A 50-year-old man presents because of a pruritic rash of several years' duration. The rash is characterized by erythematous, eczematoid patches, and raised plaques and is distributed asymmetrically over the chest and abdomen. Biopsy of the plaques reveals atypical CD41 T cells with cerebriform nuclei. Fuher marker studies lead to a diagnosis of mycosis fungoides. Which of the following is true of this disease?