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RESEARCH PRODUCT

TH-C-AUD A-07: Evaluation of the Correction Factor Due to the Lack of Full Scatter Conditions in Cs-137 and Ir-192 Brachytherapy Dosimetric Studies

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Purpose: Use of a finite phantom to derive dose rate distributions around brachytherapysources implies a lack of backscattering material near the phantom periphery. Conventional planning algorithms and newly‐developed 3D correction algorithms are based on physics data under full scatter conditions. Presently, most published Monte Carlodosimetric studies have been obtained using either a spherical phantom (15cm in radius) or a cylinder phantom (40×40cm2). The study objective was to derive a simple relationship to correlate the radial dose function, g(r), obtained for each one of these phantoms to that obtained for an unbounded phantom. Method and Materials: Assuming bare point sources of 137 Cs and 192 Ir , kerma was calculated using Monte Carlo GEANT4 code for 1) a spherical phantom of 40 cm in radius, R, which is assumed an unbounded phantom for r ⩽ 20cm, and 2) spherical phantoms of R=15cm and R=21cm. The later size mimics the scatter conditions of a 40×40cm2 cylindrical phantom for both radionuclides. From the ratio of the dose rate distributions for unbounded/bounded phantoms we derived the relationship between g(r) for both phantoms. Results: Phantom size correction results to g(r) were obtained and fit to 3rd order polynomials (R2 > 0.999) valid for r ⩽ 10cm, which is the clinical range of interest. To validate the method, published dose‐rate distributions for two 137 Cs and 192 Ir sources in bounded/unbounded phantoms were compared with the fits of this study. Agreement was typically within 0.2% over all distances studied. Conclusion: In order to compare the dose rate distributions published for different phantom sizes, a simple expression based on fits of the dose distribution ratios for bounded/unbounded phantoms was developed for 137 Cs and 192 Ir . Using these relations, it was possible to correlate g(r) between bounded and unbounded phantoms for improved accuracy and consistency of clinical dosimetry.