SU‐E‐T‐63: Small Field Verification with a New 2D Liquid Ionization Array

Purpose: A new 2D liquid ionization array with high resolution has been designed for the verification of treatments in Stereotactic Radiosurgery(SRS) and Radiotherapy (SRT). Methods: Firstly, a small prototype has been built. It is a matrix of 128 pixels arranged in a 16×8 rectangle filled with non‐ultrapure isooctane. Each detector has a 2×2 mm2 section and a thickness of 0.5 mm. This array has been characterized by studying the dependencies with dose,dose rate, field size, depth and the angle of incidence. We have used this 2Darray in the quality assurance of a linac by measuring output factors, transmission, tongue and groove effect and garden fence test. We have used it also for the verification of an IMRT treatment. Based on this characterization a final prototype has been made consisting ina square matrix of 256 detectors which covers 5×5 cm2. Each detector has a 3×3 mm2 and a thickness of 0.5 mm. This detector has been used to commission a machine model in Pinnacle treatment Planning System which corresponds to a micromultileaf from BrainLab attached to a Mevatron Primus from Siemens. Results: Dependencies of different parameters are low enough except for the angle of incidence. To overcome this problem we have calculated a correction factor depending on the angles. The output factors measurements agree within 1% with unshielded diode measurements. The measurement of the IMRT treatment has been compared with a CC01 chamber and EDR2 films showing and excellent agreement (100% passing rates for 1.8% and 1.8 mm tolerances). The final machine model accepted in Pinnacle agrees with the 2D detector considering gamma coefficient tolerances of 2% and 2 mm. Conclusions: A new 2D array detector with high spatial resolution has been characterized and used in different tests showing excellent properties for the quality assurance in radiotherapy.