The dosimetric measurements of small fields of radiation are undertaken with detectors such as: GafchromicTM films, diodes, diamond detectors and ionization chambers of small volumes. The penumbra width, e.g. the spatial distance between 80% and 20% dose, is smaller in small fields. This fact increases the curvature and hence the volume effect in the penumbra region. The accuracy in the penumbra calculation is important at the QA test for implementation of the cranial Stereotactic Radiosurgery. The objective of this work was to apply a Gradient- Based Edge Detection plugin to calculate the penumbra of a radiation beam defined with the Elekta SRS Cone Collimators for beams of 6 MV (Flattening Filter ??? FF) and 6 MV (Flattening Filter Free ??? FFF) energies using the beam profile obtained with GafchromicTM film. The results corroborate those from the literature, and they allow a quantitative evaluation of the width of dosimetric penumbra by Gradient-Based Edge Detection.Conselho Nacional de Desenvolvimento Cient??fico e Tecnol??gico (CNPq)Coordena????o de Aperfei??oamento de Pessoal de N??vel Superior (CAPES)CNPq: 301335/2016-8CAPES: 554/2018