The Multigaussian method: a new approach to mitigating spatial heterogeneities with multichannel radiochromic film dosimetry

Abstract

The main objective of multichannel radiochromic film dosimetry methods is to correct, or at least mitigate, spatial heterogeneities in the film-scanner response, especially variations in the active layer thickness. To this end, films can also be scanned prior to irradiation. In this study, the abilities of various single channel and multichannel methods to reduce spatial heterogeneities, with and without scanning before irradiation, were tested. Red, green and blue single channel models, two additive channel independent perturbation (CHIP) models and two multiplicative CHIP models were compared with the Multigaussian method. The Multigaussian method is a new approach to multichannel dosimetry, based on experimental findings. It assumes that the probability density function of the response vector formed by the pixel values of the different color channels, including irradiated and non-irradiated scans, follows a multivariate Gaussian distribution. The Multigaussian method provided more accurate doses than the other models under comparison, especially when incorporating the information of the film prior to irradiation. The relative dose differences between reference doses measured with MatriXX and film doses were examined. After applying inter-scan and lateral corrections, the lowest mean absolute errors were 0.8% and 1.0% for the Multigaussian method with and without the information of the scan before irradiation, respectively. Followed by the uniform multiplicative CHIP and red single channel models, using pixel values and net optical density, respectively, both with 1.1%.

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams

“…Calibration films and field output factor film pieces were scanned with 50 and 150 dpi resolution, respectively. Lateral corrections were derived from the unexposed films and the calibration strips …”

“…Doses were computed using the Multigaussian model for radiochromic film dosimetry implemented in Radiochromic.com v3.0, after applying lateral and inter‐scan corrections. Data analysis was carried out with the R statistical computing environment…”

Output correction factors for small static fields in megavoltage photon beams for seven ionization chambers in two orientations — perpendicular and parallel

“…The experimental equipment and methodology used in the present study are the same as those used in the first part of our study, where we gave detailed discussion about the determination of field output factors using plastic scintillator Exradin W1 (Standard Imaging, Middleton, WI, USA) and radiochromic films EBT3 (Ashland Inc., Wayne, NJ, USA), as reference detectors and output correction factors for six diodes and a microdiamond detector. 32,33 Therefore, only a brief discussion on the salient features of the methodology will be given here.…”

“…where A and B correspond to the radiation field width at half maximum (FWHM) in the in-line and cross-line directions, respectively, which were determined using EBT3 film measurements in the present work. 32,33 Full description of the methodology can be found in our previous work. 32 Medical Physics, 47 (1), January 2020…”

“…Film doses were calculated using the Multigaussian method implemented in Radiochromic.com v3.0 (Radiochromic SL, Benifai o, Spain). 33 Detailed methodology for the determination of field output factors using two reference detectors, EBT3 films and W1 plastic scintillator, is provided in the paper by Casar et al 32 For the functional presentation of output correction factors, discrete values of k…”