On-line radiotherapy imaging with an array of fiber-optic image reducers

Advanced image-guided external beam radiotherapy

The goal of radiation therapy is to eradicate tumor stem cells while sparing healthy tissue. Therefore, the first aim must be to delineate tumor from healthy tissue. Advanced imaging techniques will enable one to reduce the uncertainty of microscopic extension of disease. Ultimately, advanced functional imaging systems correlated with image-registered pathological specimens will allow one to delineate disease extent from normal tissue at the tumor periphery. When it is not possible to determine the CTV margin with reasonable certainty, the margins must remain generous and conformal avoidance methodology could and should be deployed to spare critical normal structures. Of equal importance to defining the CTV is the need to guarantee that this target is indeed treated. For this purpose, image guidance using a variety of systems including portal images, ultrasound devices, and CT scanners at the time of treatment has been implemented. Some image-guided methods, portal images for instance, are more amenable for use with rigid structures such as encountered in the sinus whereas others like ultrasound or CT scanners are able to account for nonrigid setup variations. Several strategies for preventing organ motion from degrading the precision that radiotherapy offers have been described. In particular, a CT scan at the time of treatment delivery can also be used as the basis to reconstruct the dose received by the patient. Dose reconstruction will allow the dose just delivered to be superimposed on the pretreatment CT scan and will allow one to compare the reconstructed delivered dose distribution with the planned dose distribution to assess discrepancies between these. Furthermore, reconstruction of the delivered dose distributions holds the promise of allowing one to accumulate dose delivered to the tumor and normal structures on a fraction per fraction basis. This will ultimately allow for the determination of treatment-specific tumor control probabilities and normal tissue complication probabilities.

ACCURACY OF POSITIONING THE CERVICAL SPINE FOR RADIATION THERAPY AND THE RELATIONSHIP TO GTV, CTV AND PTV

The purpose of the study was to evaluate the accuracy and precision of a rigid positioning device for repositioning the cervical spine accurately and precisely during conformal radiation therapy of dogs. Fifteen purpose bred research dogs in a radiation therapy study were included. The dogs were positioned using a head holder and a deflatable pillow attached to the treatment table. Port films were reviewed retrospectively, and repositioning precision was recorded by measurements in three orthogonal planes of the head, 2nd cervical vertebra and 1st thoracic spinous process. Mean treatment position was compared to the planning position for a measurement of systematic set-up error. Mean interfraction position variation of the 2nd cervical vertebra was 0.2, 0.1 and 0.2 cm for the ventrodorsal, caudocranial and laterolateral directions respectively, and the average systematic set up error was 0.2, 0.1 and 0.2 cm for the ventrodorsal, caudocranial and laterolateral directions respectively. Knowledge of the magnitude of reposition errors should be included when determining the margins around the tumor.

Image guidance for precise conformal radiotherapy

PURPOSE

To review the state of the art in image-guided precision conformal radiotherapy and to describe how helical tomotherapy compares with the image-guided practices being developed for conventional radiotherapy.

MATERIALS AND METHODS

Image guidance is beginning to be the fundamental basis for radiotherapy planning, delivery, and verification. Radiotherapy planning requires more precision in the extension and localization of disease. When greater precision is not possible, conformal avoidance methodology may be indicated whereby the margin of disease extension is generous, except where sensitive normal tissues exist. Radiotherapy delivery requires better precision in the definition of treatment volume, on a daily basis if necessary. Helical tomotherapy has been designed to use CT imaging technology to plan, deliver, and verify that the delivery has been carried out as planned. The image-guided processes of helical tomotherapy that enable this goal are described.

RESULTS

Examples of the results of helical tomotherapy processes for image-guided intensity-modulated radiotherapy are presented. These processes include megavoltage CT acquisition, automated segmentation of CT images, dose reconstruction using the CT image set, deformable registration of CT images, and reoptimization.

CONCLUSIONS

Image-guided precision conformal radiotherapy can be used as a tool to treat the tumor yet spare critical structures. Helical tomotherapy has been designed from the ground up as an integrated image-guided intensity-modulated radiotherapy system and allows new verification processes based on megavoltage CT images to be implemented.

Portal imaging

Portal imaging is the acquisition of images with a radiotherapy beam. Imaging theory suggests that the quality of portal images could be much higher if the efficiency of the imaging media in detecting radiation could be improved. Introduction of new media (films and electronic portal imaging devices) has confirmed this by markedly increasing the quality of portal images. Images from these devices can then be used to verify a patient's treatment. Geometric verification requires the portal image to be registered with a reference image. Dosimetric verification requires the portal imager to be calibrated for dose. This review gives a brief overview of the current areas of interest in portal imaging: imaging theory; imaging media, film and electronic portal imaging devices; image registration; and dosimetry using these devices.

Requirements for radiation oncology physics in Australia and New Zealand

This Position Paper reviews the role, standards of practice, education, training and staffing requirements for radiation oncology physics. The role and standard of practice for an expert in radiation oncology physics, as defined by the ACPSEM, are consistent with the IAEA recommendations. International standards of safe practice recommend that this physics expert be authorised by a Regulatory Authority (in consultation with the professional organization). In order to accommodate the international and AHTAC recommendations or any requirements that may be set by a Regulatory Authority, the ACPSEM has defined the criteria for a physicist-in-training, a base level physicist, an advanced level physicist and an expert radiation oncology physicist. The ACPSEM shall compile separate registers for these different radiation oncology physicist categories. What constitutes a satisfactory means of establishing the number of physicists and support physics staff that is required in radiation oncology continues to be debated. The new ACPSEM workforce formula (Formula 2000) yields similar numbers to other international professional body recommendations. The ACPSEM recommends that Australian and New Zealand radiation oncology centres should aim to employ 223 and 46 radiation oncology physics staff respectively. At least 75% of this workforce should be physicists (168 in Australia and 35 in New Zealand). An additional 41 registrar physicist positions (34 in Australia and 7 in New Zealand) should be specifically created for training purposes. These registrar positions cater for the present physicist shortfall, the future expansion of radiation oncology and the expected attrition of radiation oncology physicists in the workforce. Registrar physicists shall undertake suitable tertiary education in medical physics with an organised in-house training program. The rapid advances in the theory and methodology of the new technologies for radiation oncology also require a stringent approach to maintaining a satisfactory standard of practice in radiation oncology physics. Appropriate on-going education of radiation oncology physicists as well as the educating of registrar physicists is essential. Institutional management and the ACPSEM must both play a key role in providing a means for satisfactory staff tuition on the safe and expert use of existing and new radiotherapy equipment.

[Evolution of the use of the portal imaging device: prospective study over three years]

PURPOSE

To describe the evolution of the use of the electronic portal imaging device (EPID) over three periods.

