Dosimetry of Large Field Valencia applicators for Cobalt-60-based brachytherapy

Non-melanoma skin cancer is one of the most common types of cancer and one of the main approaches is brachytherapy. For small lesions, the treatment of this cancer with brachytherapy can be done with two commercial applicators, one of these is the Large Field Valencia Applicators (LFVA).

The aim of this study is to test the capabilities of the LFVA to use clinically ⁶⁰Co sources instead of the ¹⁹²Ir ones. This study was designed for the same dwell positions and weights for both sources.

The Penelope Monte Carlo code was used to evaluate dose distribution in a water phantom when a ⁶⁰Co source is considered. The LFVA design and the optimized dwell weights reported for the case of ¹⁹²Ir are maintained with the only exception of the dwell weight of the central position, that was increased. 2D dose distributions, field flatness, symmetry and the leakage dose distribution around the applicator were calculated.

When comparing the dose distributions of both sources, field flatness and symmetry remain unchanged. The only evident difference is an increase of the penumbra regions for all depths when using the ⁶⁰Co source. Regarding leakage, the maximum dose within the air volume surrounding the applicator is in the order of 20% of the prescription dose for the ⁶⁰Co source, but it decreases to less than 5% at about 1 cm distance.

Flatness and symmetry remains unaltered as compared with ¹⁹²Ir sources, while an increase in leakage has been observed. This proves the feasibility of using the LFVA in a larger range of clinical applications.