title: Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators.
abstract: Flattening filter-free (FFF) beams are becoming the preferred beam type for stereotactic radiosurgery (SRS) and stereotactic ablative radiation therapy (SABR), as they enable an increase in dose rate and a decrease in treatment time. This work assesses the effects of the flattening filter on small field output factors for 6 MV beams generated by both Elekta and Varian linear accelerators, and determines differences between detector response in flattened (FF) and FFF beams. Relative output factors were measured with a range of detectors (diodes, ionization cham-bers, radiochromic film, and microDiamond) and referenced to the relative output factors measured with an air core fiber optic dosimeter (FOD), a scintillation dosimeter developed at Chris O'Brien Lifehouse, Sydney. Small field correction factors were generated for both FF and FFF beams. Diode measured detector response was compared with a recently published mathematical relation to predict diode response corrections in small fields. The effect of flattening filter removal on detector response was quantified using a ratio of relative detector responses in FFF and FF fields for the same field size. The removal of the flattening filter was found to have a small but measurable effect on ionization chamber response with maximum deviations of less than ± 0.9% across all field sizes measured. Solid-state detectors showed an increased dependence on the flattening filter of up to ± 1.6%. Measured diode response was within ± 1.1% of the published mathematical relation for all fields up to 30 mm, independent of linac type and presence or absence of a flattening filter. For 6 MV beams, detector correction factors between FFF and FF beams are interchangeable for a linac between FF and FFF modes, providing that an additional uncertainty of up to ± 1.6% is accepted.
authors.affiliation: Shoalhaven Cancer Care Centre; Prince of Wales Hospital. madelaine.tyler@sesiahs.health.nsw.gov.au.
authors.forename: Madelaine K
authors.initials: MK
authors.lastname: Tyler
authors.forename: Paul Z Y
authors.initials: PZ
authors.lastname: Liu
authors.forename: Christopher
authors.initials: C
authors.lastname: Lee
authors.forename: David R
authors.initials: DR
authors.lastname: McKenzie
authors.forename: Natalka
authors.initials: N
authors.lastname: Suchowerska
mesh_terms: D005374:Filtration / Q000295:instrumentation*; D006801:Humans; D010315:Particle Accelerators / Q000295:instrumentation*; D019047:Phantoms, Imaging; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation* / Q000379:methods