Essentially an adenocarcinoma in situ involving the epidermis.
Affects scrotal inguinal area with spread to penis (rare as only site).
Classified as primary (cutaneous) or secondary (from an underlying malignancy).
Clinical: 6th –7th decades, fairly rare, but most cases are associated with an underlying carcinoma (especially bladder, urethra, or prostate).
Gross: Scaly erythematous or eczematous patches or plaques.
Intraepithelial proliferation of large atypical cells with vacuolated cytoplasm and large vesicular nuclei (Paget cell) (image A) & (image B).
Tumor cells proliferate within background normal epithelium.
Atypical cells tend to cluster at the tips of the rete pegs.
Intracytoplasmic mucin vacuoles may be demonstrated by a PAS or mucicarmine stain.
Primary Paget disease confined to epidermis has good prognosis if completely excised; outcome of secondary Paget disease depends on underlying cause.
Primary Paget disease: CK7+/CK20-.
Secondary Paget disease: Related to underlying cause – urothelial CK7+/CK20+, anorectal carcinomas CK20+, CDX2+.
DDX: (Tumor with similar intraepithelial spread or "pagetoid" growth)
Melanoma, superficial spreading: melanin pigment; express melanoma markers S100, HMB-45 or MART1; mucicarmine -; CK7 and CK20-.
Squamous cell carcinoma in situ with pagetoid growth: intercellular bridges; p63+, CK7 and CK20-.