INTRODUCTION: PET-CT scans are used to clinically stage cancers; however clinicians must know the causes of false positive and negative results to fully interpret the test results. We report a case of synchronous head and neck and lung cancer in a patient with a false positive PET scan in the larynx due to excessive muscle activity of the right vocal cord as it compensates for the paralyzed left vocal cord.
CASE PRESENTATION: A 64-year-old female smoker presented with a two month history of hoarseness and a chronic dry cough. Despite treatment with oral antibiotics her symptoms persisted. Endoscopic examination revealed a non motile left vocal cord with a mass. Biopsy revealed squamous cell carcinoma. A CT scan of the head and neck revealed no nodal disease in the neck but showed a left upper lobe lung mass. This led to a fused CT/PET (positron emission tomography) scan which showed increased fluorodeoxyglucose (FDG) uptake in the left hilar mass, mediastinal lymphadenopathy and the right vocal cord, but none in the left vocal cord where the lesion was found. Bronchoscopy with EBUS biopsy of an enlarged lower paratracheal lymph node was positive for squamous cell carcinoma. The patient was diagnosed with synchronous primary lung cancer (stage IIIA) and T1 N0 M0 laryngeal carcinoma. There was no pathologic correlate to the FDG uptake on the right vocal cord. The patient was not a candidate for lung cancer surgery and underwent chemoradiation to the lung and larynx. After three months she had an improvement in her phonation and repeat imaging revealed a decrease in the size of the lymphadenopathy
DISCUSSION: This case illustrates a false positive PET scan in the larynx due to excessive muscle activity of the functional right vocal cord as it compensates for paralysis of the left vocal cord(1). Compression of the left recurrent laryngeal nerve by the pathologic mediastinal lymphadenopathy led to left vocal cord paralysis. Symmetrically increased FDG muscle uptake in the neck and thoracic paravertebral region is attributed to physiological uptake due to symmetrical brown fat activation. Nonpathologic asymmetrical increased muscle uptake occurs with muscle contractions, uncomfortable positions after radiotracer injection, arthritis, or surgery(2). In instances of recent biopsy or in areas of muscle activity due to swallowing or vocalization, the PET scan may be falsely positive(2). False positives are seen in patients with active infection and inflammation where there is increased glycolysis. False negative PET scans are seen where there is impaired blood flow and minimal radiotracer can reach the area(2). In areas of tumor necrosis there is less metabolically active tissue so PET scans are falsely negative(2). It is reported that 14% of patients with head and neck cancers will develop lung cancer, and 31% of these will be synchronous(3). There are no pathological stains or genetic markers that identify the primary site in patients with synchronous squamous cell cancers(3). It is dependent on the clinical history and radiographic presentation of the patient that assist in determining which arose first and which is considered metastatic. This case is interesting as there is a clinical and radiologic pattern of synchronous primary squamous cell carcinomas of the larynx and lung instead of metastatic disease from one primary. Treatment involves resection of the lung cancer, if feasible, followed by treatment of the head and neck cancer with chemotherapy and concurrent radiation(3).
CONCLUSIONS: Awareness of the conditions and mechanisms by which false positive and negative results occur will assist in the interpretation of PET scans. Such knowledge may help clinicians decide whether or not acquisition of tissue is needed to confirm PET-CT scan findings suggestive of metastatic disease.
Reference #1 Lee M, Ramaswamy MR, Lilien DL, Nathan CA. Unilateral vocal cord paralysis causes contralateral false-positive positron emission tomography scans of the larynx. Ann Otol Rhinol Laryngol 2005;114:202-206.
Reference #2 Koppula D, Rajendran JG. PET-CT in head and neck cancer. Appl Radiol 2010;4:20-27.
Reference #3 Douglas WG, Rigual NR, Loree TR, Wiseman SM, Al-Rawi S, Hicks WL. Current concepts in the management of a second malignancy of the lung in patients with head and neck cancer. Curr Opin Otolaryngol Head Neck Surg 2003; 11: 85-88.
DISCLOSURE: The following authors have nothing to disclose: Luca Paoletti, Hiren Mehta, Leonie Gordon, Boyd Gillespie, Nicholas Pastis
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