On-line Portal Imaging Research Articles

Reproducibility of tangential breast fields using online electronic portal images

Summary Background Treatment verification and reproducibility plays a major role in radiotherapy to achieve better tumour control. Small uncertainties in daily repositioning of the patients and internal organ motion can lead to discrepancies between the planned and delivered radiation treatments. A factor that influences dose homogeneity and treatment volume is the accuracy of treatment setup. Small deviations in positioning the patient with regard to the beam setup could have a relatively significant impact on the treatment volume and it is imperative to control the setup error during radiotherapy. This study focuses on the importance of inter- and intra-fraction error in tangential breast radiotherapy with an electronic portal imaging device. Aim To study the variation in treatment setup due to intra-fraction and inter-fraction during tangential field breast irradiation. Materials/Methods Twelve patients of carcinoma breast were selected for this study and CT based planning was performed with simple tangential fields. The patients were treated on a 6MV linear accelerator equipped with an electronic portal imaging device (EPID). Portal images were acquired for both medial and lateral tangential fields for 10 fractions and intra- and inter-fraction studies were performed for all the patients. Parameters such as central lung distance (CLD), central beam edge to skin distance (CBESD), central irradiated width (CIW) and cranio-caudal distances (CCD) were measured on the acquired portal image. Results The average systematic differences observed for CLD, CBESD, CCD and CIW were 1.2mm, 2.8mm, 2.07mm and 3.30mm. For intra-fraction motion, the observed standard deviations for CLD, CBESD and CCD were 0.7mm, 0.73mm, and 1.36mm. Similarly the CLD, CBESD, CIW and CCD were analyzed for inter-fraction variation. Conclusions The online portal imaging device is an important tool for ensuring the proper delivery of planned dose. Our results suggest that intra-fraction motion of the breast has less impact on the treatment volume. Regular treatment verification between treatment fractions will help in reducing the normal tissue toxicity and ensures proper dose delivery to the tumour volume.

Quality Assurance and Safety of New Technologies for Radiation Oncology.

This article reviews suggested recommendations and procedures for quality assurance and safety tests of computer-controlled medical accelerators and the associated high-tech ancillary devices such as asymmetric collimators, dynamic wedge, multileaf collimators, and on-line electronic portal imaging devices for treatment verification, and technologies such as dynamic therapy and beam intensity modulation that are just coming on line. These new technologies are likely to lead to improved therapeutic ratios through the use of conformal physical dose distributions that cannot be achieved using traditional planning, delivery, and verification methods and, at the same time, provide more complete and thorough safety systems. However, these devices are not yet fully developed and thus come with some limitations and an increased potential for patient treatment error. To insure optimum use, a rigorous quality assurance program specifically designed for these technologies must be in place and the radiation oncology team should be constantly vigilant. In addition, provision should be made to provide for increased continuing staff education in the use of these technologies and complementary safety and quality assurance devices such as a record and verify system should be considered essential.

Dosimetric impact of image-guided 3D conformal radiation therapy of prostate cancer

The goal of this work is to quantify the impact of image-guided conformal radiation therapy (CRT) on the dose distribution by correcting patient setup uncertainty and inter-fraction tumour motion. This was a retrospective analysis that used five randomly selected prostate cancer patients that underwent approximately 15 computed tomography (CT) scans during their radiation treatment course. The beam arrangement from the treatment plan was imported into each repeat CT study and the dose distribution was recalculated for the new beam setups. Various setup scenarios were then compared to assess the impact of image guidance on radiation treatment precision. These included (1) daily alignment to skin markers, thus representing a conventional beam setup without image guidance, (2) alignment to bony anatomy for correction of daily patient setup error, thus representing on-line portal image guidance, and (3) alignment to the ‘CTV of the day’ for correction of inter-fraction tumour motion, thus representing on-line CT or ultrasound image guidance. Treatment scenarios (1) and (3) were repeated with a reduced CTV to PTV margin, where the former represents a treatment using small margins without daily image guidance. Daily realignment of the treatment beams to the prostate showed an average increase in minimum tumour dose of 1.5 Gy, in all cases where tumour ‘geographic miss’ without image guidance was apparent. However, normal tissue sparing did not improve unless the PTV margin was reduced. Daily realignment to the tumour combined with reducing the margin size by a factor of 2 resulted in an average escalation in tumour dose of 9.0 Gy for all five static plans. However, the prescription dose could be escalated by 13.8 Gy when accounting for changes in anatomy by accumulating daily doses using nonlinear image registration techniques. These results provide quantitative information on the effectiveness of image-guided radiation treatment of prostate cancer and demonstrate that the dosimetric impact is patient dependent.

