L Liang

Absorption spectra time evolution of EBT‐2 model GAFCHROMIC™ film

PURPOSE One of the major drawbacks of the current radiochromic film dosimetry protocols is the postirradiation waiting time. In this article, the authors study the postirradiation time evolution of the absorption spectrum of radiochromic EBT-2 GAFCHROMIC film model. METHODS Postirradiation scanning times range from 3 min to 5 days and a dose range extends from 0 to 6 Gy. The authors compare the results of absorption spectra measurements for the latest GAFCHROMIC EBT-2 film model to the absorption spectra of the previous EBT GAFCHROMIC film model. The authors also describe a method that can establish the time error constraints on the postirradiation scanning time that will still provide an acceptable dose error for clinical applications if the protocol employing the shorter postirradiation scanning time is implemented in the clinic. RESULTS The two film models experience the very same dose change in net absorbance with sensitivity of the latest EBT-2 model GAFCHROMIC film being slightly lower than its predecessor. The authors show that for two postirradiation scanning times of 30 min and 24 h, the 1% dose error can be achieved if the scanning time window is less than +/- 5 min and +/- 2 h, respectively. CONCLUSIONS 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.

Radiochromic film dosimetry of HDR 192Ir source radiation fields

PURPOSE A radiochromic film based dosimetry system for high dose rate (HDR) Iridium-192 brachytherapy source was described. A comparison between calibration curves established in water and Solid Water™ was provided. METHODS Pieces of EBT-2 model GAFCHROMIC™ film were irradiated in both water and Solid Water™ with HDR (192)Ir brachytherapy source in a dose range from 0 to 50 Gy. Responses of EBT-2 GAFCHROMIC™ film were compared for irradiations in water and Solid Water™ by scaling the dose between media through Monte Carlo calculated conversion factor for both setups. To decrease uncertainty in dose delivery due to positioning of the film piece with respect to the radiation source, traceable calibration irradiations were performed in a parallel-opposed beam setup. RESULTS The EBT-2 GAFCHROMIC™ film based dosimetry system described in this work can provide an overall one-sigma dose uncertainty of 4.12% for doses above 1 Gy. The ratio of dose delivered to the sensitive layer of the film in water to the dose delivered to the sensitive layer of the film in Solid Water™ was calculated using Monte Carlo simulations to be 0.9941 ± 0.0007. CONCLUSIONS 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.

Evaluation of EBT‐2 model GAFCHROMIC™ film performance in water

PURPOSE The authors present results of the measurements on the impact of radiochromic film immersion in water. The impact of film piece size, initial optical density, postimmersion waiting time prior to scanning, and the time film was kept in water has been investigated. The authors also investigated the pathways of water penetration into the film during the film immersion in water. METHODS To study the impact of water immersion on change in optical density, the authors used various sizes of the latest EBT-2 model GAFCHROMICTM film: 2 x 2, 4 x 4, and 8 x 8 in.2. In addition, to test any existing dependence of the films optical density on water diffusion, the authors used two sets of films: Unexposed (0 Gy) and film pieces exposed to a dose of 3 Gy. Times that film pieces were left in water ranged from 30 min to 24 h, and once the film was permanently removed from water, the authors also studied the impact of the scanning time (deltat) that ranged from 0 (films scanned right after removal from water) to 72 h postimmersion. RESULTS While the penetration depth can reach as much as 9 mm around the edges of the EBT-2 GAFCHROMIC film, the anticipated dose error due to the change in optical density due to the water immersion appears to be negligible for the short immersions of the order of 30 min. However, as the immersion time increases, the anticipated dose error may reach 22 cGy on a 2 x 2 in.2 piece of film, which corresponds to 7% dose error at 3 Gy of measured dose. CONCLUSIONS 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.

Characterization of calibration curves and energy dependence GafChromicTM XR‐QA2 model based radiochromic film dosimetry system

PURPOSE The authors investigated the energy response of XR-QA2 GafChromic™ film over a broad energy range used in diagnostic radiology examinations. The authors also made an assessment of the most suitable functions for both reference and relative dose measurements. METHODS Pieces of XR-QA2 film were irradiated to nine different values of air kerma in air, following reference calibration of a number of beam qualities ranging in HVLs from 0.16 to 8.25 mm Al, which corresponds to effective energy range from 12.7 keV to 56.3 keV. For each beam quality, the authors tested three functional forms (rational, linear exponential, and power) to assess the most suitable function by fitting the delivered air kerma in air as a function of film response in terms of reflectance change. The authors also introduced and tested a new parameter χ = netΔR·e(m netΔR) that linearizes the inherently nonlinear response of the film. RESULTS The authors have found that in the energy range investigated, the response of the XR-QA2 based radiochromic film dosimetry system ranges from 0.222 to 0.420 in terms of netΔR at K(air)(air) = 8 cGy. For beam qualities commonly used in CT scanners (4.03-8.25 mm Al), the variation in film response (netΔR at K(air)(air) = 8 cGy) amounts to ± 5%, while variation in K(air)(air) amounts to ± 14%. CONCLUSIONS Results of our investigation revealed that the use of XR-QA2 GafChromic™ film is accompanied by a rather pronounced energy dependent response for beam qualities used for x-ray based diagnostic imaging purposes. The authors also found that the most appropriate function for the reference radiochromic film dosimetry would be the power function, while for the relative dosimetry one may use the exponential response function that can be easily linearized.

