Given the difficulty and potential time- or financial-costs associated with accurate small field dosimetry, this study aimed to establish the clinical necessity of obtaining accurate small field output factor measurements and to evaluate the effects on planned doses that could arise if accurate measurements are not used in treatment planning dose calculations. Isocentre doses, in heterogeneous patient anatomy, were calculated and compared for 571 beams from 48 clinical radiotherapy treatments, using a clinical radiotherapy treatment planning system, with reference to two different sets of beam configuration data. One set of beam configuration data included field output factors (total scatter factors) from precisely positioned and response-corrected diode measurements and the other included field output factors measured using a conventional technique that would have been better suited to larger field measurements. Differences between the field output factor measurements made with the two different techniques equated to 14.2 % for the 6 \(\times\) 6 mm\(^2\) field, 1.8 % for the 12 \(\times\) 12 mm\(^2\) field, and less than 0.5 % for the larger fields. This led to isocentre dose differences of up to 3.3 % in routine clinical fields smaller than 9 mm across and and up to 11 % in convoluted fields smaller than 15 mm across. If field widths smaller than 15 mm are used clinically, then accurate measurement (or-remeasurement) of small field output factors in the treatment planning system’s beam data is required in order to achieve dose calculation accuracy within 3 %. If such measurements are not completed, then errors in excess of 10 % may occur if very small, narrow, concave or convoluted treatment fields are used.
This is a preview of subscription content, access via your institution.
Buy single article
Instant access to the full article PDF.
Tax calculation will be finalised during checkout.
Subscribe to journal
Immediate online access to all issues from 2019. Subscription will auto renew annually.
Tax calculation will be finalised during checkout.
Das IJ, Ding GX, Ahnesjo A (2008) Small fields: nonequilibrium radiation dosimetry. Med Phys 35:206–215
Benedict SH, Yenice KM, Followill D et al (2010) Stereotactic body radiation therapy: the report of AAPM Task Group 101. Med Phys 37:4078–4101
Morales JE, Crowe SB, Hill R, Freeman N, Trapp JV (2014) Dosimetry of cone-defined stereotactic radiosurgery fields with a commercial synthetic diamond detector. Med Phys 41:111702
Li S, Rashid A, He S, Djajaputra D (2004) A new approach in dose measurement and error analysis for narrow photon beams: beamlets shaped by different multileaf collimators using a small detector. Med Phys 31:2020–2032
Zhu XR, Allen JJ, Shi J, Simon WE (2000) Total scatter factors and tissue maximum ratios for small radiosurgery fields: comparison of diode detectors, a parallel-plate ion chamber, and radiographic film. Med Phys 27:472–477
Ding GX, Duggan DM, Coffey CW (2006) Commissioning stereotactic radiosurgery beams using both experimental and theoretical methods. Phys Med Biol 51:2549–2566
McKerracher C, Thwaites DI (1999) Assessment of new small-field detectors against standard-field detectors for practical stereotactic beam data acquisition. Phys Med Biol 44:2143–2160
Dieterich S, Sherouse GW (2011) Experimental comparison of seven commercial dosimetry diodes for measurement of stereotactic radiosurgery cone factors. Med Phys 38:4166–4173
Laub WU, Wong T (2003) The volume effect of detectors in the dosimetry of small fields used in IMRT. Med Phys 30:341–347
Scott AJ, Kumar S, Nahum AE, Fenwick JD (2012) Characterizing the influence of detector density on dosimeter response in non-equilibrium small photon fields. Phys Med Biol 57:4461–4476
Francescon P, Cora S, Cavedon C (2008) Total scatter factors of small beams: a multidetector and Monte Carlo study. Med Phys 35:504–513
Cranmer-Sargison G, Weston S, Evans JA, Sidhu NP, Thwaites DI (2012) Monte Carlo modelling of diode detectors for small field MV photon dosimetry: detector model simplification and the sensitivity of correction factors to source parameterization. Phys Med Biol 57:5141–5153
Cranmer-Sargison G, Weston S, Evans JA, Sidhu NP, Thwaites DI (2011) Implementing a newly proposed Monte Carlo based small field dosimetry formalism for a comprehensive set of diode detectors. Med Phys 38:6592–6602
