An experimental method to calculate the on-axis dose in small field for stereotactic radiotherapy

Talal Abdul-Hadi, Rezart Belshi, Gauthier Bouilhol, Abdulhamid Chaikh, Pascal François


Purpose: The use of small fields in advanced radiotherapy techniques has increased, in particular in stereotactic treatments. However, measuring on-axis dose in such fields is challenging. In this study, we developed an analytic model to accurately estimate the on-axis dose in small fields.

Methods: Our study was carried out using 6 MV photon beams from four linear accelerators and with three dosimeters placed in a water tank: EBT3 Gafchromic films, a 31016 PinPoint ionization chamber and a 60017 E diode. The out-of-field leakage factor defined as the ratio of the central axis dose to the off-axis dose was modeled. On-axis doses estimated from out-of-field measurements were compared with the measured ones.

Results: The experimental validation of the present method was performed for square and rectangular fields with sizes ranging from 0.5 ⨯ 0.5 cm2 to 10 ⨯ 10 cm2. We found the leakage factor exhibits an exponential decrease independent of the accelerator. This behavior can be integrated in the model to estimate the on-axis dose with an agreement better than 2% compared to EBT3 film measurements at a 10 cm depth and an 8 cm cross-plane off-axis distance.

Conclusion: We have developed an analytic model to estimate the on-axis dose in small fields based on the out-of-field leakage measurement. This model can be used to validate dose and output factor measurements.


Stereotactic radiotherapy, Small field, Dose measurement, Leakage factor

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Hassani H, Nedaie HA, Zahmatkesh MH, et al. A dosimetric study of small photon fields using polymer gel and Gafchromic EBT films. Med Dosim. 2014;39(1):102-7.

Karl O. Volumetric modulated arc therapy: IMRT in a single gantry arc. Med Phys. 2008;35:310.

Carmen CP, Ivo AO, WayneB, et al. Volumetric Modulated Arc Therapy Improves Dosimetry and Reduces Treatment Time Compared to Conventional Intensity-Modulated Radiotherapy for Locoregional Radiotherapy of Left-Sided Breast Cancer and Internal Mammary Nodes. J Radiat Oncol Biol Phys. 2010;76:287-95.

Zhu, T. C. Small Field: dosimetry in electron disequilibrium region. J Phys Conf. 2010;Ser.250012056.

Das IJ, Ding GX, Ahnesjö A. Small fields: non equilibrium radiation dosimetry. Med Phys. 2008;35(1):206-15.

Bouchard H, Seuntjens J, Duane S, et al. Detector dose response in megavoltage small photon beams. Med Phys. 2015;42:6033.

Bouchard H, Kamio Y, Palmans H, et al. Detector dose response in megavoltage small photon beams. II. Pencil beam perturbation effects. Med Phys. 2015;42:6048.

Wuerfel JU. Dose measurements in small fields. Medical Physics International. 2013;8130:04.

Wang L, Leszczynski L. Estimation of the focal spot size and shape for a medical linear accelerator by Monte Carlo simulation. Med Phys. 2007;34:85.

Edwin S, Jan S, Slobodan D, et al. Influence of focal spot on characteristics of very small diameter radiosurgical beams. Med Phys. 2008;35:3317.

Jang SY, Liu HH, Mohan R, et al. Variations in energy spectra and water-to-material stopping-power ratios in three-dimensional conformal and intensity-modulated photon fields. Med Phys. 2007;34(4):1388-97.

Dawson DJ, Schroeder NJ, Hoya JD. Penumbral measurements in water for high-energy x rays. Med Phys. 1986;13:101-4.

Wolfram U, Wong T. The volume effect of detectors in the dosimetry of small fields used in IMRT. Med Phys. 2003;30:341.

Le Roy M, De Carlan L, Delaunay F, et al. Assessment of small volume ionization chambers as reference dosimeters in high-energy photon beams. Med Phys. 2011;56(17):5637-50.

Andreo P, Burns D, Hohlfeld K, et al. Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water. IAEA Technical Report Series No 398 (Vienna: International Atomic Energy Agency). 2006.

Almond PR, Biggs PJ, Coursey BM, et al. AAPM TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. Med Phys.1999;26(9):1847-70.

Alfonso R, Andreo P, Capote R, et al. A new formalism for reference dosimetry of small and nonstandard fields. Med Phys. 2008;35(11):5179-86.

IRSN report. Mise au point d’un protocole dosimetrique pour la determination des FOC dans les mini-faisceaux utilises en radiotherapie, Report No. PRP-HOM/SDE n°2013-010. 2013.

Derreumaux S, Boisserie G, Brunet G, et al. Mesure de la dose absorbée dans les faisceaux de photons de tres petites dimensions utilises en radiotherapie stereotaxique. IRSN Report No. DRPH/SER 2008-18. 2008.

Institute of Physics and Engineering in Medicine (IPEM): Small field MV dosimetry. Report No. 103, York, England. 2010.

Dutreix A, Bjärngard B E, Bridier A, et al. Monitor unit calculation for high energy photon beams. Physics For Clinical Radiotherapy. ESTRO Booklet n°3. 1997.

Gafchromic EBT3 white paper. Available from and

Underwood TSA, Rowland BC, Ferrand R, et al. Application of the Exradin W1 scintillator to determine Ediode 60017 and microDiamond 60019 correction factors for relative dosimetry within small MV and FFF fields. Phys Med Biol. 2015:(60)6669.

Benmakhlouf H, Sempau J, Andreo P. Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: A PENELOPE Monte Carlo study. Med Phys. 2014;41:041711.

Bassinet C, Huet C, Derreumaux S, et al. Small fields output factors measurements and correction factors determination for several detectors for a CyberKnife and linear accelerators equipped with microMLC and circular cones. Med Phys.2013;40(11):117201.

Scott AJ, Kumar S, Nahum AE, et al. Characterizing the influence of detector density on dosimeter response in non-equilibrium small photon fields. Phys Med Biol. 2012;2157(14):4461-76.

Martens C, De Wagter C, De Neve W. The value of the PinPoint ion chamber for characterization of small field segments used in intensity-modulated radiotherapy. Phys Med Biol. 2000;45(9):2519-30.


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