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Fiducial Markers in Image-guided Radiotherapy of the Prostate

US Oncological Disease, 2006;1(2):75-9 DOI: http://doi.org/10.17925/OHR.2006.00.02.75

Abstract:

Fiducial markers are emerging as a standard tool for image-guided radiotherapy (IGRT). The markers are implantable devices designed to act as reliable surrogates for imaging anatomic structures of interest. Fiducial marker techniques were originally developed in the preconformal radiotherapy era for positional verification of tissues that were not easily visualized using portal X-ray film imaging for patient alignment. Soft tissue structures that were relatively mobile could be more readily seen when radio-opaque seeds or wires were implanted in or near organs of interest. However, since comparatively large margins around target volumes were used, dose delivery limitations overshadowed imaging accuracy. In addition, routine and efficient ‘online’ (at the time of treatment) implementation of patient set-up error analysis and patient repositioning was problematic in the past.

Modern radiotherapy increasingly utilizes conformal delivery techniques, which necessitate more accurate patient positioning and tumor targeting.1 Brought about by mechanical and computing advances, the ability to deliver radiation doses to the tumor while sparing normal tissue using 3-D conformal radiotherapy (3D-CRT), intensity modulated radiotherapy (IMRT) or other conformal treatments, such as proton therapy, allows the potential for reduction of toxicity and—through careful dose escalation—potentially better outcomes. It is hoped that by keeping ‘tight margins’ around target volumes these dual goals may be attained.

As a practical matter, improved dose delivery precision necessitates a higher degree of accuracy in targeting the tumor (target delineation) and aligning the patient so as to reproducibly confirm that the tumor is in the same place in 3-D space as the prescribed conformal dose distribution throughout therapy (positional verification). However, since even small geometric deviations from the treatment plan may result in substantial changes in dose distribution,2 margin reduction must be matched with IGRT techniques lest margin reduction result in poorer clinical outcomes.3

While fiducial markers have been implemented in head and neck, gastrointestinal, hepatic, and other sites, the vast majority of existing data has been generated in prostate cancer and, for the purposes of this manuscript, this organ site will be exclusively focused on. Prostate cancer demonstrates a dose–response profile such that increasing delivery of dose to tumor is associated with better outcomes.However, prostate radiotherapy presents several unique issues for which fiducial markers may be a useful tool.The prostate is difficult to image using standard Xray films, lies in proximity to hollow organs (the bladder and rectum), which are at risk of radiotherapy-induced toxicity, and may be displaced due to filling of these adjacent organs.As such, the prostate represents a ‘moving target’ as changes in bladder and bowel filling can displace the prostate significantly between, or even during, radiotherapy fractions.4 Since the prostate is not attached directly to bony anatomy, prostate motion differs substantially from pelvic bony anatomic position.Adding larger margins to the prostate increases dose to bladder and rectum, improving target coverage but also increasing the probability of toxicity: this is a less than optimal solution to ensure tumor coverage. The problem is magnified when steep dose gradients such as with 3DCRT or IMRT are employed or when dose escalation is desired.5 Since gross tumor volume and microscopic extent of disease cannot be reduced, the most reasonable mechanism for reduction of margins is increased accuracy of targeting and patient alignment.6,7