Room: AAPM ePoster Library
Purpose: To assess the ability of the PTW Octavius 4D Modular Phantom (O4D) to perform quality assurance for single-isocenter multiple brain metastasis stereotactic radiosurgery and determine what fraction of lesions are measurable on the 11 x 11 PTW 1000 SRS detector array.
Methods: The O4D is a water equivalent cylindrical phantom equipped with an insert for the 1000 SRS array. The array is composed of 977 liquid-filled ionization chambers that are spaced 2.5 mm apart in the inner 5.5 cm x 5.5 cm square and 5 mm apart in the outer 11 cm x 11 cm square. When the board rotates, it creates a cylinder with a diameter of 11 cm and a length of 11 cm. An inclinometer is placed on the gantry that allows for the O4D to rotate in synchronization with the gantry. 15 SRS patients plans with multiple targets (2 – 11) created with Brainlab Elements Multiple Brain Metastases 2.0 were recalculated on the O4D and analyzed using Verisoft with a gamma criteria of 3%/1mm. A target was considered measurable if it lay within a cylinder with a diameter of 10 cm and a length of 10 cm centered on the isocenter to allow for a 1 cm margin to cover the dose distribution of a peripheral target.
Results: There were 77 targets amongst the 15 patients. 61 of these targets fell within the required cylinder (79%). The average gamma passing rate was 98.39% with a sample standard deviation of 1.8%.
Conclusion: The O4D is of a large enough size that a majority of targets can be QA’d for single-isocenter multiple brain metastasis stereotactic radiosurgery. The close detector spacing allows for very high resolution sufficient for patient-specific dose verification.
Funding Support, Disclosures, and Conflict of Interest: Research funded in part by a vendor grant.