Analysis of variation in calibration curves for Kodak XV radiographic film using model‐based parameters

Shu-Hui Hsu,Peter L Roberson,Ravi Kulasekere

doi:10.1120/jacmp.v11i4.3172

Shu-Hui Hsu, Peter L Roberson + Show 1 more

Open Access

https://doi.org/10.1120/jacmp.v11i4.3172

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Abstract

Film calibration is time‐consuming work when dose accuracy is essential while working in a range of photon scatter environments. This study uses the single‐target single‐hit model of film response to fit the calibration curves as a function of calibration method, processor condition, field size and depth. Kodak XV film was irradiated perpendicular to the beam axis in a solid water phantom. Standard calibration films (one dose point per film) were irradiated at 90 cm source‐to‐surface distance (SSD) for various doses (16–128 cGy), depths (0.2, 0.5, 1.5, 5, 10 cm) and field sizes (5×5,10×10 and 20×20cm2). The 8‐field calibration method (eight dose points per film) was used as a reference for each experiment, taken at 95 cm SSD and 5 cm depth. The delivered doses were measured using an Attix parallel plate chamber for improved accuracy of dose estimation in the buildup region. Three fitting methods with one to three dose points per calibration curve were investigated for the field sizes of 5×5,10×10 and 20×20cm2. The inter‐day variation of model parameters (background, saturation and slope) were 1.8%, 5.7%, and 7.7% (1 σ) using the 8‐field method. The saturation parameter ratio of standard to 8‐field curves was 1.083±0.005. The slope parameter ratio of standard to 8‐field curves ranged from 0.99 to 1.05, depending on field size and depth. The slope parameter ratio decreases with increasing depth below 0.5 cm for the three field sizes. It increases with increasing depths above 0.5 cm. A calibration curve with one to three dose points fitted with the model is possible with 2% accuracy in film dosimetry for various irradiation conditions. The proposed fitting methods may reduce workload while providing energy dependence correction in radiographic film dosimetry. This study is limited to radiographic XV film with a Lumisys scanner.PACS number: 87.55.Qr

Highlights

223 Hsu et al.: Analyzing calibration curves for XV film strongly depends on the ratio of the number of photons below to the number of photons above 0.1 MeV
The variation of parameter m (0.8%) for the 8-field method is dominated by the variation of film processing conditions
One dose point close to 120 cGy is recommended in order to get a more accurate pixel values (Ps)

Summary

Introduction

223 Hsu et al.: Analyzing calibration curves for XV film strongly depends on the ratio of the number of photons below to the number of photons above 0.1 MeV (scatter-to-primary ratio). The influence of energy response could be reduced using Monte Carlo calculation to model the film response.[2,10] Suchowerska et al[8] reported that film parallel orientation (to the beam axis) has an increased response compared to perpendicular orientation, up to 14% at 25 cm depth for a 6 MV photon beam This could be attributed to the increased forward scattering of electrons in the silver halide for parallel orientation and the reduced attenuation of the beam when a gap appears between film and phantom.[8,11] It is necessary to use the same orientations for calibration and experimental films. A calibration curve (sensitometric curve) is usually acquired with each measurement to account for the influence of batch-to-batch and film processing variations

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