What is film gamma in IMRT dosimetry?

Film gamma in IMRT dosimetry refers to the evaluation of the level of agreement between the measured dose distribution in a film dosimetry system and the planned dose distribution generated by the treatment planning system (TPS). It is a critical parameter used to assess the accuracy of IMRT dose delivery.

In IMRT, multiple beams of radiation are shaped and modulated to conform to the target volume while minimizing dose to healthy tissues. The planned dose distribution is carefully designed to achieve the desired dose prescription. To verify the accuracy of the delivered dose, film dosimetry is often employed.

Film dosimetry involves the use of radiochromic films, which change color when exposed to radiation. The exposed films are then scanned and analyzed using specialized software to obtain the measured dose distribution. The comparison between the measured dose distribution and the planned dose distribution is performed using gamma analysis.

Gamma analysis involves calculating the gamma index (γ) for each point in the measured dose distribution. The gamma index is a measure of the combined deviation of a measured dose point from the corresponding planned dose point in terms of dose difference (ΔD) and distance to agreement (DTA).

The gamma evaluation criteria define the acceptable limits for both ΔD and DTA. Typically, a gamma value of 1 or less is considered acceptable, indicating that the measured dose is within the specified tolerance of the planned dose. Higher gamma values indicate larger deviations between the measured and planned dose distributions.

By evaluating the film gamma in IMRT dosimetry, physicists can assess the accuracy of the dose delivery and identify any potential errors in the treatment plan or delivery process. This helps ensure that patients receive the intended dose distribution, minimizing the risk of underdosing or overdosing critical structures.