The experience of a developing country using an electronic portal imaging device for the verification of patient positioning and dosimetry in radiotherapy for prostate cancer

Purpose: This is a retrospective study to evaluate the efficacy and safety of routine use of electronic portal imaging device (EPID) in intensity-modulated radiation therapy for localised prostate cancer. Materials and methods: Data from 20 patients with localised prostate cancer treated by radical radiotherapy using intensity-modulated technique in Habib Bourguiba Hospital were analysed to define the action levels for pretreatment planer dose distribution of 100 treatment fields and the set-up errors of 418 portal imaging. Pretreatment planar dose distribution was measured with the EPID. The additional dose from repeated portal imaging was determined with treatment planning system. Results: For all 100 fields, the predicted and the measured planar dose distribution agrees well with mean +/- standard deviation value for gamma(max) = 2.31 +/- 0.57, gamma(avg) = 0.36 +/- 0.07 and gamma(%) <= 1 = 98.94% +/- 0.71%, respectively. For the evaluation of set-up errors, the mean total errors with 1 SD in the lateral, longitudinal and vertical directions were 0.11 +/- 0.44 cm; 0.02 +/- 0.37 cm and -0.02 +/- 0.21 cm, respectively. The imaging additional dose was evaluated as 1 cGy per monitor unit. Conclusion: EPID is a useful tool to verify pretreatment dose distribution and to assess the correct field position without a significant increase in the absorbed dose due to the repetition of portal imaging.