SU‐FF‐T‐178: A Study On the Reproducibility of Tangential Breast Fields Using Online Electronic Portal Images

Purpose: To study the reproducibility of tangential breast treatment technique using online portal imaging system.Introduction: Treatment verification and reproducibility is an important step in radiotherapy to achieve a better tumor control. Care should be taken to ensure the same dose to be delivered in the same volume of irradiation. Electronic portal imaging technique plays a vital role in accomplishing the above task by studying the setup error and correct the same before the treatment delivery. Method and Materials: Twelve patients of carcinoma of breast were selected for this study and CT based planning was performed with simple tangential fields. The patients were then treated on a 6MV linear accelerator equipped with an electronic portal imaging device. Portal images were acquired for both medial and lateral tangential fields for 10 fractions and intra and the inter‐fraction studies were performed for all the patients. The parameters such as central lung distance (CLD), Central beam edge to skin distance, central irradiated width and cranio‐caudal distances were measured on the acquired portal image. In the intra‐fraction study lead markers were placed on the patient skin to study the breast movement during treatment. Results: The maximum variation of the marker during the treatment was 1.8 mm with a standard deviation of 0.575 mm. Similarly the CLD, CBESD, CIW and CCD were analyzed for the intra and inter‐fraction variation. Conclusion: Online portal imaging device is an important tool for ensuring the proper delivery of planned dose. Our result suggests that intra‐fraction motion of the breast has less impact on the treatment volume.

SU-FF-J-45: A Fluence Deformation Based Technique for Portal Image-Guided Adaptive Head-And-Neck IMRT

Purpose: Recent studies have shown significant inter-fraction patient anatomy changes over the courses of fractionated head-and-neck IMRT. Current IMRT planning uses a fixed 3D-margin around CTV to account for these changes and patient setup errors, which results in high-dose to the normal tissues in the margin and may limit the treatment. We are developing an online portal image-guided adaptive technique aimed at reducing the margin by adapting the photon fluence to these inter-fraction changes. Method and Materials: This technique uses portal images taken at each treatment gantry angle and compares them with the corresponding DRRs from the planning CT. First, a deformable registration is performed to determine a 2D transformation between the two images. This transformation is then applied to the originally optimized fluence to obtain a deformed fluence map that adapts to the detected changes. Finally, MLC sequences and deliverable fluences are re-calculated for adaptive dose delivery. Initial development used planning studies where rigid anatomy changes were simulated in the plan by shifting the isocenter, gantry angle and couch angle. Simulated DRRs were used as approximate representations of online portal images. Dose distributions and DVHs were calculated and compared to those from the originally optimized IMRT plan. Results: Preliminary results of applying this technique to head-and-neck patient data: 1) Deformed fluences calculated from transformations obtained by registering the portal images to the DRRs; 2) Comparisons of the resulting dose distribution of the adaptive technique to the one from the originally optimized plan. Conclusion: Preliminary results suggest that this fluence deformation based adaptive technique can geometrically account for simple rigid anatomic variations including 2D shifts-rotations. Evaluation of the full extent of dosimetric outcome from applying this technique and implementation of deformable registration algorithms for adapting to more complex anatomic changes, such as 3D deformation and volume change, are in progress.

956 poster Comparison of low risk prostate cancer patients positioned with and without implanted gold markers and online portal imaging based on magnetic resonance spectroscopic imaging

956 poster Comparison of low risk prostate cancer patients positioned with and without implanted gold markers and online portal imaging based on magnetic resonance spectroscopic imaging

On-line aSi portal imaging of implanted fiducial markers for the reduction of interfraction error during conformal radiotherapy of prostate carcinoma

