RESUMEN
AIM: The aim is to evaluate the use of digital megavolt imager (DMI) aS1200 in portal dosimetry with flattening filter free (FFF) beams. MATERIALS AND METHODS: Dosimetric properties of DMI is characterized at 6MV FFF beams for signal saturation, dose linearity, dependency on dose-rate and source-detector distance (SDD), signal lag (ghosting), and back scatter. Portal dosimetry is done for twenty volumetric modulated arc therapy (VMAT) based stereotactic body radiotherapy (SBRT) plans for the treatment of liver metastases and the results are compared with repeated measurements of Octavius 4D. RESULTS: The detector signal to monitor unit (MU) ratio drops drastically below 25MU. The detector linearity with dose is within 1% and no evidence of signal saturation as such. The aS1200 response variation across various dose rates and SDD is <0.4% and <0.2% respectively. The effect of ghosting increased distinctly at higher dose rate but however it is negligible (0.1%). The impact of back scatter is <0.3% because of additional shielding provided at the back of the detector. The portal dosimetry results of SBRT QA plans evaluated at the gamma criteria of 2mm/2% (DTA/DD) both under global and local mode analysis has shown an average gamma passing rate of area gamma (<1) 97.9±0.8% and 96.4±0.9%. The SBRT QA results observed in aS1200 are inline and consistent with Octavius 4D measured results. CONCLUSION: The characteristics of aS1200 evaluated at FFF beams have shown its potential ability as QA tool and can be used in SBRT QA for liver metastases with greater confidence.
Asunto(s)
Neoplasias Hepáticas/cirugía , Órganos en Riesgo/efectos de la radiación , Aceleradores de Partículas/instrumentación , Fantasmas de Imagen , Garantía de la Calidad de Atención de Salud/normas , Radiocirugia/métodos , Planificación de la Radioterapia Asistida por Computador/métodos , Algoritmos , Humanos , Neoplasias Hepáticas/secundario , Pronóstico , Radiometría/métodos , Dosificación Radioterapéutica , Radioterapia de Intensidad Modulada/métodosRESUMEN
STUDY DESIGN: Cadaveric biomechanical study. PURPOSE: We compared the “skipped segment screw” (SSS) construct with the conventional “all segment screw” (ASS) construct for cervical spine fixation in six degrees of freedom in terms of the range of motion (ROM). OVERVIEW OF LITERATURE: Currently, no clear guidelines are available in the literature for the configuration of lateral mass (LM) screwrod fixation for cervical spine stabilization. Most surgeons tend to insert screws bilaterally at all segments from C3 to C6 with the assumption that implants at every level will provide maximum stability. METHODS: Six porcine cervical spine specimens were harvested from fresh 6–9-month-old pigs. Each specimen was sequentially tested in the following order: intact uninstrumented (UIS), SSS (LM screws in C3, C5, and C7 bilaterally), and ASS (LM screws in C3–C7 bilaterally). Biomechanical testing was performed with a force of 2 Nm in six degrees of freedom and 3D motion tracking was performed. RESULTS: The two-tailed paired t-test was used for statistical analysis. There was a significant decrease in ROM in instrumented specimens compared with that in UIS specimens in all six degrees of motion (p<0.05), whereas there was no significant difference in ROM between the different types of constructs (SSS and ASS). CONCLUSIONS: Because both configurations provide comparable stability under physiological loading, we provide a biomechanical basis for the use of SSS configuration owing to its potential clinical advantages, such as relatively less bulk of implants within a small operative field, relative ease of manipulating the rod into position, shorter surgical time, less blood loss, lower risk of screw-related complications, less implant-related costs, and most importantly, no compromise in the required stability needed until fusion.