Introduction: Small fields photon dosimetry is associated with many problems. Using the right detector for measurement plays a fundamental role. This study investigated the measurement of relative output for small photon fields with different detectors. It was investigated for three-photon beam energies at SSDs of 90, 95, 100 and 110 cm. As a benchmark, the Monte Carlo simulation was done to calculate the relative output of these small photon beams for the dose in water.
Materials and Methods: 6, 10 and 15 MV beams were delivered from a Synergy LINAC equipped with an Agility 160 multileaf collimator (MLC). A CC01 ion chamber, EFD-3G diode, PTW60019 microdiamond, EBT2 radiochromic film, and EDR2 radiographic film were used to measure the relative output of the linac. Measurements were taken in water for the CC01 ion chamber, EFD-3G diode, and the PTW60019. Films were measured in water equivalent RW3 phantom slabs. Measurements were made for 1 × 1, 2 × 2, 3 × 3, 4 × 4, 5 × 5 and a reference field of 10 × 10 cm2. Field sizes were defined at 100cm SSD. Relative output factors were also compared with Monte Carlo (MC) simulation of the LINAC and a water phantom model. The influence of voxel size was also investigated for relative output measurement. Results and Discussion: The relative output factor (ROF) increased with energy for all fields large enough to have lateral electronic equilibrium (LEE). This relation broke down as the field sizes decreased due to the onset of lateral electronic disequilibrium (LED). The high-density detector, PTW60019 gave the highest ROF for the different energies, with the less dense CC01 giving the lowest ROFs.
Conclusion: These are results compared to MC simulation, higher density detectors give higher ROF values. Relative to water, the ROF measured with the air-chamber remained virtually unchanged. The ROFs, as measured in this study showed little variation due to increased SSDs. The effect of voxel size for the Monte Carlo calculations in water does not lead to significant ROF variation over the small fields studied.
If the inline PDF is not rendering correctly, you can download the PDF file here.
 Benedict SH, Yenice KM, Followill D, et al. Stereotactic body radiation therapy: The report of AAPM Task Group 101. Med Phys. 2010;37(8):4078-4101.
 Godwin GA, Mugabe K. Characterization of a dynamic multi-leaf collimator for stereotactic radiotherapy applications. Phys Med Biol. 2012;57(14):4643-4654.
 Russo S, Reggiori G, Cagni E, et al. Small field output factors evaluation with a microDiamond detector over 30 Italian centers. Phys Med. 2016;32(12):1644-1650.
 Das IJ, Ding GX, Ahnesjo A. Small fields: Nonequilibrium radiation dosimetry. Med Phys. 2008;35(1): 206-215.
 Cranmer-Sargison G, Weston S, Sidhu NP, Thwaites DI. Experimental small field 6 MV output ratio analysis for various diode detector and accelerator combinations. Radiother Oncol. 2011;100(3):429-435.
 Gagnon JC, Theriault D, Guillot M, et al. Dosimetric performance and array assessment of plastic scintillation detectors for stereotactic radiosurgery quality assurance. Med Phys. 2012;57(14):429-436.
 Scott AJ, Nahum AE, Fenwick JD. Monte Carlo modeling of small photon fields: Quantifying the impact of focal spot size on source occlusion and output factors, and exploring miniphantom design for small-field measurements. Med Phys. 2009;26(7):3132-3144.
 Herrup D, Chu J, Cheung H, Pankuch M. Determination of penumbral widths from ion chamber measurements. Med Phys. 2005;32(12):3636-3640.
 Nasir MKR, Amjad N, Razzaq A, Siddique MT. Measurement and Analysis of PDDs Profile and Output Factors for Small Field Sizes by cc13 and Micro-Chamber cc01. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology. 2017;6(1):36-56.
 Underwood TS, Winter HC, Hill MA, Fenwick JD. Detector density and small field dosimetry: Integral versus point dose measurement schemes. Med Phys. 2013;40(8):1-16.
 Sauer OA, Wilbert J. Measurement of output factors for small photon beams. Med Phys. 2007;34(6): 1983-1988.
 Fox C, Simon T, Simon B, et al. Assessment of the setup dependence of detector response functions for mega-voltage linear accelerators. Med Phys. 2010;37(2):477-484.
