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

Abstract

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

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

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

Keywords: 169Yb; 192Ir; 60Co; HDR brachytherapy; Monte Carlo; conical shielded applicator; skin therapy.