Fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography imaging in response monitoring of extra-pulmonary tuberculosis

INTRODUCTION

Cervical lymphadenopathy is the most common form of extra-pulmonary tuberculosis (EPTB), but it may also involve other lymph nodes throughout the body.[1] Due to vague clinical presentation or coexistence with malignant lesion, it is very difficult to distinguish EPTB from other benign inflammatory, infectious, or malignant lesions.[234] Now, fluorine-18 fluoro-D-glucose positron emission tomography/computed tomography (F18-FDG PET/CT) has been shown to be a promising imaging tool in infection and inflammatory conditions.[56] F18-FDG PET/CT is sensitive in detecting malignancy; however, specificity is low in differentiating the infections or inflammatory diseases from tumor.[7] Because of the similarities in the clinical and radiological presentations between TB and malignancy, diagnosis might get mislead or inconclusive.[89] A number of cases have been documented with increased FDG uptake in active TB mimicking the malignancy.[8910111213] In such cases, often the diagnosis gets delayed due to the difficulty in getting the right tissue samples for the confirmation of diagnosis and also, because of the poor yield of conventional diagnostic methods. Biopsy or sometimes surgery is required to get tissue samples for diagnosis and for managing complications. In this case, FDG PET/CT scan was found to be very helpful in localizing the active sites of infections and to guide the fine needle aspiration cytology (FNAC) site. The disease usually responds to standard anti-tuberculosis drug treatment, so FDG PET/CT also helps in monitoring the response of the treatment given.

CASE REPORT

A 53-year-old asthmatic male with a previous history of carcinoma of tongue subjected to left hemiglossectomy with suprahyoidal neck dissection 10 years back, was presented with right cervical lymphadenopathy and fever. He was normotensive and euglycemic. He denied of any weight loss, night sweats, and cough. Sputum examination was negative for acid-fast bacilli (Mycobacterium tuberculosis). C-reactive protein was negative and ultrasonography of neck had showed multiple bilateral cervical lymphadenopathy. Suspecting it to be metastatic disease, a whole body F-18 FDG PET/CT was performed which showed metabolically active cervical, mediastinal, abdominal, pelvic, left inguinal and femoral lymph nodes [Figure 1]. Diffuse low-grade uptake noted in the region of soft palate, base of tongue, and valleculae was considered to be physiological. The pattern of increased FDG uptake is not typical for squamous carcinoma for head and neck metastasis and so it raised the possibility of lymphoma. FNAC showed the Ziehl–Neelsen staining to be strongly positive for acid-fast bacilli [Figure 2]. Tuberculin skin test was positive (15 mm induration after 48 h). The patient was started on first line antitubercular medication. Six months later after the initiation of therapy, a follow-up whole body F18-FDG PET/CT scan showed a significant resolution of previously FDG avid lymph nodes but for residual hypermetabolism at right cervical level I, right paratracheal and right axillary lymph nodes [Figure 3]. The maximum standardized uptake value (SUVmax) of different lymph nodes before and after therapy is depicted in Table 1.

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Figure 1 Maximum intensity projection fluoro-D-glucose positron emission tomography/computed tomography image demonstrated multifocal fluoro-D-glucose avid generalized lymphadenopathy (maximum standardized uptake value 13.8) with the pattern of hypermetabolic lesions mimicking lymphoma

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Figure 2 Lymph node fine needle aspiration cytology showing Ziehl–Neelsen staining, strongly positive for acid-fast bacilli

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Figure 3 Maximum intensity projection fluoro-D-glucose positron emission tomography/computed tomography showing remarkable improvement of disease after 6 months of anti-tubercular treatment, with residual hypermetabolism noted at right cervical level I, right paratracheal, and right axillary lymph nodes after the treatment

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Table 1 Maximum SUVmax for EPTB lesions pre- and post-anti-tubercular treatment

DISCUSSION

The diagnosis of EPTB is sometimes very difficult, generally because of nonspecific symptoms, atypical clinical and radiological findings, inconclusive conventional TB laboratory tests, or lack of right tissue sample for biopsy.

The role of ¹⁸F-FDG PET/CT is well established in oncological imaging but higher uptake of FDG is not only specific to cancerous cells but also indicates the presence of active infections or inflammatory cells.[5] The reason behind the high uptake of FDG in active infections or inflammatory cells is due to the higher rate of glycolysis.[14] In general, FDG avid generalized lymphadenopathy suggests lymphoma but it may be due to several other benign etiologies including tuberculosis and sarcoidosis.[89101112131415] The present case report shows, how the FDG avid generalized EPTB lesion scans mimic lymphoma. However, once confirmed to be tubercular on tissue biopsy, FDG PET/CT can be a potential tool for monitoring the therapeutic response of treatment. In the present case study, a remarkable decrease in SUVmax value has been noted in previously FDG avid lesion suggesting good response to the treatment.