SU-E-T-738: The Limit of Detectability and Resolution of a Cylindrical Diode Array Phantom for VMAT and SRS Quality Assurance




To determine the limit of detectability and resolution of a cylindrical QA phantom for quality assurance of VMAT and SRS procedures in small field sizes.


Eight different square field sizes were delivered onto a cylindrical QA phantom with 1386 diodes at gantry angles of 0, 90, 180 and 270 degrees, using a 6 MV beam at a maximum dose rate of 600 MU/min on a dedicated stereotactic linear accelerator with 2.5mm and 5mm leaves. After verifying pass rates of normal square field sizes, errors were introduced into the quality-assurance analysis, including an expansion of the field size by 1 mm for all jaws on all sides, an expansion of the field size by 1 mm on one side of the X and Y jaws,MLC errors, and a table shift by 1 mm vertically and longitudinally at 270 and 90 degrees and a table shift of 1mm laterally and longitudinally for angles of 0 and 180 degrees. Treatment planning system calculated dose distributions for measured square field size dose data was imported into the QA phantom software and compared against a treatment planning system calculated 3D dose distribution. Percent pass rates were computed using gamma analysis with 2mm/2% analysis criteria with a background threshold value of 30%.


QA software comparison between measured fields and treatment planning system calculations indicate that the passing rate of square fields decreases significantly when errors are induced in the quality assurance analysis. Adding two opposing MLC leaves on opposing sides of each square field in the treatment planning system produced a 31.2% average difference from normally measured square field sizes compared between the treatment planning system and what was measured by the QA software.


This work indicates the potential for detecting errors in VMAT quality assurance in small field sizes using a cylindrical QA phantom.