Abstract
The objective of this theist is to study (1) the image resolution of an electronic portal image devices (EPID), (2)the physical characteristic of the dynamic multileaf collimator(DMLC)and the relationship between DMLC’s spatial resolution and intensity modulated capability. The materials of the experiment are (1) in house made line-pair phantom with 0.1,0.125,0.167,0.2,0.25,0.33,0.5 lp/mm respectively, (2) a DMLC created line pair template with sliding window technique. The line-pair phantom is designed as an objective phantom from which we can utilize modulation transfer function (MTF) method to quantify the image resolution of an EPID, we compared with the other test pattern and the results show consistent. The inspection on the physical characteristic of DMLC shows that the dosimetry precision by DMLC is strongly affacted by the leaves’ traveling stability, which is again affected by leaves’ tolerance setting. The results show that higher dose rate may shorter the exposure time in the expense of higher root mean square error (RMS) of the leave position. However, too tight tolerance setting (<1mm) would induce beam hold off, which cased overall dose error especially when low MU is setting. Clinically, a trade off between dosimetry accuracy and treatment is to use 2mm tolerance setting for DMLC and no more than 400 MU/min dose rate. From our line pair template we test the least spatial resolution and intensity resolution of two DMLC system. We expect these results would provide a reference for a clinical planar an idea of reachable dose gradient, which is the key point in an intensity modulated radiation treatment (IMRT).