MATERIAL AND METHODS

From 1990, as part of the quality assurance research programs, the radiotherapy department of the G.-F. Leclerc Centre of Dijon used EPID systems in a prospective fashion. During the first of the three periods (PER 1:1990-1993), the study consisted of analysis criteria determination, software efficiency improvement and a selection of patients who could benefit from the method. Eight hundred and forty-five images of 40 patients were analysed qualitatively and quantitatively. Two verifications per week were planned, and the action level for correction was 10 mm. Head and neck images were also displayed in 'cinema' presentation for internal movements analysis. From 1994 to 1995 (PER 2), off-line procedure (OLP) based upon early correction of the systematic error and the rules calculated from our previous experience were tested for checking the brain, head and neck (LOC 1: 396 images) and many of the pelvic irradiations (LOC 2: 260 images). A double-exposure procedure and/or movie loop presentation was reserved for other patients. During the last period (PER 3: 1996-1997), the OLP procedure was routinely performed in 54 patients (images: 321 LOC 1, 680 LOC 2).

RESULTS

LOC 1: deviations of < 3 mm increased from 75.5% during PER 1 to 81% during PER 2 to 83% during PER3. Conversely, deviations of 3-5 mm dropped from 19.5 to 13%, while deviations of more than 5 mm remained stable, around 5%. The actual standard error of the mean deviation observed was 2 mm. LOC 2: deviations of < 5 mm were observed in 81% of the cases during PER 1 and in 91% during PER 3 (89.5% in PER 2). These good results led to a decrease in deviation of 5 to 7 mm (11 to 6%) and also to a significant drop in deviations of more than 7 mm, 8 to 3% respectively. The actual precision obtained was 2.5 mm +/- 3 mm SD.

CONCLUSIONS

The OLP based upon the early correction of the systematic error led to a significant increase of setup accuracy of patients irradiated for the brain, head and neck, and especially for pelvic lesions.

Characteristics of metal-plate/film detectors at therapy energies. I. Modulation transfer function

Measurements of modulation transfer function (MTF) for front and back metal-plate/film portal detectors are reported for the Cobalt-60 and 10 MV spectra. The detectors consist of a double-emulsion portal film secured between plates of Al, Cu, brass, or Pb with thicknesses varying from 0 to 4.81 mm. Secondary electrons produced within the front plate generate the main signal, but the MTF decreases with an increase in front plate thickness greater than the maximum range of electrons Rmax because of photon scatter in the front plate. Because the decrease of MTF with backplate thickness ceases for backplate thickness greater than Rmax, the MTF is influenced more by the backscatter electrons than the backscatter photons.

The use of an electronic portal imaging system to measure portal dose and portal dose profiles

The dosimetric characteristics of a scanning liquid-filled ionization chamber (SLIC) electronic portal imaging device have been investigated. To assess the system's response in relation to incident radiation beam intensity, a series of characteristic curves are obtained for various field sizes and nominal energies of 6 and 10 MV photons. The response of the imaging system is dependent on incident radiation intensity and can be described to within 1% accuracy on central axis using a square root function. Portal dose measurements with the SLIC at the plane of the detector, on central axis of the beam using homogeneous attenuating phantom materials show that the imaging system is capable of measuring the portal (transmission) dose to within 3% of the ionization chamber results for homogeneous material. For two-dimensional dosimetry applications, the system is calibrated with a 10 cm Perspex block used as beam flattening material on the detector cassette to correct for variations in individual ion chamber sensitivity and the effect of nonuniform beam profiles produced by the flattening filter. Open and wedged dose profiles measured with the SLIC agreed with ion chamber measured profiles to within 3.5% accuracy.

Radiotherapy Treatment Verification

During a radiotherapy treatment, a dosimetric verification or a geometric localization can be done, in order to assess the quality of the treatment. The dosimetric verification is generally performed measuring the dose at some points inside (natural cavities) or outside the patient, and comparing it to the dose at the same points calculated and predicted by the treatment planning system. This can be done either with thermoluminescent or diodes dosimeters or with ionization chambers. The geometric localization can be done acquiring a portal image of the patient. Portal imaging can be performed either with films placed between metallic screens, or with an electronic portal imaging device such as fluoroscopic systems, solid state devices or matrix ionization chamber systems. In order to assess possible field placement errors, the portal images have to be compared with images obtained with the simulator in the same geometric conditions and/or with the digitally reconstructed radiograph (DRR) obtained with the treatment planning system. In particular, when using matrix ionization chamber systems, the portal images contain also information regarding the exit dose. This means that this kind of imaging device can be used both for geometric localization and for dosimetric verification. In this case, the exit dose measured by the portal image can be compared with the exit dose calculated and predicted by the treatment planning system. Some “in-vivo” applications of this methodology are presented.

Improvement of image quality in megavoltage computed tomography with second generation scanning mode

Megavoltage computed tomographic (CT) scanning is a topic of interest in precision radiation therapy. It is useful in verifying and improving the accuracy of the patient's positioning. For this purpose, we developed a third generation mode megavoltage CT scanner. However, insufficient spatial resolution limits its clinical usefulness. A second generation mode megavoltage scanner using a turntable has been newly developed to investigate whether improvements in spatial sampling could result in image quality high enough for clinical use. Scanning is composed of 11 rotations and 12 translations of the table. The scanning beam is a 3 MV X-ray, and the detector consists of 75 elements of cadmium tungstate crystals combined with photodiodes. A spatial resolution of 0.5 mm and contrast resolution of approximately 5% were obtained. The image quality is inferior to that of conventional diagnostic CT scanners, but is estimated to be adequate for some clinical applications of radiation therapy. Based on the satisfactory results, a new third generation megavoltage CT scanner is under investigation.

Measurement of patient positioning errors in three-dimensional conformal radiotherapy of the prostate

PURPOSE/OBJECTIVE

To determine the spatial distribution of setup errors for patients treated with six-field, three-dimensional (3D) conformal radiation therapy for prostate cancer.

METHODS AND MATERIALS

Port films for 50 patients were analyzed retrospectively. The port films were digitized and compared, using image registration software, to simulator films (representing the ideal treatment position). Patient positioning uncertainty for a given setup was determined using port films from three projections, two obliques, and one lateral. A total of 1239 port films and 300 simulator films were analyzed for the study. Patient position was analyzed for out-of-plane rotations and time trends over the course of treatment.