Sorafenib in combination with ionizing radiation has a greater anti-tumour activity in a breast cancer model.

High expression of vascular endothelial growth factor (VEGF) in patients with breast cancer has been associated with a poor prognosis, indicating that VEGF could be linked to the efficacy of chemotherapy and radiotherapy. It has also been suggested that radiation resistance is partly due to tumour cell production of angiogenic cytokines, particularly VEGF receptor (VEGFR). This evidence indicates that inhibition of VEGFR might enhance the radiation response. Sorafenib tosylate (Bay 54-9085) is an oral, small-molecule multikinase inhibitor of several targets including RAF/MEK/ERK MAP kinase signalling, VEGFR-2, VEGFR-3 and platelet-derived growth factor receptor-beta. Sorafenib has shown clinical efficacy in treating solid tumours such as renal cell and hepatocellular carcinomas. However, strategies are yet to be identified to prolong and maximize the anticancer effect of this multikinase inhibitor. The objective of this study was to determine whether a combination of Sorafenib and radiation will enhance the treatment response in vitro and in vivo. Radio-modulating effect of Sorafenib was assessed by performing clonogenic assays. In addition, cell cycle analyses as well as annexin-V apoptosis assays were performed 24 and 48 h after treatment, respectively. To confirm our in-vitro results, tumour growth delay assays were performed. Our results showed a strong and supra-additive antitumour effect of radiation combined with Sorafenib in vitro (dose enhancement factor of 1.76). The combined therapy demonstrated a strong and significant G2/M cell cycle arrest (combined treatment vs. irradiated alone: P<0.0008). Moreover, annexin-V staining showed a significant increase in the level of apoptosis (combined treatment vs. irradiated alone: P<0.0004). Study of the syngeneic model demonstrated the superior potency of the Sorafenib combined with radiotherapy. Our results demonstrate that higher antitumour activity can be achieved when radiation and Sorafenib are combined.

SU-E-T-462: Impact of the Radiochromic Film Energy Response On Dose Measurements of Low Energy Electronic Brachytherapy Sources

Purpose: We investigated the effect of the EBT3 GafChromicTM film model absorbed dose energy response when used for percent depth dose (PDD) measurements in low-energy photon beams. Methods: We measured PDDs in water from a Xoft 50 kVp source using EBT3 film, and compared them to PDD measurements acquired with a PTW-TN34013 parallel-plate ionization chamber. For the x-ray source, we simulated spectra using the EGSnrc (BEAMnrc) Monte Carlo code, and calculated Half Value Layer (HVL) at different distances from the source in water. Pieces of EBT3 film were irradiated in air and calibration curves were created in terms of air-kerma in air ((Kair)air) for different beam qualities. Pieces of EBT3 film were positioned at distances of 2–6 cm from the Xoft source in a water phantom using a custom-made holder, and irradiated at the same time. As scatter is incorporated in the measured film signal in water, measured (Kair)wat was subsequently converted into absorbed dose to water by the ratio of mass energy absorption coefficients following the AAPM TG-61 dosimetry protocol. Results: Our results show that film calibration curves obtained at beam qualities near the effective energy of the Xoft 50 kVp source in water lead to variation in absorbed dose energy dependence of the response of around 3%. However, if the calibration curve was established at MV beam quality, the error in absorbed dose could be as large as 15%. We observed agreement within 1% between PDD measurements using EBT3 film model (using a calibration curve obtained at 80 kVp, HVL=2.18 mm Al, Eeff=29.5 keV) and the parallel-plate ionization chamber. Conclusion: Accurate dose measurements using radiochromic films at low photon energies require that the radiochromic film dosimetry system be calibrated at corresponding low energies, as large absorbed dose errors are expected for calibrations performed at MV beam qualities.

