Mycosis Fungoides Is a Proliferation of Multiple Mutational Subclones.

Mycosis fungoides (MF) is a unique type of lymphoma with an excellent prognosis presenting with a wide variety of clinical and histological findings. Recent sophisticated research techniques have revealed that very slow changes in the clinical and histological pictures of one patient over time are due to intratumoral heterogeneity. This means the distribution of different subclones into individual skin lesions and the accumulation of genomic and epigenomic changes in known pathways, starting from immature T-cell precursors. MF is now recognized as the proliferation of multiple mutational subclones that originate from immature T-cell precursors with malignant potential. This may explain why there are so many case reports of MF accompanied by T-cell lymphoproliferative disorders or cutaneous T-cell lymphomas. In addition, the distinction between "transformation" and "complication" of MF may depend on where the disease branches in the T-cell lineage development and the extent of the disease entity defined by MF.