Estimation of midpoint dose for cervical cancer patients using EPID

Purpose: To estimate the midpoint dose delivered to cervical cancer patients treated by conventional technique using Electronic Portal Imaging Device (EPID).

Materials and Methods: Clinac 2100 equipped with aS500 EPID was used in this study. A methodology was developed to generate a Gy/Calibration Unit (CU) look up table for the determination of midpoint dose of patients. 25 patients of cervical cancer were included in this study and the delivered dose to the midpoint of the patients was estimated using EPID. The deviation between the prescribed and the measured dose was calculated and analysed.

Results: EPID showed a linear response with increase in Monitor unit and the Gy/CU look up table was validated for different field sizes and depth. 250 fields were measured for 25 patients, 10 measurements per patient, weekly once and for 5 weeks. The results show that out of 250 measurements, 98% of the measurements are within ±5% and 83.2% are within ±3% for with a standard deviation of 1.66%.

Conclusion: The outcome of this study proves the efficacy of this methodology for the estimation of midpoint dose using EPID with minimal effort, time and without any inconvenience to the patients unlike other in-vivo dosimeters.