SU‐E‐T‐213: Preliminary Testing of a 2‐D Fluence Measurement Prototype Device (Delta4‐AT) for In‐Vivo Patient Verification Dosimetry

Purpose: To perform preliminary sensitivity testing on the performance of a prototype device (Delta4‐AT) that measures the 2D linear accelerator photon fluence during IMRT treatments. The measured fluence is then projected onto the Delta4 phantom to assess the dosimetric impact. Methods: Two Smart‐Arc patient treatment plans were delivered on the Delta4 for pretreatment dose (PT) verification with the At‐treatment (AT) Delta4 prototype. The measurement process of Delta4‐AT is the same as the Delta4‐PT. Delta4‐AT acquires the fluence data and reconstructs a semi‐measured dose in the Delta4 geometry based on daily changes such as MLC position and output. To assess the sensitivity, MLC patterns for the two patients were modified to introduce two sets of known errors: 1) Systematic error—all leaves moved relative to isocenter, and 2) Random error— selected leaves shifted. Data was collected and an inter‐comparison of the TPS calculated dose, AT semi‐measured, and PT semi‐measured dose was performed. All data was analyzed using the Delta4 software. Results: Measurements showed a good gamma index (GI) agreement of 90.0% (3%/3mm) between the Delta4‐AT and Delta4‐PT when small discrepancies were introduced. When larger errors were present, a larger discrepancy existed between both measurement devices partly due to the decreased resolution of the prototype Delta4‐AT. Relative to the TPS dose, both showed good agreement within the GI criteria. Conclusions: The Delta4‐AT prototype together with the Delta4‐PT is a promising solution for the quantification of the dosimetric impact of daily variations in linear accelerators during IMRT delivery. The Delta4‐PT can serve as a vital, addon tool for patient dose verification.