Film Gamma in IMRT Dosimetry: Accuracy & Assessment

Film gamma analysis is the standard method for verifying IMRT (intensity-modulated radiation therapy) dose delivery accuracy using radiochromic film.

Principle

Compares measured 2D dose distribution (film) vs. planned distribution (TPS) at each point using two criteria simultaneously:

• Dose Difference (ΔD): % deviation from prescribed dose.
• Distance to Agreement (DTA): Spatial distance to a point with matching dose.

Gamma Index (γ)

γ = √[(ΔD/ΔD_crit)² + (DTA/DTA_crit)²]

Common criteria: 3%/3 mm or 2%/2 mm. γ ≤ 1 = pass; γ > 1 = fail.

Clinical Role

• Pre-treatment QA for each patient's IMRT plan.
• Identifies delivery errors (MLC calibration, dose rate, latency).
• Complements ion chamber and EPID QA.

High-resolution film (e.g., EBT3) and dedicated analysis software (e.g., OmniPro, VeriSoft) are standard. Results guide plan approval or revision.