Abstract
A scanning liquid‐filled ionization chamber electronic portal imaging device (SLIC‐EPID) and extended dose range (EDR2) films were used to evaluate transmitted dose profiles for homogeneous and inhomogeneous phantoms. Calibrated ionization chamber measurements were used to convert the pixel values acquired from the electronic portal images to dose. Because SLIC‐EPID was developed to have a uniform response for all liquid ionization chambers, the off‐axis dose values were reconstructed using a correction factor matrix, defined as the ratio of the relative EDR2 film and the corresponding EPID dose values measured in air. The transmitted dose distributions in the EPID detector layer were also modeled using a Pinnacle3 treatment planning system (TPS: Philips Radiation Oncology Systems, Milpitas, CA). The gamma function algorithm was then used to assess agreement between transmitted dose distributions measured using a SLIC‐EPID and EDR2 film, and those calculated using the TPS. For homogenous and inhomogeneous phantoms, more than 90% agreement was achieved using gamma criteria of 2% and 3 mm and 3% and 2.5 mm respectively. Our results indicate that the calibration procedure proposed in the present study should be performed if SLIC‐EPID is to be used as a reliable two‐dimensional transmitted dosimeter for clinical purposes.PACS numbers: 87.53.Tf, 87.53.Oq
Highlights
Electronic portal imaging devices (EPIDs) are used to acquire digital images for patient positioning verification
The transmitted dose maps[10,17,18,19,20,21] are back-projected to obtain either exit dose maps or midplane dose maps.[3,10,17,18,19,22] In the second approach, the transmitted dose maps measured using EPIDs are compared with predicted portal dose images, calculated using a treatment planning system (TPS) or another method leading to portal doses.[8,23,24,25,26,27,28]
After applying the correction factor matrix (CFM), we found excellent agreement between the beam fluence maps measured using EPID and those measured using EDR2 film
Summary
Electronic portal imaging devices (EPIDs) are used to acquire digital images for patient positioning verification. The dosimetric calibration procedure includes the conversion of EPID raw pixel values into dose. The term “pretreatment dose verification” is defined as the verification of dose delivery in a phantom before the first radiotherapy session.[4] In vivo dose verification is the actual patient dose measurement using EPIDs. Two methods using measured portal dose distributions called “transmitted dose maps” have been developed to verify the in vivo dose delivered to the patient. The transmitted dose maps[10,17,18,19,20,21] are back-projected to obtain either exit dose maps or midplane dose maps.[3,10,17,18,19,22] In the second approach, the transmitted dose maps measured using EPIDs are compared with predicted portal dose images, calculated using a treatment planning system (TPS) or another method leading to portal doses.[8,23,24,25,26,27,28]
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