Mycosis Fungoides — Patch to Plaque Stage

A 62-year-old man presents with a 6-year history of pruritic, slowly progressive red patches and plaques on the buttocks, lower abdomen, inner thighs, and flanks. The lesions show characteristic wrinkled, cigarette-paper atrophy and have been resistant to multiple courses of topical corticosteroids. Examination reveals asymmetric, fixed erythematous to violaceous patches with some infiltrated plaques. Notably, the lesions spare the face, dorsal hands, and distal forearms. Skin biopsy confirms mycosis fungoides with epidermotropic T-cell infiltrate and clonal T-cell receptor gene rearrangement. The distribution pattern marked **B** in the diagram is a key diagnostic clue. Which of the following best explains the clinical significance of this distribution pattern in mycosis fungoides?