TU‐E‐BRB‐04: Confidence Limit Variation for a Single System Following the TG119 Protocol

Purpose: To evaluate the consistency of TG119‐based quality assurance metrics for an IMRT system. Methods and Materials: Four planners constructed treatment plans for the five IMRT test cases described in TG119 using iPlan RT Dose 4.1 software. All plans were delivered on a Varian 6EX to a 30×30×15cm solid water phantom in one treatment session in order to minimize session‐dependent variation from phantom setup, film quality, machine performance, etc. Composite measurements utilized EDR2 film and an Exradin A1SL ionization chamber (sensitive volume of 0.056 cm³). Each film was developed and digitized immediately after exposure. Per‐field measurements were collected using a MapCHECK device at an effective depth of 5cm. All data collected were analyzed using the TG119 specifications to determine quality assurance metrics for each planner separately and then compared. Results: The mean variance of ion chamber measurements for each planner was within 1.7% of the planned dose. Resulting confidence limits were 3.77%, 1.98%, 4.50%, and 5.99%. Confidence limit values determined by composite film analysis were 26.43%, 25.02%, 21.50%, and 56.58%. Using normalization to a film point rather than to ionization chamber, these confidence limits were reduced to 12.27%, 12.73%, 6.96%, and 15.75%. Confidence limits from per‐field measurements were 1.55%, 0.00%, 0.00%, and 2.89%. Conclusions: For a single IMRT system, the accuracy assessment provided by TG119‐based quality assurance metrics showed significant variations across all composite and per‐field evaluations. Performing the TG119 evaluation a single time may not provide an adequate estimation of IMRT system accuracy.