Comparison of 192Ir, 169Yb, and 60Co high-dose rate brachytherapy sources for skin cancer treatment

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Abstract

Purpose

To evaluate the possibility of utilizing the high-dose rate (HDR) 169Yb and 60Co sources, in addition to 192Ir, for the treatment of skin malignancies with conical applicators.

Methods

Monte Carlo (MC) simulations were used to benchmark the dosimetric parameters of single 169Yb (4140), 60Co (Co0.A86), and 192Ir (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 169Yb and 60Co sources have been compared with those of the 192Ir source, for the same geometry. Then, applicator wall thickness for the 60Co 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 192Ir 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 169Yb-Stand.Appl and the 60Co-double-wall applicator were found to be similar to that of the 192Ir-Stand.Appl, with differences < 2.5%. The percentage depth doses (PDD) for the 192Ir- 169Yb- and 60Co-Stand.Appl were found to be comparable to the values with the 60Co-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 192Ir- 169Yb-Stand.Appl, and 60Co-double-wall applicators respectively. The leakage dose around the Stand.Appl for distance > 2 cm from the applicator surface was < 5% for 192Ir, < 1% for 169Yb, and < 18% for 60Co relative to the prescription dose. However, using the double-walled applicator for the 60Co source reduced the leakage dose to around 5% of the prescription dose, which is comparable with that of the 192Ir source. The optimized end-cap thicknesses for the 192Ir- 169Yb-Stand.Appl, and the 60Co-double-wall applicator were found to be 1.1, 0.6, and 3.7 mm respectively.

Conclusions

Application of the 169Yb (with Stand.Appl) or the 60Co source (with double-wall applicator) has been evaluated as alternatives to the existing 192Ir source (with Stand.Appl) for the HDR brachytherapy of skin cancer patients. These alternatives enable the clinics that may have 169Yb or 60Co sources instead of the 192Ir 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 over-dosage. The treatment times for the 60Co source remain to be determined. Additionally, for 169Yb, the source needs to be changed on monthly basis due to its limited half-life.

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