Purpose An on-line system to ensure accuracy of daily setup and therapy of the prostate has been implemented with no equipment modification required. We report results and accuracy of patient setup using this system. Methods and materials Radiopaque fiducial markers were implanted into the prostate before radiation therapy. Lateral digitally reconstructed radiographs (DRRs) were obtained from planning CT data. Before each treatment fraction, a lateral amorphous silicon (aSi) portal image was acquired and the position of the fiducial markers was compared to the DRRs using chamfer matching. Couch translation only was used to account for marker position displacements, followed by a second lateral portal image to verify isocenter position. Residual displacement data for the aSi and previous portal film systems were compared. Results This analysis includes a total of 239 portal images during treatment in 17 patients. Initial prostate center of mass (COM) displacements in the superior, inferior, anterior, and posterior directions were a maximum of 7 mm, 9 mm, 10 mm and 11 mm respectively. After identification and correction, prostate COM displacements were <3 mm in all directions. The therapists found it simple to match markers 88% of the time using this system. Treatment delivery times were in the order of 9 min for patients requiring isocenter adjustment and 6 min for those who did not. Conclusions This system is technically possible to implement and use as part of an on-line correction protocol and does not require a longer than standard daily appointment time at our center with the current action limit of 3 mm. The system is commercially available and is more efficient and user-friendly than portal film analysis. It provides the opportunity to identify and accommodate interfraction organ motion and may also permit the use of smaller margins during conformal prostate radiotherapy. Further integration of the system such as remote table control would improve efficiency.

Radiation treatment of lung cancer—Patterns of Practice in Canada

Background and purposeTo assess the patterns of practice among Canadian radiation oncologists who treat lung cancers. Patients and methodsA questionnaire detailing different aspects of radiation treatment of lung cancer was mailed to all radiation oncologists treating lung cancer in Canada. Seventy-two percent (74/103) of radiation oncologists who treat lung cancer from all 34 Canadian cancer centres replied to the questionnaire. Results(a) Radiotherapy regimens in Canadian cancer centres are in accordance with several major randomised studies. There is still some variation in treatment practice that may be due to unresolved controversies or limited resources. The most frequently used prescription dose was 40Gy/15f/3w (where f stands for fractions and w stands for weeks) for small cell lung cancer (SCLC) and 60Gy/30f/6w for non-small cell lung cancer (NSCLC). If there were no resource constraints, 30% (22/74) and 20% (15/74) would prefer to use a different dose–fractionation scheme for SCLC and NSCLC, respectively; 95% (70/74) would prefer to use 3D-conformal or intensity-modulated radiotherapy. (b) Among the various modern technologies assessed by respondents, CT (computed tomography) simulator, multi-leaf collimator, on-line electronic portal imaging and PET (positron-emission tomography) scanning were rated the highest in terms of potential patient benefit. Discrepancy between demand and availability of technology was greatest for PET scanning. ConclusionsCanadian practice in the treatment of lung cancers shows some variations although it is consistent with the trends in the literature. The lack of some modern technologies and human resources is an ongoing concern, especially the lack of PET imaging equipment.

Daily electronic portal imaging for morbidly obese men undergoing radiotherapy for localized prostate cancer

Purpose We summarize our experience with a series of morbidly obese men treated using daily online portal imaging and implanted gold markers to guide external beam radiation therapy (EBRT). Methods and materials Three consecutive morbidly obese men were treated with EBRT for localized prostate cancer. Daily electronic portal imaging was used to verify patient position. The magnitude and direction of patient positioning error were documented for each fraction. Results The absolute magnitude of positioning error was greatest in the left-right direction with a mean of 11.4 mm/fraction (median, 8 mm; range, 0–42 mm). Mean error in the superior-inferior direction was also substantial at 7.2 mm/fraction (median, 5 mm; range, 0–47 mm). Anteroposterior error was the least problematic with a mean value of 2.6 mm/fraction (median, 2.5 mm; range, 0–8 mm). Conclusions Daily electronic portal imaging combined with gold fiducial markers dramatically improves the precision of EBRT in the treatment of morbidly obese men with prostate cancer. Setup error rather than organ motion appears to be the dominant force in positioning error in obese men.