RESULTS

The distribution of systematic setup errors for the 50 patients, defined as the mean patient displacement for the treatment course, had a mean and standard deviation (SD) of (-0.1 +/- 1.9) mm, (0.4 +/- 1.4) mm, and (-0.3 +/- 1.3) mm in the mediolateral (ML), superior-inferior (SI) and anterior-posterior (AP) directions, and (-0.1 +/- 0.2) for rotational errors. The distribution of random setup errors about the mean approximated a normal distribution and the standard deviations for the population of patients in the ML, SI, and AP directions, were 2.0 mm, 1.7 mm, and 1.9 mm, respectively. The distribution of out-of-plane rotations had 1 SD of 0.9 degrees and 0.6 degrees about the SI and AP axes. Ten of the 50 patients demonstrated a statistically significant time trend in their setup position resulting in shifts ranging from 2 to 7 mm.

CONCLUSIONS

The setup verification protocol appears to minimize systematic setup errors to a level that approaches the sensitivity of the image registration technique. The random day to day fluctuations, represented by the average values of the standard deviations, are minor in comparison to the currently used margins, which further emphasizes the effectiveness of this protocol in conjunction with the use of the immobilization device.

Megavoltage imaging with a large-area, flat-panel, amorphous silicon imager

PURPOSE

The creation of the first large-area, amorphous silicon megavoltage imager is reported. The imager is an engineering prototype built to serve as a stepping stone toward the creation of a future clinical prototype. The engineering prototype is described and various images demonstrating its properties are shown including the first reported patient image acquired with such an amorphous silicon imaging device. Specific limitations in the engineering prototype are reviewed and potential advantages of future, more optimized imagers of this type are presented.

METHODS AND MATERIALS

The imager is based on a two-dimensional, pixelated array containing amorphous silicon field-effect transistors and photodiode sensors which are deposited on a thin glass substrate. The array has a 512 x 560-pixel format and a pixel pitch of 450 microns giving an imaging area of approximately 23 x 25 cm2. The array is used in conjunction with an overlying metal plate/phosphor screen converter as well as an electronic acquisition system. Images were acquired fluoroscopically using a megavoltage treatment machine.

RESULTS

Array and digitized film images of a variety of anthropomorphic phantoms and of a human subject are presented and compared. The information content of the array images generally appears to be at least as great as that of the digitized film images.

CONCLUSION

Despite a variety of severe limitations in the engineering prototype, including many array defects, a relatively slow and noisy acquisition system, and the lack of a means to generate images in a radiographic manner, the prototype nevertheless generated clinically useful information. The general properties of these amorphous silicon arrays, along with the quality of the images provided by the engineering prototype, strongly suggest that such arrays could eventually form the basis of a new imaging technology for radiotherapy localization and verification. The development of a clinically useful prototype offering high-quality images, ultimately with an approximately 52 x 52-cm2 detection surface, is anticipated.

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.

Investigation of a phase-only correlation technique for anatomical alignment of portal images in radiation therapy

A new image registration algorithm based on phase-only correlation is applied to portal images in radiation therapy to detect translational shift. The phase-only correlation shows a sharp peak in the correlation distribution as compared to the broad peak computed from conventional correlation using fast Fourier transform. In this paper, the algorithm of phase-only correlation is described and its applicability and robustness are tested when applied to portal images used in clinical radiation oncology. The results achieved give evidence that the phase-only correlation will deliver an alternative approach for image registration and image comparison, that may be applicable in routine clinical practice.

Prospective clinical evaluation of an electronic portal imaging device

PURPOSE

To determine whether the clinical implementation of an electronic portal imaging device can improve the precision of daily external beam radiotherapy.

METHODS AND MATERIALS

In 1991, an electronic portal imaging device was installed on a dual energy linear accelerator in our clinic. After training the radiotherapy technologists in the acquisition and evaluation of portal images, we performed a randomized study to determine whether online observation, interruption, and intervention would result in more precise daily setup. The patients were randomized to one of two groups: those whose treatments were actively monitored by the radiotherapy technologists and those that were imaged but not monitored. The treating technologists were instructed to correct the following treatment errors: (a) field placement error (FPE) > 1 cm; (b) incorrect block; (c) incorrect collimator setting; (d) absent customized block. Time of treatment delivery was recorded by our patient tracking and billing computers and compared to a matched set of patients not participating in the study. After the patients radiation therapy course was completed, an offline analysis of the patient setup error was planned.

RESULTS

Thirty-two patients were treated to 34 anatomical sites in this study. In 893 treatment sessions, 1,873 fields were treated (1,089 fields monitored and 794 fields unmonitored). Ninety percent of the treated fields had at least one image stored for offline analysis. Eighty-seven percent of these images were analyzed offline. Of the 1,011 fields imaged in the monitored arm, only 14 (1.4%) had an intervention recorded by the technologist. Despite infrequent online intervention, offline analysis demonstrated that the incidence of FPE > 10 mm in the monitored and unmonitored groups was 56 out of 881 (6.1%) and 95 out of 595 (11.2%), respectively; p < 0.01. A significant reduction in the incidence of FPE > 10 mm was confined to the pelvic fields. The time to treat patients in this study was 10.78 min (monitored) and 10.10 min (unmonitored). Features that were identified that prevented the technologists from recognizing more errors online include poor image quality inherent to the portal imaging device used in this study, artifacts on the portal images related to table supports, and small field size lacking sufficient anatomical detail to detect FPEs. Furthermore, tools to objectively evaluate a portal image for the presence of field placement error were lacking. These include magnification factor corrections between the simulation of portal image, online measurement tools, image enhancement tools, and image registration algorithms.

CONCLUSION

The use of an electronic portal imaging device in our clinic has been implemented without a significant increase in patient treatment time. Online intervention and correction of patient positioning occurred rarely, despite FPEs of > 10 mm being present in more than 10% of the treated fields. A significant reduction in FPEs exceeding 10 mm was made in the group of patients receiving pelvic radiotherapy. It is likely that this improvement was made secondarily to a decrease in systematic error and not because of online interventions. More significant improvements in portal image quality and the availability of online image registration tools are required before substantial improvements can be made in patient positioning with online portal imaging.

Commissioning and periodic quality assurance of a clinical electronic portal imaging device

PURPOSE

An electronic portal imaging device (EPID) was recently installed on our dual-energy linear accelerator. Commissioning and quality assurance techniques were developed for the EPID.

METHODS AND MATERIALS

A commissioning procedure was developed consisting of five parts: (a) physical operation and safety; (b) image acquisition, resolution, and sensitivity calibration; (c) image storage, analysis, and handling; (d) reference image acquisition; and (e) clinical operations.

RESULTS

The physical operation and safety tests relate to the motions of the unit, stability of the unit supports, safety interlocks, and interlock overrides. Imager contrast and spatial resolutions are monitored by imaging a contrast-detail phantom. The imager calibration procedure consists of a no-radiation image to compensate for signal offsets, as well as a "flat-field image." The flat-field image is taken with 5.0 cm of homogeneous phantom material placed at isocenter to provide some photon scatter and to approximate the presence of a patient. Daily quality assurance procedures consists of safety tests and the acquisition and inspection of images of the contrast-detail phantom. After 1 year, the frequency of the daily procedure was reduced to weekly. Quarterly QA procedures are conducted by the physicist and consist of the same procedures conducted in the weekly test. The annual QA procedure consists of a duplication of the commissioning procedure.

