title: Equivalent square formula for determining the surface dose of rectangular field from 6 MV therapeutic photon beam.
abstract: The purpose of the study was to investigate the use of the equivalent square formula for determining the surface dose from a rectangular photon beam. A 6 MV therapeutic photon beam delivered from a Varian Clinac 23EX medical linear accelerator was modeled using the EGS4nrc Monte Carlo simulation package. It was then used to calculate the dose in the build-up region from both square and rectangular fields. The field patterns were defined by various settings of the X- and Y-collimator jaw ranging from 5 to 20 cm. Dose measurements were performed using a thermoluminescence dosimeter and a Markus parallel-plate ionization chamber on the four square fields (5 × 5, 10 × 10, 15 × 15, and 20 × 20 cm2). The surface dose was acquired by extrapolating the build-up doses to the surface. An equivalent square for a rectangular field was determined using the area-to-perimeter formula, and the surface dose of the equivalent square was estimated using the square-field data. The surface dose of square field increased linearly from approximately 10% to 28% as the side of the square field increased from 5 to 20 cm. The influence of collimator exchange on the surface dose was found to be not significant. The difference in the percentage surface dose of the rectangular field compared to that of the relevant equivalent square was insignificant and can be clinically neglected. The use of the area-to-perimeter formula for an equivalent square field can provide a clinically acceptable surface dose estimation for a rectangular field from a 6 MV therapy photon beam.
authors.affiliation: Chulalongkorn University. none@none.com.
authors.forename: Lukkana
authors.initials: L
authors.lastname: Apipunyasopon
authors.forename: Somyot
authors.initials: S
authors.lastname: Srisatit
authors.forename: Nakorn
authors.initials: N
authors.lastname: Phaisangittisakul
mesh_terms: D003198:Computer Simulation; D006801:Humans; D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons / Q000627:therapeutic use*; D011874:Radiometry; D011879:Radiotherapy Dosage; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods*
references.citation: Br J Radiol Suppl. 1983;17:105-14
references.pmid: 6600115
references.citation: Med Phys. 2006 Apr;33(4):1116-24
references.pmid: 16696489
references.citation: Appl Radiat Isot. 2008 Oct;66(10):1438-42
references.pmid: 18434173
references.citation: Radiology. 1975 May;115(2):473-4
references.pmid: 806935
references.citation: Med Dosim. 1996 Fall;21(3):145-8
references.pmid: 8899678
references.citation: Med Phys. 2011 May;38(5):2619-28
references.pmid: 21776799
references.citation: Phys Med Biol. 2012 Jul 21;57(14):N267-78
references.pmid: 22750849
references.citation: Phys Med Biol. 2003 Nov 7;48(21):R107-64
references.pmid: 14653555
references.citation: Med Phys. 1983 Nov-Dec;10(6):856-61
references.pmid: 6419032
references.citation: Br J Radiol. 1964 Jul;37:544-50
references.pmid: 14196797
references.citation: Med Phys. 1997 Nov;24(11):1770-4
references.pmid: 9394284
references.citation: Jpn J Radiol. 2010 Feb;28(2):110-6
references.pmid: 20182845
references.citation: Phys Med Biol. 2004 Jul 7;49(13):N197-201
references.pmid: 15285265
references.citation: Med Phys. 1995 May;22(5):503-24
references.pmid: 7643786
references.citation: Phys Med Biol. 2006 Jul 21;51(14):3549-67
references.pmid: 16825748