For patients suffering metastatic cancerous diseases, successful management of metastases is of utmost importance. About 90% of all cancer-related deaths are due to the presence of metastases. A common therapeutic approach is radiation: more than 50% of all cancer patients receive a form of radiotherapy during their treatment course. But there are several dynamics that feature both challenges and opportunities as well as pose fundamental questions on current knowledge and understanding of radiation biology. Clinical case reports of observed regression or progression of metastatic lesions distant from the irradiation site following local radiation on the primary tumor have been increasing in recent years. This phenomenon is sometimes referred to as the abscopal effect and has been claimed to stem from immune activation as a potential result of radiation therapy. However, these effects are to date rather unpredictable events, as they have been classified with fundamentally different outcomes for patients featuring similar genetic profiles. The beneficial outcome of these systemic effects of local radiotherapy reliably triggered in clinical application could lead to the shrinkage of both treated tumor and distant untreated tumors. In this presentation we propose a mathematical model based on a McKendrick-von Foerster transport equation with a global carrying capacity shared by all tumors. We calibrate the model with experimental data of 4T1 tumors in BALB/c mice and Py230 tumors in C57BL/6 mice. This allows us to examine the interaction dynamics of primary tumor and metastases in an untreated setting.