Hypopigmented MF is a rare type of MF that is typically observed in dark skinned individuals of Asian or African origin and is mostly encountered in children.
The phenotype is predominantely CD8+ve and TCR gene rearrangement studies usually show clonal T cell population. From my limited experience, I came across two problems in the diagnosis of hypopigmented MF; both of them have something to do with vitiligo!!!

The first problem is a pitfall when you don’t have enough clinical data and diagnose a case as hypopigmented MF based on typical histopathological findings of epidermotropic CD8+ lymphocytes within the epidermis with a patchy band/band like dermal lymphocytic infiltrate and then the clinician is not convinced and asks you to see the patient for clinicopathological correlation or asks you to check the clinical photos to discover that the lesions are completely depigmented, typically vitiliginous, with inflamed erythematous sometimes raised borders!!! You feel embarrassed and you know that you made a serious mistake by frightening the parents with your diagnosis, while the case is just inflammatory vitiligo!!! Inflammatory vitiligo is a great pathologic mimic of hypopigmented MF and sometimes it adds to the confusion by presenting clinically with only a scaly erythematous patch with no depigmentation at all.

The second problem goes in the opposite way. You see a patient with hypopigmented slightly scaly patches and you think of hypopigmented MF but you get perplexed when you see associated depigmented milky white vitiliginous patches and sometimes vitiliginous rims around centrally located scaly erythematous patches. Is it hypopigmented MF or early (inflammatory) vitiligo?? You take a biopsy and it reveals typical MF findings, and you remain confused because it has been reported that hypopigmented MF may be associated with vitiliginous patches.

So what to do?? Immunophenotyping will not be helpful as epidermotropic cells in hypopigmented MF are usually CD8+ as in inflammatory conditions. TCR gene rearrangement studies are expensive and not available in all centers in some countries and may yield false positive or negative results. So again it is the clinicopathological context that will solve the dilemma. I believe that dermatopathologists should not sign out hypopigmented lesions before knowing all the details of the clinical picture including onset, course, duration, number of the lesions (single or multiple), distribution (generalized or localized), sites of affection, and response to previous therapies if any were given. I developed this routine in my department and I ask to see all cases with hypopigmented lesions before finalizing the diagnosis. Follow up is also important and it helps in settling the diagnosis as things may become clearer within few months. I like these words of Dr Lorenzo Cerroni and his colleagues said in their book “Skin Lymphoma”: “a delay of a few months in the diagnosis of early mycosis fungoides is probably better than a wrong diagnosis of “cancer” particularly in children”.