Malignancy in dermatomyositis

Clinical History

A 62-year-old male patient presented to the emergency department with progressive proximal muscle weakness and fatigue. Physical examination was unremarkable. Routine blood tests showed an elevated creatine kinase level. The patient was investigated for dermatomyositis. While awaiting a skin and muscle biopsy a CT Thorax was performed.

Imaging Findings

The CT Thorax showed a right upper lobe mass and an enlarged right para-tracheal lymph node. There was no abnormality in the skeletal muscles. A subsequent Positron Emission Tomography (PET/CT) revealed intense fluorodeoxygenase (FDG) uptake in a 15 mm spiculated nodule in the right lung apex (SUVmax 5.3), most consistent with a primary lung neoplasm with ipsilateral FDG-avid mediastinal lymph nodes, likely to be metastases. In addition there was patchy increased FDG uptake throughout the patient’s skeletal muscles. The degree of FDG uptake in skeletal muscle in a number of muscle groups (SUVmax 7.8) was greater than that in the liver (SUVmax 2.1).

Discussion

Dermatomyositis (DM) is an idiopathic inflammatory myopathy characterised by proximal muscle weakness together with a number of characteristic cutaneous manifestations. The aetiology of it is unknown. The association between DM and malignancy was first published in 1916 [1]. Population-based studies have shown an increased incidence of malignancy in DM compared to the general population [2-4]. DM is associated with a wide range of cancers including ovarian, lung, pancreatic, cervix and stomach.

DM is investigated using laboratory tests (creatine kinase, autoimmune markers) and imaging modalities (CT, MRI, electromyography (EMG), PET/CT) as well as skin and muscle biopsies. Screening for malignancy is recommended in all adults with DM. The European Federation of the Neurological Societies (EFNS) has advised that all patients should have a CT Thorax and Abdomen and those over 50 should have a colonoscopy. Male patients under 50 should have an ultrasound of the testes done while women should have an ultrasound of pelvis and mammography performed [5].

FDG PET/CT has been shown to be useful in early diagnosis and in the assessment of disease activity in a number of rheumatic diseases [6]. In DM some studies have questioned its usefulness in the evaluation of the extent of myositis. One study found the sensitivity of FDG PET/CT to detect myositis was lower than that of MRI, EMG and muscle biopsy [7]. However, studies looking at FDG PET/CT in DM have suggested that it may be useful in the detection of malignancy [8].

This patient had generalised patchy FDG uptake in skeletal muscle. FDG uptake in skeletal muscle should be low (significantly lower than liver or blood pool) as patients are asked to fast and refrain from strenuous exercise prior to the study. The aim of fasting is to lower insulin levels, thus decreasing FDG uptake. The most common causes of FDG uptake in muscle are recent exercise, with tracer uptake confined to a muscle group or high insulin levels (due to incorrect fasting or exogenous administration), which leads to generalised uptake in all muscles. When present, FDG uptake in muscles should be carefully assessed to exclude inflammatory or neoplastic conditions. Myositis results in a generalised patchy increased FDG uptake in skeletal muscles without CT abnormality. The most common causes are DM, polymyositis, statin-associated myositis and graft vs host disease. Common FDG-avid muscle neoplasms include metastases, lymphoma and rhabdomyosarcoma and these tend to have very focal FDG uptake.

Differential Diagnosis List

Final Diagnosis

Dermatomyositis with associated lung cancer

URL:	https://eurorad.org/case/12812
DOI:	10.1594/EURORAD/CASE.12812
ISSN:	1563-4086