SU‐E‐T‐50: Anomalous Dose Measurements From Small‐Volume Ion Chamber

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.