Mycosis fungoides every so often presents as erythematous patches or plaques on the trunk, but usually lacks central clearing. 

Mycosis fungoides (MF) with associated lymphomatoid papulosis (LyP) is an important analysis to consider in lesions refractory to common topicals. Biopsy or multiple biopsies should be measured for challenging diagnoses, and expert histopathological assessment should be sought, particularly given the varied histological presentations of MF.  This case is rare as both MF and LyP are uncommon conditions.

A 16-year-old girl presented with a 5-year history of an erythematous circular patch over the left buttock. Within the last 4 years, recurrent erythematous papules with necrosis and ulceration developed. On examination, a 10×15 cm circular erythematous patch with silvery papules and keratin plugs was seen. Initially, the patch had been treated as eczema, and although emollients and topical steroids had helped, the lesions persisted. Over time, the patch enlarged with prominent erythema and ulcers requiring recurrent antibiotic therapy, prompting a dermatology referral.

The patch biopsy depicted irregular chronic inflammation in the upper dermis with lichenoid features and a disrupted epidermal base. On presentation to dermatology, differential diagnoses included MF or naevoid psoriasis. MF has multiple differentials and is difficult to diagnose initially given its indolent progression.

Early stage MF is challenging, as it often lacks atypical lymphocytes, has non-specific changes consistent with other dermatoses and has multiple histological subtypes. Psoriasis generally has erythematous plaques in an extensor distribution but can predicate within the groin region similar to MF. 

Management options for MF with LyP include observation, phototherapy, corticosteroids, low-dose methotrexate or superficial low-dose radiotherapy. She was given low-dose superficial radiotherapy via 8 Gy in two fractions with initially good outcomes. She subsequently required further radiotherapy and is having ongoing specialist follow-up with dermatology. She remains systemically well with ongoing dermatology follow-up and low-dose radiotherapy.