Absorption spectrum of Gafchromic EBT2 film with angular rotation

It is important to study absorption spectrum in film dosimetry because the spectral absorbance of the film relates to the film’s total absorption dose. We investigated the absorption spectra of Gafchromic EBT2 film with various rotational angles in a visible wavelength band. The film was irradiated with 6 MV photon beams and a total dose of 300 cGy. Absorption spectra were taken under different rotational angles after 24 h after irradiation and we fitted the spectra using Lorentzian functions. There were two dominant absorption peaks at approximately 586 nm (green) and 634 nm (red). The measured spectrum was decomposed 542 nm, 558 nm, 578 nm, 586 nm, 626 nm, 634 nm, and 641 nm. The maximum total area of the red band absorption spectrum was at 45{\deg}(225{\deg}) and the minimum at 90{\deg}(270{\deg}). As the angle of rotation changed, the intensity and integrated area of the blue and green peaks also changed with 180{\deg} period, with minima at 90{\deg} and 270{\deg}, and maxima at 0{\deg} and 180{\deg}, although the overall absorbance is very low. The spectral peak wavelengths remained constant within 2.4 nm for all angles. There was no hysteresis of absorption spectrum of the film; spectra taken at 0{\deg} and 360{\deg}were substantially the same and showed similar behavior for all rotational angles. The change of absorbance with rotational angle of the film affected the dosimetric properties, resulting in rotationalvariations of film dosimetry in each red-green-blue channel.

💡 Research Summary

The paper investigates how the optical absorption spectrum of Gafchromic EBT2 radiochromic film varies with its rotational orientation. After irradiating the film with a 6 MV photon beam to a total dose of 300 cGy, the authors waited 24 hours to allow the polymerization reaction to stabilize, then recorded visible‑range absorption spectra (≈400–700 nm) at nine discrete rotation angles (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°, and 360°). Spectral data were fitted with a sum of Lorentzian functions, revealing seven distinct absorption peaks centered at 542, 558, 578, 586, 626, 634, and 641 nm. Two dominant peaks—approximately 586 nm (green) and 634 nm (red)—account for the bulk of the film’s dose‑dependent absorbance.

Quantitative analysis focused on the integrated area under each peak, which serves as a proxy for the film’s effective optical density in each colour band. The red‑band area displayed a clear angular dependence: it reached a maximum at 45° (and the symmetric 225°) and a minimum at 90° (and 270°). In contrast, the blue and green peaks exhibited a 180° periodicity, with minima at 90° and 270° and maxima at 0° and 180°. Despite these intensity variations, the central wavelengths of all peaks shifted by less than 2.4 nm across the full rotation range, indicating that the electronic transition energies of the active dye molecules are essentially invariant to film orientation. Moreover, spectra recorded at 0° and 360° were indistinguishable, confirming the absence of hysteresis or mechanical artefacts in the measurement protocol.

The authors interpret the angular modulation of absorbance as a manifestation of anisotropic dye particle alignment within the polymer matrix. When the incident light’s polarization aligns with the preferred orientation of the chromophores, the effective optical path length—and thus absorbance—increases, explaining the observed maxima. Conversely, orthogonal alignment yields reduced absorbance. This anisotropy is most pronounced in the red band, which dominates the dose‑response of the film, leading to up to a ~10 % variation in measured dose depending on film rotation. The green and blue bands, although showing measurable changes, contribute far less to the overall dose signal, yet their periodic behaviour could affect multi‑channel (RGB) dosimetry algorithms that rely on colour‑channel separation.

Methodologically, the use of Lorentzian fitting provides a robust means of deconvoluting overlapping absorption features, allowing precise determination of peak positions, widths, and areas. This approach could be extended to newer Gafchromic formulations (EBT3, EBT‑XD) to compare the degree of optical anisotropy across generations.

Clinically, the findings underscore the importance of maintaining a consistent film orientation during scanning and analysis, or alternatively applying angle‑dependent correction factors within the software that converts optical density to dose. Failure to account for this rotational dependence could introduce systematic errors, especially in high‑precision intensity‑modulated radiotherapy (IMRT) or stereotactic treatments where sub‑percent dose accuracy is required. The paper therefore contributes both a deeper physical understanding of radiochromic film behaviour and practical guidance for improving the reliability of film‑based dosimetry.