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Introduction to Mycosis Fungoides

Skin lesions in your USMLE test

USMLE skin lesion questions will appear on your test with visual pictures or in-depth descriptions. Usually, the questions will cover the pathophysiology and presentation; they may include immunologic or cytologic features of the lesion. Most of the lesions will be common, but occasionally, some rare lesions will appear, too.

USMLE quick review: Due to the sometimes complex nature of skin lesions, it is important to remember the common ones that will be tested on the USMLE. Consider reviewing their visual and microscopic appearances to help reinforce your understanding of them.

Review – common skin lesions:

  • Macule: A flat lesion with clear, circumscribed margins that differs from the surrounding skin color and is less than 1cm in size
  • Patch: Larger sized macules that are greater than 1cm and well-circumscribed (seen in MF)
  • Papule: An elevated skin lesion less than 1cm in size. Commonly thought of as basal cell carcinoma
  • Plaque: A larger papule greater than 1cm in size
  • Vesicle: A fluid-filled blister that is less than 1cm in size
  • Bulla: A larger fluid-filled blister greater than 1 cm in size
  • Pustule: A vesicle containing pus that can be of any size
  • Wheal: Smooth papule with a transient time course; usually occurs secondary to an allergic reaction and seen in cases of hives
  • Scale: Dry, flaky skin lesion that is usually seen in eczema and in squamous cell carcinoma
  • Crust: Exudate from an underlying skin infection that crusts over

A rare group of skin lesions is associated with Cutaneous T-cell lymphoma (CTCL), which can present as Mycosis Fungoides (MF) or Sezary syndrome. CTCL is characterized as a form of non-Hodgkin’s lymphoma, where neoplastic T-lymphocytes begin to localize to the skin. There are a few other forms of CTCL with provisional classifications but, for the purpose of this resource and USMLE preparation, this article will focus only on MF and Sezary syndrome.

General Overview of Mycosis Fungoides

High Yield: CTCL is CD4+ (Helper) T cell-based in 65% of cases. It is more common in patients with sub-Saharan ancestry, and there is a 2:1 predominance towards men than women.

CTCL is extremely rare in children and most commonly occurs in middle-aged to elderly patients. The skin lesions resemble other, more common conditions at first, so the average time before a diagnosis is about six years. Prognosis is poor since there is no cure for either MF or Sezary syndrome, and terminal outcomes depend upon type and stage of the disease. Differential diagnoses include allergic and irritant contact dermatitis, lichen planus, and plaque psoriasis.

Definition of Mycosis Fungoides

MF is an indolent form of non-Hodgkin’s lymphoma that presents with skin lesions in a stepwise fashion. MF is the most common presentation of CTCL and comprises 44% of all cases. The name originates from the patches of plaques that were first thought to resemble a mushroom shape.

Note: There is also some confusion where CTCL is called MF, and it should be clear that MF is a sub-form of CTCL. MF has an incidence of 0.36 per 100,000 cases in the USA. The three stepwise presentations occur in the following pattern:

  1. Patch presentation with dermatitis that is located on the trunk and often times on the buttocks, which are usually not pruritic.
  2. Plaque presentation that is characterized by intense pruritus and can be associated w/ lymphadenopathy.
  3. The Tumor phase is the last manifestation that results in lesion changes to the point of ulceration.

Diagnosis of Mycosis Fungoides

Diagnosis can be difficult since the initial presentation can be similar to many other skin-related diseases. Due to MF’s indolent nature, it is important to take a thorough history to understand lesion progression. A biopsy of the lesion will show distinct characteristics, such as brightly-colored infiltrate indicating lymphocytes and hyporeflective papillae at the dermo-epidermal junction. Immunohistochemistry staining will reveal T cell lymphocytes that express the phenotype CD4+, CD8−, and loss of the surface antigens CD5 and CD7. Diagnostic testing should include a CBC, LFTs, flow cytometry, and HIV screening. In suspected later stages of the disease, a chest X-ray and potential CT scans can be done to look for further organ involvement.

Management of Mycosis Fungoides

Management of MF depends upon the stage:

  • Stage 1 management focuses on topical therapy with antipruritic medications and retinoids.
  • Stages 2 and 3 include the use of steroids, surgical excision, and radiotherapy for advanced disease.

There is some research indicating that allogeneic stem cell transplants have helped with MF treatment. Photochemotherapy with UV-A (PUVA) has been beneficial for some patients.

Prognosis of Mycosis Fungoides

Prognosis depends on the stage of the disease. Patients diagnosed with Stage 1 MF have very good prognoses because MF is indolent and slow to progress. Those patients in later stages may have reduced survival times, especially if there is lymph node or blood involvement.

Sezary Syndrome as a Special Form of Mycosis Fungoides

Sezary syndrome is an aggressive form of CTCL that will present with rapid symptoms of generalized edematous skin, lymphadenopathy, generalized alopecia, and hyperkeratosis of palms and plantar surface of the foot. In some patients, it can present with ectropion of the eyelids and hepatosplenomegaly. It comprises 5% of all CTCL presentations.


Diagnosis is made by analyzing the Sezary cell count (cerebriform specific cellular morphology). Immune cell phenotypic expression changes with increased CD4 to CD8 ratio and a loss of CD markers 2, 3, 4, and 5. In advanced disease, a chest X-ray and CT may be indicated for further spread of the disease.


Management focuses on how fast the disease is progressing. Therapy involves immune modulation with IFN-a and antineoplastic drugs, including chlorambucil, methotrexate, and etoposide. Topical immunomodulatory therapy includes imiquimod and retinoid agents.


Prognosis is poor due to the incurable nature of the disease. Since Sezary syndrome is rapidly progressive and results in immunosuppression, patients will often have secondary systemic infections, such as Staph aureus and Pseudomonas. The median survival from the date of diagnosis is 2—4 years.

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