Benign Causes of 18FDG Uptake

Uptake of 18FDG is not specific to malignant tissue, and it is well recognised that inflammation may lead to ac cumulation in macrophages and other activated inflammatory cells [7, 8]. In oncological imaging, this inflammatory uptake may lead to decrease in specificity. For example, it may be difficult to differentiate benign postradiotherapy changes from recurrent tumour in the brain, unless the study is optimally timed or unless alternative tracers such as 11C methio-nine are used. Apical lung activity may be seen following radiotherapy for breast cancer, and moderate uptake may follow radiotherapy for lung cancer [36]. It may also be difficult to differentiate radiation changes from recurrent tumour in patients who have undergone radiotherapy for rectal cancer within six months of the study [12].

Pancreatic imaging with 18FDG may be problematic. In some cases, uptake into mass-forming pancreatitis may be comparable in degree to uptake in pancreatic cancer. Conversely, false negative results have been described in diabetic patients with pancreatic cancer. However, if diabetic patients and those with raised inflammatory markers are excluded, then 18FDG PET may still be an accurate test to differentiate benign from malignant pancreatic masses [37].

Table 14.4. Benign causes of 18FDG uptake

Figure 14.11. Coronal 18FDG scan demonstrating high uptake in lymph nodes in a patient with sarcoidosis.

Table 14.4. Benign causes of 18FDG uptake

Figure 14.11. Coronal 18FDG scan demonstrating high uptake in lymph nodes in a patient with sarcoidosis.

Organ/Type

Brain

Pulmonary Myocardium

Bone/bone marrow Inflammation

Endocrine

Disease

Postradiotherapy uptake.

Tuberculosis, sarcoidosis, histoplasmosis, atypical mycobacteria, pneumoconiosis, radiotherapy.

Heterogeneous left ventricular activity possible after myocardial infarction, increased right ventricular activity in right heart failure

Paget's disease, osteomyelitis, hyperplastic bone marrow.

Wound healing, pyogenic infection, organising haematoma, oesophagitis, inflammatory bowel disease, lymphadenopathy associated with granulomatous disorders, viral and atypical infections, chronic pancreatitis, retroperitoneal fibrosis, radiation fibrosis (early), bursitis.

Graves' disease and chronic thyroiditis, adrenal hyperplasia.

A number of granulomatous disorders have been described as leading to increased uptake of 18FDG, including tuberculosis [38], and sarcoidosis [39] (Fig. 14.11). It is often necessary to be cautious in ascribing 18FDG lesions to cancer in patients who are known to be immunocompromised. It is these patients who often have the unusual infections that may lead to uptake that cannot be differentiated from malignancy. PET remains useful in these patients despite a lower specificity, as it is often able to locate areas of disease that have not been identified by other means and that may be more amenable to biopsy [40].

A more comprehensive list of benign causes of abnormal 18FDG uptake is displayed in Table 14.4.

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