Sci-Fri PM: Delivery - 06: A generalized solution to the wide field array calibration method.

Abstract

Multi detector arrays are commonly used in radiation oncology for IMRT and Linac QA. The calibration of detector arrays is usually based on the wide field calibration technique. Unfortunately small beam changes between measurements will result in large error propagation. The present work introduces a generalized modified version of the wide field calibration method, robust against measurement to measurement variation. Our generalized framework uses an unlimited number of measurement pairs, n geometric positions providing n(n-1)/2 pairs. We solve this large over determined linear system using least squares with gradient method. Measurements were made on an Elekta synergy 6 MV beam with two IBA Matrixx detectors, each containing a 32 × 32 array (1024) of vented pixel ionization chambers. Data acquisition was by the IBA Omnipro Advance software, version 1.2 running in the "ONLINE" cine mode with a 10 sec integration time. Continuous beam sampling (10 seconds long) measured over 10 minutes demonstrated why consistent calibration using the conventional wide field calibration is a challenge. Overall signal changes of 1.6%, flatness changes of 0.3% and the beam symmetry changes of 0.2% over the full 10 minute beam-on time were observed. For the purpose of testing and demonstration of our method, we have chosen to make measurements in 5 geometric orientations relative to the beam, including 1 reference position, 2 rotations and 2 translations. With our method we were able to calibrate all 1024 detectors to better than 0.6% total uncertainty as demonstrated against inter and intra MatriXX comparison.