A BRIEF NOTE ABOUT FLATTENING FILTER-FREE (FFF) TREATMENT BEAM

Overview:·??????

In external radiotherapy (RT), the use of flattening filter-free (FFF) radiation beams obtained by removing the flattening filter (FF) in standard linear accelerators is rapidly increasing, and the benefits of clinical use are the issue of research. Advanced treatment techniques have increased the interest in operating linear accelerators in FFF mode.

The differences between the beams with non-uniform dose distribution created by removing FF and those with uniform dose distribution used as a standard were examined. These differences were compared in lung patients' treatment plans with different planning target volumes (PTV).?Removing the filter from the standard linear accelerator (LINAC) results in an increase in dose rate, reduced scattering from the filter, a decrease in beam intensity from the centre of the beam field to the edges, and a non-uniform beam profile.

Achieving a flattening filter-free (FFF) beam in radiotherapy is accomplished by using a linear accelerator (linac) that is specifically designed or modified to operate without a flattening filter. ?

·????? Flattening filter-free (FFF) beams have less variation in off-axis beam hardening

·????? FFF have less scatter introduced into the field and may be easier to model accurately in the TPS

·????? Since the flattening filter is removed, the dose rate on the machine is very high for example, 1400MU/min for 6MV

·????? Modulation of the beam at such a high dose can be done with compensator-based IMRT

·????? Using compensators with high dose rates can be an optimal combination to reduce treatment times, for example with breath-hold techniques

·????? Patient throughput is improved with less “on the table” time

?Scatter Reduction

The use of a flattening filter contributes greatly to the variation of the source of head scatter in a beam. In particular, for FFF beams, the variation in in-air output factors for field sizes between 3x3 to 40x40 is around 3%, whereas the variation with conventional beams is more than 8% for 6MV beams.

Another key scattering reduction attribute with FFF is the out-of-field dose near the treatment field (<3cm from the field edge). This small effect may be beneficial in reducing certain risks in secondary induced cancers from primary radiation treatments.

Increased Dose Rate

Since the flattening filters are composed of medium to high-Z materials and can be a few centimetres thick at the central axis, they greatly reduce the dose rate of the machine.

Benefits of a Higher Dose Rate

FFF beams shorten the treatment time. For the patient, that means less time on the table and for the institution, a potential increase in throughput. There may also be radiobiological advantages as well.

Remove the Flattening Filter

?? - In conventional radiotherapy, the flattening filter is used to even out the beam intensity to make it uniform across the field.

?? - In FFF mode, the flattening filter is physically removed from the beam path, resulting in a non-uniform (forward-peaked) intensity profile, with a higher dose in the centre and a lower dose toward the periphery.

?Adjust the Linac Parameters

?? - The dose rate of the FFF beam is significantly higher than that of a flattened beam, allowing for faster delivery of radiation.

?? - Modern linacs are equipped with settings that allow clinicians to choose between FFF and flattened modes. The machine calibrates itself to deliver the dose accurately based on the mode selected.

?Beam Energy Selection

?? - FFF beams are typically available for high-energy photon beams, such as 6 MV and 10 MV photons. These energies are commonly used in stereotactic radiosurgery (SRS) or stereotactic body radiotherapy (SBRT), where high precision and dose rates are needed.

??Optimize Treatment Planning

?? - Since FFF beams have a non-flat intensity profile, treatment planning systems (TPS) must be adjusted to handle the beam distribution appropriately. This often involves advanced optimization algorithms like intensity-modulated radiation therapy (IMRT) or volumetric-modulated arc therapy (VMAT).

?? - Treatment planning with FFF beams can often lead to faster delivery times, reduced out-of-field dose, and potentially less normal tissue toxicity.

?Commissioning and Quality Assurance

?? - As with any change in beam configuration, FFF beams must be thoroughly tested and commissioned before clinical use. This includes:

???? - Measuring the beam profiles.

???? - Verifying dose rates and dose distribution in various scenarios.

???? - Performing regular quality assurance (QA) to ensure accurate and safe beam delivery.

Dosimetric advantages of FFF beams

·????? FFF has increased dose rate, e.g., 1400 MU/min for 6 MV, 2400 MU/min for 10 MV.

·????? Flattening filter-free (FFF) beams have less variation of off-axis beam hardening.

·????? FFF has less photon head scatter and thus less field size dependence.

·????? FFF has less leakage outside of beam collimation

?Potential advantages of FFF beams

·?Fast treatment for Stereotactic Radiotherapy (SRT) and SRT plans between FF and FFF beams should be similar for small fields.

·??FFF is especially useful for SBRT, where respiration-controlled treatment delivery is compromised by a large number of MU to deliver high fraction doses.

·??Patient beam on time can be reduced for IMRT treatment, including compensator-based IMRT

Conclusions

·?Advanced treatment techniques such as SRT and IMRT have stimulated interest in FFF beam, which provides a higher dose rate, and reduced head scatter, leaf transmission, and head radiation leakage.

·?A pencil-beam kernel model is necessary to perform secondary dose per MU calculations since the conventional method is no longer valid.

·Clinical utilities of FFF beam include a treatment time reduction for SRT, SBRT, and IMRT.

References:

1.???? 6S. Stathakis, C. Esquivel, A. Gutierrez, C. R. Buckey, and N. Papanikolaou, “Treatment planning and delivery of IMRT using 6 and 18 MV photon beams without flattening filter,” Appl. Radiat. Isot. 67, 1629–1637 (2009)

2.???? 7D. Opp, K. Forster,V. Feygelman, “Commissioning compensator-based IMRT on the Pinnacle treatment planning system”, J Appl Clin Med Phys 12, 310-325, (Spring 2011).

3.???? 8Bartrum T, Bailey M, Nelson V, Grace M. Linear attenuation coefficients for compensator-based IMRT. Australas Phys Eng Sci Med. 30(4):281–87 (2007).

All rights reserved ?radiotherapyin

?

?

??