We develop a biomechanical model of an isolated stellate breast tumour under mammographic compression forces for a range of reported mechanical properties, both linear elastic and hyperelastic. We also introduce different volumes of increased density/stiffness around the tumour as well as a solid pressure effect. We show that each of these issueswell known to clinicians but ignored to date in modelshas a non-negligible effect on stresses and strains/deformations.