Out-of-field photon dose following removal of the flattening filter from a medical accelerator

Abstract

The aim of this paper is to determine the effect of removing the flattening filter from a linear accelerator on the out-of-field photon dose. A Monte Carlo model was used to simulate 6 MV and 18 MV photon beams from a Varian 2100 accelerator with the flattening filter in place and with it removed. The out-of-field photon doses and composition (head leakage, patient scatter and collimator scatter) were calculated from square open fields in a water tank as a function of distance from central axis, field size and depth. The out-of-field doses resulting from intensity-modulated radiation therapy to the prostate at 6 MV were also calculated, with and without the flattening filter, to sensitive organs in an anthropomorphic Rando phantom. Removal of the flattening filter reduced the out-of-field dose near the treatment field (<3 cm from the field edge) because of decreased collimator scatter. It increased the out-of-field dose at intermediate distances from the field edge (3-15 cm) because of increased patient scatter. At greater distances, the out-of-field dose was decreased because of reduced head leakage. For the clinical treatment examined, the out-of-field dose was generally reduced following removal of the flattening filter, particularly at large distances from the treatment field. Removal of the flattening filter may be advantageous by reducing the out-of-field dose to the patient. This could contribute to reducing the long-term risk of secondary malignancies. In general, however, the out-of-field dose depends on treatment and patient parameters, and a reduction may not always be achievable.

Accuracy and sources of error of out‐of field dose calculations by a commercial treatment planning system for intensity‐modulated radiation therapy treatments

“…In order to gain a better understanding of the source of TPS calculation inaccuracies, the accuracy of each individual component of peripheral dose was evaluated, paralleling the methodology described by Kase et al ( 1 ) and Kry et al ( 20 ) The total dose outside of the treatment field T was thereby decomposed into leakage radiation L , scatter from collimators and other beam line components C , and patient scatter P : $\mathrm{T}=\mathrm{L}+\mathrm{C}+\mathrm{P}$ …”

“…These results from Pinnacle were then compared to previously published Monte Carlo results simulated with a detailed model of a Varian 2100 Clinac, the same experimental setup conditions, and a parallel computational approach. ( 20 ) …”

Volumetric‐modulated arc therapy and intensity‐modulated radiation therapy treatment planning for prostate cancer with flattened beam and flattening filter free linear accelerators

“…The desktop software, the hardware of the linac, and the TPS influence the treatment time. Kry et al 15. showed that there is also a dependency of the PD on the field size and the amount of modulation.…”

Ion recombination corrections of ionization chambers in flattening filter‐free photon radiation

“…Without the flattening filter in the X‐ray beam path, the radiation output near the central axis and the dose rate at the treatment target have increased significantly, (2) which is especially beneficial to facilitate motion management during stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) 3 , 4 . In addition, the FFF photons provide dosimetric advantages, such as lower head scatter and lower out‐of‐field radiation 5 , 6 . For very high photon energies, it has been proposed that fewer neutrons can be produced with the FFF beams and thus unwanted exposure is reduced (7) .…”