Fiducial Markers: An Important Tool for the Cyberknife’s Motion Tracking Capability
The past several assignments have explicitly focused on evaluating aspects of the Cyberknife system, such as the treatment method’s precision, efficacy, and cost-effect ratio. What has yet to be explored among these assignments is an important tool that essentially enables the Cyberknfie to do its job and effectively treat patients with various types of cancers. This important tool is known as a fiducial marker.
One of the main aspects of the Cyberknife system that makes it unique from other cancer treatment methods is that it can correct for patient motion while it is delivering radiation to a target cancer site. What enables the Cyberknife to do this is the use of gold fiducial markers. Past scientific literature in this field has predominantly focused on the use of fiducial markers for prostate cancer. Fiducial markers are placed into patients’ prostates in either a transperineal or transrectal manner under the guidance of a transrectal ultrasound (Hellinger et al., 2015). The Cyberknife is able to offer real time movement correction when delivering radiation by monitoring the positions of the fiducial markers with the use of a digital x-ray (Hellinger et al., 2015).
Fig 1: Fiducial markers in the prostate. The markers are inserted into the prostate (2 are visibly sparkling the center of the figure) and then used by the X-ray portion of the Cyberknife to track and correct for position changes made by patients undergoing treatment. (Hellinger et al., 2015)
Variations on fiducial markers have been produced over the years as a means of developing a more accurate way of localizing the prostate so that treatment from the Cyberknife can be delivered more precisely. A few of these variations include round markers, cylindrical markers, and elongated markers (Boer et al., 2012). Research conducted in 2012 specifically sought to evaluate the number of elongated fiducial markers that would produce the most accurate localization of the prostate during treatment (Boer et al., 2012). Using either 1, 2 or 3 markers for 24 patients, the researchers saw that placing 2 markers, one on each side of the prostate, was able to accurately locate the prostate; using 1 produced a larger position tracking error and using 3 produced a tracking error (0.3-0.8mm) similar to 2 markers (0.4-1mm) (Boer et al., 2012). Furthermore, research conducted in 2008 suggested that fiducial markers enable the Cyberknife x-ray to keep the prostate in tracking range at approximately 40 seconds between consecutive x-rays (Xie et al., 2008).
Additional research has been conducted not only to understand the capacity of fiducial markers and their impact on prostate movement tracking, but on improving the Cyberknife’s ability to track prostate motion. A study published in 2011 specifically focused on acquiring a six-dimensional prostate motion tracking system (Lei et al., 2011). In contrast to research by Boer mentioned above, 4 fiducial markers were placed at least 2 cm apart in the prostate in order to obtain 6D motion tracking for Lei’s patient sample (Lei et al., 2011). In 98% of the 88 patients undergoing the Cyberknife system with the use of fiducial markers, at least one fiducial marker met the criteria for 6D correction of prostate motion (Lei et al., 2011). By obtaining 6D motion tracking, a higher dose of radiation could be imposed on a patient to ultimately reduce the number of fractions he would have to undergo in order to treat his cancer.
One incredibly important factor to consider when it comes to any form of treatment for cancer are the complications that may result from it. In addition to exploring 6D motion tracking, Lei’s article reported that patients did not experience complications higher than grade 2 (Lei et al., 2011). Additional research reviewed 270 fiducial markers placed in 77 patients; 31 of these implants were placed into the prostate (Kim et al., 2012). 21% of the patients experienced minor complication rates either before and during treatment; 1% experienced severe complications (Kim et al., 2012). 2.2% of fiducial markers migrated from initial implantation sites. 96.7% of the implants were considered successful (Kim et al., 2012). In another study by Gill, the authors noted that out of the 234 patients assessed, 32% reported at least one new symptom post treatment (Gill et al., 212). However, most of the symptoms presented were considered either grade 1 or grade 2, such as urinary frequency and mild rectal bleeding, and often lasted less than 2 weeks (Gill et al.,2012).
Overall, this paper focused on evaluating the importance of fiducial markers for the use of the Cyberknife system for prostate cancer patients. The articles provided information on different types of fiducial markers, how they aid in motion tracking, ways of improving motion tracking with their use, and ultimate safety and efficacy were all assessed. It appeared that the fiducial markers had a high success rate for 6D correction and often did not result in incredible severe complications post treatment. Despite this, it is important to note that 32% of patients undergoing radiation therapy paired with fiducial markers experienced some grade level of complication as a result of the fiducial markers and treatment.
References
Boer, J. D., Herk, M. V., & Sonke, J. (2012). The Influence Of The Number Of Implanted Fiducial Markers On The Localization Accuracy Of The Prostate [Abstract]. IOP Science, 57(19).
Gill, S., Li, J., Thomas, J., Bressel, M., Thursky, K., Styles, C., Foroudi, F. (2012). Patient-reported complications from fiducial marker implantation for prostate image-guided radiotherapy. The British Journal of Radiology, 85(1015), 1011-1017.
Hellinger, J. C., Blacksberg, S., Haas, J., & Melnick, J. (2015, September). Interventional uroradiology in the management of prostate cancer. Applied Radiology, 40-41.
Kim, J. H., Hong, S. S., Kim, J. H., Park, H. J., Chang, Y., Chang, A. R., & Kwon, S. (2012). Safety and Efficacy of Ultrasound-Guided Fiducial Marker Implantation for CyberKnife Radiation Therapy. Korean Journal of Radiology, 13(3), 307-313.
Lei, S., Piel, N., Oermann, E. K., Chen, V., Ju, A. W., Dahal, K. N., Collins, S. P. (2011). Six-Dimensional Correction of Intra-Fractional Prostate Motion with CyberKnife Stereotactic Body Radiation Therapy. Frontiers in Oncology, 1, 1-12.
Xie, Y., Djajaputra, D., King, C. R., Hossain, S., Ma, L., & Xing, L. (2008). Intrafractional Motion of the Prostate During Hypofractionated Radiotherapy. International Journal of Radiation Oncology*Biology*Physics, 72(1), 236-246.