Charles PH, Crowe SB, Kairn T et al (2013) Monte Carlo-based diode design for correction-less small field dosimetry. Phys Med Biol 58:4501–4512
Charles PH, Cranmer-Sargison G, Thwaites DI et al (2014) Design and experimental testing of air slab caps which convert commercial electron diodes into dual purpose, correction-free diodes for small field dosimetry. Med Phys 41:101701
Ralston A, Liu P, Warrener K, McKenzie D, Suchowerska N (2012) Small field diode correction factors derived using an air core fibre optic scintillation dosimeter and EBT2 film. Phys Med Biol 57:2587–2602
Kairn T, Crowe SB, Kenny J, Trapp JV (2011) Investigation of stereotactic radiotherapy dose using dosimetry film and Monte Carlo simulations. Radiat Meas 46:1985–1988
Pappas E, Maris TG, Zacharopoulou F et al (2008) Small SRS photon field profile dosimetry performed using a PinPoint air ion chamber, a diamond detector, a novel silicon-diode array (DOSI), and polymer gel dosimetry. Analysis and intercomparison. Med Phys 35:4640–4848
Kairn T, Taylor ML, Crowe SB et al (2012) Monte Carlo verification of gel dosimetry measurements for stereotactic radiotherapy. Phys Med Biol 57:3359–3369
Belec J, Patrocinio H, Verhaegen F (2005) Development of a Monte Carlo model for the Brainlab microMLC. Phys Med Biol 50:787–799
Kairn T, Kenny J, Crowe SB et al (2010) Technical note: Modelling a complex micro-multileaf collimator using the standard BEAMnrc distribution. Med Phys 37:1761–1767
Kairn T, Charles PH, Crowe SB, Langton CM, Trapp JV (2015) Clinical use of diodes and micro-chambers to obtain accurate small field output factor measurements. Australas Phys Eng Sci Med 38:357–367
Kawrakow I (2005) egspp: the EGSnrc C++ class library, NRCC Report PIRS-899, National Research Council of Canada
Alfonso R, Andreo P, Capote R, Huq MS, Kilby W, Kjall P, Mackie TR, Palmans H, Rosser K, Seuntjens J, Ullrich W, Vatnitsky S (2008) A new formalism for reference dosimetry of small and nonstandard fields. Med Phys 35(11):5179–5186
Standard Imaging (2013) Exradin Detectors (product brochure), accessible from http://www.standardimaging.com/exradin/micro-ion-chambers/
Crowe SB, Kairn T, Middlebrook N et al (2013) Retrospective evaluation of dosimetric quality for prostate carcinomas treated with 3D conformal, intensity-modulated and volumetric-modulated arc radiotherapy. J Med Radiat Sci 60:131–138
Kairn T, Crowe SB, Kenny J, Knight RT, Trapp JV (2014) Predicting the likelihood of QA failure using treatment plan accuracy metrics. J Phys Conf Ser 489:012051
Crowe SB, Kairn T, Kenny J et al (2014) Treatment plan complexity metrics for predicting IMRT pre-treatment quality assurance results. Australas Phys Eng Sci Med 37:475–482
Ezzell GA, Burmeister JW, Dogan N et al (2009) IMRT commissioning: multiple institution planning and dosimetry comparisons, a report from AAPM Task Group 119. Med Phys 36(11):5359–5373
Kairn T, Charles P, Crowe SB, Trapp JV (2014) Effects of small field output factors on IMRT optimisation and dose calculation. Australas Phys Eng Sci Med 37(1):214–215
Charles PH, Cranmer-Sargison G, Thwaites DI et al (2014) A practical and theoretical definition of very small field size for radiotherapy output factor measurements. Med Phys 41:041707
Experimental measurements used in this work were obtained with assistance from Greg Pedrazzini, Richard Knight, George Warr and Trent Aland, based on advice provided by Gavin Cranmer-Sargison. This work was supported by the Australian Research Council, the Wesley Research Institute, Premion (Genesis Cancer Care Queensland) and the Queensland University of Technology (QUT), through linkage Grant no. LP110100401.
Preliminary aspects of this study, relating to the evaluation of ‘worst-case-scenario’ treatments, were originally presented at the Engineering and Physical Sciences in Medicine (EPSM) conference, Perth, Australia, 2012.
About this article
Cite this article
Kairn, T., Charles, P., Crowe, S.B. et al. Effects of inaccurate small field dose measurements on calculated treatment doses. Australas Phys Eng Sci Med 39, 747–753 (2016). https://doi.org/10.1007/s13246-016-0461-y
- Stereotactic radiosurgery
- Treatment planning
- Radiation therapy