The use of an aSi-based EPID for routine absolute dosimetric pre-treatment verification of dynamic IMRT fields

Background and purposeIn parallel with the increased use of intensity modulated radiation treatment (IMRT) fields in radiation therapy, flat panel amorphous silicon (aSi) detectors are becoming the standard for online portal imaging at the linear accelerator. In order to minimise the workload related to the quality assurance of the IMRT fields, we have explored the possibility of using a commercially available aSi portal imager for absolute dosimetric verification of the delivery of dynamic IMRT fields. Patients and methodsWe investigated the basic dosimetric characteristics of an aSi portal imager (aS500, Varian Medical Systems), using an acquisition mode especially developed for portal dose (PD) integration during delivery of a—static or dynamic-radiation field. Secondly, the dose calculation algorithm of a commercially available treatment planning system (Cadplan, Varian Medical Systems) was modified to allow prediction of the PD image, i.e. to compare the intended fluence distribution with the fluence distribution as actually delivered by the dynamic multileaf collimator. Absolute rather than relative dose prediction was applied. The PD image prediction was compared to the corresponding acquisition for several clinical IMRT fields by means of the gamma evaluation method. Results and conclusionsThe acquisition mode is accurate in integrating all PD over a wide range of monitor units, provided detector saturation is avoided. Although the dose deposition behaviour in the portal image detector is not equivalent to the dose to water measurements, it is reproducible and self-consistent, lending itself to quality assurance measurements. Gamma evaluations of the predicted versus measured PD distribution were within the pre-defined acceptance criteria for all clinical IMRT fields, i.e. allowing a dose difference of 3% of the local field dose in combination with a distance to agreement of 3 mm.

Design of a graticule indicating central axis position for use with portal imaging

This technical note describes a port film graticule that can be used to determine the central axis of a portal image when used with megavoltage film or on-line electronic portal imaging. The construction and quality control of the portal film graticule are highlighted.

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.

Improvement in dose escalation using off-line & on-line image feedback in the intensity modulated beam design for prostate cancer treatment

Purpose: To test the capability of dose escalation in the IMRT process where the organ/patient temporal geometric variation, measured using either off-line or on-line treatment CT and portal images, are adapted for the optimal design of intensity modulated beam. Materials and Methods: Retrospective study was performed on five prostate cancer patients with multiple CT scans (14∼17/patient) and daily portal images obtained during the treatment course. These images were used to determine the displacements of each subvolume in the organs of interest caused by the daily patient setup and internal organ motion/deformation. The temporal geometric information was processed in order of treatment time and fed into an inverse planning system. The inverse planning engine was specifically implemented to adapt the design of intensity modulated beam to the temporal subvolume displacement and patient internal density changes. Three image feedback strategies were applied to each patient and evaluated with respect to the capability of safe dose escalation. The first one is off-line image feedback, which designs the beam intensity based on the patient images measured within the first week of treatment. The second is an on-line ’the target of the day’ strategy, which designs the beam intensity in daily bases by using ’the image of the day’ alone. The last one is also the on-line based. However, it designs the instantaneous beam intensity based on also dose distribution in each organ of interest received prior to the current treatment. For each of the treatment strategies, the minimum dose delivered to the CTV was determined by applying the identical normal tissue constraints of partial dose/volumes. This minimum dose was used to represent the treatment dose for each patient. Results: The off-line strategy appears feasible after 5 days of image feedback. The average treatment dose among the patients can be 10% higher than the one in the conventional IMRT treatment (Table) where the inverse planning was performed based on the predefined planning target volume (CTV + 1cm margin. The capability of safe dose escalation increases, on average, extra 15% in the first on-line strategy. The second on-line feedback strategy shows only a marginal improvement on the total treatment dose compared to the first one. However, it shows further improvement on the dose distribution in the normal organs. Conclusions: The capability of safe dose escalation can be significantly enhanced in both the off-line and the on-line image feedback strategies. In the off-line image feedback strategy, patient/organ geometric information obtained from the first week of treatment appears adequate to guide the inverse planning design for remaining treatment. In the on-line image feedback strategy, the knowledge of organ dose distribution received from the previous treatment improves the design of beam intensity. Supported in part by NCI-CA71785 & Deutsche Krebshilfe. Patient# Off-line vs Conventional IMR On-line vs Off-line 1 6.1% 16% 2 6.4% 23% 3 15.8% 14% 4 15.9% 5.9% 5 7.7% 14.5% MEAN 10.4% 14.7%

Positioning errors and prostate motion during conformal prostate radiotherapy using on-line isocentre set-up verification and implanted prostate markers

