On-line Portal Imaging System Research Articles

Imaging of 1.0-mm-diameter radiopaque markers with megavoltage X-rays: an improved online imaging system

Purpose : To improve an online portal imaging system such that implanted cylindrical gold markers of small diameter (no more than 1.0 mm) can be visualized. These small markers would make the implantation procedure much less traumatic for the patient than the large markers (1.6 mm in diameter), which are usually used today to monitor prostate interfraction motion during radiation therapy. Methods and Materials : Several changes have been made to a mirror-video based online imaging system to improve image quality. First, the conventional camera tube was replaced by an avalanche-multiplication-based video tube. This new camera tube has very high gain at the target such that the camera noise, which is one of the main causes of image degradation of online portal imaging systems, was overcome and effectively eliminated. Second, the conventional linear-accelerator (linac) target was replaced with a low atomic number (low-Z) target such that more diagnostic X-rays are present in the megavoltage X-ray beam. Third, the copper plate buildup layer for the phosphor screen was replaced by a thin plastic layer for detection of the diagnostic X-ray components in the beam generated by the low-Z target. Results : Radiopaque fiducial gold markers of different sizes, i.e., 1.0 mm (diameter) × 5 mm (length) and 0.8 mm (diameter) × 3 mm (length), embedded in an Alderson Rando phantom, can be clearly seen on the images acquired with our improved system. These markers could not be seen on images obtained with any commercial system available in our clinic. Conclusion : This work demonstrates the visibility of small-diameter radiopaque markers with an improved online portal imaging system. These markers can be easily implanted into the prostate and used to monitor the interfraction motion of the prostate.

Retrospective analysis of the reproducibility of the daily set-up of the irradiation fields of cancer of the rectum using a megavoltage imaging system

To evaluate a ionization chamber on-line portal imaging system in routine clinical radiotherapy of rectum cancers. Megavolt portal images were obtained using a fast electronic megavoltage radiotherapy imaging system in 13 cases of pelvic fields. A total of 208 portal images and 13 simulator films were used to determine the values of set-up deviations in the X-Z-directions of a fixed co-ordinate system, and of the rotation fields (R). Mean standard deviations of the difference between simulation and treatment images were 3.2 mm and 0.9 mm for X and Z, 3.6 mm for the rotation fault. The standard deviations were, respectively, 7.1 mm, 7.1 mm and 1.5 mm. The cumulative frequency distributions revealed that 80% and 95% of the absolute differences were less than 10 mm and 17.5 mm, respectively. These results indicate the difficulty of a reproducible daily set-up. A weekly control could be proposed in order to increase the quality of pelvic site treatment. The introduction of masks to improve the set-up is also discussed.

Intra- and interfractional reproducibility of tangential breast fields: A prospective on-line portal imaging study

Purpose : A perception exists that weekly verification films accurately reflect the setup of the tangential breast portals. This prospective study was undertaken to assess patient movement during treatment and setup reproducibility of tangential breast fields using electronic on-line portal imaging. Methods and Materials : Thirteen patients with carcinoma of the breast were treated on a linear accelerator equipped with an on-line portal imaging system. Patients were immobilized daily with an alpha cradle. The medial and lateral tangential fields were imaged and 139 fractions, 225 portal fields, and 4450 images were obtained. Images were then analyzed off line and 22,250 measurements were made from these images. Anatomical features recorded include the lung area (LA), central lung distance (CLD), central breast distance (CBD), central flash distance (CFD), and inferior central margin (ICM). Intrafractional variations were calculated for every portal field and fraction for each patient. Interfractional variations were determined by finding the variance of intrafractional means for each patient. A population standard deviation for each of the five parameters for intra- and interfractional variations were determined. The simulation to treatment setup errors were calculated for all five variables. Results : Lung area variation was 1.50 and 4.19 cm 2 [1 standard deviation (SD)] for intra- and interfractional movement. Intrafractional variation for the other four variables ranged from 0.85 mm for ICM to 2.1 mm (1 SD) for CBD, while interfractional variations ranged from 3.2 to 6.25 mm for CBD and ICM, respectively. The simulation-to-treatment setup variation was greater than the interfractional variation for three of the five variables and was similar for the other two. Conclusions : On-line verification of intrafractional variation shows a moderate deviation from the treatment setup position for all five parameters studied, while interfractional variation showed even greater deviations for these five parameters. To cover the breast target in 95% of cases, margins of 7.70, 7.70, and 10.30 mm corresponding to the CLD, CFD, and ICM distances, respectively, are required.

