Malignant neoplasms are one of the most dangerous diseases. Within the framework of the well-established Theory of Porous Media (TPM), a multi-constituent model is derived. The model is mathematically formulated by a set of coupled partial differential equations which are solved within the well-known framework of the finite-element method. The general TPM model is applied to basal-cell carcinoma in the avascular skin and representative numerical examples show the capabilities of the model.