Purpose: To evaluate treatment errors from set-up and inter-fraction prostatic motion with port films and implanted prostate fiducial markers during conformal radiotherapy for localized prostate cancer. Methods: Errors from isocentre positioning and inter-fraction prostate motion were investigated in 13 men treated with escalated dose conformal radiotherapy for localized prostate cancer. To limit the effect of inter-fraction prostate motion, patients were planned and treated with an empty rectum and a comfortably full bladder, and were instructed regarding dietary management, fluid intake and laxative use. Field placement was determined and corrected with daily on-line portal imaging. A lateral portal film was taken three times weekly over the course of therapy. From these films, random and systematic placement errors were measured by matching corresponding bony landmarks to the simulator film. Superior-inferior and anterior-posterior prostate motion was measured from the displacement of three gold pins implanted into the prostate before planning. A planning target volume (PTV) was derived to account for the measured prostate motion and field placement errors. Results: From 272 port films the random and systematic isocentre positioning error was 2.2 mm (range 0.2–7.3 mm) and 1.4 mm (range 0.2–3.3 mm), respectively. Prostate motion was largest at the base compared to the apex. Base: anterior, standard deviation (SD) 2.9 mm; superior, SD 2.1 mm. Apex: anterior, SD 2.1 mm; superior, SD 2.1 mm. The margin of PTV required to give a 99% probability of the gland remaining within the 95% isodose line during the course of therapy is superior 5.8 mm, and inferior 5.6 mm. In the anterior and posterior direction, this margin is 7.2 mm at the base, 6.5 mm at the mid-gland and 6.0 mm at the apex. Conclusions: Systematic set-up errors were small using real-time isocentre placement corrections. Patient instruction to help control variation in bladder and rectal distension during therapy may explain the observed small SD for prostate motion in this group of patients. Inter-fraction prostate motion remained the largest source of treatment error, and observed motion was greatest at the gland base. In the absence of real-time pre-treatment imaging of prostate position, sequential portal films of implanted prostatic markers should improve quality assurance by confirming organ position within the treatment field over the course of therapy.

Investigation of a direct conversion flat panel imager for portal imaging.

Flat-panel based x-ray imaging is an emerging new technology that could be used to significantly improve the quality of on-line portal imagers. There are two types of flat panel imagers: direct and indirect conversion. Previous experimental work on flat panel detectors for portal imaging application used indirect-conversion imagers. In this paper, a direct-conversion amorphous-selenium flat panel imager is investigated for application in portal imaging. The imager has an active imaging area of 14 in. X 17 in., i.e., 3072 X 2560 pixels each with dimensions 139 microm X 139 microm. The spatial frequency dependent detective quantum efficiency of the imager has been measured for a 6 MV beam and found to be amongst the best area detectors investigated for on-line portal imaging. Effects of changing pixel size, as well as possible improvements to both the image quality and convenience of operation are discussed. Comparison with an indirect conversion flat panel imager is also included.

128 invited Implementation of a patient set-up verification and correction protocol with an on-line portal imaging in a IMRT procedure for localised prostate cancer

128 invited Implementation of a patient set-up verification and correction protocol with an on-line portal imaging in a IMRT procedure for localised prostate cancer

Improvement in dose escalation using the process of adaptive radiotherapy combined with three-dimensional conformal or intensity-modulated beams for prostate cancer

