Heterogeneity describes a set of entities  none of which resembles the other.
Heterogeneity is applied by medicine to describe the diversity of tumors.
Medicine postulates  that  cancer is a genetic disease and each mutation causes a different cancer.  For instance hitherto breast cancer was regarded as one disease with heterogeneous manifestations.  Today  each manifestation is regarded as a different breast cancer!!.
Breast cancer is regarded as  a disease conglomerate.  Initially  breast tumors were classified into three different diseases. Recent study describes ten breast cancer diseases.
Hitherto medicine was  perplexed by  inter-tumor heterogeneity. Recent studies reveal another heterogeneity source,
intra-tumor heterogeneity.
Genetic studies fragment classical disease into sub types which are regarded as genuine cancers.