1. A. Nisbet 1,2, A. Dimitriadis 1,2,3, A.L. Palmer 1,4, C.H. Clark 2,3
IS EBT-XD FILM SUITABLE FOR LINAC AND GAMMA KNIFE RADIOSURGERY DOSIMETRY VERIFICATION AND AUDIT?
A. Nisbet 1,2, A. Dimitriadis 1,2,3, A.L. Palmer 1,4, C.H. Clark 2,3
1 University of Surrey, Faculty of Engineering and Physical Sciences, Guildford, United Kingdom 2 St. Luke’s Cancer Centre, Department of Medical Physics, Royal Surrey County Hospital, Guildford, United Kingdom 3 National Physical Laboratory, Acoustics and Ionising Radiation, Teddington, United Kingdom 4 Radiotherapy Physics, Queen Alexandra Hospital, Portsmouth Hospitals NHS Trust, Portsmouth, United Kingdom
Purpose/objectives:
The validation of radiotherapy treatments by dosimetric measurement is essential for the introduction of new techniques, pre-treatment verification and dosimetry audit. Film dosimetry has the advantage of high spatial resolution, low energy dependence and water equivalence. A new film (EBT-XD) has been assessed for its suitability for the dosimetry of stereotactic radiosurgery (SRS) applications.
Materials/Methods:
Glass plate compression
Black template for consistent film positioning
EPSON 10000XL scanner
96dpi resolution
Triple channel correction
Film QA Pro
Film-Dose linear scaling method
Calibration curves for red, green and blue channels were created in the range of cGy for EBT-XD and its predecessor EBT-3. Ten film pieces were irradiated in a nominal 6MV linac. The film was scanned using an EPSON Expression 10000XL scanner and the analysis was performed in FilmQA Pro software (Ashland ISP Inc, NJ, USA). Film dosimetry uncertainties were assessed for typical SRS fields, including lateral scanner effects at high doses. Both EBT-XD and EBT-3 films were used in-phantom for treatment dose verification of typical Linac-based and Gamma Knife (GK) stereotactic radiosurgery within the STE2EV anthropomorphic phantom (CIRS, VA, USA). The dosimetry methodology for a forthcoming UK dosimetry audit of SRS treatment was utilised.
Figure 1: Setup used for scanning film
Results:
EBT-XD film has lower optical density than EBT-3 throughout the dose range tested. EBT-XD was more suitable for high-dose applications because of lower lateral scanner uncertainty. For the width of the film sizes that will be used in the SRS audit (50 mm) and the typical doses measured, the lateral scanner effect was estimated to be of the range of 0.5% for EBT-XD and 3% for EBT-3. Higher agreement between TPS and film dose distributions was seen for EBT-XD using both single and triple channel dosimetry (Table 1). An example is shown in figure 3, of EBT-XD showing a 98.3% gamma passing rate for a GK radiosurgery plan at 3% (local), 1.5 mm criteria.
Table 1: Gamma passing rates, at various criteria, comparing TPS-planned dose and film-measured dose for EBT-XD and EBT3 films, using triple-channel and single (red)-channel dosimetry, with film-dose linear scaling via reference dose films unless otherwise stated, mean data over three separate treatment irradiations with 2500 cGy maximum dose.
Extracted from: Palmer A L, et al (2015)
4A )
4B )
Figures 4A and 4B: Calibration curves and lateral scanner effect evaluation for the two films
Extracted from: Palmer A L, et al (2015)
Figure 2: Film inserted into STEEV phantom and displayed post-irradiation
Figure 3: Isodose distribution comparison between EBT-XD (thin lines) and TPS (thick lines) for a GK treatment.
Conclusions:
We have evaluated the use of a new film, EBT-XD, for SRS dosimetry verification and demonstrated its suitability for a forthcoming audit of radiosurgery services in the UK. EBT-XD is less susceptible to lateral scanner effects and shows better agreement to TPS dose distributions than EBT-3 in linac-based radiosurgery dose verifications. EBT-XD also showed excellent agreement with TPS dose distributions in GK radiosurgery.
References: Palmer A L, Dimitriadis A, Nisbet A and Clark C H (2015) Evaluation of Gafchromic EBT-XD film, with comparison to EBT3 film, and application in high dose radiotherapy verification. Phys. Med. Biol –52