Poster — Thur Eve — 55: An automated XML technique for isocentre verification on the Varian TrueBeam

Isocentre verification tests, such as the Winston-Lutz (WL) test, have gained popularity in the recent years as techniques such as stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments are more commonly performed on radiotherapy linacs. These highly conformal treatments require frequent monitoring of the geometrical accuracy of the isocentre to ensure proper radiation delivery. At our clinic, the WL test is performed by acquiring with the EPID a collection of 8 images of a WL phantom fixed on the couch for various couch/gantry angles. This set of images is later analyzed to determine the isocentre size. The current work addresses the acquisition process. A manual WL test acquisition performed by and experienced physicist takes in average 25 minutes and is prone to user manipulation errors. We have automated this acquisition on a Varian TrueBeam STx linac (Varian, Palo Alto, USA). The Varian developer mode allows the execution of custom-made XML script files to control all aspects of the linac operation. We have created an XML-WL script that cycles through each couch/gantry combinations taking an EPID image at each position. This automated acquisition is done in less than 4 minutes. The reproducibility of the method was verified by repeating the execution of the XML file 5 times. The analysis of the images showed variation of the isocenter size less than 0.1 mm along the X, Y and Z axes and compares favorably to a manual acquisition for which we typically observe variations up to 0.5 mm.

MO‐F‐214‐04: Protoyping Compositions of Novel Radiochromic Film Types: Towards Complete Absorbed Dose Energy Independence

Purpose: To develop a new generation energy‐independent radiochromic film over a wide energy range of 50 kVp to 18 MV. The films are preferably symmetric in structure and can be used for measurements in complex spectral distributions. Methods: The overall energy dependence and intrinsic energy dependence were measured for EBT2 and several prototype films of known compositions. The intrinsic energy response is quantified through a measurement of total energy response divided by the Monte Carlo calculated absorbed dose energy response. The measurements consisted of delivering an exact dose of 2 Gy to the sensitive layer of the film at both orthovoltage energies (50kVp, 120kVp, and 180kVp) and 60Co beam. Thus far, two iterations have been performed, whereby; numerical simulations are used to obtain an optimized absorbed dose energy response that will undo for the film intrinsic energy response. DOSRZnrc user‐code of the EGSnrc Monte Carlo code was used for all simulations. AAPM TG51 and TG61 were used to determine the dose‐to‐water and air‐kerma in air in megavoltage and orthovoltage beams, respectively, while Monte Carlo simulated corrections were used to convert these results to the desired dose to film. Results: For EBT2 films the overall energy dependence was found to vary by 35% over 50 kVp to Co‐60 energy range. The latest prototype measured in this work has shown a total net variation of 8% over the same range. High atomic number elements (Chlorine and bromine) were found to affect absorbed dose response drastically. The iterative optimization technique has placed a large focus in accurately determining the elemental composition. Conclusions: We have quantified the intrinsic energy dependence of radiochromic films, and have used the data to numerically optimize the composition of the active layer to produce an energy‐ independent film with an 8% energy variation over a wide energy range. Funding: This work was supported by the Natural Sciences and Engineering Research Council of Canada contract No. 386009. Conflict of interest: ISP is a manufacturing company of GAFCHROMICTM films.

SU‐E‐T‐113: Measurement of the Cutout Output Factor on Ortho‐Voltage Unit Using EBT‐2 Model GafchromicTM Film

Purpose: In this study, we evaluate feasibility of using EBT‐2 model GafchromicTM film to measure patient specific cutout output factors on an ortho‐voltage treatment machine. Methods: A Gulmay Orthovoltage D3225 unit was used to produce clinical beams in kV range from 50 kVp to 220 kVp. Previously established air kerma based radiochromic film dosimetry protocol for low energy x‐ray beams was used. A closed‐end applicator with 10 × 10 cm2 field size and SSD 50 cm was employed. Cutouts were made from a lead sheet of 1.8 mm in thickness. A 6‐cm thick slab of Solid WaterTM was used as a backscatter material for output factor measurement. In‐house made MatLab routines were used to analyze scanned images of exposed and unexposed films and a PTW 23342 parallel‐plate ion chamber (PP IC) was used to measure the cutout output factors for comparison. Results: Air‐Kerma based calibration curve was created in the 1–300 cGy range. Film dosimetic accuracy was verified for known doses to water of 1.00 and 2.00 Gy, respectively, with maximum observed difference of 0.4%. Two‐dimensional dose maps and dose profiles were obtained for various cutout sizes ranging from 0.8 cm diameter circle to 10 × 10 cm2 square. Comparisons of output factors between film and pp ion chamber measurements show that there are no differences for large cutouts but the chamber measurements are up to 11 % higher for small cutout size. Conclusions: We demonstrated that the air‐Kerma based radiochromic film dosimetry protocol using EBT‐2 model GafChromicTM film for low energy x‐ray beam is adequate for patient specific cutout output measurements on orthovoltage units, especially for small size cutout measurements, where IC measurements give higher output factor than films due to ICs over responses to electron contaminations from lead sheets.

SU‐E‐T‐378: Radiochromic Film Based Reference Dosimetry System for 192Ir Brachytherapy Sources

Purpose: Comparison between radiochromic film reference dosimetry system calibration curves established in water and Solid Water(TM) for high dose rate (HDR) 192‐Iridium brachytherapysource 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 brachytherapysource 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.