Online

ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
9319
Poster
Physics
00:00 - 00:00
SBRT/SRS patient-specific QA using GAFchromic EBT3 e FilmQA software
Giuseppe STELLA, Italy
PO-1405

Abstract

SBRT/SRS patient-specific QA using GAFchromic EBT3 e FilmQA software
Authors: Elisa Bonanno.(HUMANITAS-Centro Catanese di Oncologia, Department of Medical Physics, Catania, Italy), Nina Cavalli.(HUMANITAS-Centro Catanese di Oncologia, Department of Medical Physics, Catania, Italy), Andrea Girlando.(HUMANITAS-Centro Catanese di Oncologia, Department of Radiotherapy, Catania, Italy), Anna Maria Gueli.(University of Catania, Department of Physics and Astronomy "E. Majorana", catania, Italy), Carmelo Marino.(HUMANITAS-Centro Catanese di Oncologia, Department of Medical Physics, Catania, Italy), Giuseppe STELLA.(HUMANITAS-Centro Catanese di Oncologia, Department of Medical Physics, Catania, Italy)
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Purpose or Objective

SRS/SBRT treatments require typically pre-treatment (patient specific) procedures to verify, for each patient, the agreement between the measured and calculated dose distribution. In this context, film dosimetry with GafchromicTM is a reference method thanks to high spatial resolution, excellent energy independence and tissue equivalence. The aim of this work is to verify the use of GafchromicTM EBT3 and FILMQATM software for SBRT/SRS patient-specific QA for a routine clinical procedure.

Material and Methods

SBRT/SRS treatment, related to Brain, Lung and Nodes, were calculated using the AcurosXB 13.6.23 algorithm and TPS Eclipse Varian Medical Systems (13.6), VMAT and Flattening Filter Free (FFF) mode (6X-FFF and 10X-FFF beams). GafchromicTM EBT3 calibration was obtained using 6X-FFF and 10X-FFF photon beams and VARIAN TRUE BEAM 2.5 (0-25 Gy); they were digitized with EPSON EXPRESSION 10000 XL, one scan method, transmission mode and analyzed with FILMQA software using triple channel method. To avoid scan-to-scan variability and uncertainty, EBT3 films, exposed to each VMAT arc, were digitized in a single scan with two reference films: a film exposed to 80% dose max of the calculated dose frequency distribution and an unexposed film. To obtain a linear dose scaling using the calibration curve and a more accurate dose map, each film was associated to a calibration curve created using 0 Gy, 20% and 40% of calculated max dose and finally the reference film (80% of calculated max dose). The agreement between calculated and measured dose distributions were evaluated in terms of ɣ passing rate. Criteria were: 3%3mm, 2%2mm, 2%1.5mm, 1.5%1.5mm, 3%1.5mm. Results obtained were then compared with the pre-treatment verification procedures normally used: EPID and EPIQA software.


Fig. 1: mean values and relative errors of the ɣ passing rate, for each criteria, obtained both with the EPID-EPIQA and GAF-FILMQA method.


Fig. 2: results obtained, by relating the modulation coefficient and the ɣ passing rate (criteria 3%1.5mm), with the two verification methods.
Results

For GAF/FILMQA method, mean ɣ passing rate greater than 95% with 2%1.5mm criteria were obtained (better than 7% compared to EPID/EPIQA method results) (fig. 1). GAF/FILMQA method shows to be independent from modulation values obtained from planning, unlike the EPID/EPIQA method (fig. 2).

Conclusion

GAF/FILMQA method fully respects the accuracy and time-consuming criteria for a routine clinical use for SBRT/SRS patient specific QA.