Portal Verification Research Articles

160 MR image-guided portal verification for brain treatment field

160 MR image-guided portal verification for brain treatment field

Quantitative VS. Subjective portal verification using digital portal images

Off-line, computer-aided prescription (simulator) and treatment (portal) image registration using chamfer matching has been implemented on PC based viewing station. The purposes of this study were (a) to evaluate the performance of interactive anatomy and field edge extraction and subsequent registration, and (b) to compare observer's perceptions of field accuracy with measured discrepancies following anatomical registration. Prescription-treatment image pairs for 48 different patients were examined in this study. Digital prescription images were produced with the aid of a television camera and a digital frame grabber, while the treatment images were obtained directly from an on-line portal imaging system. To facilitate perception of low contrast anatomical detail, on-line portal images were enhanced with selective adaptive histogram equalization prior to extraction of anatomical edges. Following interactive extraction of anatomical and field border information by an experienced observer, the identified anatomy was registered using chamfer matching. The degree of conformity between the prescription and treatment fields was quantified using several parameters, which included relative prescription field coverage and overcoverage, as well as the translational and rotational displacements as measured by chamfer matching applied to the boundaries of the two fields. These quantitative measures were compared with subjective evaluations made by four radiation oncologists. All the images in this series that included a range of the most commonly seen treatment sites were registered and the conformity parameters were found. The mean treatment/prescription field coverage and overcoverage were approximately 95 and 7%, respectively before registration. The mean translational displacement in the transverse and cranio-caudal directions were 2.9 and 3.4 mm, respectively. The mean rotational displacement was approximately 2 degrees. For all four oncologists, the portals classified as unacceptable, in terms of the field placement, exhibited significantly higher (p < 0.03) translational errors in the transverse direction. The field coverages were significantly lower (p < 0.05) and the translational errors in the cranio-caudal direction were significantly higher (p < 0.05) for the portals rated as unacceptable by two of the oncologists. From the parameters that were used to quantify the degree of conformity between the prescription and treatment fields, the translational error in the transverse direction correlated best with the oncologists' assessments on the field placement. Field coverage and translational error in the cranio-caudal direction correlated well with assessments of only two out of the four participating oncologists. This can be explained by the fact that for the majority of treatment sites included in the study the positioning of field borders was more critical for the transverse direction. A conclusion for the design of future quantitative and automated on-line portal verification systems is that they will have to model different perceived significances of different types of localization errors intrinsic to oncologist evaluation of portal images.

93 Development of a computerized portal verification scheme for pelvic treatment fields

93 Development of a computerized portal verification scheme for pelvic treatment fields

189 Factors influencing treatment field accuracy in fractionated radiotherapy

<h3>Purpose</h3> Field placement errors can substantially contribute to local recurrences in radiotherapy. Demands for use of advanced treatment techniques have lead to improved patient fixation devices and improved portal verification systems. But fractionated treatment without patient fixation is still daily routine in most departments of radiotherapy. We evaluated the factors influencing treatment field accuracy in patients with fractioned radiotherapy without the use of fixation devices. <h3>Material and methods</h3> From 9/93 until 12/93 61 patients with isocentric opposing fields or single stationary fields of the thoracic or abdominal region were examined in respect of treatment field deviations from simulation film. We performed weekly portal verification films for each patient. The extend of treatment field deviation from simulation fiIrns were correlated to objective patient data (age, weight), subjective patient data (Karnofsky score, self assessed pain and anxiety scores) treatment intent (palliative vs. curative) and to daily workload. <h3>Results</h3> Deviations of treatment field borders varied from 0cm to 3.8cm (medium: 0.8cm). The percentage of field placement errors >1cm was significantly increased in patients treated in palliative intent (<i>P</i><0.025). Factors influencing treatment field accuracy were the absolute number of patients per treatment machine and day, the selfassessed pain score and the self assessed anxiety score during simulation or treatment. <h3>Conclusions</h3> Modem treatment techniques (linear accelerators, 3Dtreatment planning, etc.) allow to reduce planning and treatment volumes. Therefore, an accurate treatment field set-up must be warranted in order not to compromise treatment results. To keep treatment field deviations as small as possible patient fixations should be used, the radiotherapist must adequately care for the patient's medical and psychological welfare and sufficient daily set-up time must be given.

