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.     

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.     

Quality parental decision making and distress

OBJECTIVES: To prove that the model of perspective projection allows precise registration of intra-oral radiographs regardless of whether they have been acquired with or without individual adjustment aids and independent of the human observer or computer algorithm marking corresponding landmarks in the images and, based on in vivo radiographs, to introduce and evaluate a model-based registration method. METHODS: Five observers (three experts and two non-experts) were asked to define corresponding points in 24 pairs of in vivo dental radiographs from the same region of the same patient. The landmarks were used to fit the model of perspective projection applying the least squares method. Misplaced landmarks were detected and suppressed by analysing the quality of all subsets of landmarks with respect to the minimal residual (leaving one out method). In addition, local correlation was used to optimize the quality of registration as well as observer independence. RESULTS: Using six or more corresponding landmarks in both radiographs the correlation of the images registered was > 0.95 (S.D. < 0.063) irrespective of the observers' expertise. CONCLUSIONS: Perspective projection is a reliable model for sequentially acquired intra-oral radiographs. The co-ordinates of anatomical landmarks are useful for determining the parameters of perspective projection. Local correlation and leaving one out techniques improve the geometrical adjustment as well as observer independence. Registration is nearly independent of the actual position of the landmarks and hence independent of the observer. Our algorithm will also be useful for registration techniques based on automatically detected landmarks.     

Solution structure of a DNA complex with the fluorescent bis-intercalator TOTO modified on the benzothiazole ring

The aim of the present study was to evaluate and compare the reproducibility of cephalometric landmarks on (1) conventional films, and images acquired by storage phosphor digital radiography both on (2) hardcopy and (3) monitor-displayed versions. The material consisted of 19 cephalograms for each image modality. The phosphor plates were scanned in an image reader and the 10-bit normalized, raw data digital images were converted to 8-bit TIFF images for PC monitor-display. The digital hardcopies were produced in a laser printer. Six observers were asked to record 21 cephalometric landmarks on each conventional film, hardcopy, and monitor-displayed image. For the films and hardcopies, the landmark co-ordinates were recorded via a digitizing tablet. For the monitor-displayed images, the co-ordinates were recorded directly from the monitor using a dedicated Windows-based cephalometric program. Reproducibility was defined as an observer's deviation (in mm) from the mean between all observers. Differences between the image modalities and between the observers were tested by two-way analysis of variance for each landmark. There was a statistically significant difference between the reproducibility of film, hardcopy and monitor-displayed images in 11 of the 21 landmarks. There was no unequivocal trend that one modality was always the best. For a full cephalometric recording (the sum of all 21 landmarks), the monitor-displayed images (mean = 25.3 mm) had a lower precision than film (P < 0.005) and hard-copy (P < 0.02). There was no significant difference between film (mean = 21.8 mm) and hardcopy (mean = 22.8 mm). The lower reproducibility seen for the monitor-displayed images is most probably of little clinical significance.     

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.     

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.     

Feasibility of megavoltage portal CT using an electronic portal imaging device (EPID) and a multi-level scheme algebraic reconstruction technique (MLS-ART)

Although electronic portal imaging devices (EPIDs) are efficient tools for radiation therapy verification, they only provide images of overlapped anatomic structures. We investigated using a fluorescent screen/CCD-based EPID, coupled with a novel multi-level scheme algebraic reconstruction technique (MLS-ART), for a feasibility study of portal computed tomography (CT) reconstructions. The CT images might be useful for radiation treatment planning and verification. We used an EPID, set it to work at the linear dynamic range and collimated 6 MV photons from a linear accelerator to a slit beam of 1 cm wide and 25 cm long. We performed scans under a total of approximately 200 monitor units (MUs) for several phantoms in which we varied the number of projections and MUs per projection. The reconstructed images demonstrated that using the new MLS-ART technique megavoltage portal CT with a total of 200 MUs can achieve a contrast detectibility of approximately 2.5% (object size 5 mm x 5 mm) and a spatial resolution of 2.5 mm.     

