Differential diagnosis of lymphomatoid papulosis

	LyP	Systemic ALCL	Mycosis fungoides	Pityriasis lichenoides	Arthropod bite	Scabies
Clinical	Self-healing papulonodular skin lesions	Frequent secondary skin lesions, lymphadenopathy B symptoms	Scaling erythematous patches, plaques, tumors, +/– lymphadenopathy	Scaling erythematous hemorrhagic lesions	Exposure	Itchy lesions Responds well to scabies treatment
Histopathology/ immunophenotype	Atypical CD30, CD4+ cells surrounded by inflammatory cells	Lack of epidermotropic cerebriform cells ALK+, EMA+; ALK– in patients over 30	Epidermotropism of cerebriform cells CD30+ with large cell transformation	Interface dermatitis, necrotic keratinocytes, extravasated erythrocytes CD8>CD4, few CD30+ cells	Punctum, insect parts Polymorphous Inflammation CD30+ cells may be present	Presence of mite CD30+ cells and B cells present
Genetics	Clonal in 60% Aneuploidy of type A Lack of t(2;5) Recurrent rearrangement of 6p25.3	t(2;5) often present No IRF4 translocation Clonal TCR rearrangement	Lack of t(2;5) Complex karyotype, clonal or oligoclonal TCR rearrangement	Clonal TCR rearrangement in 50 percent	No abnormalities	No abnormalities

ALK: anaplastic lymphoma kinase; EMA: epithelial membrane antigen; TCR: T cell receptor.

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