The use of aSi EPID for in vivo dosimetry in photon beams: clinical experience

摘要

In vivo dose verification is used to prevent major deviations between the prescribed dose and the dose really delivered to the patient. This work presents a quick and simple alternative method for verification of dose delivered to the patient using photon beams. During the treatment session, a transit dose is measured with the EPID and the dose in the patient is estimated from back projection of the portal dose. This method was validated by phantom measurement using an ionisation chamber. Central axis doses estimated by this formalism were compared with measured dose. The feasibility of the method and its applicability in clinical use has been evaluated on patients treated with conformal therapy (18 patients, 325 treatment beams over 8 months) and with IMRT (9 patients, 165 treatment beams). Using this amount of data, the definition of action levels became possible. The uncertainties are within the accepted tolerance of classical in vivo dosimetry (1SD = 3.5% for conformational beams and 1SD = 3.6% for IMRT). The proposed method for in vivo dose verification is very simple to implement and to use in clinics. Measurements can be repeated during several sessions giving the opportunity to built new strategies for the validation by statistical evaluation of the data. The trending of in vivo dose along the treatment becomes also possible.