Abstract
To describe the patient specific QA (Quality Assurance) protocol implemented for plan integrity verification, in stereotactic radiosurgery (SRS) and radiotherapy (SRT) treatments performed on a novel dedicated frameless image guided radiosurgery system using conical collimator, and a combination of yoked gimbals to cover near2pi solid angle. Startup and commissioning results obtained for planned vs measured dose distributions with several detectors are presented. The patient specific QA protocol includes: A) Review of the approved plan based on AAPM TG 275 report recommendations B) 1D verification using small volume ionization chambers inside an anthropomorphic head phantom C) 2D dose verification of a coronal and sagittal dose plan performed with high resolution 2D Array and respective phantom apparatus D) In-house independent MU (Monitor Unit) calculation using the same formalism of the vendor TPS (Treatment Planning Systems) RayTracing dose-calculation algorithm E) Monte Carlo based secondary dose check & plan QA. A retrospective analysis of the results of the first 15 patients treated is presented, focusing on plan complexity vs QA results. The 1D results obtained for the 15 SRS plans were within ±5% for all reported cases, with a mean percent difference of -1,25%, confirming an overall good agreement and, as expected, a partial volume effect in plans with smaller collimators. For the 2D dose verifications, with a 10% dose threshold, gamma passing rates of 97,5% (coronal) and 96,78 (sagittal) with 3% 1mm criteria, 95,1% (coronal) and 94,3% (sagittal) for 2% 1 mm criteria and 90,7% (coronal) and 90,0% (sagittal) for 1% 1mm criteria were obtained. Moreover, the results showed a correlation between lesion volume or number of collimators used with gamma passing rates. All MU verification results were within ±0,3% and provides an efficient risk mitigation approach for this new delivery technique. The verification results of the first 15 treated SRS plans confirmed point dose and planar measurements in agreement with TPS calculations, with superior results for planes with smaller lesions and fewer collimators. This also represents an integral validation of the image-based alignment system and fine treatment couch movements, as treatments are intrinsically multi-isocentric.
Published Version
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