Idiopathic Follicular Mucinosis or Mycosis Fungoides? Classification and Diagnostic Challenges
In recent years, the distinction between idiopathic follicular mucinosis (FM) and lymphoma-associated follicular mucinosis (LAFM) has been made through assessment of T-cell receptor gene rearrangement, flow cytometry, and immunohistochemistry. These methods, among others, have mostly identified monoclonality as a defining characteristic of LAFM; however, this finding cannot be considered conclusive, as monoclonality also has been described in benign inflammatory dermatoses such as lichen planus and idiopathic FM. Pure histologic diagnosis also is unreliable in many cases, as the histologic patterns of idiopathic FM and LAFM overlap. In this article, we discuss the importance of close clinical follow-up in patients with patch-stage mycosis fungoides (MF) or FM who have had a nondiagnostic histopathologic evaluation. We also highlight the value of ancillary testing, including T-cell receptor gene rearrangement, flow cytometry, and immunohistochemistry, as a component in the diagnostic process rather than the sole diagnostic moiety. Diagnosis and classification of idiopathic FM and LAFM continue to pose challenges for dermatologists, oncologists, and pathologists, and no single diagnostic tool is sufficient in providing diagnostic certainty; rather, a collective evaluation of pathologic, molecular, and clinical criteria is required. Currently, classification of idiopathic FM and LAFM incorporates clinical information and histologic assessment, but little consideration is given to the implications of the diagnosis from the patient’s perspective. Revisiting histologic classification of these entities while incorporating the patient’s perspective may prove beneficial to dermatologists as well as patients.
Practice Points
- An isolated patch in the head or neck area is much more likely to be follicular mucinosis (FM) than mycosis fungoides (MF).
- Monoclonality does not reliably distinguish FM from MF.
- Younger patients are more likely to have FM with spontaneous remission, and older patients are more likely to develop MF.
- None of the clinicopathologic features of FM or MF are without overlap.
Other clinical factors that may be helpful in the diagnosis of MF are the presentation of lesions in non–sun-exposed areas of the skin and multiple lesions, as unilesional MF is exceedingly uncommon.21 No histologic features have been proven to predict which early patch- or plaque-stage MFs will progress to full-blown CTCL versus benign idiopathic FM; thus, great caution should be taken in patients with early-stage disease to ensure they are not prematurely and/or incorrectly classified as CTCL. Such a diagnosis has medical, social, and economical ramifications that should not be overlooked.
If idiopathic FM and LAFM were considered distinct disease processes, the ambiguity in making a definitive diagnosis should give the physician pause, and a diagnosis of LAFM may only be appropriate when there is unequivocal clinicopathologic evidence. Otherwise, a lymphoma diagnosis is somewhat superfluous and potentially harmful. Definitive diagnosis of LAFM also is complicated by reports of other hematologic malignancies presenting with FM-like histopathologic findings, such as chronic myelogenous leukemia, leukemia-associated eosinophilic folliculitis, and acute myeloblastic leukemia.23,24 Although MF is the most common malignancy associated with FM, it is important to consider other less common malignancies that also may be present.
Diagnosis: Patient Consequences
Accurate diagnosis of idiopathic FM versus LAFM is critical, as the ramifications of a cancer diagnosis can have broad implications. For example, patients who receive cancer diagnoses often experience emotional trauma and social stigma, even when adequate patient education has been provided. The incidence of depression and anxiety also can increase following a cancer diagnosis and can be complicated by medical treatments (eg, systemic steroids, interferon),25 which are known to increase the frequency of these psychological disturbances. Health insurance premiums likely will be higher if a patient is diagnosed with cancer versus a benign inflammatory condition. Hesitation of the pathologist to assign a cancer diagnosis when unequivocal evidence is not present should not be regarded as trickery, malpractice, or deceit of the health care bylaws, as benign language with suggestion of close clinical follow-up in the setting of diagnostic uncertainty will “first, do no harm” and secondly, serve as a vehicle for patient advocacy.
If there is a definitive distinction between idiopathic FM and LAFM, it requires further research before it can be fully understood. Currently, the WHO does not recognize a diagnosis of FM-associated MF (or LAFM) and acknowledges that folliculotropic MF is not always associated with FM.16,26 Given uncertainty and repercussions associated with a cancer diagnosis, however indolent, it may be morally responsible and medically favorable for physicians to consider FM in the differential diagnosis when applicable rather than making a diagnosis of MF outright. Given the importance of both clinical and histologic factors, it may be beneficial for definitive diagnosis of FM versus MF to lie with the clinician, while the pathologist serves as an adjunct in the diagnostic process. Because the prognosis of idiopathic FM often is marred by possible transformation into MF or other CTCLs, therapeutic decisions should be dictated by close clinical follow-up. Additionally, stage of disease, patient age, treatment compliance, comorbidities, and possible side effects of medications should all be considered when evaluating potential therapeutic regimens.27
Conclusion
Research is underway to more accurately identify patients with FM who are at risk for progression to LAFM versus those with benign remitting FM. Once the required diagnostic criteria are established to accurately classify these patients, with an emphasis on prognosis and suggested treatments, it might be necessary to establish new, less debated terminology so pathologists and clinicians alike can improve patient care. Continued histopathologic scrutiny, use of sophisticated molecular techniques, and knowledge of other currently undiscovered modalities will shed light on this important disease process and aid in proper disease management, which may be advantageous to both patients and physicians.
