Variable course of patients with plaque psoriasis: lack of transformation into tumorous mycosis fungoides

The question of whether plaque psoriasis (PP) must be assessed as a premalignant state of mycosis fungoides (MF) is still a matter of intense controversy.1-3 Muche et al4 pointed the way, reporting that clonal T cells have been found in high percentages in the blood of patients with small plaque psoriasis (SPP), indicating the potential of malignant transformation of this disease into T-cell lymphoma, whereas there were no T-cell clones found in the blood of controls with contact dermatitis. But in the skin of patients with SPP, no T-cell clones were detected, and Muche et al suggested that the clinical and histologic findings in SPP might be a successful antitumor response to cutaneous T-cell lymphoma (CTCL). Our own clinical and histopathologic data strongly support Muche et al's view.

In our study, 30 patients (29 males, 1 female) diagnosed with PP had been admitted to the university‘s dermatology department in Munich from 1962 to 1985. We were able to follow them up for a long time period. The observation time was 1 to 5 years for 13 patients, 6 to 10 years for 8 patients, and more than 10 years for 9 patients; the maximum observation period was 23 years.

The clinical type of PP was classified according to Bonvalet et al.5 No patient in our study group suffered from poikilodermatous PP. Bonvalet’s classification was extended to the mixed type of plaque psoriasis (MTPP) for those patients with different types of plaques and for those who changed from one type to another during the period of observation. The different types of PP and their age-related distribution are listed in the Table.

Some of the patients improved during the observation period, but none of them showed complete clearing of all skin lesions. PP is not a static disease but is characterized by a rather dynamic clinical course. In only 12 of 30 cases was the diagnosis maintained throughout the observation period. Different types of eczema were discussed in 8 patients. In 12 patients a manifest MF was diagnosed and histopathologically confirmed, and in 5 patients a transformation into MF was considered.

Altogether 56 histopathologic specimens were taken from the 30 patients at the beginning of the disease and/or during its course. The histopathologic criteria were grading of the exocytosis, density of the dermal infiltration, the presence of microabscesses, and the extent of spongiosis present in the epidermis. In 18 specimens taken during the course of the disease, the diagnosis of PP has been dropped in favor of eczema. Transformation into MF has been found in 4 specimens, and a manifest MF was diagnosed histologically in 14 specimens. But none of the patients who had been observed for at least 5 years developed the clinical findings of terminal, tumorous MF or died from this disease.

Our findings indicate that PP often shows a changing feature during its course of disease both clinically and histopathologically. None of the patients developed terminal MF, but the fact that in more than 30% of the patients the diagnosis of MF was made at least once during the course of PP and that in 17% a transformation into MF was considered clearly indicates the problems in arriving at a definite assessment of PP for nearly 1 century, because it was described for the first time about 100 years ago. The investigator's uncertainty cannot be the only explanation; the histological specimens often do show signs that are typical for MF in the infiltrative state, such as microabscesses or exocytosis.

Our study was based on a close clinical and immunohistochemical follow-up, and no studies on clonal rearrangements were performed with these patients. But modern technologies such as polymerase chain reaction that locate clonal cell populations provide greater opportunity for distinguishing malignant changes from benign ones in the skin and blood when CTCL is suspected. Although transformation to MF seemed to be present in a large number of our patients, none of them developed the terminal stage of MF. That would indicate in patients with PP the presence of a very strong factor with the ability to fight lymphoma. A similar possibility was discussed by Muche et al. Identification of this antitumorous factor would be one of the most interesting discoveries that could be achieved in CTCL research.