Comprehensive Review: Linear Accelerators and FFF in Radiation Therapy

This report provides a comprehensive review of linear accelerators, specifically focusing on the application of flattening filter free (FFF) beams in radiation therapy. It begins with an introduction to the topic, highlighting the increasing use of FFF beams and the standard methodologies in radiation therapy. The report then delves into the characteristics of FFF beams, comparing their dosimetric properties with conventional flattening filter (FF) beams, including dose rates, surface dose, and scatter factors. Clinical applications, such as stereotactic radiosurgery (SRS) and brain tumor treatment, are discussed, along with the advantages of FFF beams in terms of target coverage and treatment time. The report also addresses the limitations of FFF beams, such as photon target burn-up and modulation issues. Finally, the conclusion summarizes the findings, emphasizing the promising future of FFF beams in radiation therapy while acknowledging the need for addressing radiation safety and modulation challenges. The report is supported by references to relevant research papers.

A REVIEW ON THE LINEAR ACCELERATORS WITH FLATTENING FILTER IN THE ADMINISTRATION OF
RADIATION THERAPY
INTRODUCTION
The linear accelerators with flattening filter free beam capabilities are now gaining traction in the
radiation therapy arena. There are standard methodologies that have been used to administer the
radiation therapy. In this brief report, a special focus is on the FFF beams where they are assessed by
comparing their dosimetric characteristics with the conventional FF beams. In other words, what special
features does the FFF beam have that makes them unique in the radiation therapy administration. Xiao1
asserts that the FFF are used to absorb radiations so as to direct it to the spots where cancerous cells
can be annihilated. The photons are absorbed, scattered or produced in an ordered fashion. Therefore,
the flattening filters act as attenuators, hardeners and scatters in the beam path2.
THE FLATTENING FILTERS
The FFF has been in use in the past due to its facilitation of the delivery time so that what took so long to
deliver the treatment (for SRS) was considerably shortened by the method. However, concerns have
been raised on its efficiency as sometimes it leaves behind traces of radiation that can further cause
other cancer types1.
The differences in dosimetry properties
The total dose rate for FFF is 2.46 times higher than the FF beams. Ultimately, the dose rate increase
facilitates elimination of a larger portion of photons from the main beam center. Connectedly,
treatment delivery efficiency has also been improved courtesy of the higher dose rates registered in FFF
beams3.
The surface dose of FFF beams are at a higher level compared with FF beams1. This is mainly attributed
to the increased incident contaminant charged particle and low energy photons being propagated.
On the side of the scatter factor, the FF beams are mainly attributed to the dispersion that is often
exhibited when streamed3. The FFF beams tend to be denser at their penumbra hence recording
relatively lower scatter factors. Therefore, in treating cancer, it even gets better with the use of FFF
beams. The beam profiles of FFF and FF also show a marked difference. The FFF beams have relatively
smaller penumbra width than FF beams; this is mainly due to softer beam spectrum.
CLINICAL APPLICATIONS
FFF beams were initially developed and used in the treatment of the SRS (small field stereotactic
radiosurgery). Later, their use has been extended to areas such as annihilation of brain tumors4. On
target coverage, the FFF beams capture a larger portion of the target unlike FF beams. Besides, the time
taken to complete treatment has greatly been improved with the use of FFF beams.
LIMITATION OF THE FFF beams
The beam often experiences a phenomenon technically referred to as photon target burn-up. Problems
of modulation has also arisen especially since it is the target is distance-dependent and sometimes off-
axis modulation may result into missing of the target hence can be another source for additional
unsolicited radiations.

CONCLUSION
The review above ostensibly discussed about the FFF beams. Furthermore, comparison with other
conventional beams has been done. The focus was on how the mentioned beams differ with the normal
radiation beams hence the difference in dosimetry and other relevant properties have been relooked.
Certainly, as mentioned earlier, the future of these beams in the radiation therapy administration is still
bright. However, a number of issues must be fixed such as the radiation safety during administration of
the therapy; for instance, are there residual radiation after administration? Besides, the modulation and
propagation challenges cited must be addressed in a continuous fashion.

REFERENCES
1. Xiao Y. Flattening filter-free accelerators: a report from the AAPM Therapy Emerging Technology
Assessment Work Group. 2015; pp 2-3. Available from:
http://onlinelibrary.wiley.com/doi/10.1120/jacmp.v16i3.5219/pdf
2. Sunil Dutt Sharma. Unflattened photon beams from the standard flattening filter free
accelerators for radiotherapy: Advantages, limitations and challenges. Journal of Medical
Physics. 2011. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3159217/
3. Simon Duane. Dosimetry for Flattening Filter Free (FFF) linac beams and small fields (SF).
National Physics laboratory. 2013. Available from: http://www.npl.co.uk/upload/pdf/20131202-
duane.pdf
4. Matthias Kretschmer et al. The impact of flattening-filter-free beam technology on 3D conformal
RT. 2013. Available from: https://ro-journal.biomedcentral.com/articles/10.1186/1748-717X-8-
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