Assessment of geometric treatment accuracy using time-lapse display of electronic portal images

[Quality control during radiation therapy]

The aim of quality control procedures during radiation therapy is to check the consistency between actual and prescribed treatments. Given the technical complexity of modern radiotherapy, stricter policies are necessary to meet increasing requirements for quality and safety. Among the various tools available, electronic imaging systems play an increasing role in patient-beam position checking and in vivo dose measurements. Written procedures will have to be established in order to describe the control modalities and frequency, as well as the rules for error corrections according to the treatment intent. Non medical staff will be devoted to new tasks, under the radiation oncologist's responsibility. A special attention should be directed at electronic archives, since the present technology is unlikely to meet the legal requirement to keep medical records accessible for at least 30 years.

Guide to clinical use of electronic portal imaging

The Electronic Portal Imaging Device (EPID) provides localization quality images and computer-aided analysis, which should in principal, replace portal film imaging. Modern EPIDs deliver superior image quality and an array of analysis tools that improve clinical decision making. It has been demonstrated that the EPID can be a powerful tool in the reduction of treatment setup errors and the quality assurance and verification of complex treatments. However, in many radiation therapy clinics EPID technology is not in routine clinical use. This low utilization suggests that the capability and potential of the technology alone do not guarantee its full adoption. This paper addresses basic considerations required to facilitate clinical implementation of the EPID technology and gives specific examples of successful implementations.

Observation study of electronic portal images for off-line verification

PURPOSE

The goals of this study were to evaluate the use of electronic portal imaging device (EPID) paper images as off-line verification tools and to assess the feasibility of replacing portal films by EPID printed images.

MATERIALS AND METHODS

Electronic portal images were acquired using a video-based imaging system. After contrast enhancement, these images were printed and compared to portal films when prescribed, and judged about their usefulness for off-line verification. A total of 2025 images were acquired from 322 fields on 137 patients. The images were shown to eight radiation oncologists and two senior residents in radiation oncology, each one of them judging fields relevant to his (her) daily practice. The questions asked were related to the choice of important anatomical structures and the visibility of such structures, the usefulness of the printed images, the comparison with portal films and the possible replacement of such films by paper images.

RESULTS

Answers to the different questions were treated as quantitative scores. For the visibility question, means and standard deviations were calculated for each individual structure, then a global score was obtained for a given treatment site. Means and standard deviations were also computed for the comparison question. Proportions and confidence intervals were used for the other questions. The results show that EPID paper images are useful for some treatment sites such as breast, thorax, prostate, abdomen, pelvis (other than rectum) and axilla. The image quality remains insufficient for some other sites such as head and neck and spine.

CONCLUSION

Although global anatomical landmarks scores are good, the usefulness score is not always as high because some essential anatomical structures scores must be taken into account. There is also a strong habit factor related to acceptance of EPID printed images as verification tools. As long as they see more and more images, radiation oncologists can more easily visualize anatomical structures and are less stringent when evaluating the efficiency of EPID paper images as off-line verification tools.

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.

Target volume definition in radiation therapy

Target volume definition in radiation therapy is a broad field of interdisciplinary research. We give a brief history of clinical research in this field and outline some remarkable steps which led to the well-defined target volume concepts. The challenges in target volume definition for high-precision conformal radiation therapy are discussed, and possibilities of improving target volume definition, such as the integration of modern imaging modalities and the use of computer-based systems to support the radiation oncologist are indicated, as well as novel techniques for increasing the accuracy of patient positioning. All these tools should be evaluated with regard to their potential for increasing the therapeutic ratio and, as appropriate, should be implemented in clinical practice. However, target volume definition is a complex process influenced by many factors, currently under investigation. While questions remain in this field, and the impact of the influencing factors is not defined, the process of target volume definition should remain the subject of clinical research.

Consistency of three-dimensional planning target volumes across physicians and institutions

PURPOSE

Three-dimensional treatment planning depends upon exact and consistent delineation of target volumes. This study tested whether different physicians from different institutions vary significantly in their creation of planning target volumes (PTVs).

METHODS AND MATERIALS

Eight physicians from three different institutions created partial planning target volumes for nine clinical test cases. Their target volumes were evaluated qualitatively and quantitatively. Quantitative results were tested for significant differences.

RESULTS

Qualitative analysis showed the physicians to vary in (a) the margin placed around the clinical target volume, (b) the margin used near critical structures, and (c) handling of concavities in the clinical target volume. Quantitative analysis showed these variations to result in statistically significant differences in the measured volume of the physicians' planning target volumes.

