Room: Track 3
Purpose: We present an HDR source tracking model (STM) for 3D spatial position detection based on a 2D plane measurement. This model has been utilized for the development of a comprehensive source quality assurance methodology using radiochromic film dosimetry.
Methods: Initially, 2D dose maps of the source were generated based on the TG-43 formalism. Then, STM was developed to incorporate the unique geometric features of all isodose lines (IDL) within any given 2D dose map away from the source. The STM normalized the dose map to its maximum, then quantified the IDLs using blob analysis based on features such as area, perimeter, weighted-centroid, elliptic orientation, and circularity. The Pearson correlation coefficients (PCC) between these features and 3D source coordinates (x, y, z, ?y, ?z) were calculated. To capture real 2D dose distributions, EBT3 film pieces were positioned in a Solid Water® phantom: three above and three below the source and they were exposed simultaneously.
Results: The source center (x,y) was significantly correlated with the low IDL-weighted centroid (PCC=0.99), while the distance to source (z) was correlated with the IDL areas (PCC=0.96) and perimeters (PCC=0.99). The source orientation ?y was correlated with the difference between high and low IDL-weighted centroids (PCC=0.98), while ?z was correlated with the elliptic orientation of the 60-90% IDL (PCC=0.97) for a maximum distance of z = 5 mm. Beyond 5 mm, IDL circularity was significant, therefore limiting the determination of ?z (PCC=0.48). The measured positional errors from the film sets, above and below the source were -0.19 mm and 0.29 mm, respectively.
Conclusion: Isodose line features of a 2D map away from a source can reveal its 3D coordinates. Combined with dosimetry, this technique offers a novel source quality assurance method and has the potential to be used for quality assurance of commercial and customized applicators.
Funding Support, Disclosures, and Conflict of Interest: The authors have no conflicts to disclose. This work was supported partially by the Natural Sciences and Engineering Research Council of Canada Grant No. 386009 and No. 432290. S.A. is a Ph.D. Candidate supported by the scholarship program at King Faisal Specialist Hospital & Research Centre.