Dose verification of proton beam therapy using the Gafchromic EBT film

In proton beam therapy, dosimetric verification is important because proton beams are stopped within targets, and almost no dose proceeds beyond those targets. Patient-specific quality assurance (QA) checks the beam-delivery system using passive-scattering techniques, including portal calibrations and verification of beam-shaping devices, because there are many components to be checked, such as the range shifter, the aperture, and the compensator, in the beam-delivery system. Although the conventional measurement of dose distribution by scanning the ion chamber gives an accurate estimate of the dose, it requires considerable time and effort. While a mechanical check is useful to physically check the milling condition of the compensator, this capability is limited with respect to the investigation of the dose distribution. We investigated the usefulness of EBT films for their applications in the area of high-precision clinical QA of proton beams. Proton beams were delivered to films in a transverse (in solid water) or a longitudinal (in water) direction, with respect to the beam axis, and information about the aperture and the compensator shapes was obtained from the dose distributions measured with either film orientation. Quantitative comparisons between the treatment planning system (TPS) and the EBT film data were carried out using both distance-to-agreement and gamma index procedures. The percentage of points exceeding the acceptance criterion of the gamma index (i.e. 3%, 3 mm) was, on average, 2.4% and 8.3% in films oriented transversely and longitudinally to the beam axis, respectively. The results of transverse irradiations of EBT films were in good agreement with those of the TPS, although consistent disagreements in longitudinal irradiation were observed near the distal edge of the spread-out Bragg peak (SOBP), presumably due to differences in sensitivity in the linear energy transfer (LET). In addition to range, uncertainty (∼3 mm) caused by the non-water-equivalence of the EBT film was observed in longitudinal irradiation. The under-response of the EBT film in the high-LET region improved remarkably after we corrected the SOBP of the EBT film, using a weight function based on the Bragg peaks. Although our correction data obtained using the EBT film did not match the result of the chamber after the distal fall-off region, the EBT film could be a useful QA tool for both dose and range verification of aperture and compensator, after careful correction.

► Patient-specific QA checks the beam-delivery system using EBT film.
► We corrected the SOBP of the EBT film, using a weight function.
► The under-response of the EBT film in the high-LET region improved remarkably.
► The EBT film could be a useful QA tool for both dose and range verification.