[en] A prototype image detector has been designed and built for patient moniorting during radiotherapy treatment with high energy photon beams. A matrix of DC operated ionisation chambers was used to detect the transmitted radiation through a patient. The detector was developed in order to replace the current method, in which film radiography is used. Various detector configurations have been tested, and some results will be presented. Image enhancement was achieved by computer processing of the data. Theoretical possibilities of the detector system have been studied. (Auth.)

[en] Purpose: To determine, in 3-D, the difference between prostate delineation in MRI or CT for radiotherapy treatment planning. Patients and Methods: Eighteen patients with localized prostate cancer were scanned by means of CT and axial, coronal, and (in seven cases) sagittal MRI. The MRI scans were matched in 3-D on the planning CT using chamfer matching. Three observers outlined the prostate (i.e. no seminal vesicles) in all scans. No observer had knowledge of the contours outlined by the other observers. The volumes were measured and, to quantify differences related to the scan modality, the observer encompassing volume (i.e. the smallest volume encompassing all the volumes outlined by one observer in all scans for that patient) and the observer common volume (i.e. the largest volume common to all the volumes outlined by one observer in all scans for that patient) were determined. Likewise, the observer variation was calculated by determining the scan encompassing and common volume. The spatial difference between CT and MRI and the interobserver variation in a scan were quantified three-dimensionally, by the difference and variation in the distance between the center of gravity and the edge of the delineated prostate in each direction phi and θ (polar coordinates). Results: Inter-scan variation: CT volumes were larger than the axial MRI volumes in 52 out of 54 outlined volumes. The average ratio between these volumes was 1.4, significantly different from 1 (p<0.005). Only small differences were observed between the volumes of the various MRI scans although the coronal MRI was smallest. A large ratio (compared to 1.4) was found between observer encompassing and observer common volume (2.3, 2.5 and 2.6 for each observer), mostly due to variation in location of the MRI volumes. Furthermore, CT derived volumes never entirely encompassed the MRI volumes (Fig. 1). The average distance between CT and axial MRI is shown in Fig. 1, the CT derived contours extended on average 7.5 mm. (SD: 2 mm.) further from the center than the axial MRI at the base of the seminal vesicles (Fig. 1). The apex was located 6.5 mm. (SD: 3 mm.) more superior than on CT. Similar maps could be obtained for the other MRI scans. Inter-observer variation: The average ratio between the volume derived by one observer for a particular scan and patient and the average volume were 0.95, 0.97 and 1.08 (SD: 0.1). The ratio between scan encompassing and common volume was 1.5 and similar for all imaging modalities and directions (SD: 0.2-0.4). From a similar map, as presented in Fig. 1, it could be concluded that the spatial inter-observer variation for CT and axial MRI was similar in both scans and located mainly at the base of the seminal vesicles (max. SD in this area: 2.6 mm.). At the apex the inter-observer variation was less, SD: 2.1 mm. Conclusions: CT derived prostate volumes are larger than MRI derived volumes; this difference is mainly located at the apex and at the base of the seminal vesicles. The inter-observer variation is similar in all directions and both imaging modalities. The inter-scan variation is larger than the inter-observer variation, consequently the prostate GTV is more dependent on the scan modality than on the observer

[en] A complicating factor in the analysis of portal images is the presence of out-of-plane rotations. The purpose of this investigation was to determine the frequency and magnitude of out-of-plane rotations for patients treated for prostate cancer. In a first study, 4 repeat CT scans of 11 patients were made in treatment position on weeks 2, 4 and 6 of treatment. By using 3D chamfer matching of the pelvic bone, the difference in orientation of the pelvis between repeat scans could be measured accurately. In addition, differences in orientation of the femurs were measured by matching of the femoral heads. In a second study, electronic portal images from AP, left lateral and right lateral beams were analysed using a new interactive matching tool in which DRRs, that are computed with high speed, are compared with portal images. Because in the repeat CT study full 3D data is applied, it is more accurate than the portal image based method. However, the CT scans might not be fully representative for the situation during treatment. In both studies, similar results were found for the distribution of out-of-plane rotations. The magnitude of the out-of-plane rotations is about 1 degree standard deviation, where the rotation around the L-R axis is larger than the rotation in other directions. However, we found some outlier patients in the portal imaging study with significantly higher rotations, of up to 7 degrees. The measured out-of-plane rotations of the pelvis showed a correlation with the orientation differences of the femurs. It can be concluded that out-of-plane rotation for this group of patients is only a small effect. However, attention should be paid to the orientation of the legs. In addition, larger out-of-plane rotations might occur for a few patients