Quality of portal verification images using GLP7 film

To improve portal verification radiographs, we tested the application of GLP7 film. The quality of the portal verification radiograph using the XV cassette-GLP7 film combination and the XL cassette-GLP7 film combination was investigated. The XV cassette-XV2 film combination and the SA cassette-GS screen-XTL film combination also were used for comparison. The characteristic curves showed that the relative speeds were 0.32 and 0.47 for the XV-GLP7 and XL-GLP7 combinations and that the average gradients were 1.93, 2.14, and 1.32 for the XV-GLP7, XL-GLP7, and XV-XV2 combinations, respectively. In the experiment using Burger's phantom, the smallest visible volumes were 11.8, 11.3, 28.7, and 19.6 mm3 for the XV-GLP7, XL-GLP7, XV-XV2, and SA-GS-XTL combinations, respectively. In the lower dosage treatment in the clinic, there were no marked differences between the GLP7 film and XV2 film. However, in the higher dosage treatment, the GLP7 film had a better quality than did the XV2 film. Portal verification radiographs using GLP7 film are of sufficient quality for clinical use.

Radiotherapy portal verification: an observer study

In many radiotherapy facilities radiotherapy portal verification is currently a subjective process based on the visual comparison of a treatment or portal image with a prescription or simulation image. The reliability of this process is unknown. We describe here a study in which 16 observers (oncologists, physicists and therapists) independently evaluated the geometric accuracy of 530 treatment fields on 45 patients. The treatment images were acquired by the BEAMVIEW on-line portal imaging system (Siemens Medical Laboratories, Concord, CA, USA). Illustrative examples of the large variation in observers' assessments of the same field are given. The kappa statistic is used to evaluate the degree of agreement between observers and between on-line (at the treatment unit) and off-line (in a quiet viewing room) assessments. The best interobserver agreement was between the four oncologists contributing to the study although this level of agreement was rated only as "fair". Comparison of on-line and off-line decisions made by therapists exhibited "poor" agreement. This study has provided statistical confirmation of the suspicions of many workers in the field of radiotherapy portal verification, viz that the subjective evaluation of field accuracy is unreliable. However, the degree of unreliability is surprisingly large. The inconsistencies between observers documented in this study need to be clearly acknowledged in the development of protocols for the clinical use of on-line portal imaging systems. Acceptable reliability in radiotherapy portal verification will only be achieved when subjective decision making is eliminated.

155 An automated portal verification system for the tangential breast portal field

155 An automated portal verification system for the tangential breast portal field

Systematic analysis of errors in target localization and treatment delivery in stereotactic radiosurgery (SRS)

Purpose : To systematically analyze the spatial uncertainties associated with each step of the stereotactic radiosurgery (SRS) procedure and the overall spatial accuracy in the treatment delivery. Methods and Materials : A special test device has been designed and fabricated to permit accurate simulation, localization and treatment portal verification of a SRS target. This device with simulated targets dispersed in 3-D space can be rigidly attached to the BRW CT or angio frame for localization, to the BRW phantom base for target coordinate determination, and to the floor stand for portal verification. The setup allows evaluation of the accuracy of each procedure separately as well as the overall accuracy in the delivery of SRS treatment. Results : Biplanar film localizations reveal a systematic error in the phantom base pointer of the order of 0.1 mm that is applied as a correction to the measured target coordinates. Targets localized by planar film angiography had a mean positional error of 0.38 ± 0.1 mm compared to 1.0 ± 0.7 mm for digital angiography localization. The positional accuracy associated with CT localization was superior when a 2 mm vs. 4 mm slice thickness was used (0.9 ± 0.3 mm vs. 1.6 ± 0.5 mm error). Cumulative mean errors, including inaccuracies associated with treatment setup, were 1.0 ± 0.4 mm for radiographic localization, 1.2 ± 0.5 mm for CT with a 512 × 512 matrix and 2 mm slice thickness, and 2.0 ± 0.6 mm for CT at a 4 mm slice thickness (256 × 256 matrix). Larger errors would be expected in the clinical setting. Conclusion : Spatial errors in stereotactic radiosurgery are best estimated using a systematic approach to isolate independent contributing factors. The accuracy in target localization determines the overall accuracy of SRS procedure, provided the mechanical accuracy of the treatment apparatus is assured. Biplanar treatment portal verification with a fiducial localization frame is an accurate method of verifying the target coordinates before delivering treatment.

