Multi-parametric Improvements in the CCD Camera-based EPID for Portal Dosimetry

Dosimetric verification of radiation treatment has recently been extended by the introduction of electronic portal imaging devices (EPIDs). Detailed dose response specifications of EPID should be addressed prior to any dosimetric application. The present study evaluates improvements of dosimetric properties of the low elbow camerabased EPID Theraview (Cablon Medical, Leusden, The Netherlands) equipped with a cooled charge coupled device (CCD) for portal dosimetry. The dose response, warmup behavior, stability over long and shortterm scales (throughout a day) were studied. The field size dependency of the EPID response was also investigated and compared with ion chamber measurements under the same conditions. The EPID response without saturation for doses up to 2 Gy was linear for both beam qualities (6 and 15 MV). There was no evident warmup characteristic. The detector sensitivity showed excellent stability in short term [standard deviation (SD) 0.38%]. In longterm stability (over a period of approximately 3 months), a negligible linear decline of 0.01% per day was observed. It was concluded that the cooled CCD camerabased EPID could be used for portal dosimetry, after accurate corrections for the field size dependency and sensitivity loss.