MO-A-BRB-02: Facts and Fiction of Flattening Filter Free (FF-FFF) X-Rays Beams

The primary purpose of the FFF X-rays is to provide much higher dose rates available for treatments. For example, FFF X-rays from Varian TrueBEAM can deliver 1400 MU/minute for 6 MV X-rays and 2400 MU/minutes for 10 MV X-rays. Higher dose rates have definite clinical benefits in organ motion management. For example, larger dose fractions can be delivered in a single breath-hold or gated portion of a breathing cycle. In SRS or SBRT treatments, large MUs are often required and FFF X-ray beams can deliver these large MUs in much shorter "beam-on" time. With shorten treatment time, these FFF X-rays improve patient comfort and dose delivery accuracy. FFF X-ray beams may become one of the necessary equipment configurations for SBRT and/or SRS treatments, in the future. This presentation will address some unique issues dealing with the FFF X-rays: (1) FIELD SIZES flattening Filter Free (FFF) X-ray beam has been in clinical use for quite some time. However, not until recently, these FFF beams are used in limited, small field sizes, for example, in Tomotherapy and CyberKnife machines.Varian TrueBEAM allows the FFF X-ray beam to have up to 40 × 40 cm field sizes for both 6 and 10 MV X-rays (15 MV FFF X-rays are not yet released for clinical use). For large treatment fields, the dose uniformity within an irradiated treatment field will need to be "modulated" by MLC movements (IMRT) to cut down the higher beam intensity near the central portion of the FFF X-ray beam. Thus, larger MUs are required compared with a conventional (flattened) X-ray beam. Or, MLC movements (EVIRT) are now being used to "flatten" the FFF X-rays to provide dose uniformity within those large PTVs. The high dose rates from the FFF X-rays are now being off-set by the larger MUs requirements. Therefore, FFF X-rays can bring clinical advantages over conventional X-rays when used with small field sizes, such as in SBRT and/or SRS applications. (2) DOSEVIETRY MEASUREMENT EQUIMENT: Because of the more than 2 to 4 fold increases in dose rate (MU/minute), the radiation measurement equipment and techniques need to be carefully evaluated, such as ionization chamber characteristics, electrometers, scanning equipment. First comes to mind is the ion-recombination characteristics of the ionization chamber (P-ion). This will determine the accuracies of the measured percentage depth doses and penumbra of these FFF X-ray fields. And, it will also affect the absolute dose measurements (Gy/MU) using the TG-51 formulations. The measured PDDs and profiles should be corrected for the P-ion effect. However, it is not a simple task for physicists to perform the P-ion corrections for PDDs and profiles using the presently available methods associated in commercial 3-D scanning equipment and algorithm. It may become necessary for physicists to adapt and get accustom to the use of "standard beam data" provided by manufacturers in the future. In addition, because of the use of FFF X-rays are focused on SBRT and/or SRS applications, beam data acquisition, scanning techniques, and beam modeling are vitally important. There are many publications addressing the "output factors" from small fields, but none pay enough attention to the penumbra characteristics of these small X-ray beams. Because of the proximity to critical organs, the penumbra characteristics of small fields are often more clinically important than output factors. FFF X-rays play an important role in SBRT and SRS applications. Therefore, careful penumbra measurements should be addressed. Again, it may become necessary for physicists to adapt the use of "standard beam data" provided by manufacturers. (3) RADIO-BIOLOGICAL QUESTION: Though there is a lack of controlled clinical studies with FFF X-ray beams, there are several scientific articles addressing the radiobiological concerns of high dose rate deliveries, especially when it is used to deliver large doses per fraction, such as 10 Gy/faction. This type of dose per fraction is often used in SBRT or SRS treatments. Radiobiological concerns are not in the cell kill effect within the target volume. It is the normal tissue damages surrounding the target. There are concerns about the late toxicities of these high dose rate and high dose per fraction deliveries using FFF X-ray beams. (4) SKIN (ENTRANCE) DOSE DISCUSSIONS: In conventional X-ray beams, the low energy components of the X-ray beam are removed by the in-line X-ray flattening filter. But, in Flatten Filter Free X-ray beams, these low energy components are exiting the X-ray collimators. This is clearly documented by the difference in the percentage depth doses for these FFF X-ray beams. The FFF X-ray beams have a lower "effective energy" compared to conventional X-rays. Therefore, it is important to study the skin (entrance) dose from these FFF X-rays. In the literature, reported skin (entrance) doses from different linear accelerator manufacturers vary widely. Skin doses from Varian TrueBEAM have been studied and have found to be marginally higher than the conventional X-rays. However this margin increase is not clinically significant.(5) Summary / conclusions / discussions: The FFF X-rays improve the treatment delivery by their very high dose rates (1400 and 2400 MU/minute) and shortened treatment time. FFF X-ray beams are most applicable and the high dose rates are most advantageous when the treatment field sizes are small. The dosimetry of FFF-X-rays is made more complex by the P-ion determination and necessary corrections to X-ray beam percentage depth doses and profiles. There are radiobiological concerns about late toxicity of normal tissue irradiated by FFF X-rays when large dose per faction treatment applications are used. There are wide ranges of skin doses from these FFF X-rays reported in the literature.

1. Flattening Filter Free X-rays have been in clinical use for many years but mostly for small field sizes. 2. Flattening Filter Free X-rays have significantly shorter treatment time if is used for small field applications, such as: SBRT or SRS. 3. Because of the high dose rate (MU/minute), dosimetric properties of these FFF X-rays need to be carefully studied. Ion-recombination of ionization chambers is a concern. Beam data acquisition, beam modeling, and absolute dosimetry need to be done with great care, especially in small field applications. 4. Late toxicities of normal tissue may be a concern and need to be studied by organized clinical protocols.