Image registration for portal verification and rectification

Image registration for portal verification and rectification

Utilizing MRI for radiation oncology treatment portal verification and treatment planning in pediatric brain tumors

Utilizing MRI for radiation oncology treatment portal verification and treatment planning in pediatric brain tumors

Contrast enhancement of portal images by selective histogram equalization.

Because of the high energy of the treatment beam, contrast of portal verification films is very poor. A simple contrast enhancement technique is described which we have labeled selective histogram equalization (SHE), to improve visualization of double-exposure portal images and this facilitate the beam verification process. The technique performs separate histogram equalization on the treatment- and open-field sections of double-exposure portal images. Delineation of the treatment field edge and separation into two regions is performed automatically for off-line portal radiographs by a strategic combination of Sobel filtration and morphological processes. Analyses of images processed by SHE and other adaptive histogram equalization techniques indicate that SHE produces improved contrast enhancement with minimal addition of noise or artifacts, thus simplifying the beam verification procedure. The simple implementation of an automatic SHE process with on-line portal systems is also discussed.

Portal radiography practice in Canada

In recent decades most of the effort directed toward ensuring that a treatment prescription is followed has involved the development of accurate dosimetric standards and computational techniques which, under most situations, permit the absolute radiation dose to be known at point with respect to the beam position to within about 5%. More recently, there has been increasing emphasis placed on confirming that the second component of the prescription is followed, viz that the target volume specified by the physician is that irradiated and, in particular, that neighboring radiosensitive tissues are appropriately shielded. Although quantitative clinical evidence of the consequences of inappropriate field placement is severely limited it is a reasonable axiom that the outcome of a course of radiotherapy is influenced by the accuracy with which the prescribed field is actually treated. Verification of treatment field accuracy has benefitted from many technical advances in the methods of portal imaging. These advances have ranged from the development of new screens for the conventional film/screen approach to the construction of computer based systems which can present quality portal images in real time. Achieving these advances has been technically challenging by itself. However, if these efforts are to be translated into a real benefit for the patient the further challenge of developing effective portal verification programs must be met. In preparation for the development of such programs, it is appropriate to review current practice and, in particular, to attempt to identify those areas which may hinder the effective use of portal imaging in the radiotherapy clinic. To this end, a 7-page questionnaire was prepared and sent to 26 radiotherapy facilities in Canada in August 1989. With a little prompting, 25 centers had responded by the end of November 1989. Of these, 25 one did not use portal radiography at all and hence the observations which follow are based on responses from 24 centers. In this brief report we present the main findings of the survey and discuss their significance for the expanding field of portal verification. On average, 67% or more of patients receiving radical treatment at each center have field placement accuracy checked using portal radiography. With the exception of mantle and inverted Y treatments, less than 67%, on average, of palliative treatments are supplemented with portal films. As participating centers were not requested to furnish absolute patient numbers, the survey cannot yield information on the percentage of treatments of a particular type which was accompanied by portal radiography across the country. Rather, the numbers above refer to the average practice over the centers polled. It was also noted, not surprisingly, that there was a very wide variability in practice between centers. To identify the reasons different groups of patients might receive portal radiography, the questionnaire invited respondents to identify, for each site, one or more of eight alternatives. Mantle and inverted Y treatments which are both highly successful and require intricate field shapes are most frequently checked radiographically on all patients (> 90% of centers). The majority of centers (55%) perform routine portal radiography on all their patients treated with radical intent. Cobalt-60 is still widely used in Radiotherapy but, due to its larger penumbra, produces portal radiographs which are difficult to interpret. This survey did not indicate any significant reluctance to perform portal radiography on 6oCo units in spite of the poor resolution films which result. The frequency with which films were taken on those patients who were radiographed on a routine basis was also covered in the study. For those patients who routinely receive portal radiography at the start of treatment (i.e. within the first three days) there is a clear and reasonable trend to repeat the procedure following shielding modification. However, a measurable number of centers do

