A comparison of phantom scatter from flattened and flattening filter free high-energy photon beams

Abstract

Flattening filter free (FFF) photon beams have different dosimetric properties from those of flattened beams. The aim of this work was to characterize the collimator scatter (S_c) and total scatter (S_cp) from 3 FFF beams of differing quality indices and use the resulting mathematical fits to generate phantom scatter (S_p) data. The similarities and differences between S_p of flattened and FFF beams are described. S_c and S_cp data were measured for 3 flattened and 3 FFF high-energy photon beams (Varian 6 and 10 MV and Elekta 6 MV). These data were fitted to logarithmic power law functions with 4 numerical coefficients. The agreement between our experimentally determined flattened beam S_p and published data was within ± 1.2% for all 3 beams investigated and all field sizes from 4 × 4 to 40 × 40 cm². For the FFF beams, S_p was only within 1% of the same flattened beam published data for field sizes between 6 × 6 and 14 × 14 cm². Outside this range, the differences were much greater, reaching − 3.2%, − 4.5%, and − 4.3% for the fields of 40 × 40 cm² for the Varian 6-MV, Varian 10-MV, and Elekta 6-MV FFF beams, respectively. The FFF beam S_p increased more slowly with increasing field size than that of the published and measured flattened beam of a similar reference field size quality index, i.e., there is less Phantom Scatter than that found with flattened beams for a given field size. This difference can be explained when the fluence profiles of the flattened and FFF beams are considered. The FFF beam has greatly reduced fluence off axis, especially as field size increases, compared with the flattened beam profile; hence, less scatter is generated in the phantom reaching the central axis.

Introduction

Megavoltage flattening filter free (FFF) treatment beams are typified by the forward-peaked nature of their fluence distributions and have recently become treatment options offered on modern linear accelerators. The requirement to have a flattened beam profile for treatment delivery is no longer paramount when intensity-modulated radiotherapy or intensity-modulated arc therapy is used. The patient dose distribution can instead be sculpted by the multileaf collimator (MLC) movement and dose rate control to create the desired clinical effect. The advantages of a FFF beam are reduced head scatter and head leakage and a more uniform photon energy spectrum away from the central axis. Further advantages of FFF beams are the intrinsically high dose rate at which treatment can be delivered, which can lead to reduced treatment times. This is especially important where large doses per fraction are prescribed, e.g., stereotactic ablative body radiotherapy or where intrafraction organ/patient motion might affect the efficacy of the delivery or both.

The FFF beam produced by an Elekta Agility linac is generated by replacing the conical flattening filter with a flat attenuating disk made of 2-mm thick stainless steel in the beam line below the level of the x-ray target. Varian units use a 1-mm thick steel plate. This filter serves a number of functions. Firstly, it prevents direct exposure of the patient to the electron beam if there should be a catastrophic target failure. Secondly, it generates a flux of secondary electrons at the level of the monitor ionization chambers, making the beam feedback mechanism sufficiently responsive to maintain energy and symmetry. Finally, it removes low-energy photons that only increase the superficial dose and not contribute to the target dose. The energy of a particular megavoltage photon beam is a matter of departmental choice and is usually influenced by previous treatment machines. If the same linac operating parameters, e.g., bending magnet current and microwave power, were used for a flattened and FFF beam, then the quality index of the 2 would be quite different because of the beam-hardening effect of the flattening filter. The 6-MV FFF beams on Elekta linacs are initially set up to best match the conventional flattened 6-MV beam percentage depth dose at 100-cm source-to-surface distance (SSD), with collimator setting of 10 × 10 cm². To obtain a FFF treatment beam with similar depth dose characteristics requires a higher initial beam energy provided by the bending magnet and magnetron power characteristics closer to that of an 8-MV beam. The philosophy of Varian is different in that the same beam generating parameters are used for flattened and FFF beams. The FFF beam is therefore much softer than its flattened counterpart. FFF beams in general are characterized by a forward-peaked intensity profile and a small variation in energy spectrum off axis.

Dosimetric data requirements for treatment planning systems and independent monitor unit check systems usually include variation of descriptors with field size. These may include the total scatter (S_cp) output factors that are defined as the ratio of dose for the field of interest to that of a reference field for the same delivered monitor units measured under full scatter conditions in a large water tank at the reference depth, e.g., 10-cm deep isocentrically. Collimator scatter (S_c) factors, also known as head scatter factors, account for the variation in beam output with field size from changes in direct and indirect radiation from the head of the linac.¹ It is recommended that they are determined in a miniphantom that has a smaller size than the smallest radiation field to be measured² but provides sufficient charged particle equilibrium from the side walls.

It has been noted by Sauer³ that the conventional notion of equivalent square field size for high-energy FFF photon beams is different from that of conventional flattened beams. Data for phantom scatter (S_p) in conventional flattened beams have been published in the literature⁴ as a function of both field equivalent square and beam quality index. There is a need for similar dosimetric reference data for FFF beams. If these reference data are to be generally applicable, then they should be measured for a standard dosimetry system such as that advocated by European SocieTy for Radiotherapy and Oncology (ESTRO)⁵ and more recently the American Association of Medical Physicists,⁶ e.g., isocentrically at 90-cm SSD and 10-cm deep.