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• W2765194803 abstract "Arguing against the Proposition is Brent C. Parker, Ph.D. Dr. Parker earned his M.S. and Ph.D. degrees in Medical Physics at the University of Texas-Houston Health Science Center Graduate School of Biomedical Sciences, Houston, Texas while he was working as a Graduate Research Assistant in the M.D. Anderson Cancer Center, Houston. He subsequently worked as Medical Physicist at The University of Texas Medical Branch, Galveston, TX from 2004–2007 and the Mary Bird Perkins Cancer Center, Baton Rouge, LA from 2007–2011, after which he returned to The University of Texas Medical Branch, Galveston, where he is currently Director, Division of Physics and Engineering, and Associate Professor in the Department of Radiation Oncology. Dr. Parker is certified in Therapeutic Radiologic Physics by the ABR and has served as the President of the AAPM Southwest Chapter. His major research interests include stereotactic radiosurgery, and radiotherapy treatment planning, delivery and quality assurance, on which he has published over 20 papers in refereed journals. Considering these drawbacks of FMs, kilovoltage cone-beam CT (CBCT) without FMs may provide a better alternative. One advantage of CBCT includes good quality soft tissue image resolution compared to EPI. Studies have shown that CBCT provides localization accuracy comparable to that with FMs.10 CBCT also provides better visualization than EPI of the prostate, seminal vesicles, and the adjacent structures at the time of treatment. Given the low risk of alternative techniques, there are no reasons why FMs are still needed for prostate IGRT. One definition of fiducial is “taken as a standard of reference”.11 In radiation oncology, implanted prostatic fiducials are frequently used as the standard for target position during patient setup. With 2D orthogonal imaging used for patient positioning, fiducials allowed for 3D position corrections. With the advent of cone-beam computed tomography (CBCT), however, 3D volumetric imaging information could be directly used in patient positioning. This may lead one to conclude that fiducials are no longer needed in prostate radiotherapy. Indeed, studies have shown that the use of implanted fiducials imaged with orthogonal planar imaging is not superior to CBCT for patient positioning.12 Thus, there is intrinsically no “need” for fiducials in the traditional use of initial patient positioning. The flip side to that position, though, is that neither is CBCT superior to fiducials. In that situation, the decision becomes a matter of other issues. Instead, I argue that the “need” for fiducials depends on how the prostate is to be treated and how the fiducials are to be used after the initial patient setup. While CBCT may negate the need for the use of fiducials in initial target positioning, it does not address intrafraction prostate displacement. This intrafraction motion can be determined by imaging of radiopaque fiducials or acquisition of data from transponder fiducials. For target tracking, implanted transponder fiducials (e.g., Calypso, Varian Medical Systems) allow for continuous, real-time tracking of intrafraction prostate displacement without the need to interrupt treatment for volumetric imaging.13 X-ray IGRT systems can allow for imaging of radiopaque fiducials during treatment delivery to evaluate prostate displacement as a function of time.14 Studies have shown that beacon and radiographic fiducials provide comparably accurate intrafraction prostate motion measurements.15 While intrafraction motion is typically small, it can be clinically significant at times, leading to treatment deliveries that do not meet clinical goals depending on the specifics of the treatment plan (e.g., margin size).16, 17 Additionally, we may expect these displacements to increase in magnitude with an increase of the overall fraction delivery time.17 In these cases, prostate position will need to be corrected back to its nominal position or larger margins will be required to ensure adequate target coverage. However, larger margins will lead to increased normal tissue doses and possible increases in complications. With the increasing popularity of hypofractionated prostate radiotherapy, the ability to reduce margins while ensuring adequate target coverage becomes even more important.18 This will require accurate real-time measurement of intrafraction prostate displacement using fiducials to determine if treatment intervention is required. In conclusion, while prostate fiducials are not needed for initial patient setup, they will play an important role in the evolution of adaptive and hypofractionated radiation therapy of the prostate. I agree with Dr. Parker that FMs are not superior to noninvasive techniques such as CBCT in the initial setup alignment of prostate patients.12 Dr. Parker went further to present an argument for an alternative role for FMs whereby they are used to monitor prostate intrafraction motion. However, the data do not justify this. For example, studies have found that intrafraction motion only becomes clinically significant for long duration treatments.19-21 Langen et al.21 reported that, from the initial setup, only 13% of patients have displacements above 3 mm by 5 min, rising to 25% by 10 min. Furthermore, such movements are accounted for in the treatment planning margins. Dr. Parker implied that intrafraction monitoring is critical for hypofractionated regimens because of increased fraction time and the need for reduce treatment margins. The increased fraction time is true with CyberKnife, where prostate treatments typically take up to 45 min per fraction.22 With the implementation of volumetric modulated arc therapy (VMAT) on traditional linacs; however, the treatment time for prostate cancer has significantly decreased to a mean of 4.6 min.20 In addition, with flattening filter-free (FFF) treatment delivery with dose rates up to 2400 MU/min, it possible to deliver hypofractionated doses within a few minutes.23, 24 Hence, there is not enough time for clinically significant prostate motion.24 Dr. Parker also indicated that intrafraction motion, if not corrected, will necessitate higher treatment margins. However, the greatest contributors to prostate treatment margins are systematic errors such as target delineation, not random errors such as intrafraction motion.25 Studies have also cautioned against excessive margin reduction around CTVs when using IGRT.26 It is therefore logical to conclude that for prostate cancer radiotherapy, FMs are no longer required. My colleague has made a number of arguments against the use of fiducial markers, and I will address them sequentially. While I agree that FMs are not necessary for initial prostate positioning, they can provide essentially real-time intrafraction displacement data. This may allow for smaller margins and potentially reduced normal issue complications. We have no conflicts of interest to disclose." @default.
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• W2765194803 date "2017-11-15" @default.
• W2765194803 modified "2023-09-28" @default.
• W2765194803 title "Implanted fiducial markers are no longer needed for prostate cancer radiotherapy" @default.
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