PET-CT A valuable tool for detection and follow up of subcutaneous involvement of extranodal marginal zone lymphoma

Background: Extranodal marginal zone lymphoma (ENMZL), also known as mucosal-associated lymphoid tissue (MALT) lymphoma, can involve virtually any body site. Gastric MALT lymphoma is the most common ENMZL, whereas subcutaneous involvement is considered rare. The value of PET/CT in evaluating ENMZL is controversial. The various locations, as well as the clinical and radiological heterogeneity of ENMZL, require particular attention. Subcutaneous involvement is especially difficult to detect at ENMZL diagnosis, and can be easily overlooked when the disease is diagnosed at another site. Left unnoticed, subcutaneous involvement at diagnosis may have consequences on the outcomes of therapy and disease management. Our aim was to investigate the radiological characteristics of subcutaneous ENMZL.

Methods: We identified subcutaneous lesions in PET-CT scans of patients with histologically confirmed ENZML in sites other than subcutaneous tissue, as diagnosed at our institution during January 2008 to January 2017.

Results: We reviewed 571 PET/CT studies of 178 patients with histologically confirmed MZL. Subcutaneous lesions were found in 20 patients (11%), median age 66 years (range 21-78); 12 females. These 20 patients underwent a median of 4 (1-13) PET/CT scans. Fourteen (70%) had subcutaneous lesions on their first PET/CT scan. Thirteen (65%) had 3 or fewer subcutaneous lesions; of them, 6 had a single. The distribution of 58 subcutaneous lesions was mainly along lateral aspects of the buttocks, the thigh and lower back (n=29; 50%), the upper back, flank and the lateral aspect of the shoulders (n=19; 33%) arms (n=4, 7%) and chest (n=4, 7%). Single lesions also presented in the scalp and axilla. The subcutaneous lesions were located alongside soft tissue structures, with a mean long axis 2.0 cm (range 1.1-6.0) and mean short axis 0.8 cm (range 0.3-2.0). The mean 18F- FDG uptake was 2.3 SUV max (range: 0.9-7.6). In 12 (60%) of the patients the disease was diagnosed at an early stage. Fifteen patients (75%) had lymph node involvement, 10 (50%) had other extranodal involvement. None of the patients had gastric involvement, one had spleen involvement and only 2 (3%) had surface skin involvement.

Conclusions: Whole body PET/CT is useful for the detection of subcutaneous ENMZL, which have a unique lentiform shape and low FDG avidity, with a typical distribution. Detection of subcutaneous lesions may affect disease staging, and decisions regarding follow up and treatment.