Room: Exhibit Hall
Purpose: The STEEV anthropomorphic skull phantom (CIRS, Norfolk, VA) allows for a clinically realistic simulation of the SRS treatment process. The goal was to utilize this phantom for creation of an end-to-end testing method for our linac-based cranial SRS program. Further, Brainlab SRS cones were commissioned and validated using this phantom.
Methods: Radiochromic films were utilized for spatial and dosimetry accuracy of treatment plan delivery, as well as for output factor measurements for Brainlab SRS cones. Cone factors were measured on NovalisTx 6MV-SRS beam, at 1.5cm depth, 100SAD with a 2.0x2.0 cm jaw size. They were validated against published data as well as measurements using the SunNuclear Edge diode. 3 different treatment planning scenarios were created using the STEEV phantom: Simple cylinder, C-shape high-dose gradient target, and a small target using the 4mm cone. Exactrac was used for treatment positioning. Film measurements were taken for all plans. Chamber measurements were taken for the cylinder and C-shaped targets only.
Results: Brainlab SRS cone factors measured using radiochromic film were (0.602,0.715,0.758,0.816,0.854,0.864) for (4,6,7.5,10,12.5,15)mm cones respectively. All factors were within 2% of diode measurements after applying small field output-correction factors from IAEA-AAPM 483. Exactrac/radiation isocenter coincidence was determined to be <0.5mm in all directions prior to treatments, and intra-fractional positioning verifications were all <0.5mm. For the cylinder and C-shaped targets, chamber measurements were between 0.3-1.3% higher than planned doses and film gamma pass rates were >99.0% at 2%/2mm. The 4mm cone target peak dose was 0.5% lower than planning, and film gamma pass rates >95% at 3%/0.5mm.
Conclusion: An end-to-end testing method was created, evaluated, and implemented for our SRS QA program integrating the STEEV phantom. Dosimetric results using this method showed good accuracy and precision of our SRS delivery, leading to a higher clinical confidence using these treatment techniques.