Radiosurgery target point alignment errors detected with portal film verification

Stereotactic radiosurgery with a linear accelerator requires an accurate match of the therapeutic radiation distribution to the localized target volume. Techniques for localization of the target volume using CT scans and/or angiograms have been described. Alignment of the therapeutic radiation distribution to the intended point in stereotactic space is usually accomplished using precision mechanical scales which attach to the head ring. The present work describes a technique used to verify that the stereotactic coordinates of the center of the intended radiation distribution are in agreement with the localized target point coordinates. This technique uses anterior/ posterior and lateral accelerator portal verification films to localize the stereotactic coordinates of the center of the radiation distribution with the patient in the treatment position. The results of 26 cases have been analyzed. Alignment errors of the therapeutic radiation distribution in excess of 1 mm have been found using the portal film verification procedure.

Efficiency and decision making in on-line portal verification

Efficiency and decision making in on-line portal verification

Radiotherapy Verification Film for Estimating Cumulative Entrance Skin Exposure for Fluoroscopic Examinations

Measurement of skin entrance exposures during fluoroscopic procedures is complicated by the use of automatic exposure control devices and the presence of contrast media. Due to variability in positioning spot films from patient to patient, standard dosimeters, such as thermoluminescent, cannot be properly placed on the skin prior to examination. Prepackaged film of the type used for portal verification in radiation therapy held next to the patient's skin in a specially modified patient examination gown was found to be useful for determining the entrance skin exposure from both fluoroscopy and spot films during air contrast barium enema exams. The usable sensitivity range of this film has been found satisfactory for exposure measurements at exposures and kVps typically used for gastrointestinal fluoroscopic procedures. Errors in exposure estimates due to changes in film speed and contrast with kVp are less than 5% for the range of kVps used. Errors from variations in beam quality due to the adjacency of scattering material are approximately 5%. Entrance exposures determined with film agreed with those determined from TLD measurements to within 21%, with an average difference of 9%.

Verification of electron beam therapy with storage phosphor images: precision of field placement.

Portal verification images were generated from the photon contamination in electron beams produced by a linear accelerator during treatment of patients receiving high-energy electron radiation therapy (8-14 MeV). An experimental storage phosphor system was used to record the images and display them on laser-printed film. Images were obtained from four or more treatment fractions from 21 cases of head and neck cancer. Precision in field placement was estimated by determining the position of a selected anatomic landmark relative to the center of the field for each series of images. The average standard deviation in the field-position measurements was 3.8 mm. Several procedural problems were also detected and corrected after review of the verification images. The results indicate that the emphasis placed on monitoring and control of field-positioning error in high-energy electron treatments should be similar to the emphasis placed on this aspect of error in photon treatment.

Verification of electron beam therapy with conventional and storage phosphor images: Preliminary experience

Portal verification images were generated by the photon contamination in electron beams produced by a linear accelerator during treatment of patients receiving high-energy electron radiation therapy. Both conventional and storage phosphor methods yielded projection radiographs in which anatomy of the irradiated and surrounding tissue was demonstrated. Exposed phantoms were used to confirm that the images represent a true projection of the radiation field. A preliminary series of 22 cases was evaluated by two radiotherapists and judged subjectively to be of clinical value. Geometric error, or more importantly, the lack thereof, during high-energy electron treatments was easily confirmed with this method. In three cases, the treatment protocol was corrected based on the images obtained. Because the readout process of storage phosphor images allows for gain adjustments and post-processing, the images obtained with this method were found to delineate anatomy in the treated and surrounding tissues somewhat more consistently than could conventional images.

