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Experimental Validation of SRS VMAT in Small Volumes. Tolerance Criteria for Plan-Specific QA Verification

M Adria-Mora1*, N Carrasco Vela1 , J Bonaque Alandi1 , F Ballester2,3 , J Gimeno Olmos1,3 , V Carmona Meseguer1,3 , F Lliso Valverde1,3 , J Perez-Calatayud1,3 , (1) Hospital Universitari i Politecnic La Fe, Valencia, Spain (2) University of Valencia, Burjassot, Spain (3) Grupo de Investigacion en Fisica Medica IRIMED, UV - Hospital La Fe, Valencia, Spain


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: The aim was to experimentally validate intracranial Stereotactic Radiosurgery (SRS) with Volumetric Modulated Arc Therapy (VMAT) for small volumes through ionization chamber dose differences and 2D gamma analysis for radiochromic films. These plans were also acquired with Octavius 4D 1000SRS (PTW-Freiburg, Germany) in order to establish tolerance criteria for routine patient-specific QA verifications.

Methods: Plans corresponding to small intracranial benign lesions for five SRS VMAT patients were evaluated. The target volumes ranged from 0.16 to 1.62 cm³ and the prescribed doses varied from 12.5 to 19 Gy. All plans were calculated with Varian Eclipse treatment planning system (v.13) and were delivered by a Varian TrueBeam STx (Varian, CA, USA) equipped with a HD120 MLC. Lucy QA Phantom (Standard Imaging, WI, USA) was employed to evaluate, on one hand, dose differences between planned and measured dose with PinPoint 3D (PTW-Freiburg) ionization chamber and, on the other hand, 2D global gamma 2%/2mm with EBT3 radiochromic films (Ashland, NJ, USA). Finally, 3D local gamma 2%/2mm and 1%/1mm were obtained from Octavius 4D 1000SRS (PTW-Freiburg) verification system.Additionally, for 30 patients, Octavius 4D 1000SRS pre-treatment verification results have been analyzed with the referred comparison criteria.

Results: For PinPoint 3D, relative differences between planned and measured dose ranged from -0.26 to 3.32%. 2D global gamma 2%/2mm calculated from radiochromic films analysis fluctuated from 92.5 to 98.1%. Finally, mean 3D local gamma 2%/2mm and 1%/1mm pass rate based on Octavius 4D measurements was (99.4±0.8)% and (93.1±3.4)%, respectively. The average gamma values for the 30 patients studied are (99.2±1.1)% for 2%/2mm and (93.8±4.7)% for 1%/1mm.

Conclusion: SRS VMAT planning for small volumes has been experimentally validated by means of ionization chamber and planar radiochromic films measurements, and tolerance criteria for plan-specific QA verification have been established for Octavius 4D 1000SRS.


Stereotactic Radiosurgery, Quality Assurance, Intensity Modulation


TH- External beam- photons: intracranial stereotactic/SBRT

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