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Clinical Dose Response Characterization of the PTW OCTAVIUS Detector 1000 SRS

Joseph Driewer

J Driewer*, B Morgan, D Zheng, S Zhou, University of Nebraska Medical Center, Omaha, NE

MO-B-Salon EF-5 Monday 10:00:00 AM - 12:00:00 PM Room: Salon EF

Purpose: The PTW OCTAVIUS 1000 SRS is a novel planar array of 977 liquid ionization chambers on fine grid spacing. In this study we examine the dose response of the 1000 SRS and demonstrate its use for 2D IMRS QA.

Methods: The 1000 SRS was exposed by a Novalis 6MV beam for dosimetric tests. Response was characterized by: reproducibility, linearity, nominal dose rate, field size and depth, and angular dependence, and TMR measurements. Finally, a 15 beam IMRS plan was computed and delivered on a 1000 SRS phantom.

Results: A reproducible and linear dose response over 5 cGy to 6 Gy was observed. Response per cGy was within 0.2%. A dose rate dependence of nearly 3.5% from 160 MU/min to 800 MU/min was seen, whether the device was cross calibrated at 160 or 480 MU/min. Response dependence on field size and depth was observed and smallest at shallow depths. The angular dependence was about 2% when the beam entered from the back of the detector and was asymmetric between left and right side irradiations. Gamma analysis on measured and planned IMRS dose distributions yielded 95.6% passing rate at 2%/2mm and 99% at 3%/3mm, which compared favorably with standard film and ionization chamber measurements.

Conclusion: A linear, reproducible dose response over a wide dose range indicates that the detector would be suitable for varying dose/segment and dose levels often seen in IMRS patient specific QA. Dense detector packing contributed to acceptable QA results on a small IMRS field. A nearly 3.5% difference over a range of nominal dose rates, as well as field size and depth dependence, indicates that the choice of cross calibration field size and dose rate should be driven by clinical practice. Angular dependence may be mitigated by orienting the detector perpendicular to the beam central axis.

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