By comparing the resultant change in net absorbance between the latest EBT-2 and previous EBT GAFCHROMIC film models, the authors conclude that the addition of the yellow marker dye to the sensitive layer does not affect dosimetric properties of the latest film model. The authors also describe a procedure by which one can establish an acceptable time window around chosen postirradiation scanning time protocol that would provide an acceptable dose error for practical purposes.

A radiochromic film based dosimetry system using only the green color channel of a flatbed document scanner showed superior precision if used alone in a dose range that extends up to 50 Gy, which greatly decreases the complexity of work. In addition, Solid Water™ material was shown to be a viable alternative to water in performing radiochromic film based dosimetry with HDR (192)Ir brachytherapy sources.

In this work, the authors report on an undoubted impact of radiochromic film immersion in water on the measured change in optical density, which may lead to systematic errors in dose measurements if the film is kept in water for longer periods of time. The magnitude of the impact depends on many parameters: Size of the film piece, initial optical density, postimmersion waiting time prior to scanning (defined by the current radiochromic film dosimetry protocol in. place), and the time film was kept in water. The authors also suggested various approaches in correcting for the change in netOD due to water penetration into the film, but the authors believe that the use of the control film piece would be the most appropriate.

Purpose: To investigate the feasibility of decomposition of differential uptake volume histograms (UVH) derived from FDG‐PET and CT data for uncovering tumor sub‐volumes as a novel approach for defining biological target volumes (BTV) for use in radiotherapy treatment planning. Methods: For a cohort of 27 histopathologically proven non‐small cell lung carcinoma (NSCLC) patients, background uptake values were sampled within contra‐lateral healthy lung over PET slices containing tumor and then scaled by the ratio of tissue densities between healthy lung and tumor derived from CT data. Signal‐to‐background uptake values within volumes of interest encompassing the tumor were scored from which differential uptake volume histograms were constructed. These were subsequently decomposed into the minimum number of analytical functions that yielded acceptable net fits, as assessed by chî2 values. Results: Based on the assumption that each function used to decompose the UVH may correspond to a single sub‐volume comprising the volume of interest sampled, at least four sub‐volumes consistently evolved for our patient population. Furthermore, if crossing points between adjacent functions are interpreted as threshold values that differentiate sub‐volumes, average threshold values between the four sub‐volumes were found to be 0.80±0.21, 1.56±0.48, and 2.96±1.04 for adenocarcinomas, 0.89±0.48, 1.60±0.40, and 2.85±0.75 for large cell carcinomas, and 0.84±0.31, 1.70±0.58, and 3.72±1.68 for squamous cell carcinomas. Conclusions: Our study suggests that FDG‐based PET data could be used to identify biological sub‐volumes within tumor in NSCLC patients. Significant fluctuations in threshold values throughout the patient cohort can be explained as a consequence of large variability in physiological status of the tumor volume for each patient at the time of the PET/CT scan. This further suggests that BTV threshold values may be rather patient‐specific, and could be determined by creation and curve fitting of differential uptake volume histograms on a patient‐specific basis.

The method of (18)F-FDG-PET-based dUVH decomposition described in this work may lead to BTV segmentation in tumours.

Purpose: Comparison between radiochromic film reference dosimetry system calibration curves established in water and Solid Water(TM) for high dose rate (HDR) 192‐Iridium brachytherapy source is described and assessed. Accordingly, a new reference dosimetry system protocol for HDR 192‐Iridium using the latest EBT‐2 model GAFCHROMIC(TM) film is suggested. Methods: Calibration curves were established in water and Solid Water(TM) using pieces of EBT‐2 GAFCHROMIC(TM) film irradiated with HDR 192‐Iridium brachytherapy source for a dose range from 0 to 50 Gy. A parallel‐opposed beam setup was specially designed to allow the positioning of the HDR source into two channels (catheters) with the film piece positioned mid‐way between them. This setup increases the dose homogeneity region over the film piece and reduces the positional uncertainty with respect to the radiation source. Responses of dosimetry systems were compared for irradiations in water and Solid Water(TM) by scaling the dose between media through Monte Carlo‐calculated conversion factor simulated for the two setups.Results: Monte Carlo calculated conversion factor, which converts dose delivered to the sensitive layer of the film in water to a dose delivered to the sensitive layer of the film in Solid Water(TM), was found to be 0.9941±0.0007. The EBT‐2 GAFCHROMIC(TM) film based reference dosimetry system described in this work can provide an overall one‐sigma uncertainty in measured dose of 2% for doses above 1 Gy Conclusions: Experimental confirmation of the Monte Carlo‐calculated factor that shows dose difference between measurements in water and Solid Water(TM) was provided. The remaining 0.6% difference between measurements in both media shows that Solid Water(TM) is a viable alternative to water as a reference medium in establishing the calibration curve at 192‐Iridium energy. Response curves utilizing green color channel only has shown superior precision if used alone in dosimetry for dose range that extends up to 50 Gy. This work was supported by the Natural Sciences and Engineering Research Council of Canada contract No. 386009. Saad Aldelaijan would like to acknowledge Saudi Food & Drug Authority for financial support during his graduate studies.

Purpose: A new EBT‐2 GAFCHROMIC™ model radiochromic film has been released recently with addition of a yellow marker dye within sensitive layer that enables correction of non‐uniformity of the active layer of the film. Performance of the EBT‐2 was compared with former EBT model in terms of absorption spectra. Since one of the major drawbacks of the current radiochromic film dosimetry protocols is the post‐irradiation waiting time, we studied the impact of post‐irradiation scanning time on the dose measurements accuracy if shorter times are to be adopted. Method and Materials: Post‐irradiation scanning times employed range from 3 minutes to 5 days and dose range extends from 0 to 6 Gy. Absorption spectra of film samples were measured using a Perkin Elmer Lambda 650 spectrophotometer over the spectral range from 400 nm to 800 nm. Changes in absorption spectra of the samples irradiated to various doses were determined as the net difference between measurement and control film pieces which accounts for changes due to environmental conditions. Results: Both film models experience similar dose change in net absorbance. However, the sensitivity of the latest EBT‐2 model GAFCHROMIC™ film is slightly lower than its predecessor. We show that for two post‐irradiation scanning times of 30 minutes and 24 hours the 1% dose error can be achieved if the scanning time window is less than ±5 minutes and ±2 hours, respectively. Conclusion: By comparing the resultant change in net absorbance between the latest EBT‐2 and previous EBT GAFCHROMIC™ film models we conclude that the addition of the yellow marker dye to the sensitive layer does not affect dosimetric properties of the latest film model. We also describe a procedure by which one can establish an acceptable time window around chosen post‐irradiation scanning time protocol that would provide an acceptable dose error for practical purposes.

No abstract