, realtime imaging sequences can be obtained and used to analyse anatomical movement within a single treatment field. The technique is employed here to investigate movement in radical lung, breast, abdomen, pelvis and thyroid treatments. Our results show that the protocol is vital for treatment sites involving the lungs; changes up to 5 mm have been observed in the maximum lung depth for breast treatments, and displacements up to 16 mm for radical lung treatments. It is also useful in other anatomical sites for ensuring that no movement occurs.     

Initial performance evaluation of an indirect-detection, active matrix flat-panel imager (AMFPI) prototype for megavoltage imaging

PURPOSE: The development of the first prototype active matrix flat-panel imager (AMFPI) capable of radiographic and fluoroscopic megavoltage operation is reported. The signal and noise performance of individual pixels is empirically quantified. Results of an observer-dependent study of imaging performance, using a contrast-detail phantom, are detailed and radiographic patient images are shown. Finally, a theoretical investigation of the zero-frequency detective quantum efficiency (DQE) performance of such imagers, using a cascaded systems formalism, is presented. METHODS AND MATERIALS: The imager is based on a 508-microm pitch, 26 x 26 cm2 array which detects radiation indirectly via an overlying copper plate + phosphor screen converter. RESULTS: Due to its excellent optical coupling, the imager exhibits sensitivity superior to that of video-based systems. With an approximately 133 mg/cm2 Gd2O2S:Tb screen the system is x-ray quantum-noise-limited down to approximately 0.3 cGy, conservatively, and extensions of this behavior to even lower doses by means of reduced additive electronic noise is predicted. The observer-dependent study indicates performance superior to that of conventional radiotherapy film while the patient images demonstrate good image quality at 1 to 4 MU. The theoretical studies suggest that, with a 133 mg/cm2 Gd2O2S:Tb screen, the system would provide DQE performance equivalent to that of video-based systems and that almost a factor of two improvement in DQE is achievable through the incorporation of a 400 mg/cm2 screen. CONCLUSION: The reported prototype imager is the first megavoltage AMFPI having performance characteristics consistent with practical clinical operation. The superior contrast-detail sensitivity of the imager allows the capture of high-quality 6- and 15-MV images at minimal dose. Moreover, significant performance improvements, including extension of the operational range up to full portal doses, appear feasible. Such capabilities could be of considerable practical benefit in patient localization and verification.     

Population dynamics of the Wolbachia infection causing cytoplasmic incompatibility in Drosophila melanogaster

PURPOSE: The use of escalated radiation doses to improve local control in conformal radiotherapy of prostatic cancer is becoming the focus of many centers. There are, however, increased side effects associated with increased radiotherapy doses that are believed to be dependent on the volume of normal tissue irradiated. For this reason, accurate patient positioning, CT planning with 3D reconstruction of volumes of interest, clear definition of treatment margins and verification of treatment fields are necessary components of the quality control for these procedures. In this study electronic portal images are used to (a) evaluate the magnitude and effect of the setup errors encountered in patient positioning techniques, and (b) verify the multileaf collimator (MLC) field patterns for each of the treatment fields. METHODS AND MATERIALS: The Phase I volume, with a planning target volume (PTV) composed of the gross tumour volume (GTV) plus a 1.5 cm margin is treated conformally with a three-field plan (usually an anterior field and two lateral or oblique fields). A Phase II, with no margin around the GTV, is treated using two lateral and four oblique fields. Portal images are acquired and compared to digitally reconstructed radiographs (DRR) and/or simulator films during Phase I to assess the systematic (CT planning or simulator to treatment error) and the daily random errors. The match results from these images are used to correct for the systematic errors, if necessary, and to monitor the time trends and effectiveness of patient imobilization systems used during the Phase I treatment course. For the Phase II, portal images of an anterior and lateral field (larger than the treatment fields) matched to DRRs (or simulator images) are used to verify the isocenter position 1 week before start of Phase II. The Portal images are acquired for all the treatment fields on the first day to verify the MLC field patterns and archived for records. The final distribution of the setup errors was used to calculate modified dose-volume histograms (DVHs). This procedure was carried out on 36 prostate cancer patients, 12 with vacuum-molded (VacFix) bags for immobilization and 24 with no immobilization. RESULTS: The systematic errors can be visualized and corrected for before the doses are increased above the conventional levels. The requirement for correction of these errors (e.g., 2.5 mm AP shift) was demonstrated, using DVHs, in the observed 10% increase in rectal volume receiving at least 60 Gy. The random (daily) errors observed showed the need for patient fixation devices when treating with reduced margins. The percentage of fields with displacements of < or = 5.0 mm increased from 82 to 96% with the use of VacFix bags. The rotation of the pelvis is also minimized when the bags are used, with over 95% of the fields with rotations of < or = 2.0 degrees compared to 85% without. Currently, a combination of VacFix and thermoplastic casts is being investigated. CONCLUSION: The systematic errors can easily be identified and corrected for in the early stages of the Phase I treatment course. The time trends observed during the course of Phase I in conjunction with the isocenter verification at the start of Phase II give good prediction of the accuracy of the setup during Phase II, where visibility of identifiable structures is reduced in the small fields. The acquisition and inspection of the portal images for the small Phase I fields has been found to be an effective way of keeping a record of the MLC field patterns used. Incorporation of the distribution of the setup errors into the planning system also gives a clearer picture of how the prescribed dose was delivered. This information can be useful in dose-escalation studies in determining the relationship between the local control or morbidity rates and prescribed dose.     

