Introduction. Volumetric modulated arc therapy technique (VMAT) is now widely used in radiotherapy. Verification of the dose delivered to the patient is performed prior to the treatment (pre-treatment mode). However, during the therapeutic session, only the patient's position is verified and monitored. EPID’s (electronic portal imaging device) matrices can measure the intensity of radiation passing through the patient, but the calculation of the dose distribution from this measurement is limited due to the lack of reliable algorithms and software. Therefore, it seems promising to develop a method to estimate the dose in the patient's body based on the measured CU values ​​(calibration units).

Material and methods. The material consists of 53 patients treated for prostate cancer with the VMAT technique. The CU signal is measured during the treatment and its value is then transformed according to the self-developed algorithm into a dose. This delivered dose is then compared with the planned dose in the target.

Results. The performed measurements of the CU’s and preliminary calculations indicate that it is possible to estimate the dose that the patient receives during the therapeutic session. The mean difference between the prescribed and measured dose values is less than 1%, however, there are differences of 17%.

Conclusions. The proposed method can be used in clinical practice for actual dose estimation. The uncertainty of the proposed method was estimated at 5%. In the event of differences above 10%, the treatment realization should be verified by additional tests including patient positioning and technical tests of accelerator.