Investigation of the AGFA high dose cassette for quality assurance in radiotherapy

Abstract

Introduction The Agfa CR system is comprised by the CR 35-X digitizer, the NX workstation and the CR RT1.5 high dose cassette (HDC). The HDC has a nominal tolerance of about 400 MU and is designated for portal imaging in radiotherapy, during irradiation of the patient with the full field dose. Purpose To investigate the characteristics of the HDC and its applicability to radiotherapy quality assurance (QA) tests. Materials and methods The HDC was irradiated with doses in the range of 0.02–4.1 Gy, using different: (a) field sizes, (b) irradiation geometries and, (c) beam energies (6 MV, 15 MV, wedged fields), in order to determine the dependence of its response on the above conditions. Results An exponential equation was found to accurately reproduce dose from pixel values, for single irradiated 10 × 10 cm 2 fields. However, this equation was not accurate for irradiations made using other field sizes, multiple and complex field irradiations. Typical errors were within ±20% but larger errors were observed in some extreme irradiation conditions. The main problems identified were: the increased response of the HDC to lower energy scattered photons and the non-uniformities observed across its area. However, concerning geometry, the accuracy of distance measurements was better than ±0.2 mm. The HDC was also proven useful for testing the dynamic multileaf collimator and the enhanced dynamic wedges. Conclusion The HDC is very useful as a first level QA tool, mainly for geometrical tests but also for dosimetry tests, when relative rather than absolute dosimetry issues are under investigation.