CONCLUSION

The procedures discussed in this article were applied to an ionization-chamber device. They have been useful in identifying difficulties with the EPID operation, including the need for recalibrating and monitoring the accelerator output stability.

The cumulative verification image analysis tool for offline evaluation of portal images

PURPOSE

Daily portal images acquired using electronic portal imaging devices contain important information about the setup variation of the individual patient. The data can be used to evaluate the treatment and to derive correction for the individual patient. The large volume of images also require software tools for efficient analysis. This article describes the approach of cumulative verification image analysis (CVIA) specifically designed as an offline tool to extract quantitative information from daily portal images.

METHODS AND MATERIALS

The user interface, image and graphics display, and algorithms of the CVIA tool have been implemented in ANSCI C using the X Window graphics standards. The tool consists of three major components: (a) definition of treatment geometry and anatomical information; (b) registration of portal images with a reference image to determine setup variation; and (c) quantitative analysis of all setup variation measurements. The CVIA tool is not automated. User interaction is required and preferred. Successful alignment of anatomies on portal images at present remains mostly dependent on clinical judgment. Predefined templates of block shapes and anatomies are used for image registration to enhance efficiency, taking advantage of the fact that much of the tool's operation is repeated in the analysis of daily portal images.

RESULTS

The CVIA tool is portable and has been implemented on workstations with different operating systems. Analysis of 20 sequential daily portal images can be completed in less than 1 h. The temporal information is used to characterize setup variation in terms of its systematic, random and time-dependent components. The cumulative information is used to derive block overlap isofrequency distributions (BOIDs), which quantify the effective coverage of the prescribed treatment area throughout the course of treatment. Finally, a set of software utilities is available to facilitate feedback of the information for treatment plan recalculation and to test various decision strategies for treatment adjustment.

CONCLUSIONS

The CVIA tool provides comprehensive analysis of daily images acquired with electronic portal imaging devices. Its offline approach allows characterization of the nature of setup variation for the individual patient that would have been difficult to deduce using only a few daily or weekly portal images. Distribution of the tool will help establish an important database of setup variation from many clinics. The information derived from CVIA can also serve as the foundation to integrate treatment verification, treatment planning, and treatment delivery.

The effects of out-of-plane rotations on two dimensional portal image registration in conformal radiotherapy of the prostate

PURPOSE

Rotations of the patient out of the image plane can significantly degrade the accuracy of two-dimensional (2D) image registration. This study determines the magnitude of the geometric errors introduced by 2D image registration as a result of out-of-plane rotations, and analyzes the dosimetric effects of these errors.

METHODS AND MATERIALS

The magnitude of the errors introduced by 2D registration were determined by comparing orthogonal view portal images of a rotated phantom to simulator reference images of the same phantom without rotation. Dosimetric effects were calculated for three-dimensional (3D) conformal prostate treatments by applying the registration errors to patient treatment plans. The calculations were performed using a modified version of the dose calculation software used in our Cancer Center for 3D treatment planning based on computed tomography (CT). A method to detect out-of-plane rotations, specific to pelvic treatments, is introduced that uses the relative displacement of the centers of gravity of the acetabula in lateral images.

RESULTS

The inherent uncertainty in the registration algorithm was 0.6 +/- 0.5 mm in translation and 0.7 +/- 0.8 degree in rotation within the image plane. For a 2 degrees out-of-plane rotation, the errors increase to 2.3 +/- 1.0 mm and 1.2 +/- 1.1 degrees. In some clinically realizable treatment scenarios it was observed that the errors introduced by the registration procedure could result in an overdosing of the rectal wall. The method to detect out-of-plane rotations was found to have an accuracy of better than 1 degree for rotations of less than 10 degrees.

CONCLUSIONS

The errors introduced to the patient position by 2D image registration have dosimetrically significant consequences for out-of-plane rotations of 2 degrees or more. However, when used in conjunction with the method to detect out-of-plane rotations, 2D registration software was found to cause insignificant dose errors and, thus, become a more reliable and accurate clinical tool.

A heuristic approach to edge detection in on-line portal imaging

PURPOSE

Portal field edge detection is an essential component of several postprocessing techniques used in on-line portal imaging, including field shape verification, selective contrast enhancement, and treatment setup error detection. Currently edge detection of successive fractions in a multifraction portal image series involves the repetitive application of the same algorithm. As the number of changes in the field is small compared to the total number of fractions, standard edge detection algorithms essentially recalculate the same field shape numerous times. A heuristic approach to portal edge detection has been developed that takes advantage of the relatively few changes in the portal field shape throughout a fractionation series.

METHODS AND MATERIALS

The routine applies a standard edge detection routine to calculate an initial field edge and saves the edge information. Subsequent fractions are processed by applying an edge detection operator over a small region about each point of the previously defined contour, to determine any shifts in the field shape in the new image. Failure of this edge check indicates that a significant change in the field edge has occurred, and the original edge detection routine is applied to the image. Otherwise the modified edge contour is used to define the new edge.

RESULTS

Two hundred and eighty-one portal images collected from an electronic portal imaging device were processed by the edge detection routine. The algorithm accurately calculated each portal field edge, as well as reducing processing time in subsequent fractions of an individual portal field by a factor of up to 14.

CONCLUSIONS

The heuristic edge detection routine is an accurate and fast method for calculating portal field edges and determining field edge setup errors.

Three-dimensional treatment planning and conformal radiation therapy: preliminary evaluation

Preliminary clinical results are presented for 209 patients with cancer who had treatment planned on our three-dimensional radiation treatment planning (3-D RTP) system and were treated with external beam conformal radiation therapy. Average times (min) for CT volumetric simulation were: 74 without or 84 with contrast material; 36 for contouring of tumor/target volume and 44 for normal anatomy; 78 for treatment planning; 53 for plan evaluation/optimization; and 58 for verification simulation. Average time of daily treatment sessions with 3-D conformal therapy or standard techniques was comparable for brain, head and neck, thoracic, and hepatobiliary tumors (11.8-14 min and 11.5-12.1, respectively). For prostate cancer patients treated with 3-D conformal technique and Cerrobend blocks, mean treatment time was 19 min; with multileaf collimation it was 14 min and with bilateral arc rotation, 9.8 min. Acute toxicity was comparable to or lower than with standard techniques. Sophisticated 3-D RTP and conformal irradiation can be performed in a significant number of patients at a reasonable cost. Further efforts, including dose-escalation studies, are necessary to develop more versatile and efficient 3-D RTP systems and to enhance the cost benefit of this technology in treatment of patients with cancer.

