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

PURPOSE

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

METHODS

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

CONCLUSIONS

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