A mathematical contouring algorithm that automatically determines the planning volume of a sarcoma prior to designing a brachytherapy treatment plan. The algorithm, utilizing computational geometry, numerical interpolation and artificial intelligence (AI) techniques, returns the planning volume in digitized and graphical forms in a matter of minutes. Such an automatic procedure reduces labor time and provides a consistent and objective method for determining planning volumes. In addition, a definitive representation of the planning volume allows for sophisticated brachytherapy treatment planning approaches to be applied when designing treatment plans, so as to maximize local tumor control and minimize normal tissue complications.