DISCUSSION

Subcutaneous panniculitis-like T cell lymphoma (SPTCL) is a rare cytotoxic T cell lymphoma that accounts for <1% of non-Hodgkin lymphomas. It occurs in males and females equally. It has a broad age range, although it is more common in young adults. The aetiology is unknown.1 The tumour commonly presents as subcutaneous nodules of the extremities and trunk, ranging from 0.5 cm to several centimetres in diameter.2 Lymphadenopathy and hepatomegaly are usually absent.

On biopsy, the tumour involves the subcutaneous fat lobules and spares the septa and the overlying dermis and epidermis. The neoplastic cell can range from medium sized with round nuclei and inconspicuous nucleoli to larger cells with hyperchromatic nuclei and a moderate amount of pale staining cytoplasm. The diagnostic feature is the presence of neoplastic cells rimming the individual fat cell, known as adipotropism.3 Reactive vacuolated histiocytes may also be seen, and karyorrhexis, cytophagocytosis and fat necrosis are common.4 Importantly, interface dermatitis and dermal connective tissue mucin are largely absent. The tumour cells show an alpha-beta (αβ) T cell phenotype by immunohistochemistry or flow cytometry, are CD8 positive, and contain cytotoxic molecules such as granzyme B, perforin and T cell intracellular antigen-1. Characteristic T cell markers such as CD5, CD7 and CD2 may be lost. The neoplastic cells are Epstein-Barr virus (EBV) negative and demonstrate a T cell receptor (TCR) gene rearrangement.5

The main differential diagnostic considerations include cutaneous natural killer cell (NK)/T cell lymphoma, cutaneous gamma-delta (γδ) T cell lymphoma and lupus panniculitis. The presence of a dermal infiltrate with angioinvasion/angiodestruction, CD56 positivity, and EBV positivity, and absence of TCR γ or β expression by immunohistochemistry or flow cytometry suggests cutaneous NK/T cell lymphoma rather than SPTCL. The lesional cells of cutaneous NK/T cell lymphoma are often medium sized with an immature chromatin pattern, unlike the cells of SPTCL. Fat rimming is not a prominent feature in cutaneous NK/T cell lymphoma (although it can happen), and epidermal involvement and ulceration are often seen due to the underlying vascular involvement. T cell markers are missing (except for cytoplasmic CD3 by immunohistochemistry) in cutaneous NK lymphoma, but can be present in the so-called NK/T cell lymphoma.

Likewise, epidermal and dermal involvement, lack of CD56 and EBV expression, and expression of TCR γ by immunohistochemistry or flow cytometry support cutaneous γδ T cell lymphoma over SPTCL. Before the advent of specific gamma protein typing by flow cytometry/immunohistochemistry, cutaneous γδ T cell lymphomas were often combined with SPTCL, giving this group an overall dismal clinical outcome. Careful epidemiological studies, combined with increasingly advanced diagnostic tools, allowed for the creation of the provisional category of γδ T cell lymphoma (WHO 2008 classification) and distinction between these two entities. Distinction between lupus panniculitis and a low-grade form of SPTCL can be difficult; however, lupus panniculitis often has lymphoid aggregates and plasma cells at the periphery of the lobular panniculitis, a feature that is not prominent in SPTCL. CD4-expressing and CD8-expressing T cells tend to both be present, unlike SPTCL, which is largely a neoplasm of cytotoxic CD8-expressing T cells.6 As with all cases of cutaneous lymphoma, clinical correlation is absolutely crucial in arriving at the appropriate diagnosis, in excluding secondary cutaneous involvement by systemic lymphoma, and in excluding other types of primary cutaneous T cell lymphoma, including transformed mycosis fungoides and primary cutaneous peripheral T cell lymphoma, not otherwise specified.

Dissemination of disease is rare but can be seen later in the disease course. Combination chemotherapy including cyclophosphamide, doxorubicin, vincristine and prednisone can be used to treat these patients. Aggressive cases require an anthracycline-based regimen.5 The 5-year median survival is 80%; however, prognosis is poor when coupled with haemophagocytic syndrome and lymph node involvement.6