Technique for verifying treatment fields using portal images with diagnostic quality

The image quality of portal films for megavoltage photon beams, when using the double-exposure technique, is poor compared to diagnostic quality, X ray images. A technique is described to record on a single film a megavoltage portal image superimposed upon a diagnostic X ray image, which provides the radiotherapist with “diagnostic quality” portal images. The technique uses a commercially available X ray tube mounted on the head of a 60Co unit. The alignment procedure, which uses a leveling device to ensure that the X ray focal spot and 60Co source are at the same location for each exposure, is confirmed by registering on film the image of an alignment marker. An evaluation of film-screen combination showed therapy verification film in a rare earth intensifying screen cassette to be best suited for this technique. The relationship between off-axis dose and the penumbral region of the portal image has been evaluated and should be useful in the interpretation of portal verification film relative to the treatment volume.

A new technique for the precise localization of the rectum in treatment planning and portal verification for prostate tumors

A new technique for the precise localization of the rectum in treatment planning and portal verification for prostate tumors

An External Lead Shutter System for Supervoltage Roentgenography

Medical radiography in the supervoltage range has recently become an area of clinical and investigative interest for specialized diagnostic procedures. These include exploration of soft-tissue densities in the head, neck, and thorax, and portal verification in supervoltage therapy (1–5). In our Radiation Clinic, we employ a resonance transformer generator operating at 1,000 kv., with 1 mm. lead filter, h.v.l. 3.9 mm. Pb, TSD 70 cm., 3 ma, and an output of 60 r/min. A posteroanterior radiograph of the chest on industrial type C film requires a dose of 1.5 r, measured at the back of the patient, and an exposure time of three seconds. Conventional timers on therapy machines are regarded as unsatisfactory because they permit radiation exposure to the patient during the “build-up” time of the generator. Tuddenham, Hale, and Pendergrass (5) have suggested the use of an external lead shutter mechanism activated by a conventional impulse timer and out of direct contact with the generator. We have designed and constructed such a lead shutter system for use with our 1,000-kv. therapy machine. The use of this device reduces the dose to the patient from 1.5 r to 1.1 r, or 26.6 per cent. It may be easily constructed and adapted for use with other supervoltage or orthovoltage x-ray therapy equipment. In the horizontal position most x-ray heads, like ours, are perfectly counterbalanced. Any attempt to install a shutter system on the face of the port would cause an imbalance requiring considerable engineering for correction. Furthermore, repeated impact from constant use might injure the tube. For these reasons, a separate shutter mechanism had to be constructed out of direct contact with the generator. In Figure 1 are shown the relative positions of the shutter and the x-ray machine. Figure 2 is a schematic drawing of the system. Basically the shutter operates as follows: the lead shutter, weighing 25 lbs. (6 × 6 × 2 inches), is raised manually by a crank located on the top of the frame and connected to the shutter by wire supports (Fig. 2). An A.C./D.C. converter switch is thrown, activating a brake which holds the entire shutter mechanism at whatever height is selected. When peak x-ray voltage is reached, a push-button switch outside the therapy room releases the brake, allowing the shutter to fall freely by gravity until it makes contact with an actuator located near the bottom of the frame. The actuator makes contact with a microswitch, which again energizes the brake, stopping the fall of the lead shutter. In the event of a power failure, a spring system absorbs the fall of the shutter. Figure 3 illustrates the electrical construction. The cost of the parts is modest, and a skilled technician can readily assemble and wire the unit. Summary A simplified external lead shutter system for supervoltage roentgenography is described.