Microsatellite instability and loss of heterozygosity at DNA mismatch repair gene loci occurs during hepatic carcinogenesis

OBJECTIVES: The purpose of this study was to compare D-speed film, E-speed film, and the Soredex Digora system with respect to the detection of periradicular pathosis. STUDY DESIGN: Radiographic images of 100 cadaver jaws were made with E-speed film, D-speed film, and the Soredex Digora. Each set of 100 images was interpreted by four observers, with 30 days separating each of three viewing sessions from the next. The presence or absence of pathologic (inflammatory) periradicular bone resorption was determined by histologic examination of the samples. The observer performance was compared with the true histologic findings and evaluated with receiver operating characteristic and corrected receiver operating characteristic analysis. RESULTS: No statistically significant differences were found in diagnostic performance among the three radiographic techniques. In addition, no imaging technique was a good indicator of pathosis as determined by histologic analysis. CONCLUSION: Under the conditions of this study, it was determined that D-speed film, E-speed film, and the Soredex Digora were equivalent diagnostic imaging modalities with regard to the detection of pathologic periradicular bone resorption. No technique predictably indicated inflammatory resorption.     

Digital images in the diagnosis of wound healing problems

The use of digital wound images could allow remote consultation among patients, physicians, or other care-givers located at quite distant sites by means of the Internet. To evaluate the efficacy and validity of digital images for the evaluation of wounds, the ability and reliability of surgeons to diagnose and make treatment suggestions using digital images of several types of wounds were compared. Twenty-four wound images on 35-mm slides were selected for use in this study. Each slide image was digitized at 24-bit color with a resolution of 640 pixels horizontal by 425 pixels vertical and stored as a JPEG file. These images were then presented as a slide show on a video monitor, with resolution set at 640 x 480. Six physicians examined the images, first in digital format and later in the original slide form. Each observer assessed each wound and possible treatment options by filling out a questionnaire using a series of yes/no questions. For all observers, there was an 87 percent agreement between digital and slide images (p = 0.004). The agreement between the digital and slide images was measured for each individual observer using a kappa coefficient. The agreement level corresponded to the experience of the observer, with the kappa values ranging from greater than 0.8 (almost perfect agreement) for the attending plastic surgeon to just greater than 0.5 (moderate agreement) for the intern. With this study, the feasibility of distance wound consultation using digital images of a quality consistent with consumer-grade digital photography was demonstrated.     

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.     

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%.     

Selective MR angiography of the liver

OBJECTIVE: Two-dimensional time-of-flight MR angiography was done with a 1.0 T whole-body imaging system. METHODS: The 10 mm thick presaturation slab was positioned between two sagittal imaging slices of the liver. Images were obtained through the right lobe of the liver by moving the slab and slices together. Each image was acquired during a breath-holding interval of 16 s. RESULTS: Since the directions of the portal and hepatic venous flows are opposite to each other in the right lobe, these two venous systems could be visualized on separate images by the interleaved presaturation slab. On the reconstructed angiograms, separation between the two venous systems was complete and even the fourth and fifth branches were demonstrated clearly. These images facilitate clear understanding of the structure of the intrahepatic blood vessels. CONCLUSION: Although this technique is limited to volunteer studies and works only on the right lobe of the liver, it will provide valuable information for evaluating the location and vascular involvement in various liver diseases.     

Palisaded encapsulated neuroma of the nasal fossa

The aim of the study was to assess the potential application of teleradiology in the neonatal intensive care unit (NICU) by ascertaining whether any decrease in conspicuity of anatomic detail or interventional devices in the chest radiographs of premature infants is caused by picture archiving and communication system (PACS)-based soft copy interpretation of 10 : 1 compressed images. One hundred digital chest radiographs of low-birthweight infants were obtained in the NICU using a storage phosphor system. Laser-printed images were interpreted and the data set for each radiograph was then irreversibly compressed by a 10 : 1 ratio. Four radiologists with extensive PACS experience used a five-point grading system to score laser-printed hard copy images for the visibility of six parameters of anatomic landmarks and interventional devices in the chest. Compressed soft copy images displayed on 2K PACS workstation were subsequently scored using the same approach. Statistical manipulation demonstrated no loss of anatomic detail in five of the six parameters scored, with minimal difference in one landmark, the retrocardiac lung assessment. While further study is required to assess the clinical impact of the variance noted when evaluating lung parameters, the preservation or improvement of information in the remaining parameters following irreversible compression and soft copy interpretation is promising for the potential use of teleradiology in this population.     