[en] One of the difficulties associated with the application of CT for treatment planning of prostate cancer is the poor contrast between the apex of the prostate and the surrounding tissues. On MRI, these tissues are well differentiated, but due to the lack of geometric accuracy applications of MRI for treatment planning have been limited so far. We have previously developed an automatic 3D image correlation technique based on chamfer matching, which is routinely applied for head and neck patients. It is the purpose of this work to extend this technique for matching of pelvic CT and MR. Some of the problems encountered for this particular application are poor segmentation of the bone due to fat shift and geometric distortions in the MRI. However, a reasonable accurate match is obtained for the anatomy within the pelvis. The computation time is about 2 min for a full automatic correlation. However, due to the poor segmentation of the pelvic bones in MRI, the method sometimes fails and requires manual adjustment. Using matched CT and MRI, the position of the apex of the prostate could be easily determined and transferred to the CT scan for further treatment planning. It can be concluded that 3D image matching is a useful tool for treatment planning of the prostate

[en] Purpose. To determine the changes of rectum and bladder structures during conformal therapy of prostate cancer and the accuracy of DVHs and NTCPs of these organs, based on the planning CT scan only. Methods. For 11 conformally treated prostate cancer patients, 3 repeat CT scans were made during treatment. The internal and external surfaces of rectum and bladder were contoured. Three volumes were calculated: solid organ (including filling), filling and wall volume. DVHs and NTCPs were calculated for all structures. Results. The solid organ and filling volumes varied considerably between patients and within one patient and decreased with increasing treatment time. The variations of rectum and bladder wall volumes were 9% and 17% (1 SD), respectively, with no time trend. The changes of the high dose (> 80% or 90% of the dose) volumes of the rectum in response to rectum filling differences, were proportional to the whole rectum volume changes. The variation of the high dose rectum wall volume was relatively small (14%, 1 SD). As a result the NTCPs of rectum and rectum wall were overall the same and the variation of the NTCPs during treatment was about 14% (1 SD) and not correlated with rectum filling. The variation of the high dose bladder volumes (about 14%, 1 SD) was smaller than the variation of the whole bladder volumes (30%, 1 SD). The high dose bladder wall volume decreased significantly due to wall distention as the bladder filling increased. As a result of this complex pattern, the variation of NTCPs of bladder (85%, 1 SD) and bladder wall (88%, 1 SD) during treatment was large and significantly correlated with bladder filling. Conclusions. The planning CT scan overestimates rectum and bladder filling during treatment. Furthermore, the variation of filling is so large that only the wall structures have relatively constant volumes. For the rectum wall, the DVHs and NTCPs as estimated from the initial scan, are representative for the whole treatment, since no correlation was seen between these parameters and organ filling. For the bladder wall, however, such a correlation was present and consequently, the initial bladder wall DVHs and NTCPs can only be representative for the whole treatment, if the bladder filling can be kept reasonably constant during treatment

[en] It is very important to have a daily verification of patient setup during radiotherapy. Therefore, we have developed an on-line imaging device for high energy photons. It consists of a matrix of 128 X 128 liquid filled ionisation chambers and has a field of view of 320 mm X 320 mm. This device has an extremely flat cassette-like housing for easy handling and for application with existing radiotherapy equipment. A dedicated microcomputer is used to measure the currents of the 16384 ionisation chambers at high speed. The same computer is used to restore and process the images. With an imaging time of 3.1 s, an image quality comparable to film is obtained. Images of high and low contrast phantoms and of patients are presented. With this device, high quality portal images will be available within only a few seconds after the start of the treatment. This allows an almost instantaneous decision on the approval of patient setup. In addition, it enables observation of organ or patient motion during a single treatment. Analysis of these images at high speed will be an interesting new area of research. 14 refs.; 7 figs

