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To perform a dosimetric verification with EBT-XD film of a non-coplanar plan for multiple brain metastases monitored by a commercial optical surface tracking system (OST).

A 3-camera OST system was used (Catalyst HD^TM, C-RAD, Sweden) on a Varian Truebeam STx linac with a 6DoF couch. The set-up accuracy and agreement between the OST system and cone beam CT (CBCT) and kV-MV imaging at respectively couch angles 0^o and 270^o were examined. Then dosimetric film measurements at 3 depths in the Rando-Alderson phantom were performed based on a single isocenter non-coplanar VMAT plan containing 4 brain lesions. Initial set-up of the phantom was performed with CBCT at couch 0° and subsequently monitored by Catalyst HD^TM at all other couch angles. The absolute measured film doses were compared to the calculated TPS doses.

For repeated tests with the Rando-Alderson phantom, the deviations between rotational and translational isocenter corrections for CBCT and OST system are always within 0.2° (pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0° and 270°, respectively. Dose deviations between the absolute measured and calculated doses in the center of the 4 PTVs irradiated with a non-coplanar plan monitored by the OST system are -1.2%, -0.1%, -0.0% and -1.9% for PTV-1, PTV-2, PTV-3 and PTV-4, respectively. The dosimetric agreement between film and TPS presented by a local gamma evaluation criterion of 2%/2mm and 3%/1mm (both with cut-off dose value of 20%) yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for PTV-1, PTV-2, PTV-3, and PTV-4 respectively.

Patient positioning for a non-coplanar single isocenter VMAT treatment of multiple brain metastases can be monitored with an OST system with submillimeter and subdegree accuracy.



Figure: A five arc single isocenter non-coplanar VMAT plan with 4 PTVs, verified using film dosimetry with EBT-XD films in the first 3 slices of the Rando-Alderson phantom head.



Table:The dosimetric agreement between film and TPS doses is presented by the deviations between measured and calculated doses in the center of the 4 PTVs and by a local gamma evaluation criterion of 2%/2mm as well as 3%/1mm for the 4 PTVs irradiated with single isocenter non-coplanar VMAT and monitored using Catalyst HD^TM . To put these numbers into perspective, also the results for PTV-1 irradiated in a co-planar set-up (couch 0˚) are given (highlighted in bold).

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