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Specific Patient Exit Dose Verification Base On Treatment Planning System and Experimental Data

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S Saboori

S Saboori1*, M Schmidt2, R Mueller3, M Mohammadi4, (1) Universitatklinikum Erlangen, Erlangen, Deutschland (DEU), (2) ,,,(3) ,,,(4) Royal Adelaide Hospital,

SU-E-T-58 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
In our center we have developed an exit dose verification technique enable us to check two dimensional patient exit dose at EPID level. This system works base on collected patient portal dose images. A series of pre calculated calibration factors which depends on patient thickness, field size and air gap between patient and detector were developed.

Methods:
Two dimensional transmitted portal dose images (PDI) utilized to compare them with predicted portal dose maps calculated using treatment planning system (TPS).
This study investigates the effect of phantom thickness, air gap size between the phantom and the EPID, field size on portal transmitted dose images. A model performed to find the optimal calibration conditions, for which the field size dependence minimized. To validate the model two phantom models including: a cubic solid water and a patient model phantoms were used. To determine the calibration parameters of EPID model, several measurements have been performed using a two dimensional ionisation chamber detectors array (Matrixx by IBA) and the results were compared with those calculated dose images using treatment planning system (Pinnacle3 V9). For the acquisition of portal images of IMRT fields, an in-house software (MATLAB) was used gamma method to compare portal dose images and predicted portal dose maps.

Results:
The EPID model succeeded in predicting the EPID response for various field sizes and offsets within 3% of measurements.
This method is still in early stages, but for the first few anal cancer IMRT patients evaluated an average of 93% of points passed the criteria of gamma method with the criterion of deviation of 5% and 5mm.

Conclusions:
Although we are in developing phase, the result showed that it could be used for clinical routine patient dose verificatio.

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