[en] A new fast method is presented for the quantification of patient set-up errors during radiotherapy with external photon beams. The set-up errors are described as deviations in relative position and orientation of specified anatomical structures relative to specified field shaping devices. These deviations are determined from parameters of the image transformations that make their features in a portal image align with the corresponding features in a simulator image. Knowledge of some set-up parameters during treatment simulation is required. The method does not require accurate knowledge about the position of the portal imaging device as long as the positions of some of the field shaping devices are verified independently during treatment. By applying this method, deviations in a pelvic phantom set-up can be measured with a precision of 2 mm within 1 minute. Theoretical considerations and experiments have shown that the method is not applicable when there are out-of-plane rotations larger than 2 degrees or translations larger than 1 cm. Inter-observer variability proved to be a source of large systematic errors, which could be reduced by offering a precise protocol for the feature alignment. (author)

[en] An inverse filter method was investigated for digital restoration of partial images and values of parameters of the filter determined for various clinical irradiation conditions. The main factors affecting the LSF were the radiation source size and the isocentre to film distance. The photon energy had only a minor influence. The thickness of the phantom and the field size, however, did influence the contrast in the image and therefore determined, together with the digitising system, the noise to signal ratio parameter of the inverse filter to a great extent. Application of the inverse filter improved significantly the visual appearance of anatomical details in the portal images. In addition to image restoration, the contrast was enhanced by the application of a low frequency cut-off filter. (author)

[en] Experiments with an ionisation detector were performed in order to determine whether it was possible to obtain high energy photon beam images for radiotherapy treatment verification. A small prototype detector with a field of view of 78 mm x 78 mm and constructed from printed circuit boards was used. The imaging area was a matrix ionisation chamber, filled with air or liquid (2,2,4-trimethylpentane). A minicomputer was used to control the data acquisition electronics and to reconstruct and restore the images. The images were displayed on a viewing console for computed tomography images. The liquid filled detector with a front-rear board separation of 1.0 mm gave the best results. The spatial resolution was about 3.8 mm with a density resolution of 0.5% for a data acquisition time of 120 s. Comparison of the liquid detector images with corresponding metal screen-film detector images showed that the image qualities were the same. An important advantage of the ionisation detector image is that grey scale modification, sharpening and smoothing by digital processing can easily be performed. (author)

[en] Purpose: To evaluate the ability of MRI during intracavitary brachytherapy to visualize the tumor extension in relationship to the intracavitary applicator and to compare the actual dose distribution in the Gross Tumor Volume (GTV) as identified on the MRI with the prescribed dose in 'Manchester' point A. Methods and Materials: In 7 patients with cervical cancer stage Ib (n = 2) and IIb (n = 5), both CT and MRI were performed during intracavitary brachytherapy, using a CT-MRI compatible gynaecological applicator. The GTV was drawn on the MRI and subsequently matched in 3-D with the CT-scan using chamfer matching. Isodose distributions were calculated and displayed on the CT-scan. Dose-volume histograms of GTVs as identified on the MRI were obtained and compared with the prescribed dose in point A. Results: Although delineation of the macroscopic tumor on the MRI was possible in all 7 patients, the tumor was better visible in the 2 patients who had brachytherapy after 10 Gy external radiation than in the 5 patients who had brachytherapy after 46 Gy. The GTV varied from 8 cm³ to 44 cm³ with a mean of 22 cm³. In all 7 cases the 'treated volume' (volume encompassed by the reference isodose surface) was considerably larger than the GTV ranging from 95 cm³ to 122 cm³ with a mean of 101 cm³. However, only in 3 patients the reference isodose surface fully covered the GTV. The minimum tumor dose in these patients was 100%, 131% and 136% of the reference dose in point A. In the other 4 patients the percentage of the GTV receiving a dose equal to or higher than the reference dose was 58%, 91%, 98% and 98%. The minimum dose in the GTV in these patients was 44%, 49%, 85% and 91% of the prescribed dose respectively. Conclusion: Using a CT-MRI compatible applicator artefact-free MR images can be obtained during intracavitary brachytherapy allowing good visualization of the tumor. In many patients there is a large discrepancy between the prescribed dose in point A and the actual dose in the GTV as identified on MRI. On average the 'treated volume' is about 5 times as large as the GTV. Work to further evaluate MRI as a tool to optimize and individualize the treatment is ongoing