The dual classification of cancer as localized or metastatic disease is one of the key point in the elaboration of the best therapy for each patient. Neverthe-less, many studies reveal that part of these localized diseases are already metastatic. The presence of undetectable or micro-metastases explains the necessity of adjuvant chemotherapies after resection of the primary tumor even for some T1N0M0 cancer. There is probably a continuum between these two stages. We expose here how a mathematical model of growing metastases could reflect this continuum of the disease and how such a model could help the oncologists in the choice of the treatment. This phenomenological model is based on a structured transport equations with non local boundary condition describing the evolution of the density of metastasis. Thanks to this model, we forge a new numerical index, that we call Metastatic Index, able to reveal either the micro-metastatic state of the patient, or the visible metastatic one. Numerical illustrations show how this new index can be used.