TU‐C‐108‐07: Transmission Detector Array System for RT Delivery Monitoring: Validation Testing

Abstract

Purpose: To quantify the influence of the gantry‐mounted Delta4‐AT detector array on beam quality, and the detection sensitivity to machine output and MLC position deviations. Methods: A Delta4‐AT prototype was attached to the accessory holder mount of a Varian 21EX linear accelerator. The system utilizes an array of ∼4,000 transmission diodes (40 per MLC leaf, every 2.5mm along the direction of motion) to measure the fluence of photon beams during treatment deliveries. To quantify the influence from the detector array on beam quality, the percentage depth dose curves were measured with an IBA PFD3G diode in water and compared between with and without detector array. A static field with comb‐shaped MLC pattern was used to evaluate the systems ability to detect machine output and MLC position deviations. 1–5% MU deviations and 1–5 mm MLC offsets were introduced to the static plan. The modified plans were then delivered and the measurements by Delta4‐AT were compared to the original delivery. Results: For the 6MV beam tested in this study, the transmission reduction due to the presence of the detector array is ∼1% at both dmax and 10 cm depths for multiple field sizes. The skin dose measured at 1mm depth increases by < 0.5% relative to dose at dmax for field sizes up to 10cm, and by 4% and 7% for 20×20cm and 34×34cm fields, respectively. For plans with introduced MU deviations, the detected MU errors matched the planned errors within 0.8%. MLC position errors of 1‐5 mm are also identifiable through measured dose profiles by Delta4‐AT. The measured leaf position deviation is within 0.5 mm from the actual offsets introduced. Conclusion: The prototype Delta4‐AT transmission detector array has small influence on the beam quality, and successfully detected machine output deviations and MLC position offsets as low as 1% and 1mm, respectively.This research was partially supported by a grant from Scandidos Inc.