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

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%.

Grid patterns, spatial inter-scan variations and scanning reading repeatability in radiochromic film dosimetry.

PURPOSE: When comparing different scans of the same radiochromic film, several patterns can be observed. These patterns are caused by different sources of uncertainty, which affect the repeatability of the scanner. The purpose of this work was to study these uncertainties. METHODS: The variance of the scanner noise, as a function of the pixel position, was studied for different resolutions. The inter-scan variability of the scanner response was analyzed taking into account spatial discrepancies. Finally, the distance between the position of the same point in different scans was examined. RESULTS: The variance of noise follows periodical patterns in both axes, causing the grid patterns. These patterns were identified for resolutions of 50, 72 and 96dpi, but not for 150dpi. Specially recognizable is the sinusoidal shape with a period of 8.5mm that is produced with 72dpi. Inter-scan variations of the response caused systematic relative dose deviations larger than 1% in 5% of the red channel images, 9% of the green and 51% of the blue. No systematic deviation larger than 1% was found after applying response corrections. The initial positioning and the speed of the scanner lamp vary between scans. CONCLUSIONS: Three new sources of uncertainty, which influence radiochromic film dosimetry with flatbed scanners, have been identified and analyzed in this work: grid patterns, spatial inter-scan variations and scanning reading repeatability. A novel correction method is proposed, which mitigates spatial inter-scan variations caused by deviations in the autocalibration of the individual Charge Coupled Device detectors.

On multichannel film dosimetry with channel-independent perturbations.

PURPOSE: Different multichannel methods for film dosimetry have been proposed in the literature. Two of them are the weighted mean method and the method put forth by Micke et al. ["Multichannel film dosimetry with nonuniformity correction," Med. Phys. 38, 2523-2534 (2011)] and Mayer et al. ["Enhanced dosimetry procedures and assessment for EBT2 radiochromic film," Med. Phys. 39, 2147-2155 (2012)]. The purpose of this work was to compare their results and to develop a generalized channel-independent perturbations framework in which both methods enter as special cases. METHODS: Four models of channel-independent perturbations were compared: weighted mean, Micke-Mayer method, uniform distribution, and truncated normal distribution. A closed-form formula to calculate film doses and the associated type B uncertainty for all four models was deduced. To evaluate the models, film dose distributions were compared with planned and measured dose distributions. At the same time, several elements of the dosimetry process were compared: film type EBT2 versus EBT3, different waiting-time windows, reflection mode versus transmission mode scanning, and planned versus measured dose distribution for film calibration and for γ-index analysis. The methods and the models described in this study are publicly accessible through IRISEU. Alpha 1.1 (http://www.iriseu.com). IRISEU. is a cloud computing web application for calibration and dosimetry of radiochromic films. RESULTS: The truncated normal distribution model provided the best agreement between film and reference doses, both for calibration and γ-index verification, and proved itself superior to both the weighted mean model, which neglects correlations between the channels, and the Micke-Mayer model, whose accuracy depends on the properties of the sensitometric curves. With respect to the selection of dosimetry protocol, no significant differences were found between transmission and reflection mode scanning, between 75 ± 5 min and 20 ± 1 h waiting-time windows or between employing EBT2 or EBT3 films. Significantly better results were obtained when a measured dose distribution was used instead of a planned one as reference for the calibration, and when a planned dose distribution was used instead of a measured one as evaluation for the γ-analysis. CONCLUSIONS: The truncated normal distribution model of channel-independent perturbations was found superior to the other three models under comparison and the authors propose its use for multichannel dosimetry.

Gafchromic EBT2 film dosimetry in reflection mode with a novel plan-based calibration method.

PURPOSE: A dosimetric system formed by Gafchromic EBT2 radiochromic film and Epson Expression 10000XL flatbed scanner was commissioned for dosimetry. In this paper, several open questions concerning the commissioning of radiochromic films for dosimetry were addressed: (a) is it possible to employ this dosimetric system in reflection mode; (b) if so, can the methods used in transmission mode also be used in reflection mode; (c) is it possible to obtain accurate absolute dose measurements with Gafchromic EBT2 films; (d) which calibration method should be followed; (e) which calibration models should be used; and (f) does three-color channel dosimetry offer a significant improvement over single channel dosimetry. The purpose of this paper is to help clarify these questions. METHODS: In this study, films were scanned in reflection mode, the effect of surrounding film was evaluated and the feasibility of EBT2 film dosimetry in reflection mode was studied. EBT2's response homogeneity has been reported to lead to excessive dose uncertainties. To overcome this problem, a new plan-based calibration method was implemented. Plan-based calibration can use every pixel and each of the three color channels of the scanned film to obtain the parameters of the calibration model. A model selection analysis was conducted to select lateral correction and sensitometric curve models. The commonly used calibration with fragments was compared with red-channel plan-based calibration and with three-channel plan-based calibration. RESULTS: No effect of surrounding film was found in this study. The film response inhomogeneity in EBT2 films was found to be important not only due to differences in the fog but also due to differences in sensitivity. The best results for lateral corrections were obtained using absolute corrections independent of the dose. With respect to the sensitometric curves, an empirical polynomial fit of order 4 was found to obtain results equivalent to a gamma-distributed single hit model based on physical assumptions. Three-channel dosimetry was found to be substantially superior to red-channel dosimetry. CONCLUSIONS: Reflection mode with Gafchromic EBT2 radiochromic film was found to be a viable alternative to transmission mode. The same methods that are used in transmission mode can be followed in reflection mode. A novel plan-based method was developed for calibration and multichannel dosimetry. This novel method offers increased robustness against film response inhomogeneities and reduces considerably the time required for calibration.

Anal cancer chemoirradiation with curative intent - a single institution experience.

UNLABELLED: Results of radiochemotherapy in 50 patients with squamous cell carcinoma of the anal canal, treated with radical radiochemotherapy between January 2003 and September 2007, at the Institute of Oncology Ljubljana are presented. The treatment schedule consisted of 3-D conformal external beam radiotherapy (45 Gy in 25 fractions), with two cycles of concurrent chemotherapy (5-fluorouracil (5-FU) / Mitomycin C), followed by brachytherapy or external beam boost (15-30 Gy) to the primary tumor. Locoregional control (LRC), disease-free survival (DFS), disease-specific survival (DSS), overall survival (OS) and colostomy-free survival (CFS) rates and the rate of acute and chronic side-effects were estimated. The impact of individual tumor- and therapy-related factors on treatment outcome was assessed.

Treatment was completed according to the protocol in 72% of patients. The median follow-up time of 40 survivors was 22 months (range 1.7-53.2 months). At 2 years, LRC, DFS, DSS, OS and CFS rates were 68%, 67%, 87%, 76% and 85%, respectively. In the multivariate analysis, nodal stage was identified as an independent prognostic factor for LRC, DSS and CFS and application of Mitomycin C for OS. The most frequent acute side-effect of treatment was radiodermatitis (grade 3 in 66% of patients, grade 4 in 2%). Late anal stenosis, chronic ulceration and grade 2-3 incontinence developed in 3 (6 %), 2 (4 %) and 5 (10 %) of colostomy-free survivors, respectively.

Radiotherapy with concurrent 5-FU / Mitomycin C chemotherapy is feasible, with acceptable toxicity. The presented treatment outcome is comparable to other published results.