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

Abstract

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. 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. 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. 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.