Purpose: Two‐dimensional (2D) detector arrays for QA of volumetric‐modulated‐arc‐therapy (VMAT) plans show angular dependence. An angular correction‐factor‐table (CFs) minimize this effect, but assumes: (1) the response of all ion chambers (IC) is identical per angle, (2) the ICA response from 0–180° = 180–360°, and (3) the ICA response is independent of the direction of rotation. We investigated whether these assumptions are correct and developed improved CFs. Methods: Measurements were done with a 2D ICA (Matrixx®, IBA Dosimetry). Dose was calculated with the AAA algorithm (Eclipse v8.6, Varian Medical Systems). Default angular CF tables were available for 0–180° and 0–360°. Custom CFs were created by dividing the calculated dose to each IC by the measured dose from static open‐fields delivered every 5°, for both coronal (couch included) and 2 sagittal (couch avoided) orientations. The CFs were tested with open‐field arcs and VMAT plans, comparing results using gamma analysis (3%, 3mm). Additionally, measurements of 1cm‐wide fields delivered clockwise (CW) vs. counter‐clockwise (CCW) were compared to examine effects of ICA structure. Results: The angular response of individual IC significantly varies (1stdev<=4.6%). The response from 0–180° vs. 180–360° is significantly different (paired t‐test, p<0.0001). Custom CFs generated greater agreement between measurement and calculation for open‐field arcs and VMAT plans than the default CFs. Custom CFs also improved agreement compared to no CF for open‐field arcs, but not for VMAT plans. The CW vs. CCW tests demonstrated that the dose in the penumbra region of narrow fields is affected by the direction of rotation (<=1%); the cumulative effect of this may be significant for VMAT. Conclusion: Custom CF tables, using measurements from 0–360° and individual chamber CFs rather than a global CF per angle, can improve the measurement accuracy of an ICA. Even with these thorough corrections, discrepancies were still observed for VMAT plans.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging