Pergamon Int. J. Radiation Oncology Biol. Phys., Vol. 28, No. 4, pp. 1043-1045, 1994

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??Editorial

PROGRESS IN THE EVALUATION OF ELECTRONIC PORTAL IMAGING-TAKING ONE STEP AT A TIME

SHLOMO SHALEV, PH.D.

Manitoba Cancer Treatment and Research Foundation, Winnipeg, MB R3E 0V9, Canada

There is increasing interest in the quality assurance of radiation therapy, particularly with regard to the quan- tification of field displacements due to inaccuracies in pa- tient set-up. Although most studies are still based on the analysis of portal films, a number of recent reports have used electronic portal imagers (3, 4, 7, 10, 13, 21). In some studies the systematic set-up errors are dominant, while in others random day-to-day errors are also signif- icant. Their relative magnitudes appear to depend on the treatment site, patient obesity, immobilization technique, and other conditions and procedures specific to each cen- ter. Electronic portal imaging (EPI) provides not only a more effective way of determining field displacements than film, but the immediate availability of the images opens up new possibilities for the reduction of set-up errors during the course of treatment. It is of interest to ask what is the current status of EPI, and what are the prospects for its routine application in the clinical setting?

The study by Herman et al. (15) in this issue examines the clinical efficacy of EPI by asking whether the digital images are as good as portal films for identifying set-up errors. They found that in 63% they were as good, and in 7% were better than film. Improvements in image quality and larger detectors to fully accommodate pelvic fields will answer some of the concerns raised in this study. Of greater interest was the finding that visual comparison of the portal image with the simulator film was not a reliable indicator of set-up error, and 14% of the accepted fields still had set-up errors of 5- 10 mm as determined by ret- rospective analysis. Indeed, 4% were misaligned by greater than 10 mm, even though the operators had judged them to be acceptable. While there are some difficulties with the study protocol (on-line set-up was judged by a sim- ulator-portal comparison, and off-line analysis used the first portal for comparison), this report points to the prob- lems encountered when image alignment is operator de- pendent, and indicates the need for automatic image reg- istration and analysis, perhaps along the lines proposed for pelvic fields by van Herk et al. ( 14).

The range of potential applications of EPI technology is far greater than simply replacing visualinspection of weekly portal films by the visual inspection of daily digital images. Automatic analysis of field size and shape can eliminate gross errors due to incorrect, misplaced or miss- ing blocks, or improper collimator settings (19). The placement of lung attenuators in total-body irradiation can be verified (13), and the location of dosimeters has been determined using EPI (16). However, most current interest is concentrated on the determination and reduc- tion of set-up errors, and the procedures required for this task. Systematic set-up errors can be reduced by daily portal imaging and intertreatment set-up corrections prior to the following fraction, in much the same way that portal films have been used for many years, except that the sam- pling frequency is much higher. The determination of limits of acceptability and decision criteria for imple- menting corrections is still the subject of debate (2,822). On the other hand, the reduction of random set-up errors implies intratreatment corrections, as reported by Herman et al. and previously by groups in Belgium (6), Canada (1 l), and Great Britain (12). This interventional tech- nique, in which each treatment is verified in the first few seconds and, if necessary, corrected, is expensive in time and personnel, and can be justified by only the most strin- gent requirements for accuracy. Certainly one would want to ensure first that the best immobilization procedures are being used, and that systematic errors have been identified and minimized as far as possible. Only then would intra- treatment correction be worthwhile for selected treatment sites and patient populations.

In the short term we can expect most centers using EPI in routine clinical applications to view the digital display during each treatment to check against any gross errors in field size, shape, or placement, and to make subsequent off-line comparisons with the digitized simulator film to obtain a quantitative measure of field displacements, using either in-house software or programs which will be pro- vided by the equipment manufacturers. Data accumulated

Reprint requests to: Shlomo Shalev, Ph.D. Accepted for publication 10 January 1994.

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1044 I. J. Radiation Oncology 0 Biology 0 Physics Volume 28, Number 4, 1994

on the distribution of set-up errors will be used to modify structures suitable for image alignment, and implanted treatment plan protocols, or to recalculate a plan during radiopaque markers or wires may be required, since the course of treatment in consequence of significant set- markers on the immobilization cast can be quite mis- up errors (2 1). leading (9).

The long-term challenge facing EPI is to verify radiation treatments using noncoplanar conformal beams, either in the form of multiple static ports or as dynamic treatments under computer control. The advent of high resolution computed tomography (CT) and magnetic resonance im- aging (MRI) scanners has revolutionized tumor localiza- tion, and concurrent advances in treatment planning pro- vide the ability to conform the treatment fields to beams- eye-views of the target. Recent studies of three-dimen- sional conformal radiation therapy by Armstrong et al. for lung cancer (l), and by Vijayakumar et al. for prostate cancer (24) confirm the potential of this methodology, and several dose-escalation trials are in progress to eval- uate whether increased local control can be attained with- out increased morbidity. However, the possible advantages of conformal therapy may be limited by the ability to deliver the prescribed dose accurately and reliably to the intended target volume, day after day, over a protracted course of fractionated treatments. The use of immobili- zation devices has been shown to be beneficial for breast and pelvic treatments, but movement due to breathing and organ distention must also be considered. While EPI has the potential to improve the accuracy of dose delivery to meet the requirements of conformal radiotherapy, there are numerous difficulties still awaiting solutions. Small conformal fields may have few recognizable anatomical

Conformal therapy will be carried out using multileaf collimators, and their “scalloping” effect on beam pen- umbra may be a problem for portal imaging, since ac- curate field edge detection is essential for image alignment. It remains to be seen how current algorithms (5, 20, 23) deal with this situation. It is still an open question whether real-time verification of dynamic treatments will be pos- sible with EPI, or merely preliminary verification of the patient’s position before the start of treatment.

Most effort on developing EPI technology is being di- rected towards geometric verification of the treatment fields, even though one of the first applications of EPI was as an areal dosemeter ( 18). More recent studies have been reported on dosimetric verification (17, 25), which will become an essential requirement for the verification of intensity-modulated conformal beams.

In spite of all these challenges and possibilities for the future of EPI, Herman et al. (15) are correct in taking one step at a time. Unless digital images can be shown to be at least as good as conventional film, their acceptance in the routine clinical setting will be problematic, at best. The technology must be demonstrated to be efficacious, nonintrusive, cost effective, and reliable, and to have a positive impact on overall patient management. This will require the combined efforts of all those involved in the development and the evaluation of this technology.

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