The Department of Biomedicine

BBB Seminar: Dietmar Georg

Flattening filter free photon beams - a new standard for advanced photon beam therapy techniques?

Dietmar Georg
Division of Medical Radiation Physics, Department of Radiotherapy, Medical University of Vienna/AKH Vienna, Austria

Flattening filters (FFs) have been considered as an integral part of the treatment head of a medical accelerator for more than 50 years. The reasons for the longstanding use are, however, historical ones. Advanced treatment techniques, such as stereotactic radiotherapy or intensity modulated radiotherapy have stimulated the interest in operating linear accelerators in a flattening filter free (FFF) mode. The presentation will start with a review of treatment head physics of FFF beams, followed by a description of their dosimetric characteristics. In brief, the number of dosimetric benefits range from increased dose rate and dose per pulse to favorable output ratio in-air variation with field size, reduced energy variation across the beam, and reduced leakage and out-of-field dose, respectively. Finally, the softer photon spectrum of unflattened beams has implications on imaging strategies and radiation protection. These dosimetric characteristics of FFF beams have an effect on treatment delivery, patient comfort, dose calculation accuracy, beam matching, absorbed dose determination, treatment planning, machine specific quality assurance, imaging, and radiation protection. When considering conventional C-arm linacs in a FFF mode, preliminary results on treatment planning studies on SBRT (Stereotactic Body Radiation Therapy) and IMRT (Intensity Modulated Radiation Therapy) indicated the same or improved treatment plan quality. New treatment units are already on the market that operate without a FF or can be operated in a dedicated clinical FFF mode. Due to the convincing arguments of removing the FF, it is expected that more vendors will offer dedicated treatment units for advanced photon beam therapy in the near future. Several aspects related to standardization, dosimetry, treatment planning, optimization and radiobiology need to be addressed in more detail in order to facilitate the clinical implementation of unflattened beams.

Chair: Odd Harald Odland, Center for Nuclear Medicine/PET, Haukeland University Hospital