Clinical implementation of an objective computer-aided protocol for intervention in intra-treatment correction using electronic portal imaging

In order to test the feasibility of a protocol for intra-fractional adjustment of the patient position, during radiation therapy treatment in the pelvic region, a two-fold study is carried out. The protocol involves an objective quantitative measurement of the error in positioning starting from the comparison of a portal image with a reference image. The first part of the study applies the protocol to determine the efficacy of adjustment using subjective determination of the positioning errors by a clinician by measuring the residual errors after adjustment. A group of 13 patients was followed extensively throughout their treatment, analyzing 240 fields. In the second part the measurement itself determines the extent of readjustment of the position. Throughout the procedure elapsed time is measured to determine the extra time involved in using this procedure. For this part a group of 21 patients was followed yielding statistics on 218 fields. Using this computer aided protocol it is shown that systematic as well as random errors can be reduced to standard deviations of the order of 1 mm. The price to pay however is additional treatment time up to 58% of the treatment time without the protocol. Time analysis shows that the largest part of the added time is spent on the readjustment of the patients' position adding a mean of 37% of time to the treatment of one field. This is despite the fact that the readjustment was performed using a remote couch controller. Finally a statistical analysis shows that it is possible to select patients benefiting from the use of such a protocol after a limited number of fractions.

Reproducibility of patient positioning during routine radiotherapy, as assessed by an integrated megavoltage imaging system

A portal imaging system has been used, in conjunction with a movie measurement technique to measure set-up errors for 15 patients treated with radiotherapy of the pelvis and for 12 patients treated with radiotherapy of the brain. The pelvic patients were treated without fixation devices and the brain patients were treated with individually-moulded plastic shells. As would be expected the brain treatments were found to be more accurate than the pelvic treatments. Results are presented in terms of five error types: random error from treatment to treatment, error between mean treatment position and simulation position, random simulation error, systematic simulator-to-treatment errors and total treatment error. For the brain patients the simulation-to-treatment error predominates and random treatment errors were small (95% < or = 3 mm, 77% < or = 1.5 mm). Vector components of the systematic simulation-to-treatment errors were 1-2 mm with maximal random simulation error of +/- 5 mm (2 S.D.). There is much interest in the number of verification films necessary to evaluate treatment accuracy. These results indicate that one check film performed at the first treatment is likely to be sufficient for set-up evaluation. For the pelvis the random treatment error is larger (95% < or = 4.5 mm, 87% < or = 3 mm). The systematic simulation-to-treatment error is up to 3 mm and the maximal random simulation error is +/- 6 mm (2 S.D.). Thus corrections made solely on the basis of a first day check film may not be sufficient for adequate set-up evaluation.

Real-time beam monitoring in dynamic conformation therapy

PURPOSE

Although portal imaging is a promising method of verification during static multiport irradiation, it cannot be applied directly to dynamic irradiation such as rotational conformation with multileaf collimator movement. A real-time beam monitoring system based on megavoltage computed tomography scanning has been developed to establish a verification method for the rotational conformation technique.

METHODS AND MATERIALS

Exit beam through the patient is extracted by the same detector unit as used for megavoltage scanning during the actual treatment. Beam edge is defined as the 50% level of the maximum dose of the detector array. Megavoltage computed tomography is done after patient setup and just prior to the actual irradiation. Detected beam pathways are overlaid on this image approximately every 1 s. Therapists can monitor correlation between the target and actual beam pathways on a real-time computer display.

RESULTS

The accuracy of field edge detection has been proven to be less than 2 mm from various measurements. Real-time monitoring is more useful in rotational conformation than in static multiport irradiation due to dynamic movement of the collimator. Field errors were identified in two of 54 sessions using this method.

CONCLUSIONS

Although several limitations remain to be solved, the method presented is a useful tool for treatment verification of high accuracy radiation therapy, particularly rotational conformation irradiation.

Design of a fully integrated three-dimensional computed tomography simulator and preliminary clinical evaluation

PURPOSE

We describe the conceptual structure and process of a fully integrated three-dimensional (3-D) computed tomography (CT) simulator and present a preliminary clinical and financial evaluation of our current system.

METHODS AND MATERIALS

This is a preliminary report on 117 patients treated with external beam radiation therapy alone on whom a 3-D simulation and treatment plan and delivery were carried out from July 1, 1992, through June 30, 1993. The elements of a fully integrated 3-D CT simulator were identified: (a) volumetric definition of tumor volume and patient anatomy obtained with a CT scanner, (b) virtual simulation for beam setup and digitally reconstructed radiographs, (c) 3-D treatment planning for volumetric dose computation and plan evaluation, (d) patient-marking device to outline portal on patient's skin, and (e) verification (physical) simulation to verify portal placement on the patient. Actual time-motion (time and effort) recording was made by each professional involved in the various steps of the 3-D simulation and treatment planning on computer-compatible forms. Data were correlated with the anatomic site of the primary tumor being planned. Cost accounting of revenues and operation of the CT simulator and the 3-D planning was carried out, and projected costs per examination, depending on case load, were generated.

RESULTS

Average time for CT volumetric simulation was 74 min without or 84 min with contrast material. Average times were 36 min for contouring of tumor/target volume and 44 min for normal anatomy, 78 min for treatment planning, 53 min for plan evaluation/optimization, and 58 min for verification simulation. There were significant variations in time and effort according to the specific anatomic location of the tumor. Portal marking of patient on the CT simulator was not consistently satisfactory, and this procedure was usually carried out on the physical simulator. Based on actual budgetary information, the cost of a volumetric CT simulation (separate from the 3-D treatment planning) showed that 1500 examinations per year (six per day in 250 working days) must be performed to make the operation of the device cost effective. The same financial projections for the entire 3-D planning process and verification yielded five plans per day. Some features were identified that will improve the use of the 3-D simulator, and solutions are offered to incorporate them in existing devices.

CONCLUSIONS

Commercially available CT simulators lack some elements that we believe are critical in a fully integrated 3-D CT simulator. Sophisticated 3-D simulation and treatment planning can be carried out in a significant number of patients at a reasonable cost. Time and effort and therefore cost vary according to the anatomic site of the tumor being planned and the number of procedures performed. Further efforts are necessary, with collaboration of radiation oncologists, physicists, and manufacturers, to develop more versatile and efficient 3-D CT simulators, and additional clinical experience is required to make this technology cost effective in standard radiation therapy of patients with cancer.

Is weekly port filming adequate for verifying patient position in modern radiation therapy?

PURPOSE

The objective of this study is to use daily electronic portal imaging to evaluate weekly port filming in detecting patient set-up position.

