Surface dose variations in 6 and 10 MV flattened and flattening filter-free (FFF) photon beams.
As the use of linear accelerators operating in flattening filter-free (FFF) modes becomes more widespread, it is important to have an understanding of the surface doses delivered to patients with these beams. Flattening filter removal alters the beam quality and relative contributions of low-energy X-rays and contamination electrons in the beam. Having dosimetric data to describe the surface dose and buildup regions under a range of conditions for FFF beams is important if clinical decisions are to be made. An Elekta Synergy linac with standard MLCi head has been commissioned to run at 6 MV and 10 MV running with the flattening filter in or out. In this linac the 6 MV FFF beam has been energy-matched to the clinical beam on the central axis (D10). The 10 MV beam energy has not been adjusted. The flattening filter in both cases is replaced by a thin (2 mm) stainless steel plate. A thin window parallel plate chamber has been used to measure a comprehensive set of surface dose data in these beams for variations in field size and SSD, and for the presence of attenuators (wedge, shadow tray, and treatment couch). Surface doses are generally higher in FFF beams for small field sizes and lower for large field sizes with a crossover at 10 × 10 cm2 at 6 MV and 25 × 25 cm2 at 10 MV. This trend is also seen in the presence of the wedge, shadow tray, and treatment couch. Only small differences (< 0.5%) are seen between the beams on varying SSD. At both 6 and 10 MV the filter-free beams show far less variation with field size than conventional beams. By removing the flattening filter, a source of contamination electrons is exchanged for a source of low-energy photons (as these are no longer attenuated). In practice these two components almost balance out. No significant effects on surface dose are expected by the introduction of FFF delivery.
University Hospital Birmingham NHS Foundation Trust. jason.cashmore@uhb.nhs.uk.
Jason
J
Cashmore
D004583:Electrons; D005374:Filtration / Q000295:instrumentation*; D006801:Humans; D010315:Particle Accelerators / Q000295:instrumentation*; D017785:Photons; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation*; D012542:Scattering, Radiation
J Appl Clin Med Phys. 2012 Sep 06;13(5):3758
22955642
Radiother Oncol. 1986 Mar;5(3):235-44
3085172
Radiother Oncol. 2007 Sep;84 Suppl 1:S1-304
18183644
Appl Radiat Isot. 2008 Oct;66(10):1438-42
18434173
Med Phys. 2005 May;32(5):1281-92
15984680
J Appl Clin Med Phys. 2006 Aug 24;7(3):21-7
17533341
Med Phys. 1975 Jan-Feb;2(1):14-9
805358
Med Phys. 1988 Mar-Apr;15(2):246-9
3386598
Med Phys. 2010 Jan;37(1):322-8
20175495
Med Phys. 2010 Jul;37(7):3595-606
20831067
Med Phys. 2008 Mar;35(3):950-8
18404931
Med Phys. 2005 Jan;32(1):286-98
15719980
Med Phys. 1990 Jan-Feb;17(1):17-26
2106611
Acta Radiol Oncol. 1981;20(6):401-15
6278863
Phys Med Biol. 2008 Apr 7;53(7):1933-46
18364548
J Appl Clin Med Phys. 2003 Winter;4(1):1-7
12540813
J Appl Clin Med Phys. 2015 Nov 08;16(6):376–385
26699592
Med Phys. 1987 Mar-Apr;14(2):210-5
3587140
Med Phys. 1998 Jun;25(6):860-6
9650173
Med Phys. 1992 May-Jun;19(3):641-8
1508103
Radiat Med. 2008 Nov;26(9):539-44
19030962
Phys Med Biol. 2011 Jul 7;56(13):3809-24
21628779
Phys Med Biol. 2006 Apr 7;51(7):1907-17
16552113
J Appl Clin Med Phys. 2012 May 10;13(3):3734
22584171
Radiother Oncol. 2000 May;55(2):191-8
10799732
Med Phys. 1986 Mar-Apr;13(2):191-5
3702815
Int J Radiat Oncol Biol Phys. 1994 Aug 30;30(1):211-9
8083116
Phys Med Biol. 1998 May;43(5):1133-46
9623645
Med Phys. 2010 Jan;37(1):49-53
20175465
Med Phys. 2005 Feb;32(2):483-7
15789595
Int J Radiat Oncol Biol Phys. 2012 Jun 1;83(2):e281-5
22414287
Med Phys. 1990 Nov-Dec;17(6):1041-4
2280733
Radiother Oncol. 2009 Oct;93(1):141-6
19592123