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. 2019 Nov-Dec;24(6):585-592.
doi: 10.1016/j.rpor.2019.09.007. Epub 2019 Oct 18.

Measurement of percentage dose at the surface for a 6 MV photon beam

O O Galván De la Cruz  1   2 , M A Rodríguez-Ávila  3 , T Rivera-Montalvo  2 , O A García Garduño  4
Affiliations

Measurement of percentage dose at the surface for a 6 MV photon beam

O O Galván De la Cruz et al. Rep Pract Oncol Radiother. 2019 Nov-Dec.

Abstract

Aim: To evaluate if a radiochromic film (RF) Gafchromic EBT3 is suitable for surface dose measurements of radiotherapy treatments performed with a 6 MV linear accelerator. Two aspects of RF were analyzed, beam energy dependence and surface dose determination.

Background: The measurements done at the surface or near the radiation source are done without charged electronic equilibrium and also have contribution of electron contamination. The detectors used for these measurements should not alter the dose to the target. To counteract these dosimetric problems it is proposed to do the measurements with radiochromic films which are thin detectors and have tissue equivalent properties.

Materials and methods: The measurements were done using a Novalis linear accelerator (LINAC) with nominal energy of 6 MV. To determine the surface dose, the total scatter factors (TSF) of three different field sizes were measured in a water phantom at 5 cm depth. Energy dependence of EBT3 was studied at three different depths, using a solid water phantom. The surface measurements were done with the RF for the same field sizes of the TSF measurements. The value of the percentage depth dose was calculated normalizing the doses measured in the RF with the LINAC output, at 5 cm depth, and the TSF.

Results: The radiochromic films showed almost energy independence, the differences between the curves are 1.7% and 1.8% for the 1.5 cm and 10 cm depth, respectively. The percentage depth doses values at the surface measured for the 10 cm × 10 cm, 5 cm × 5 cm and 1 cm × 1 cm were 26.1 ± 1.3%, 21.3 ± 2.4% and 20.2 ± 2.6%, respectively.

Conclusions: The RF-EBT3 seems to be a detector suitable for measurements of the dose at the surface. This suggests that RF-EBT3 films might be good candidates as detectors for in vivo dosimetry.

Keywords: Energy independence; Radiochromic film EBT3; Surface dose.

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Figures

Fig. 1
Fig. 1
Total scatter factor (TSF) obtained with two different ionization chambers. Semiflex for 10 cm × 10 cm and 5 cm × 5 cm and CC01 for 1 cm × 1 cm. The values measured are compared with the ones reported by Ding et al. and Monte Carlo (MC) simulation. There is a good agreement between MC and the TSF, for the 1 cm × 1 cm measured with the daisy chain method. The TSFs measured by Ding et al do not show a good agreement and this may be due to the way the fields are defined, which is different from this work.
Fig. 2
Fig. 2
Measurements of the same doses at different depths with interval dose range from 0 to 10 Gy. It can be seen that the fitted curves have a very good agreement. For doses less than 10 Gy there is no difference of response for a beam of 6 MV as a function of depth. The curve measured at 5 cm depth was taken as reference according to the IAEA protocol. The uncertainties associated with the optical densities and defined by Eq. 3 are avoided in the Figure to facilitate their interpretation. The values have an average of 1.11% (min:0.49%, max:2.33%), 1.14 (min:0.48%, max:3.40%) and 1.05% (min:0.47%, max:2.44%) for the 10 cm, 5 cm, 1.5 cm depth curves, respectively.
Fig. 3
Fig. 3
Doses measured in the surface of the solid water phantom.
See this image and copyright information in PMC

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