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摘要
Purpose: To evaluate the possibility of utilizing the high-dose rate (HDR) Yb-169 and Co-60 sources, in addition to Ir-192, for the treatment of skin malignancies with conical applicators. Methods: Monte Carlo (MC) simulations were used to benchmark the dosimetric parameters of single Yb-169 (4140), Co-60 (Co0.A86), and Ir-192 (mHDR-V2) brachytherapy sources in a water phantom and compared their results against published data. A standard conical tungsten alloy Leipzig-style applicator (Stand. Appl) was used for determination of the dose distributions at various depths with a single dwell position of the HDR sources. The HDR sources were modeled with its long axis parallel to the treatment plane within the opening section of the applicator. The source-to-surface distance (SSD) was 1.6 cm, which included a 0.1 cm thick removable plastic end-cap used for clinical applications. The prescription depth was considered to be 0.3 cm in a water phantom following the definitions in the literature for this treatment technique. Dose distributions generated with the Stand. Appl and the Yb-169 and Co-60 sources have been compared with those of the Ir-192 source, for the same geometry. Then, applicator wall thickness for the Co-60 source was increased (doubled) in MC simulations in order to minimize the leakage dose and penumbra to levels that were comparable to that from the Ir-192 source. For each source-applicator combination, the optimized plastic end-cap dimensions were determined in order to avoid over-dosage to the skin surface. Results: The normalized dose profiles at the prescription depth for the Yb-169-Stand. Appl and the Co-60-double-wall applicator were found to be similar to that of the Ir-192-Stand. Appl, with differences < 2.5%. The percentage depth doses (PDD) for the Ir-192-, Yb-169- and Co-60-Stand. Appl were found to be comparable to the values with the Co-60-double-walled applicator, with differences < 1.7%. The applicator output-factors at the prescription depth were also comparable at 0.309, 0.316, and 0.298 (cGy/hU) for the Ir-192-, Yb-169-Stand. Appl, and Co-60-double-wall applicators respectively. The leakage dose around the Stand. Appl for distance > 2 cm from the applicator surface was < 5% for Ir-192, < 1% for Yb-169, and < 18% for Co-60 relative to the prescription dose. However, using the double-walled applicator for the Co-60 source reduced the leakage dose to around 5% of the prescription dose, which is comparable with that of the Ir-192 source. The optimized end-cap thicknesses for the Ir-192-, Yb-169-Stand. Appl, and the Co-60-double-wall applicator were found to be 1.1, 0.6, and 3.7 mm respectively. Conclusions: Application of the Yb-169 (with Stand. Appl) or the Co-60 source (with double-wall applicator) has been evaluated as alternatives to the existing Ir-192 source (with Stand. Appl) for the HDR brachytherapy of skin cancer patients. These alternatives enable the clinics that may have Yb-169 or Co-60 sources instead of the Ir-192 source to perform the skin brachytherapy and achieve comparable results. The conical surface applicators must be used with a protective plastic end-cap to eliminate the excess electrons that are created in the source and applicator, in order to avoid skin surface overdosage. The treatment times for the Co-60 source remain to be determined. Additionally, for Yb-169, the source needs to be changed on monthly basis due to its limited half-life.
- 出版日期2017-9
