Verification of intensity modulated profiles using a pixel segmented liquid-filled linear array

Abstract

A liquid isooctane (C8H18) filled ionization chamber linear array developed for radiotherapy quality assurance, consisting of 128 pixels (each of them with a 1.7 mm pitch), has been used to acquire profiles of several intensity modulated fields. The results were compared with film measurements using the gamma test. The comparisons show a very good matching, even in high gradient dose regions. The volume-averaging effect of the pixels is negligible and the spatial resolution is enough to verify these regions. However, some mismatches between the detectors have been found in regions where low-energy scattered photons significantly contribute to the total dose. These differences are not very important (in fact, the measurements of both detectors are in agreement using the gamma test with tolerances of 3% and 3 mm in most of those regions), and may be associated with the film energy dependence. In addition, the linear array repeatability (0.27% one standard deviation) is much better than the film one ( approximately 3%). The good repeatability, small pixel size and high spatial resolution make the detector ideal for the real time profile verification of high gradient beam profiles like those present in intensity modulated radiation therapy and radiosurgery.

Characterisation of a two-dimensional liquid-filled ion chamber detector array using flattened and unflattened beams for small fields, small MUs and high dose-rates

“…Pardo et al (2005) designed and validated a liquid filled ionisation chamber (LIC) array for use in high dose gradients. The high spatial resolution and repeatability of LICs makes them an ideal detector for intensity modulated fields (Pardo et al 2006). The ability of the LIC to perform profile and penumbra measurements accurately has been assessed by González-Castaño et al (2011).…”

Characterization of a two-dimensional liquid-filled ion chamber detector array used for verification of the treatments in radiotherapy

“…Pardo et al . [18] investigated the verification of intensity-modulated profiles using a pixel segmented liquid-filled ion chamber linear array and concluded that good repeatability, small pixel size, and high spatial resolution make the detector ideal for profile verification of high-gradient beams utilized in IMRT and SBRT. Pena et al .…”

Radiochromic film is superior to ion chamber arrays for IMRT quality assurance

“…If higher resolution is required, it has been shown that simple mechanical systems can be used to make small shifts to the whole array to easily give 1 mm resolution. 11,12 Arrays have been extensively validated using both point detector scans 8,9 and film techniques, 13 and the conclusion of those authors is that ion chamber arrays measure dose distributions correctly. From the data presented, it would seem that an array can also be used for daily output checks with the central ion chamber of the array replacing a separate measurement of ion chamber in phantom.…”