On-line portal imaging: image quality defining parameters for pelvic fields--a clinical evaluation

PURPOSE: A test of several image enhancement techniques, performed on on-line portal images in real clinical circumstances, is presented. In addition a score system enabling us to evaluate image quality on pelvic fields is proposed and validated. METHODS AND MATERIALS: Localization images (n = 546) generated by an on-line portal imaging system during the treatment of 13 patients on pelvic fields were obtained by delivering a radiation dose of 6-8 cGy by an 18 MV photon beam, and recorded with a silicon intensified target video camera with adjustable gain, kV- and black level. Set-up errors were corrected before continuing irradiation. A scoring system based on the number of visible bone-soft tissue edges and transformed to a scale 0 to 5 was developed to judge image quality. A validation of this classification of images was performed with the use of transsectional bone-densities (bone-density*radiological path length) specified at the score defining landmarks. A high pass filter was used on all images, additional on-line open field subtraction was performed on 242 fields. Off-line study was performed in which a panel consisting of two groups (one composed of three radiation oncologists, the other of three radiotherapy technologists), scored 470 pelvic fields without further enhancement, and the same images with Contrast Limited Adaptive Histogram Equalization (CLAHE) (Pizer et al.). Two different clipping levels (3.0 and 5.0) were studied. RESULTS: Gender and transsectional bone-densities were the most defining patient-related factors influencing image quality. Camera settings, gantry angle, and image post-processing were important non-patient-related factors. All investigators judged CLAHE to ameliorate low contrast images and to deteriorate good quality images (p < 0.001).     

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.     

Initial clinical experience with computer-controlled conformal radiotherapy of the prostate using a 50-MeV medical microtron

PURPOSE: We have described previously a model for delivering computer-controlled radiation treatments. We report here on the implementation and first year's clinical experience with such treatments using a 50 MeV medical microtron. METHODS AND MATERIALS: The microtron is equipped with a multileaf collimator and is capable of setting up and treating a sequence of fixed fields called segments, under computer control. An external computer derives machine parameters for the segments from a three-dimensional treatment planning system, transfers them to the microtron control computer, checks the machine settings before allowing dose delivery to begin, and records the treatment. We describe the patient treatment methodology, portal film acquisition, electronic portal imaging, and quality assurance. RESULTS: Patient treatments began in July 1992, comprising six-segment conformal treatments of the prostate. Using the recorded treatment data, the system performance has been examined and compared to other treatment machines. The average treatment time is 10 min, of which 4 min is for computer-controlled setup and irradiation; the remaining time is for patient positioning and checking of clearances. Long-term reproducibility of computer-controlled setup of the gantry and multileaf position is better than 0.5 degrees and 1 mm, respectively. Termination due to a machine fault has occurred in 5.5% of treatments, improving to 2.5% in recent months. CONCLUSION: Our initial experience indicates that computer-controlled segmental therapy can be performed reliably on a routine basis. Treatment times with the microtron are significantly shorter than with conventional linacs, and setup accuracy is consistent with that needed for conformal therapy. We believe that treatment times can be further improved through software upgrades and integration of electronic portal imaging.     