Purpose: Advances in technology allow the creation of complex treatment plans with tightly conforming doses. However, variations in positioning of the organ/patient with respect to treatment beams necessitate the use of an appreciable margin, potentially limiting dose escalation in many patients. To ( 1) reduce this margin and ( 2) test the hypothesis that the achievable level of dose escalation is patient dependent, a patient-specific, confidence-limited planning target volume (cl-PTV) was constructed using an adaptive radiotherapy (ART) process for prostate cancer treatment developed in-house. The potential dose escalation achievable with this ART process is quantified for both conformal radiotherapy (CRT) delivery and intensity-modulated radiotherapy (IMRT) delivery. Material and Methods: Patients with organ confined prostate cancer were entered prospectively into an ART process developed in-house. This ART process has been designed to improve accuracy and precision of dose delivery, consequently enhancing dose escalation. In this process, a cl-PTV is constructed for each patient in the second week of treatment based upon on-line portal and CT images acquired during the first week of treatment. The treatment prescription dose, defined as the minimum dose to the cl-PTV, is selected based on predefined dose-volume constraints for rectum/bladder and derived from the pretreatment planning CT image. In addition, the treatment modality (CRT or IMRT) is determined based on the level of dose escalation achievable and the risk of inaccurate targeting. The potential for both dose escalation and the application of IMRT was evaluated by comparing the prescription doses delivered using the ART process, with the cl-PTV, to those in the traditional treatment process, with a conventional generic PTV. In addition, the distributions of potential geometric target underdosing and normal tissue overdosing were also calculated to evaluate the quality of the conventional treatment plans. Results: One hundred and fifty patients have been treated with the ART process. When compared to the treatment dose delivered with the conventional treatment process (generic PTV), an average 5% (2.5–10%) more dose could be delivered using the ART process with CRT, and 7.5% (2.5–15%) more dose could be delivered with IMRT. Of the 150 patients, 70% were treated to a minimum cl-PTV dose ≥ 77.4 Gy (81.3 Gy ICRU isocenter dose). Dosimetric analysis revealed that 81 Gy to the cl-PTV (or 86.7 Gy ICRU) could be prescribed to at least 50% of patients if IMRT was applied using the ART process. In contrast, IMRT did not yield an obvious dose escalation gain if patients were treated using the generic PTV. Our results also demonstrate that the cl-PTV is significantly smaller than the conventional generic PTV for most patients, with a mean volume reduction of 24% (range, 5–43%). Conclusion: These results support our hypothesis that the achievable level of dose escalation using ART is patient dependent. By using the ART process to develop a cl-PTV, one can ( 1) optimize the dose level, ( 2) increase the applicability of IMRT, and ( 3) improve the quality of dose delivery. The ART process provides the foundation to identify a suitable option (CRT or IMRT) for the delivery of a safe treatment and dose escalation. It is now our standard of practice for prostate cancer treatment.

Imaging in radiotherapy

Radiotherapy, more then any other treatment modality, relies heavily and often exclusively on medical imaging to determine the extent of disease and the spatial relation between target region and neighbouring healthy tissues. Radically new approaches to radiation delivery are inspired on CT scanning and treat patients in a slice-by-slice fashion using intensity modulated megavoltage fan beams. For quality assurance of complex 3-D dose distributions, MR based 3-D verificative dosimetry on irradiated phantoms has been described. As treatment delivery becomes increasingly refined, the need for accurate target definition increases as well and sophisticated imaging tools like image fusion and 3-D reconstruction are routinely used for treatment planning. While in the past patients were positioned on the treatment machines based exclusively on surface topography and the well-known skin marks, such approach is no longer sufficient for high-accuracy radiotherapy and special imaging tools like on-line portal imaging are used to verify and correct target positioning. Much of these applications rely on digital image processing, transmission and storage, and the development of standards, like DICOM and PACS have greatly contributed to these applications. Digital imaging plays an increasing role in many areas in radiotherapy and has been fundamental in new developments that have demonstrated impact on patient care.

Automatic on-line electronic portal image analysis with a wavelet-based edge detector.

A fully automatic method for on-line electronic portal image analysis is proposed. The method uses multiscale edge detection with wavelets for both the field outline and the anatomical structures. An algorithm to extract and combine the information from different scales has been developed. The edges from the portal image are aligned with the edges from the reference image using chamfer matching. The reference is the first portal image of each treatment. The matching is applied first to the field and subsequently to the anatomy. The setup deviations are quantified as the displacement of the anatomical structures relative to the radiation beam boundaries. The performance of the algorithm was investigated for portal images with different contrast and noise level. The automatic analysis was used first to detect simulated displacements. Then the automatic procedure was tested on anterior-posterior and lateral portal images of a pelvic phantom. In both sets of tests the differences between the measured and the actual shifts were used to quantify the performance. Finally we applied the automatic procedure to clinical images of pelvic and lung regions. The output of the procedure was compared with the results of a manual match performed by a trained operator. The errors for the phantom tests were small: average standard deviation of 0.39 mm and 0.26 degrees and absolute mean error of 0.31 mm and 0.2 degrees were obtained. In the clinical cases average standard deviations of 1.32 mm and 0.6 degrees were found. The average absolute mean errors were 1.09 mm and 0.39 degrees. Failures were registered in 2% of the phantom tests and in 3% of the clinical cases. The algorithm execution is approximately 5 s on a 168 MHz Sun Ultra 2 workstation. The automatic analysis tool is considered to be a very useful tool for on-line setup corrections.

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.