Sequence analysis of thyroid transcription factor-1 gene reveals absence of mutations in patients with thyroid dysgenesis but presence of polymorphisms in the 5'' flanking region and intron

The purpose of this study was to determine the optimal scanning technique for lesion detection in a small bowel phantom and to evaluate the virtual endoscopy (VE) technique in patients. A small bowel phantom with a fold thickness of 7 mm and length of 115 cm was prepared with nine round lesions (3 x 1 mm, 2 x 2 mm, 2 x 3 mm, 2 x 4 mm). Spiral CT parameters were 7/7/4, 3/5/2, 3/5/1, 1.5/3/1 (slice thickness/table feed/reconstruction interval). VE was done using volume rendering technique with 1 cm distance between images and 120 degrees viewing angle. Two masked readers were asked to determine the number and location of the lesions. Seven patients underwent an abdominal CT during one breathhold after placement of a duodenal tube and filling of the small bowel with methyl cellulose contrast solution. VE images were compared with the axial slices with respect to detectability of pathology. With the 7/7/4 protocol only the 4-mm lesions were visualised with fuzzy contours. The 3/5/2 protocol showed both 4-mm lesions, one 3-mm lesion and one false positive lesion. The 3/5/1 protocol showed both 4-mm and both 3-mm (one uncertain) lesions with improved sharpness, and no false positive lesions. One 2-mm and one 1-mm lesion were additionally seen with the 1.5/3/1 protocol. Path definition was difficult in sharp turns or kinks in the lumen. In all patients, no difference was found between VE and axial slices for bowel pathology; however, axial slices showed 'outside' information that was not included in VE. We conclude that the 3/5/2 protocol may be regarded as an optimal compromise between lesion detection, coverage during one breathhold, and number of reconstructed images in patients; round lesions of 4 mm in diameter can be detected with high certainty.     

Pelvic irradiation of the obese patient: a treatment strategy involving megavoltage simulation and intratreatment setup corrections

This study involves a fractionated course of external radiation therapy for a 42 year old female weighing 150 kg, diagnosed with stage IIb cancer of the cervix. The patient could not be simulated in the conventional sense due to weight restrictions on the simulator couch, and body casts or molds were impractical. Using an on-line portal imaging device, treatment fields were established during the first session, and intratreatment verification was used before every subsequent treatment to measure and, when necessary, to correct the patient setup. Two courses of treatment were prescribed with a total dose of 60 Gy delivered by a four field box technique (A/P, P/A, and two lateral fields). Out of a total of 108 treatment fields monitored, 12 anterior fields and 1 posterior field were corrected (exclusive of the first, or simulation fraction). Without corrections, 10% of the initial setup displacements would have had displacements greater than 10 mm, 21% greater than 7 mm, and 41% greater than 5 mm. With the application of intratreatment corrections, only 2% of the displacements were greater than 10 mm, 11% were greater than 7 mm, and 32% were greater than 5 mm. It was also found that the second field treated in a parallel opposed pair (i.e., anterior/posterior or left/right lateral) had lower setup displacements and did not require verification or correction.     

ArF-excimer laser-induced secondary radiation in photoablation of biological tissue

Secondary radiation, emitted during and after the irradiation of corneal, dermal, and dental tissue by an ArF-excimer laser (193 nm), was qualitatively and quantitatively characterized. Emission of secondary radiation was found in the range of 200-800 nm. The intensity of secondary radiation in the range of 200-315 nm (UVC and UVB) is approximately 20% of the total intensity at high laser fluences (> 2 J/cm2), and approximately 50% at moderate laser fluences (< 500 mJ/cm2); 10 muJ/cm2 in the UVC and UVB were measured at the sample surface, at fluences (< 1J/cm2) which are of relevance for clinical procedures on soft tissues. In dental tissue processing, very high fluences (> 5 J/cm2) are required. As a consequence, laser-induced plasma formation can be observed. Secondary radiation can be used as a visible guide for selective removal of carious altered tissue. The data we have found might be of assistance in estimating potential hazards for future mutagenic studies in the field.     

Optical CT reconstruction of 3D dose distributions using the ferrous-benzoic-xylenol (FBX) gel dosimeter

In recent years, magnetic-resonance imaging of gelatin doped with the Fricke solution has been applied to the direct measurement of three-dimensional (3D) radiation dose distributions. However, the 3D dose distribution can also be imaged more economically and efficiently using the method of optical absorption computed tomography. This is accomplished by first preparing a gelatin matrix containing a radiochromic dye and mapping the radiation-induced local change in the optical absorption coefficient. Ferrous-Benzoic-Xylenol (FBX) was the dye of choice for this investigation. The complex formed by Fe3+ and xylenol orange exhibits a linear change in optical attenuation (cm-1) with radiation dose in the range between 0 and 1000 cGy, and the local concentration of this complex can be probed using a green laser light (lambda = 543.5 nm). An optical computed tomography (CT) scanner was constructed analogous to a first-generation x-ray CT scanner, using a He-Ne laser, photodiodes, and rotation-translation stages controlled by a personal computer. The optical CT scanner itself can reconstruct attenuation coefficients to a baseline accuracy of < 2% while yielding dose images accurate to within 5% when other uncertainties are taken into account. Optical tomography is complicated by the reflection and refraction of light rays in the phantom materials, producing a blind spot in the transmission profiles which, results in a significant dose artifact in the reconstructed images. In this report we develop corrections used to reduce this artifact and yield accurate dosimetric maps. We also report the chemical reaction kinetics, the dose sensitivity and spatial resolution (< 1 mm3) obtained by optical absorption computed tomography. The article concludes with sample dose distributions produced by "cross-field" 6 MV x-ray beams, including a radiosurgery example.