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• W2093385096 abstract "Arguing against the Proposition is Lei Dong, Ph.D. Dr. Dong obtained his Ph.D. in Biomedical Sciences/Medical Physics from the University of Texas Graduate School of Biomedical Sciences, Houston, Texas. After a brief period at Baylor College of Medicine, he moved to the Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, where he attained the rank of Professor before moving to the Scripps Proton Therapy Center, San Diego in 2012, where he is currently Director and Chief Medical Physicist. Dr. Dong is certified by the American Board of Radiology in Therapeutic Radiation Physics, is a Fellow of the AAPM, and a Senior Associate Editor for the International Journal of Radiation Oncology, Biology, and Physics. He is active on several committees in the AAPM and is Chairman of the Imaging for Treatment Verification Work Group. His major research interests include IMRT, IGRT, deformable image registration, proton therapy, and dose/volume modeling, for which he has obtained numerous grants and patents and has published well over 100 papers in refereed journals. The fundamental tenet of radiotherapy is the delivery of a high dose of radiation to the tumor while limiting the dose to the surrounding normal tissues. To achieve this goal, all uncertainties in the multifaceted radiation therapy process have to be minimized. These include radiation output, geometric, and tumor delineation uncertainties.1 Developments broadly termed “image guided radiation therapy” have recently been introduced to reduce geometric uncertainties.2 There is growing evidence that these techniques have improved tumor localization in the treatment room.3 However, my key contention is that any improvement gained by the application of IGRT may be negated by inaccuracy in tumor delineation. Tumor delineation has been identified as the weakest link in the search for accuracy in radiation therapy.4,5 A high degree of uncertainty in target volume has been demonstrated for most cancer sites. For example, the target contours drawn for the same case by different physicians, or by the same physician on different days, show enormous differences. Weiss and Hess, for example, went as far as saying that “inter-observer variability in tumor delineation is a major - for some tumor locations probably the largest - factor contributing to geometric inaccuracy.”6 Major sources of variations in tumor volume delineation are: visibility of the target, including its extensions (impact of imaging protocol), disagreement on target extension, and interpretation (or lack of) delineation protocols.5,6 It is reasonable to assume that variation in tumor delineation increases the probability of geometric miss of parts of the tumor, thus increasing the risk of recurrence. Errors in tumor delineation (contouring) generate systematic errors that remain constant during the course of radiation therapy and, therefore, can have a large impact on outcome.5 No level of image guidance can eliminate this error. This also brings home the difference in precision and accuracy. Proper contouring of the target volume improves accuracy whereas image guidance improves precision. In other words, you can consistently hit a wrong target (high precision, poor accuracy). But what is required is to consistently hit the right target (high precision, high accuracy) during the course of treatment. Hence, accurate radiation therapy involves knowing exactly where the tumor is at the time of treatment. Finally, treatment margins may be overly reduced because IGRT gives a false sense of security in target localization. However, cancer has a complicated and mostly poorly understood disease spread and biology. It is likely that the incidental dose around the margins, the feathering effect from setup errors and generous planning target volume (PTV) margins, is needed to take care of the microscopic disease spread that may be present but not visible by current imaging techniques. Hence, IGRT is only as good as the accuracy with which the target is known. So, I suggest that the improvement in accuracy rendered by IGRT is limited by the accuracy of target delineation. I am arguing against the proposition because image guidance is so critical for radiation therapy that it is almost unethical not to use image guidance technology if it is available. Biological effects will be produced whenever tissues (tumors or normal organs) receive sufficiently high radiation doses.7 Geometric accuracy, therefore, is essential if high doses are to be delivered where they are needed and avoided where they are not. It is true that that IGRT requires accurate tumor delineation. Without defining a target, IGRT is impossible. I believe that the argument is whether or not the accuracy of target delineation is good enough to make IGRT clinically valuable. Interobserver contouring studies have, indeed, demonstrated large differences between radiation oncologists in target delineation for some treatment sites.8,9 Nevertheless, the value of precise target delineation is difficult to demonstrate clinically because treatment outcome depends on many other factors, including patient setup errors. Regardless of these uncertainties in target delineation, we should not ignore the other important aspect of image guidance in radiation therapy, which is to protect normal tissues and improve the patient's quality of life after radiation therapy. Compared to target delineation, it is fair to say that the accuracy of normal organ delineation is adequate. The use of IGRT in protecting normal tissue deserves its own clinical value. For example, a recent study of patients treated for localized prostate cancer demonstrated that the use of IGRT reduced grade 2 or higher urinary toxicity from 20.0% to 10.4% when compared with a group of similarly treated patients without using daily IGRT.10 The authors believed that the enhanced accuracy associated with IGRT caused a reduction in the volume of bladder or