Radiochromic EBT2 film dosimetry for low-energy protontherapy

In protontherapy, accessories are used in order to adapt the beam to the surface and to the depth of the target. They are positioned close to the patient in order to avoid perturbation effects related to proton scattering. The level of contamination of the beam caused by these accessories may be assessed by examining the dose maps in parallel planes to the beam incidence. The EBT2 radiochromic film is a suitable tool for this task, as it can be cut into small pieces and immersed in water. Prior to use the EBT2 film for dose measurements, its response when exposed to a proton beam must be analysed.

The measurements were performed at the Centre Antoine Lacassagne, using the hospital-based MEDICYC isochronous cyclotron which provides 65 MeV protons. Monoenergetic as well as polyenergetic beams were used. Small pieces of EBT2 films were irradiated with proton beams in a small water phantom. Films were exposed at various angles close to the beam incidence and received doses ranging from 0.25 to 500 Gy. The optical density (OD) was studied as a function of angle, dose and linear energy transfer (LET).

The effective atomic number of the active layer of the film is close to that of water which prevents disturbances of the measurement. However, the high density and the significant thickness of the Mylar substrate surrounding the active layer affect the use of the film in a parallel orientation to the beam. Therefore, the substrate layer may totally or partially slow down the protons. The measurement is then no longer representative to what happens in water. The measurement errors can be corrected by applying a tilt angle of at least 5° between the film and the beam. The dose analysis reveals that the green channel is the most sensitive in the dose range from 1 to 100 Gy. The OD is accurately described by a Weibull function of the dose with four free parameters. The Weibull function is valid for both monoenergetic and polyenergetic beams if the LET is limited to values below 15 MeV g(- 1) cm(2). When using a film orientation close to the beam incidence angle, increasing LET values are encountered throughout the film axis gradually with the protons slowing down in water. The EBT2 films show an underestimated response for higher LET values. The comparison of data from the present study to data obtained by other authors for EBT films allows modelling the underestimated response as a function of the LET. The definition improvement of the link between OD and LET requires to integrate more closely the beam energy characteristics.

The EBT2 film is a suitable dosimeter for analysing dose plans in planes nearly parallel to the beam orientation by compensating the underestimated dose response due to LET.