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.

177 The accuracy of patient positioning in irradiation of rectum cancer

Purpose Evaluation of correct positioning using an ionisation chamber on-line portal imaging system in routine clinical radiotherapy of rectum cancer. Methods and Materials In 13 patients with pelvic irradiation, portal images were obtained using a fast electronic megavoltage radiotherapy imaging system. A total of 208 portal images and 13 simulator films were used to determine the values of setup deviations in the X-Y-directions and the rotation of fields in a fixed coordinate system, and the accuracy of the manually positioned blocks. Results Mean standard deviations of the difference between simulation and treatment images were in mm: 7.33 and 7. 13 for X and Y, 3.55 for the rotation fault, and 4.55 for the position of blocks. Maximal variations of standard deviations were respectively 2.29–11.91; 2.39–7.83; 1.84–7.07; 1.17–10.09. Conclusion The errors of field positions summed up to a mean of up to 11.91 mm in one patient. Maximal errors counted in single fields up to 30 mm. The mean error of manual block positioning was not acceptable. Thus, mechanically fixed blocks are now used after obtaining the results of this study. A daily control should be considered for difficult patients.

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.

A semiautomatic method for registration of portal images.

A method to register pairs of portal images quickly and accurately has been developed. The approach uses a cross-correlation operator to find the optimal match between corresponding anatomic regions that have been selected by a user on pairs of portal images. The cross-correlation operator determines the translation that best registers each pair of anatomic features independently, and then the images are translated, rotated, and scaled so that the least squares difference between the coordinates of all of the paired regions is minimized. Tests using simulated images have shown that the accuracy of the algorithm is dependent on (i) the size and shape of the structures within the paired regions; (ii) the subject contrast of the anatomic features being matched; (iii) the rotational difference between the anatomic features; and, (iv) the noise in the image. Tests using contrast-detail phantom images have shown that the semiautomated registration algorithm is only slightly less accurate than human observers, but is considerably faster. The semiautomated algorithm shows great promise as a method of quickly and accurately estimating discrepancies in patient positioning when used in conjunction with an on-line portal imaging system.

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 online 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) (Pfizer 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 rho-theta technique for treatment verification in radiotherapy and its clinical applications.

A new technique based on a rho-theta coordinate system for determining differences in patient position between portal and simulator images is presented. Unlike the conventional point matching method, which requires the fiducial points to be labeled in pairs before the registration, the rho-theta technique avoids this manual procedure. It accomplishes the treatment verification in two major steps; image alignment and field displacement analysis. For the same number of fiducial points in the simulator and portal images, it first finds the corresponding paired points if the points are not distributed symmetrically about their centroid. This is followed by alignment of these paired points using the least squares matching method to find the optimal two-dimensional rigid body transformation parameters (shift, rotation, and scaling factor). The transformation parameters are then used to transform the portal field edge into the simulator image, so that the portal field can be compared with the prescribed field on the simulator image. A number of parameters were explored to describe the field displacement errors, including treatment field size, under/over irradiated size, the shift in center of gravity of the field, the field edge shift, and rotation of the field. The rho-theta technique as implemented is both fast and accurate. Experiments on the registration of radiological phantom portal images acquired with an on-line portal imaging system mounted on a linear accelerator indicate an accuracy on the order of 1 mm in detecting the shift of the field's center of gravity and approximately 1 degree in detecting the field rotation. The results of a clinical trial are also presented.(ABSTRACT TRUNCATED AT 250 WORDS)

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 I3 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 I5 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.I%). 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 ≤ 5 mm were not adjusted. In obese patients, random errors may be surprisingly large.

On-line portal imaging: computer-assisted error measurement.

A microcomputer-based system was implemented in an on-line portal imaging system to determine portal misalignment in radiation therapy. Reference markers derived from a patient simulation film are superposed on the daily on-line portal images for visual evaluation of placement accuracy. The ability to check the alignment after administration of 3-6 cGy of radiation and the minimal operator interaction required make this system useful in radiation therapy delivery.

Evaluation of immobilization devices for the head and neck region using an on-line portal imaging system

Evaluation of immobilization devices for the head and neck region using an on-line portal imaging system