bladder neck exposed to the high prescription dose of 86.4 Gy used in the treatments. A Cox regression analysis also identified IGRT as one of the predictors for PSA relapse-free survival in the high-risk cohort. This was a retrospective analysis, however, and not a randomized clinical trial (RCT). Direct comparison of IGRT vs non-IGRT is rare. It will be morally difficult to conduct prospective randomized trials for such head-to-head comparisons because of the potential risk of underdosing the target and/or increasing normal tissue doses in the non-IGRT group. Finally, IGRT plays a role in providing quality assurance of daily patient setup.3 Large setup errors can be visualized and corrected before each treatment. IGRT makes treatment more consistent and eliminates one of the key uncertainties that will affect treatment outcome. Tight quality control of daily treatment will eventually improve our target delineation because we can better differentiate the outcome of “a correct target treated consistently” and “an incorrect target treated consistently.” The experience learned will help us to define the target better in the future. Therefore, I conclude that IGRT has great clinical value both now and for the future. Clinical value can be measured by reduction in early and late toxicity and improved disease control. As Dr. Dong has rightly pointed out, it will be impossible, or rather unethical, to conduct a RCT to measure these values for IGRT, as well as it is unethical to carry out a RTC to demonstrate the effects of tumor delineation inaccuracy. However, the lack of evidence of IGRT's deleterious effects is not proof of its clinical value. Radiation therapy should be evidence based, and history has shown that not all scientifically sound developments have clinical value or improve previous approaches.11 It is for this reason that many commentaries in scientific journals have cautioned against the blind application of technology without clinical evidence of its efficacy.12 It can also be pointed out that margins that are applied to account for delineation uncertainties can be reduced if tumor delineation accuracy is improved. Since tumor abuts normal tissue, it is fair to assume that if the tumor is delineated accurately, so also will be the normal tissues. While Dr. Dong points to retrospective studies documenting the clinical utility of IGRT, there are reports casting doubts on IGRT's clinical effectiveness. For example, Engels et al.13 demonstrated increased biochemical failure in patients with distended rectum on the planning CT, in spite of image guidance by implanted markers. Dr. Dong has failed to also address the fact that there is a plethora of techniques lumped under the umbrella of IGRT, with different precisions in target localization.14 For instance, the residual error for prostate localization using CBCT and fiducial markers was less than 2 mm, while it was approximately 4 mm for daily ultrasound. Consequently, as we try to tease out the value of accurate tumor delineation on IGRT, we should be cognizant of the fact that the diversity in IGRT techniques may mask the true benefit of IGRT. I do not doubt that IGRT has potential clinical value for radiation therapy, but such benefit will be harvested only if there is improved accuracy in tumor delineation. More research is required to improve target delineation using advancements in imaging techniques such as molecular imaging, and also to identify which IGRT technique is best for which cancer type. Dr. Njeh has brought several important points to the debate. The incidental dose around the geometric margin is an important one that people tend to ignore. The incidental dose creates a “dosimetric margin” which depends on the particular treatment technique. The dosimetric margin, instead of the geometric margin, ultimately determines the success of radiation therapy; of course, assuming that the target is appropriately defined. We should be mindful when changing practice. IMRT is a good example where the sharp dose falloff near the PTV and critical structure boundary represents a dosimetric risk for underdosing the target. Fortunately, geometric setup margins have not been reduced in most clinical practices when IGRT has been introduced. A combination of sharp dosimetric penumbra and aggressive reduction of setup margin with IGRT can result in treatment failures.13 I agree that physicists should not get the false impression that the geometric margin is the only concern when designing the PTV. On the other hand, we should not expect the unreliable incidental dose to take care of the microscopic disease. Microscopic disease extension should be explicitly included in the clinical target volume (CTV) definition, rather than in the PTV. Unfortunately, defining CTV is not an easy job. Despite some reports of interobserver variability in defining the CTV,8,9 target delineation uncertainties are clearly site and application-specific. It is unfair to state that target delineation is the weakest link for all treatment disease sites or treatment techniques. For example, for early-stage disease, some well-defined small tumors are commonly treated with stereotactic body radiation therapy (SBRT). Target delineation is relatively accurate in this case while organ motion and setup error are the biggest challenges, especially for mobile lung or liver cancers.15–17 IGRT plays a critical role and perhaps is the enabling technology in delivering accurate doses to these small targets with good results.16,17 In general, I agree with Dr. Njeh that target delineation is challenging. As Sir William Osler once said, “Medicine is a science of uncertainty and an art of probability.” Obviously, IGRT has made improvements in target delineation accuracy a high priority for the future." @default.
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• W2093385096 title "IGRT has limited clinical value due to lack of accurate tumor delineation" @default.
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