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On the Dosimetric Effect of Heterogeneities and Finite Patient Dimensions on 60CO HDR Brachytherapy

      Purpose

      Brachytherapy has recently advanced towards patient specific dosimetry with the commercial availability of treatment planning systems incorporating image-based dose calculation algorithms for Ir-192. This marks an improvement over TG-43 based dosimetry that relies on source specific data pre-calculated in a standard sized geometry of homogeneous water, hence disregarding patient-specific radiation scatter conditions and the radiological differences of tissue or applicator materials from water. The aim of this work is to perform a comparative study of the effect of these factors in Co-60 and Ir-192 HDR brachytherapy.

      Materials and Methods

      Ten clinical cases were studied for each of three disease sites considered (gynaecological, esophagus and breast). Two different treatment plans were prepared for each case using an Ir-192 (Ir2.A85-2) and a Co-60 (Co0.A86) HDR source with the TG-43 option of a commercially available system (SagiPlan) and the same planning aim. The plans were exported from the treatment planning system in dicom RT format. Corresponding, patient specific, dosimetry data were obtained from Monte Carlo (MC) simulations using the MCNP6 code. Monte Carlo input files were prepared automatically from the parsing of information in the dicom RT data using a custom software tool (BrachyGuide). Personalized and TG-43 based dose distributions were compared in the 3D anatomical space of each patient using isodose distributions, % dose difference maps, Dose Volume Histograms and relevant indices of clinical interest. The statistical and clinical significance of differences between patient specific (MC) and TG-43 based dosimetry in Co-60 HDR brachytherapy was examined, and evaluated relative to corresponding results for Ir-192.

      Results

      Comparison of 60Co and 192Ir dose distributions versus corresponding MC calculations indicated that the effect of tissue heterogeneities and patient specific scatter conditions is less for Co-60 than for Ir-192 HDR treatments. Indicative results on the same plane of a breast brachytherapy patient are presented in the figure in the form of a colormap of percentage dose differences relative to MC. In general, Co-60 and Ir-192 sources of identical shape and construction have been found to deliver clinically comparable dose distributions despite definite differences in the physical characteristics of the isotopes. This is confirmed in terms of patient specific dosimetry in this study. A lower dose to critical organs close to the target was observed in Co-60 treatment plans, along with a small increase in the overdose volume (V150 to V400). The choice of isotope was not found to have an impact on the prescribed dose.

      Conclusions

      Co-60 may be used as an effective alternative to Ir-192 for HDR brachytherapy, producing similar plans of equivalent target coverage. Besides logistical advantages owing to the longer half life, another advantage associated with the use of Co-60 sources is the reduced effects of tissue heterogeneities and patient dimensions which obviate the need for image based dosimetry algorithms beyond TG-43.

      Acknowledgement

      Research supported in part by Eckert & Ziegler BEBIG.
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