Cutaneous mastocytosis (CM) is characterized by the aberrant proliferation and accumulation of mast cells in the skin without any evidence of extra-cutaneous organ involvement . There is no clear pathologic mechanism that has been presented to explain the development of cutaneous mastocytosis. We present a case of CM following adjuvant radioiodine ablation therapy for papillary thyroid carcinoma. There are few case reports of cutaneous mastocytosis arising within a localized area of external beam radiation therapy. To our knowledge there are no previously reported cases of CM associated with systemic radiotherapy.There was a prominent firm mass on her right neck. No adenopathy was noted. Darier’s sign was negative, but dermatographia was evident. KOH (potassium hydroxide) preparation was negative for dermatophytosis or candidiasis. Our differential diagnosis included an irritant or allergic contact dermatitis, radiationinduced dermatitis, histiocytosis, and mycosis fungoides. A punch biopsy of lesional skin revealed increased numbers of mast cells throughout the dermis without evidence of spongiosis, and stained positively with mast cell tryptase (MCT), and Giemsa stains.The patient denied any systemic symptoms such as flushing, hypotension, tachycardia, anaphylaxis, dizziness, headaches, abdominal pain, and diarrhea.