METHODS AND MATERIALS

A computer-based portal alignment method was used to quantify the field displacements on 191 digitized weekly port films and 848 daily electronic portal images in 21 radiation therapy patients. An electronic portal image data set as a control for actual daily treatment position was used to evaluate weekly port films with respect to same-day field displacement, rate of field placement error detection, and prediction of subsequent daily field displacements.

RESULTS

The field displacements measured on a port film frequently deviated from the corresponding field displacements on the electronic portal image obtained in the same treatment set-up. A linear regression analysis showed that the curves fitted to the same-day field displacements had slopes that differed significantly from unity (p < 0.001). Overall, the respective frequencies of field placement error, beyond clinical tolerance limits of 5, 7, and 10 mm (corresponding to head and neck, thoracic, and pelvic sites) for port filming and electronic portal imaging were 11% and 14% (p = 0.4) in the X-direction (lateral or anteroposterior) and 24% and 13% (p = .0001) in the Y-direction (caphalad-caudad). When the data were broken down by anatomical region, this discrepancy was found to be mainly due to the differences in the thorax, and head and neck image data sets. For thoracic fields, error in Y-shifts was 28% by port filming, but only 9% by portal imaging (p = 0.01). In the head and neck region, 18% of the port films exceeded tolerance, whereas only 6% of the electronic portal images did (p = 0.0001). Field displacements on the treatment set-ups between the acquisition of port films were not predicted by those films.

CONCLUSION

There are discrepancies between the field displacements and field placement errors detected by weekly port films and daily electronic portal images. This study suggests that improved methods of treatment verification may be necessary in modern radiation therapy.

A randomised trial of patient repositioning during radiotherapy using a megavoltage imaging system

Effectiveness of radiotherapy is dependent on the accuracy of beam alignment. Recent developments in megavoltage imaging allow real-time monitoring of beam placement. Maximum gains from this new technology can only be made if the information is utilised to correct patient positioning prospectively before the majority of a treatment fraction is delivered. We have developed and utilised an integrated megavoltage imaging system to perform a randomised trial demonstrating significant improvements in accuracy using treatment intervention techniques for pelvic radiotherapy. The mean field-placement accuracy improved from 4.3 mm to 2 mm and the proportion of treatments given with a field-placement error of > or = 5 mm decreased from 69% to 7%. This improvement in accuracy may enable smaller margins around the target volume to be chosen whilst ensuring complete target coverage at each treatment fraction.

A method and contrast-detail phantom for the quantitative assessment of radiotherapy portal imaging systems

Several designs of electronic portal imaging systems for radiotherapy have been reported or are now commercially available, as well as the familiar radiographic screen-film combinations. In order to evaluate imaging performance of these, and future, systems a prototype contrast-detail test object has been developed in conjunction with a method for determining contrast from a model of the X-ray spectra produced by linear accelerators. Several existing test objects rely on qualitative or semi-quantitative estimates of contrast. In this technique, quantitative estimates of contrast may be determined for each detail size from realistic estimates of the X-ray spectrum by using a spectral model either in conjunction with narrow-beam attenuation measurements on the linear accelerator used, or by using the nominal value of maximum photon energy. This technique should facilitate the comparison of imaging systems used with linear accelerators of different energies as well as providing a quantitative quality control tool for regular measurements of imaging performance. Examples of the use of the test object in the evaluation of several commercial screen-film combinations are given.

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.

An evaluation of two methods of anatomical alignment of radiotherapy portal images

PURPOSE

Two techniques have been developed at our institution to allow anatomical registration of digitized portal images to a simulation film. Accuracy of the portal image alignment methods is tested and single intrauser and multiple interuser variation is examined using each technique.

METHODS AND MATERIALS

Method one requires the identification of anatomical fiducial points on a simulation image and its corresponding portal image. The parameters required to align the corresponding points are calculated by a least squares fit algorithm. Method two uses an anatomical template generated from the simulation image and superimposing it upon a portal image. The template is then adjusted by a computer mouse to obtain the best subjective anatomical fit on the portal image. Megavoltage portal images of a skull phantom with various known shifts and eight clinical image files were aligned by each method. Each data set was aligned several times by both a single user and multiple users.

RESULTS

Alignment of the anatomical phantom portal images demonstrates an accuracy of less than 0.8 +/- 0.9 mm and 0.7 +/- 1.0 degrees with either method. As out of plane rotation increased from 0 to 5 degrees, simulating out of plane malpositioning, alignment orthogonal to the plane of rotation worsened to 1.5 +/- 1.1 mm with the point method and 2.4 +/- 1.6 mm with the template method. Alignment parallel to the axis of the gantry rotation was insensitive to this change and remained constant as did the rotational alignment parameters. For the clinical image files the magnitude of variation for a single user is typically less than +/- 1 mm or +/- 1 degree. The magnitude of variation of alignment increased when multiple users aligned the same image files. The variation was dependent upon anatomical site and to a lesser degree the method of alignment used. The root mean square deviation of translational shifts range from +/- 0.68 mm when using the template method in the pelvis to as high as +/- 2.94 mm with the template method to align abdominal portal images. In the thorax and pelvis translational alignments along the horizontal axis were more precise than along the vertical axis. Multiple user variability was in part due to poor image quality, user experience, non rigidity of the anatomical features, and the difficulty in locating an exact point on a continuous anatomical structure.

CONCLUSION

In well controlled phantom studies both the fiducial point and template method provide similar and adequate results. The phantom studies show that alignment error and variance increase with distortion in anatomical features secondary to out of plane rotations. In clinical situations intrauser variation is small, however, multiple interuser variation is larger. The magnitude of variation is dependent upon the anatomical site aligned.

Advances in 3-dimensional radiation treatment planning systems: room-view display with real time interactivity

PURPOSE

We describe our 3-dimensional (3-D) radiation treatment planning system for external photon and electron beam 3-D treatment planning which provides high performance computational speed and a real-time display which we have named "room-view" in which the simulated target volumes, critical structures, skin surfaces, radiation beams and/or dose surfaces can be viewed on the display monitor from any arbitrary viewing position.

METHODS AND MATERIALS

We have implemented the 3-D planning system on a graphics superworkstation with parallel processing. Patient's anatomical features are extracted from contiguous computed tomography scan images and are displayed as wireloops or solid surfaces. Radiation beams are displayed as a set of diverging rays plus the polygons formed by the intersection of these rays with planes perpendicular to the beam axis. Controls are provided for each treatment machine motion function. Photon dose calculations are performed using an effective pathlength algorithm modified to accommodate 3-D off-center ratios. Electron dose calculations are performed using a 3-D pencil beam model.

