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Anomalous Dose Measurements From Small-Volume Ion Chamber

S Tanny

S Tanny*, N Sperling, D Pearson, E Parsai, University of Toledo Medical Center, Toledo, OH

SU-E-T-50 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: A micro-chamber was used to scan electron profiles to verify flatness and symmetry of the scan plane prior to small-field commissioning measurements. This verification scan showed peak on one field edge and a large tail of out-of-field dose in the scan direction parallel to the ion chamber. Their appearance is inconsistent with the stem effect. This anomaly is not present when a larger volume ion chamber is used. We proposed further investigation to determine the cause and any impacts it could have on small-field dosimetry.

Methods: Scans were acquired using a Wellhofer water phantom. All scans were taken at 100 cm SSD using various size electron applicators. Scans were taken for multiple energies at R90/2 depth in in-line and cross-line directions. The phantom was rotated 90° and the scans were repeated. Scans were acquired with an A-16 and a CC-13 ion chamber using Varian TrueBeam where the effect was observed first and later with an Elekta SL-25 linac to confirm this anomaly.

Results: The dose anomaly is highly reproducible for both accelerators when A-16 chamber is used. The maximum dose-difference between a standard chamber profile and a micro-chamber profile is ~5%. Symmetry values differ by 5.7% when compared to scans taken with a standard scanning chamber. The effect is reproduced when the orientation of the ion chamber is rotated by 90° with the same magnitude and shape. An increase in measured out-of-field dose appears as distant as 8 cm from the field edge.

Conclusions: We have discovered an unexpected anomaly that occurs when performing profile measurements with small-volume chambers. It appears to be an artifact of the chamber orientation and the measurement environment. The measurement environment must be carefully considered before conducting any high-precision QA measurements when utilizing a commercial scanning phantom, particularly with small-volume chambers.

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