CONCLUSIONS

Individual physicians and institutions differ significantly in their creation of planning target volumes, suggesting individual and institutional differences in the working definition for the PTV. Implications of this fact are discussed, along with areas where standardization can be improved.

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.

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.

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.

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 multipurpose phantom for use with electronic portal imaging devices

A simple, low-cost, multipurpose phantom has been designed for use with electronic portal imaging devices (EPIDS). Making use of the high spatial resolution of an EPID, together with the built in software tools for measuring distances, it is possible to verify x-ray/light field size and congruence, at any gantry angle, for fields up to 200 mm x 200 mm. Being of an accurate construction, it can help analyse the distance measuring capabilities of an EPID, ensuring that they are accurate and remain so with time. It can also be used to quantify, and monitor, image displacement and rotation with gantry angle.

The use of an electronic portal imaging device for exit dosimetry and quality control measurements

PURPOSE

To determine ways in which electronic portal imaging devices (EPIDs) could be used to (a) measure exit doses for external beam radiotherapy and (b) perform quality control checks on linear accelerators.

METHODS AND MATERIALS

When imaging, our fluoroscopic EPID adjusts the gain, offset, and frame acquisition time of the charge coupled device (CCD) camera automatically, to allow for the range of photon transmissions through the patient, and to optimize the signal-to-noise ratio. However, our EPID can be programmed to act as an integrating dosemeter. EPID dosemeter measurements were made for 20 MV photons, for different field sizes and thicknesses of unit density phantom material placed at varying exit surface to detector distances. These were compared with simultaneous Silicon diode exit dose measurements. Our exit dosimetry technique was verified using an anthropomorphic type phantom, and some initial measurements have been made for patients treated with irregularly shaped 20 MV x-ray fields. In this dosimetry mode, our EPID was also used to measure certain quality control parameters, x-ray field flatness, and the verification of segmented intensity modulated field prescriptions.

RESULTS

Configured for dosimetry, our EPID exhibited a highly linear response, capable of resolving individual monitor units. Exit doses could be measured to within about 3% of that measured using Silicon diodes. Field flatness was determined to within 1.5% of Farmer dosemeter measurements. Segmented intensity modulated fields can be easily verified.

CONCLUSIONS

Our EPID has the versatility to assess a range of parameters pertinent to the delivery of high quality, high precision radiotherapy. When configured appropriately, it can measure exit doses in vivo, with reasonable accuracy, perform certain quick quality control checks, and analyze segmented intensity modulated treatment fields.

Clinical applications of composite and realtime megavoltage imaging

The versatility of electronic portal imaging devices (EPIDs) is best demonstrated by their ability to perform novel megavoltage imaging protocols, which are still pertinent to good radiotherapy practice. This paper examines two such techniques: composite and realtime imaging. Our EPID can be programmed to acquire and manipulate images very easily, allowing images from segmented treatment protocols to be mixed and displayed, giving a composite image of the effective treatment result. Its use for verifying the efficacy of spinal shielding using a segmented, offset collimator technique is described. By acquiring images very quickly, realtime imaging sequences can be obtained and used to analyse anatomical movement within a single treatment field. The technique is employed here to investigate movement in radical lung, breast, abdomen, pelvis and thyroid treatments. Our results show that the protocol is vital for treatment sites involving the lungs; changes up to 5 mm have been observed in the maximum lung depth for breast treatments, and displacements up to 16 mm for radical lung treatments. It is also useful in other anatomical sites for ensuring that no movement occurs.

Measurement possibilities using an electronic portal imaging device

A vital role in the quality control of radiotherapy is the use of portal imaging for verifying field size, shape, orientation and patient set-up. Coincidence of treated volume and target volume is imperative. Electronic portal imaging devices are effective at providing this verification. However, these devices are versatile enough to be used in other ways pertinent to the delivery of high quality, high precision radiotherapy. This paper examines two such ways: in assessing the reproducibility of a multileaf collimator system, and in determining exit doses in vivo. Configured as a dosimeter, the system shows a linear response with good dynamic range. Its high spatial resolution was used to show that leaf positioning was reproducible to within 0.5 mm for all tested gantry and collimator angles. Our preliminary results from this exit dosimetry technique demonstrate that, under specific conditions, doses can be determined to within 2.5% of that measured using silicon diodes or ion chambers.