RESULTS

Dose distribution information can be displayed as 3-D dose surfaces, dose-volume histograms, or as isodoses superimposed on 2-D gray scale images of the patient's anatomy. Tumor-control-probabilities, normal-tissue-complication probabilities and a figure-of-merit score function are generated to aid in plan evaluation. A split-screen display provides a beam's-eye-view for beam positioning and design of patient shielding block apertures and a concurrent "room-view" display of the patient and beam icon for viewing multiple beam set-ups, beam positioning, and plan evaluation. Both views are simultaneously interactive.

CONCLUSION

The development of an interactive 3-D radiation treatment planning system with a real-time room-view display has been accomplished. The concurrent real-time beam's-eye-view and room-view display significantly improves the efficacy of the 3-D planning process.

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).

A comprehensive system for the analysis of portal images

In recent years, several techniques for the processing and analysis of portal images have been developed. It is the aim of this study to integrate some of these techniques into one comprehensive system. An advantage of this approach is that clinical experience can be obtained with more than one technique and a comparison of the techniques becomes possible. The portal image analysis procedure is implemented in the following steps: preparation of the reference image, portal image field edge detection, field edge match, anatomy match and the presentation of the results. For most of these steps, several alternative methods (e.g., interactive and automatic) are implemented. In addition, two new visualisation techniques have been incorporated. The first is a method for combining the results of the analysis of multiple fields in two dimensions, e.g., large and boost fields. The second is a method for three-dimensional reconstruction of beam setup data, as derived from portal image analysis, on arbitrary reconstructed slices of a CT scan. With the latter method, the effect of setup errors on complex treatments (e.g., matching fields) can be studied. The new system has been in clinical use in our institution for two years and has been used to analyse about 5000 clinical portal images. The operators could choose freely from several matching methods. For 83% of the images our automatic matching algorithm was used. When required, the result of this method was corrected using the interactive drawing on image match. Significant corrections (more than 1 mm translation or 1 degree rotation) were applied to 27% of the automatically analysed images.(ABSTRACT TRUNCATED AT 250 WORDS)

A verification procedure to improve patient set-up accuracy using portal images

The purpose of this study was to establish which level of geometrical accuracy can be obtained during radiotherapy, using portal image analysis, with a minimum number of patient set-up measurements and corrections. A set-up verification and correction procedure using decision rules for improving the set-up of a patient during radiotherapy was investigated by means of a computer simulation. In this simulation study, set-up deviations were assumed to be the sum of random and systematic deviations and varying ratios of random and systematic deviations were studied. The distribution of random deviations (SD equal to sigma) was assumed to be equal for all patients of a specific treatment site. Set-up deviations are measured during the first N consecutive fractions after the start of the treatment or after a patient set-up correction. A set-up is corrected when the deviation averaged over these measurements is larger than an N-dependent action level. This action level is specified by alpha/square root of N, in which alpha is a variable initial action level parameter. After the start of the treatment or after each correction, Nmax measurements are made to decide on a possible (further) correction. By varying alpha and Nmax, the relation between the overall accuracy and the workload has been analyzed. It was possible to obtain a resulting overall accuracy level which is almost independent of the initial distribution of systematic deviations.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of on-line image verification to estimate the variation in radiation therapy dose delivery

PURPOSE

On-line radiotherapy imaging systems provide data that allow us to study the geometric nature of treatment variation. It is more clinically relevant to examine the resultant dosimetric variation. In this work, daily beam position as recorded by the on-line images is used to recalculate the treatment plan to show the effect geometric variation has on dose.

METHODS AND MATERIALS

Daily 6 MV or 18 MV x-ray portal images were acquired using a fiberoptic on-line imaging system for 12 patients with cancers in the head and neck, thoracic, and pelvic regions. Each daily on-line portal image was aligned with the prescription simulation image using a template of anatomical structures defined on the latter. The outline of the actual block position was then superimposed on the prescription image. Daily block positions were cumulated to give a summary image represented by the block overlap isofrequency distribution. The summary data were used to analyze the amount of genometric variation relative to the prescription boundary on a histogram distribution plot. Treatment plans were recalculated by considering each aligned portal image as an individual beam.

RESULTS

On-Line Image Verification (OLIV) data can differentiate between systematic and random errors in a course of daily radiation therapy. The data emphasize that the type and magnitude of patient set-up errors are unique for individual patients and different clinical situations. Head and neck sites had the least random variation (average 0-100% block overlap isofrequency distribution width = 7 mm) compared to thoracic (average 0-100% block overlap isofrequency distribution width = 12 mm) or pelvic sites (average 0-100% block overlap isofrequency distribution width = 14 mm). When treatment delivery is analyzed case by case, systematic as well as random errors are represented. When the data are pooled by anatomical site, individuality of variations is lost and variation appears random. Recalculated plans demonstrated dosimetric deviations from the original plans. The differences between the two dosimetric distributions were emphasized using a technique of plan subtraction. This allowed quick identification of relative "hot and cold spots" in the recalculated plans. The magnitude and clinical significance of dosimetric variation was unique for each patient.

CONCLUSIONS

OLIV data are used to study geometric uncertainties because of the unique nature for individual patients. Dose recalculation is helpful to illustrate the dosimetric consequences of set-up errors.

Development of a second-generation fiber-optic on-line image verification system

PURPOSE

We have previously reported the development of a fiber-optic fluoroscopic system for on-line imaging on radiation therapy machines with beam-stops because of space limitation. While the images were adequate for clinical purposes in most cases, an undesirable grid artifact existed and distracted visualization. The resolving power of the system, limited by the 1.6 mm x 1.6 mm dimension of the input fibers, appeared insufficient in some cases. This work identifies solutions to reduce grid artifact and to improve the resolution of the system.

METHODS AND MATERIALS

In the clinical system, it was found that the scanning mechanism of the newvicon camera was deflected differently at various gantry positions because of the different orientation of the earth's magnetic field. The small image misregistration produced grid artifact during image normalization, particularly near boundaries of the fiber bundles. One approach taken to reduce magnetic field effects was to shield the camera with mu-metal. Alternatively, a charged-coupled-device camera was used instead of the newvicon camera. As for improving spatial resolution, fibers with smaller input dimension were used. A 20 cm x 20 cm high resolution fiber-optic prototype consisting of 250 x 250 fibers, each with an input dimension of 0.8 mm x 0.8 mm was constructed. Its performance was tested using several phantoms studies.

RESULTS

Both shielding the newvicon camera with mu-metal or replacing it with a charge-coupled-device camera reduced grid artifact. However, optimal shielding could not be made for our clinical system because of the space limitation of its housing. High contrast resolution was improved, the 30% value of the modulation transfer function occurred at 0.3 linepairs per mm for the clinical system and at 0.7 linepairs per mm for the high-resolution prototype. However, because of the larger degree of transmission non-uniformity of the prototype, it was less effective using the current setup in detecting low contrast objects.

