Opening Statement

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.