Room: Exhibit Hall
Purpose: Quality Assurance checks of HDR afterloader units required by NRC (part 35, subpart H) includes source positioning accuracy to within Â±1 millimeter. Common techniques include use of calibrated gafchromic film or direct observation on a source position check ruler. The purpose of this work is to take advantage of the slimness (1Î¼m) of PTWâ€™s microDiamond detector for an accurate source position check QA.
Methods: Two microDiamond detectors were used. The tips of the detectors were placed in contact, face-to-face, along the direction of source travel. The reference source position was pre-determined to be 1490mm. The location of the tips coincided with this dwell position of the source (Figure 1). Inaccuracies of +/- 1mm, and +/- 2mm in the position of the source were introduced in the treatment console to test the response of the detectors. 30s of integral charge was measured after the source was in position. Three readings were taken at each position and the mean value was used in the evaluation. The difference in charge between the two detectors for each location was analyzed.
Results: The correct calibration factors were applied to the detectors. The difference in charge between the two detectors when the source was at the reference position was 0.15%. Introducing a 1mm error in position resulted in a mean difference of 3.11%, and a 2mm error led to a mean difference of 6.08%. Detector response was not symmetric for proximal versus distal inaccuracies; it was also nonlinear (Figure 2). But the difference in readings over the region of interest was linear, characterized by a linear equation with RÂ² = 99.6% (Figure 3).
Conclusion: Diamond detectors are sufficiently sensitive to detect small errors in source position, and will be used with an appropriate jig as part of a solution that can quantify source position QA.