Flattening filter free beams in SBRT and IMRT: dosimetric assessment of peripheral doses

Abstract

Purpose: Recently, there has been a growing interest in operating medical linear accelerators without a flattening filter. Due to reduced scatter, leaf transmission and radiation head leakage a reduction of out-of-field dose is expected for flattening filter free beams. The aim of the present study was to determine the impact of unflattened beams on peripheral dose for advanced treatment techniques with a large number of MUs.

Material and methods: An Elekta Precise linac was modified to provide 6 and 10 MV photon beams without a flattening filter. Basic beam data were collected and implemented into the TPS Oncentra Masterplan (Nucletron). Leakage radiation, which predominantly contributes to peripheral dose at larger distances from the field edge, was measured using a Farmer type ionisation chamber. SBRT (lung) and IMRT (prostate, head&neck) treatment plans were generated for 6 and 10 MV for both flattened and unflattened beams. All treatment plans were delivered to the relevant anatomic region of an anthropomorphic phantom which was extended by a solid water slab phantom. Dosimetric measurements were performed with TLD-700 rods, radiochromic films and a Farmer type ionisation chamber. The detectors were placed within the slab phantom and positioned along the isocentric longitudinal axis.

Results: Using unflattened beams results in a reduction of treatment head leakage by 52% for 6 and 65% for 10 MV. Thus, peripheral doses were in general smaller for treatment plans calculated with unflattened beams. At about 20 cm distance from the field edge the dose was on average reduced by 23 and 31% for the 6 and 10 MV SBRT plans. For the IMRT plans (10 MV) the average reduction was 16% for the prostate and 18% for the head&neck case, respectively. For all examined cases, the relative deviation between peripheral doses of flattened and unflattened beams was found to increase with increasing distance from the field.

Conclusions: Removing the flattening filter lead to reduced peripheral doses for advanced treatment techniques. The relative difference between peripheral doses of flattened and unflattened beams was more pronounced when the nominal beam energy was increased. Patients may benefit by decreased exposure of normal tissue to scattered dose outside the field.