CONCLUSIONS

The results are encouraging and demonstrate successful reduction of grid artifact and improvement of high contrast spatial resolution using the proposed methods. The less effective low contrast detection was related to reduced light collection efficiency due to use of prototype fibers whose productions were not closely monitored. The findings are being considered in our construction of a second generation clinical fiber-optic on-line image verification system.

Quality assurance using portal imaging: the accuracy of patient positioning in irradiation of breast cancer

PURPOSE

To study the accuracy of patient positioning in irradiation of breast cancer.

METHODS AND MATERIALS

Megavolt portal images were obtained using a fast electronic megavoltage radiotherapy imaging system in 17 breast cancer patients immobilized with plastic fixation masks on a flat board with arm support and in 14 patients positioned without a mask on either a flat or a wedge-shaped board. Quantitative analysis of 510 megavolt portal images and comparison to 66 digitized simulation films was performed. Differences between the positioning techniques were evaluated.

RESULTS

For the position of the patient in the field, standard deviations of the difference between simulation and treatment images were 3.2 mm and 4.6 mm for irradiation with and without masks, respectively. Larger standard deviations were found for the field width and length (5-7 mm), for collimator rotation (1.5-2 degrees), and for the position of the lung shielding block for patients positioned on the flat board (10-16 mm). The changes in field size and collimator rotation appeared to be largely due to the inclination of the technologists to slightly adapt fields in order to obtain a seemingly better congruity of the field with the skin or mask markings. Comparison of the accuracy of patient positioning with and without masks yielded similar error rates; standard deviations and extremes tended to be somewhat larger in positioning without a mask. The wedge-shaped board was preferred because of the ease of patient set-up and because the use of a lung block is avoided. The transition from simulation to treatment set-up yielded larger deviations than repeated treatment set-ups.

CONCLUSION

These results emphasize again the continuous need for focusing attention on the accuracy of patient positioning in order to achieve maximal precision in radiotherapy. The electronic portal imaging system is very suitable for both quick on-line treatment verification and off-line analyses.

Interactive use of on-line portal imaging in pelvic radiation

We have evaluated a fluoroscopic on-line portal imaging system in routine clinical radiotherapy, involving the treatment of 566 pelvic fields on 13 patients. The image was typically generated by delivering a radiation dose of 6-8 cGy. Comparison between portal image and simulator film was done by eye and all visible errors were corrected before continuing irradiation. If possible, these corrections were performed from outside the treatment room by moving the patient couch by remote control or by changing collimator parameters. Adjustments were performed on 289/530 (54.5%) evaluable fields or 229/278 (82.4%) evaluable patient set-ups. The lateral couch position was most frequently adjusted (n = 254). The absolute values of the adjustments were 6.8 mm mean (SD 6.6 mm) with a maximum of 40 mm. All absolute values of adjustments exceeding 25 mm were recorded in one patient and those exceeding 15 mm were observed in two patients. Both patients were obese females. Adjustments exceeding 5 mm were observed in all 13 patients. Related to the use of on-line portal imaging, treatment time was increased by a median of 36.5% (mean 45.8%; SD 42.1%). The range was 7.7 to 442%. The fraction of the total treatment time to perform corrections was 22.7% median (mean: 26.0; SD: 11.8%). Statistically significant systematic in-plane errors were found in 7/13 patients. A systematic error was detected on the lateral position of the field in five patients. In one patient a systematic error of the longitudinal field position and in one patient a rotational error was detected. For adjustments in the lateral direction the present method does not allow to detect lateral shifts of less than 2 mm. For adjustments in the longitudinal direction the sensitivity could not be estimated but the available data suggest that 80% of errors < or = 5 mm were not adjusted. In obese patients, random errors may be surprisingly large.

A megavoltage CT scanner for radiotherapy verification

We have further developed a system for generating megavoltage CT images immediately prior to the administration of external beam radiotherapy. The detector is based on the scanner of Simpson (Simpson et al 1982)--the major differences being a significant reduction in dose required for image formation, faster image formation and greater convenience of use in the clinical setting. Attention has been paid to the problem of ring artefacts in the images. Specifically, a Fourier-space filter has been applied to the sinogram data. After suitable detector calibration, it has been shown that the device operates close to its theoretical specification of 3 mm spatial resolution and a few percent contrast resolution. Ring artefacts continue to be a major source of image degradation. A number of clinical images have been presented. The next stage of this work is to use the system to make clinical measurements of patient set-up inaccuracies building on our work making such measurements from digital portal images (Evans et al 1992).

Routine clinical on-line portal imaging followed by immediate field adjustment using a tele-controlled patient couch

We have evaluated the fluoroscopic on-line portal imaging (OPI) system developed by Siemens (Beamview-1, Concord, CA, U.S.A.) in routine clinical radiotherapy, involving the treatment of 883 fields (559 patient set-ups for treatment) on 21 patients. The image was typically generated by delivering 10 monitor units when used in single exposure or 1-2 monitor units on a large open field followed by 8-10 monitor units on the actual field when double exposure was used. Comparison between the portal image and the simulator film was done by eye. A region of tolerance was drawn on the simulator film and the field edges on the portal image had to project within this region. If this criterion was not met, adjustments followed by verification portal images were done before the remaining field dose was delivered. If possible, these adjustments were performed by moving the patient couch by remote control. The image quality was insufficient for evaluation in 75/883 (8.5%) fields. The abovementioned criterion was not met in 95/808 (11.8%) of the evaluable fields (26/559 patient set-ups were not evaluable). Of the 533 evaluable patient set-ups, 92 had to be adjusted (17.2%) including three (pelvic irradiations) set-ups that were adjusted on both field irradiated during the same radiotherapy session. In one case an incorrect tray (with wrong blocks) was detected and replaced. In one case (a 5.5 x 6.0 cm rectangular larynx field) the x and y axis of the field were interswitched. In one case incorrect focusing of a block was shown by the portal image. To make adjustments, the couch longitudinal position was changed 20 times (range -10 to +15 mm). The lateral position was changed 73 times (range -15 to +16 mm). The height position was changes 6 times (range -7 to +6 mm). Diaphragma rotation changes were performed 5 times (1 degree). The fraction of treatment time that was related to the use of OPI was 30.7% median (mean 32.4%, S.D. 14.1%). The range was 4.1 to 78.6%. On the basis of calculations assuming no OPI would have been used, field treatment time was increased by a median of 44.2% (mean 55.8%; S.D. 41.2%) by using OPI. The fraction of monitor units (fraction of the dose) to generate a satisfactory image was 10% median.(ABSTRACT TRUNCATED AT 400 WORDS)