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SU‐E‐T‐113: Measurement of the Cutout Output Factor on Ortho‐Voltage Unit Using EBT‐2 Model GafchromicTM Film
Author(s) -
Liang L,
Tomic N,
Devic S,
Seuntjens J,
Deblois F
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3612064
Subject(s) - kerma , dosimetry , materials science , ionization chamber , calibration , voltage , backscatter (email) , nuclear medicine , optics , percentage depth dose curve , scanner , physics , ion , computer science , medicine , telecommunications , quantum mechanics , wireless , ionization
Purpose: In this study, we evaluate feasibility of using EBT‐2 model GafchromicTM film to measure patient specific cutout output factors on an ortho‐voltage treatment machine. Methods: A Gulmay Orthovoltage D3225 unit was used to produce clinical beams in kV range from 50 kVp to 220 kVp. Previously established air kerma based radiochromic film dosimetry protocol for low energy x‐ray beams was used. A closed‐end applicator with 10 × 10 cm2 field size and SSD 50 cm was employed. Cutouts were made from a lead sheet of 1.8 mm in thickness. A 6‐cm thick slab of Solid WaterTM was used as a backscatter material for output factor measurement. In‐house made MatLab routines were used to analyze scanned images of exposed and unexposed films and a PTW 23342 parallel‐plate ion chamber (PP IC) was used to measure the cutout output factors for comparison. Results: Air‐Kerma based calibration curve was created in the 1–300 cGy range. Film dosimetic accuracy was verified for known doses to water of 1.00 and 2.00 Gy, respectively, with maximum observed difference of 0.4%. Two‐dimensional dose maps and dose profiles were obtained for various cutout sizes ranging from 0.8 cm diameter circle to 10 × 10 cm2 square. Comparisons of output factors between film and pp ion chamber measurements show that there are no differences for large cutouts but the chamber measurements are up to 11 % higher for small cutout size. Conclusions: We demonstrated that the air‐Kerma based radiochromic film dosimetry protocol using EBT‐2 model GafChromicTM film for low energy x‐ray beam is adequate for patient specific cutout output measurements on orthovoltage units, especially for small size cutout measurements, where IC measurements give higher output factor than films due to ICˈs over responses to electron contaminations from lead sheets.