**Core Concept**
Mycosis fungoides is a type of cutaneous T-cell lymphoma characterized by patches, plaques, or tumors on the skin, often progressing from a benign to malignant stage. The disease is managed with various treatments depending on its stage, with tumor stage indicating a more aggressive form requiring specialized care.

**Why the Correct Answer is Right**
In the tumor stage of mycosis fungoides, the treatment of choice is typically systemic chemotherapy or targeted therapy to control the disease progression. The use of extracorporeal photopheresis, a form of immunotherapy, is also a preferred option. This treatment modality involves the removal of blood from the patient, exposure to ultraviolet light, and reinfusion of the treated blood, which helps to modulate the immune system's response to the cancer cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because topical treatments are generally used for early-stage mycosis fungoides, not for the tumor stage. Topical agents like topical corticosteroids or nitrogen mustard may be used for localized lesions but are insufficient for widespread disease.

**Option B:** This option is incorrect because radiation therapy, although useful for localized lesions, is not the treatment of choice for the tumor stage of mycosis fungoides. Radiation can cause significant side effects and may not effectively control the disease in its advanced stages.

**Option C:** This option is incorrect because phototherapy, although beneficial for some patients, is not the primary treatment for the tumor stage of mycosis fungoides. Phototherapy is often used in combination with other treatments or for patients with early-stage disease.

**Clinical Pearl / High-Yield Fact**
The Sézary syndrome, a leukemic variant of mycosis fungoides, is characterized by the presence of circulating Sézary cells in the blood. This variant is often more aggressive and has a poorer prognosis than the classical form of mycosis fungoides.

**Correct Answer:** C. Extracorporeal photopheresis.