Gafchromic EBT3 film provides equivalent dosimetric performance to EBT-XD film for stereotactic radiosurgery dosimetry

Abstract

The accurate assessment of film results is highly dependent on the methodology and techniques used to process film. This study aims to compare the performance of EBT3 and EBT-XD film for SRS dosimetry using two different film processing methods. Experiments were performed in a solid water slab and an anthropomorphic head phantom. For each experiment, the net optical density of the film was calculated using two different methods; taking the background (initial) optical density from 1) an unirradiated film from the same film lot as the irradiated film (stock to stock (S-S) method), and 2) a scan of the same piece of film taken prior to irradiation (film to film (F-F) method). EBT3 and EBT-XD performed similarly across the suite of experiments when using the green channel only or with triple channel RGB dosimetry. The dosimetric performance of EBT-XD was improved across all colour channels by using an F-F method, particularly for the blue channel. In contrast, EBT3 performed similarly well regardless of the net optical density method used. Across 21 SRS treatment plans, the average per-pixel agreement between EBT3 and EBT-XD films, normalised to the 20 Gy prescription dose, was within 2% and 4% for the non-target (2-10 Gy) and target (> 10 Gy) regions, respectively, when using the F-F method. At doses relevant to SRS, EBT3 provides comparable dosimetric performance to EBT-XD. In addition, an S-S dosimetry method is suitable for EBT3 while an F-F method should be adopted if using EBT-XD.

Keywords: Audit; Dosimetry; EBT-XD; EBT3; Film; Radiochromic film; Stereotactic radiosurgery.

Fig. 1

Example calibration curves for the RGB colour channels for an EBT3 and EBT-XD film set

Fig. 2

Example sensitivity curves for the RGB colour channels comparing EBT3 and EBT-XD film types. EBT3 is shown as dashed lines and EBT-XD is shown as solid lines, with colours representing the respective channels

Fig. 3

Distribution of check-film scale factors from the check-film analysis (N = 54 pairs of EBT3 and EBT-XD films from N = 17 independent experiments). S-S, stock to stock; F-F, film to film

Fig. 4

Absolute difference from agreement between film and Farmer measurements of delivered dose. Significant differences between S-S and F-F methods were observed for EBT-XD, but not EBT3, for all channels ((a) red channel, adjusted P = 0.025; (b) green channel, adjusted P = 0.001; (c) blue channel, adjusted P = 0.0004; (d) RGB average, adjusted P = 0.0004). When using an F-F method, the performance of EBT3 was 1.0% worse compared to EBT-XD (adjusted P = 0.025) (a). The performance of EBT3 (S-S and F-F) was comparable to EBT-XD for green (b) and triple channel dosimetry (d). Dose range did not impact EBT3 performance (e). * adjusted P < 0.05, ** adjusted P < 0.01, *** adjusted P < 0.001. S-S, stock to stock; F-F, film to film

Fig. 5

Absolute percentage difference from agreement between microDiamond and film-measured doses in an anthropomorphic head phantom (N = 21 pairs of EBT3 and EBT-XD films, N = 6 audits) for red (a), green (b) and blue (c) channels and for triple channel dosimetry (d). A significant difference was observed for the blue channel only when comparing S-S and F-F netOD methods for EBT-XD (adjusted P = 0.022). The difference from agreement was comparable for EBT-XD and EBT3 for green and triple channel dosimetry. * adjusted P < 0.05. S-S, stock to stock; F-F, film to film

Fig. 6

Scoring metrics for audit films (N = 21 pairs of EBT3 and EBT-XD films, N = 6 audits) against the planned dose distribution using gamma criteria and mean distance to agreement. The distribution of results for gamma pass rates for 5%/1 mm (a) and 3%/3 mm (b) were similar between all film-type and netOD combinations, except for EBT-XD when using an S-S method. The dotted line in panel (a) indicates the nominal 90% gamma (5%/1 mm) pass-rate threshold used to score the audit film as passing or out-of-tolerance

Fig. 7

Per pixel dose-difference, normalised to the prescription dose, between sandwiched EBT3 and EBT-XD audit films (N = 21 pairs of EBT3 and EBT-XD films, N = 6 audits) irradiated in an anthropomorphic head phantom. The average normalized per pixel dose-difference between film types was within 2% and 4% of the prescription dose (20 Gy) in non-target (a) and target (b) regions, respectively, when using the F-F netOD method. Dose-differences between EBT3 and EBT-XD were larger when comparing films using the S-S method, as illustrated by a representative pair of films (c). Large dose differences between S-S and F-F methods were more common for EBT-XD, while EBT3 film showed little to no dose-difference between S-S and F-F methods (d). Colour bar shows the average per pixel dose-difference, normalized to the prescription dose (20 Gy), as described by Eq. (6). S-S, stock to stock; F-F, film to film