SU‐DD‐A2‐06: EPID Portal Dosimetry for IMRT Validation

Purpose: To evaluate portal dosimetry using EPID as an alternative method for patient specific IMRT pre‐treatment verification. Dose profiles obtained using EPID and EDR2 film are compared and analyzed. Method and Materials: A total of 40 fields from patients' IMRT plans were retrospectively analyzed. 35 of which were planned with 23MV and 5 fields with 6MV. Hybrid plan was created using Eclipse TPS for each patient. All fields were measured using EPID and EDR2 film at gantry angle of 0°. Film was placed transversely into a solid water phantom at the depth of 10.3cm. EPID imager was set to 105cm. Dose profiles for all fields were compared. γ index proposed by Low D.A. et al was used to quantify the disagreement between measured and planned dose distributions. Criteria used for γ index was set at 3%/3mm. To exclude the data outside of treatment fields, only dose higher than 20% of maximum dose was included for calculation. γ values were grouped with different dose ranges and averaged for both and EPID and EDR2 film. Results: Both EPID and EDR2 film data show good correlation between measured and planned dose. Average γ value was 0.63±0.03 for EPID and 0.61±0.03 for film dosimetry. It is statistically insignificant between the results from EPID and EDR2 film using 6MV and 23MV. However, a significant difference of γ index for the dose less than 40% of maximum dose ( p <0.05) is observed using EPID and EDR2 film. Conclusion: Use of EPID to measure IMRT pre‐treatment QA appears a comparable result to film dosimetry. In advent of digital imaging, EPID dosimetry would be a quick and easy alternative for IMRT planning verification.