Quality assurance of serial tomotherapy for head and neck patient treatments

PURPOSE: A commercial serial tomotherapy intensity-modulated radiation therapy (IMRT) treatment planning (Peacock, NOMOS Corp., Sewickley, PA) and delivery system is in clinical use. The dose distributions are highly conformal, with large dose gradients often surrounding critical structures, and require accurate localization and dose delivery. Accelerator and patient-specific quality assurance (QA) procedures have been developed that address the localization, normalization, and delivery of the IMRT dose distributions. METHODS AND MATERIALS: The dose distribution delivered by serial tomotherapy is highly sensitive to the accuracy of the longitudinal couch motion. There is also an unknown sensitivity of the dose distribution on the dynamic mutlileaf collimator alignment. QA procedures were implemented that assess these geometric parameters. Evaluations of patient positioning accuracy and stability were conducted by exposing portal films before (single exposure) and after (single or double exposure) treatments. The films were acquired with sequential exposures using the largest available fixed multileaf portal (3.36 x 20 cm2). Comparison was made against digitally reconstructed radiographs generated using independent software and appropriate beam geometries. The delivered dose was verified using homogeneous cubic phantoms. Radiographic film was used to determine the localization accuracy of the delivered isodose distributions, and ionization chambers and thermoluminescent dosimetry (TLD) chips were used to verify absolute dose at selected points. Ionization chamber measurements were confined to the target dose regions and TLD measurements were obtained throughout the irradiated volumes. Because many more TLD measurements were made, a statistical evaluation of the measured-to-calculated dose ratio was possible. RESULTS: The accelerator QA techniques provided adequate monitoring of the geometric patient movement and dynamic multileaf collimator alignment and positional stability. The absolute delivered dose as measured with the ionization chamber varied from 0.94 to 0.98. Based on these measurements, the delivered monitor units for both subsequent QA measurements and patient treatments were adjusted by the ratio of measured to calculated dose. TLD measurements showed agreement, on average, with the ionization chamber measurements. The distribution of TLD measurements in the high-dose regions indicated that measured doses agreed within 4.2% standard deviation of the calculated doses. In the low-dose regions, the measured doses were on average 5% greater than the calculated doses, due to a lack of leakage dose in the dose calculation algorithm. CONCLUSIONS: The QA system provided adequate determination of the geometric and dosimetric quantities involved in the use of IMRT for the head and neck. Ionization chamber and TLD measurements provided accurate determination of the absolute delivered dose throughout target volumes and critical structures, and radiographic film yielded precise dose distribution localization verification. Portal film acquisition and subsequent portal film analysis using 3.36 x 20 cm2 portals proved useful in the evaluation of patient immobilization quality. Adequate bony landmarks were imaged when carefully selected portals were used.     

Plasticity of hippocampal corticosteroid receptors during aging in the rat

OBJECTIVE: The purpose of this study was to describe and validate an image-quality phantom to be used in dental radiography for comparison of film and digitally acquired images. STUDY DESIGN: An aluminum block of 12 steps, with 7 holes in each step, was covered by acrylic blocks. This phantom was radiographed with Kodak Ultra-speed and Ektaspeed Plus films at 70, 65, and 60 kVp with the whole exposure range available. All together, 50 dental films were randomly sequenced and presented to 7 observers. The average number of perceptible holes from all steps was plotted against exposure for each tube voltage and film type, generating a modified perceptibility curve. The tentative optimum exposure level was determined from perceptibility curves in each experimental condition and compared with that determined by means of the standard aluminum stepwedge and the preset time of the x-ray machine. The density range of this phantom at the optimum exposure was compared with that of clinical dental radiographs. Validity of the phantom was evaluated according to the optimum exposure level from the modified perceptibility curves and the overall density range. Finally, the average maximum numbers of perceptible holes at the tentative optimum exposure level were compared for each tube voltage and film type. The statistical test used was a 2-way factorial analysis of variance. RESULTS: The exposure at the perceptibility curve peak approximated that obtained by means of the standard aluminum step-wedge and the time preset by the manufacturer. The overall density range at the perceptibility curve peak covered the clinical density range for each tube voltage and film type. There were no statistically significant differences between film types or among tube voltages. CONCLUSIONS: The x-ray attenuation range for this phantom seemed to approximate clinical conditions. In addition, differences in image quality could be quantitatively evaluated by means of the number of the holes seen in the phantom.