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EBT-XD Gafchromic Films for Multiple Mets Intra-Cranial VMAT SRS Dosimetric Verification

A Hussain

A Hussain1*, A Hudson2 , N Ploquin2 , (1) Western Manitoba Cancer Centre, Brandon, MB, (2) Tom Baker Cancer Centre, Calgary, AB


SU-I-GPD-T-535 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: GAFChromic EBT-XD film is a relatively new type of radiochromic film, suitable for SRS and SBRT applications due to its wide dynamic range from 40 to 4000cGy, improved calibration curve gradients over higher doses, and reduced off-axis scanner effect compared to more common EBT3 films. The purpose of this study was to assess the use and suitability of EBT-XD for dosimetric verification of a variety of intra-cranial SRS plans for multiple mets, with single isocentre and to compare the results to Octavius® 1000-SRS 4D measurements.

Methods: A calibration curve, ranging from 25cGy to 3200cGy was created using single, red-channel dosimetry. Twelve plans were created with 6MV-FFF radiation beam, 1400 MU/min dose rate and 1mm dose grid size plans for patients with multiple brain mets of different sizes (0.1 cc to 36 cc). All the plans were delivered on a TrueBeam linac with single isocentre. Films were placed in StereoPHAN™ during exposure. The 3%/3mm, 3%/2mm, 3%/1.5mm and 3%/1mm 2D-gamma criteria were calculated with reference to global doses. A dose threshold of 10% to the maximum was used in gamma analysis.

Results: All the 12 plans measured with EBT-XD passed the 3%/3mm and 3%/2mm, and 3%/1.5mm gamma criteria. At 3%/1mm, gamma passing rate was 83%(10). Globally determined gamma passing rates for Octavius® 4D were 100%(12) at 3%/3mm, 3%/2mm and 3%/1.5mm, whereas 92%(11) for 3%/1mm. A reasonable agreement (SD = 3.8%) was found between the EBT-XD films and Octavius® 4D results.

Conclusion: We have successfully assessed the feasibility of the EBT-XD films for dosimetric verification of simultaneous treatment of multiple intracranial mets on a TrueBeam linac. Based on our findings a 3%/1.5mm globally determined 2D-gamma with EBT-XD is a reasonable criteria for assessing the dose delivery accuracy of such complex plans. Dosimeteric results from Octavius® 4D also support the films measurement accuracy.

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