 Low D, Morgan J, Dempsey J, et al. Dosimetry tools and techniques for IMRT. Med Phys. 2011;38(3):1313-1338.
 Tyler MK, Liu PZ, Lee C, et al. Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators. J Appl Clin Med Phys. 2016;17(3):223-235.
 Pai S, Das IJ, Dempsey JF, et al. TG-69: Radiographic film for megavoltage beam dosimetry. Med Phys. 2007;34(6):2228-2258.
 Andres C, Castilo A, Tortosa R, et al. A comprehensive study of the Gafchromic EBT2 radiochromic film. A comparison with EBT. Med Phys. 2010;37(12):6271-6278.
 Mayles P, Nahum A, Rosenwald J. Handbook of Radiotherapy Physics: Theory and Practice. Taylor and Francis group, 2007.
 ISP. Gafchromic EBT2 Self-developing film for radiotherapy dosimetry. 1361 Alps Road Wayne, New Jersey, USA, 2009.
 Aland T, Kairn T, Kenny J. Evaluation of a Gafchromic EBT2 film dosimetry system for radiotherapy quality assurance. Australas Phys Eng Sci Med. 2011;34(2):251-260.
 Chetty IJ, Charland PM. Investigation of Kodak Extended Dose Range (EDR) Film for Megavoltage Photon Beam Dosimetry. Phys Med Biol. 2002;47(20):3629-3641.
 Shi C, Papanikolaou N, Yan Y, et al. Analysis of the Sources of Uncertainty for EDR2 Film-Based IMRT Quality Assurance. J App Clin Med Phys. 2006;37(2):1-8.
 Lewis D, Micke A, Yu X, Chan MF. An Efficient Protocol for Radiochromic Film Dosimetry Combining Calibration and Measurement in a Single Scan. Med Phys. 2012;39(10):6339-6350.
 Alfonso R, Andreo P, Capote R, et al. A New Formalism for Reference Dosimetry of Small and Nonstandard Fields. Med Phys. 2008;35(11):5179-5186.
 Hu Y, Wang Y, Fogarty G, Liu G. Developing a Novel Method to Analyse Gafchromic EBT2 Films in Intensity Modulated Radiation Therapy Quality Assurance. Australas Phys Eng Sci Med. 2013;36(4):487-494.
 Mendez I, Peterlin R, Hudej R, et al. On Multichannel Film Dosimetry with Channel-Independent Perturbations. Med Phys. 2014;41(1):11705.
 Micke A, Lewis DF, Yu X. Multichannel Film Dosimetry with Nonuniformity Correction. Med Phys. 2011;38(5):2523-2534.
 Kawrakow I, Mainegra-Hing E, Rogers DWO, et al. The EGSnrc code system: Monte Carlo simulation of electron and photon transport,” NRCC PIRS-701, 2013.
 Rogers DWO. Fifty years of Monte Carlo simulations for medical physics. Phys Med Biol. 2006l;51(13):R287–R301.
 Oderinde OM, du Plessis FCP. Technical note: A new wedge-shaped ionization chamber component module for BEAMnrc to model the integral quality monitoring system®. Radiat Phys Chem. 2017;141:346-351.
 Ulmer W, Kaissl W. The inverse problem of a Gaussian convolution and its application to the finite size of the measurement chambers/detectors in photon and proton dosimetry. Phys Med Biol. 2003;48(6):707-727.
 García-Vicente F, Delgado JM, Rodríguez C. Exact analytical solution of the convolution integral equation for a general profile fitting function and Gaussian detector kernel. Phys Med Biol. 2000;45(3):645-650.
 García-Vicente F, Delgado JM, Peraza C. Experimental determination of the convolution kernel for the study of the spatial response of a detector. Med Phys. 1998;25(2):202-207.
 Scott AJ, Kumar S, Nahum AE, Fenwick JD. Characterizing the influence of detector density on dosimeter response in non-equilibrium small photon fields. Phys Med Biol. 2012;57(14):4461-4476.
 Laub WU, Wong T. The volume effect of detectors in the dosimetry of small fields used in IMRT. Med Phys. 2003;30(3):341-347.