British Journal of Radiology (2004) 77, 257-260
© 2004 British Institute of Radiology
doi: 10.1259/bjr/69516821
False positive fluorine-18 fluorodeoxy-D-glucose positron emission tomography finding caused by osteoradionecrosis in a nasopharyngeal carcinoma patient
S-H Liu, MD1,
J T Chang, MD2,
S-H Ng, MD3,
S-C Chan, MD1 and
T-C Yen, MD, PhD1
Departments of 1 Nuclear Medicine, 2 Radiation Oncology and 3 Diagnostic Radiology, Chang Gung Memorial Hospital and University, 5 Fu-Hsing Street, Kweishan Taoyuan 333, Taiwan
Correspondence: Dr Tzu-Chen Yen
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Abstract
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Nasopharyngeal carcinoma (NPC) is treated by radiotherapy with or without chemotherapy. It is not uncommon to find the residual/recurrent lesion in the skull base area. For patients who had received radiotherapy, it is difficult to differentiate the skull base tumour from post-treatment change in the CT or MRI. 18F-2-fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) provides an alternative diagnostic choice in this situation for head and neck cancer including NPC especially when there is inconclusive CT/MRI finding. This report of an NPC patient who received radiotherapy 18 months previously, describes the misdiagnosis of tumour recurrence at the skull base found in both MRI and FDG PET scan. Histopathological studies showed osteoradionecrosis of the debrided tissue and follow-up PET showed complete regression of the skull base lesion. Therefore, a false positive result in FDG PET caused by osteoradionecrosis was confirmed. To the best of our knowledge, this is the first case report in the literature.
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Introduction
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Nasopharyngeal carcinoma (NPC) is an endemic disease in Taiwan with documented rates of 8.86 and 3.54 cases per 100 000 populations of male and female, respectively, in 1998 (www.tpih.tpg.gov.tw). Treatment strategy for NPC patients primarily depended on radiotherapy. However, the high rate of locoregional recurrence has been reported in those who treated with radiation alone [1]. Clinically, patients with residual/recurrent NPC invading the basal skull after radiotherapy need further concurrent chemoradiotherapy [2]. Early detection of residual/recurrent tumours results in the early appropriate therapy and improves prognosis [3]. However, inflammatory change of the basal skull region and even osteoradionecrosis (ORN) would occur months or years after radiotherapy [4–6]. CT and MRI play an important role for monitoring treatment effects in NPC patients [4, 7], but limitations were observed in distinguishing post-irradiation changes from local recurrence/residual tumour [4, 8]. However, it is essential to get accurate information to help the clinicians take early action to achieve better salvage result [9].
18F-2-fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) is a potential tumour-scanning method to evaluate glucose metabolism in head and neck cancer. For inconclusive CT/MRI results in NPC patients after radiotherapy, it may provide an alternative diagnostic choice for confirmation of possible recurrence [10, 11]. However, some false-positive findings were also observed in inflammatory tissue [10, 12]. This report describes an NPC patient who was considered to have local recurrence by both MRI and FDG PET images. Final histopathological results showed ORN.
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Case report
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A 59-year-old male had NPC (TNM staging: T1N0M0), diagnosed in May 2001 (poorly differentiated squamous cell carcinoma). He received radiotherapy (totalling 64 Gy) to the gross tumour area and all clinical suspicious target areas. The radiation dose was delivered 5 days per week in 2 Gy fractions. Brachytherapy (8 Gy) was also prescribed to cover all the initial gross tumour area. Immediate post-treatment follow-up examinations showed disappearance of the lesion. Unfortunately, in July 2002, he suffered from intermittent severe headaches. MRI scan showed a soft tissue lesion in the nasopharynx with bony destruction (Figure 1
). For further evaluation, he had an FDG PET scan (ECAT EXACT HR+; Siemens, Knoxville, TN). The whole body scan performed 40 min after intravenous injection of 370 MBq (10 mCi) of 18F-FDG revealed a focal area of FDG-6-phosphate accumulation around the right basal skull region (maximum standard uptake value [SUVmax]=5.1). An additional delayed imaging 3 h after injection also showed increased SUVmax value (up to 5.3). Therefore, local recurrence was confirmed (Figure 2). However, the subsequent endoscopy showed crusts overlying the nasopharynx without a visible tumour mass. In order to check the nature of the lesion, he underwent nasopharyngeal debridement. Final pathology of the debrided tissue showed necrosis, secondary to post-irradiation changes. Following the diagnosis of ORN, he received hyperbaric oxygen therapy. FDG PET scan was performed again 2 months later. The previous lesion in the nasopharyngeal roof was not observed (Figure 3). No treatment for cancer was prescribed between the two scans. Therefore, a false positive result with high SUVmax caused by ORN was confirmed. So far, this patient has been free of recurrence at regular follow up.
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Figure 1. The contrast-enhanced MRI images, (a) coronal view, (b) transverse view, revealed a soft tissue lesion in the nasopharynx with bony destruction in the skull base (arrowhead). The lesion was more prominent than before. Therefore, recurrent tumour was highly suspected.
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Figure 2. The whole body FDG PET scan was performed at 40 min after intravenous injection of 370 MBq (10 mCi) of 18F-FDG. One focal area of significant FDG-6-phosphate accumulation in the right skull base region was noted (arrow). The delayed imaging was performed 3 h after injection. (a) Coronal view, (b) transverse view; SUVmax in early and delayed images were 5.1 and 5.3, respectively. The lesion was later debrided and proved to be osteoradionecrosis without malignant cells histologically.
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Figure 3. In order to rule out the possibility of inaccurate siting of the surgery, FDG PET scan was performed again 2 months after debridement, (a) coronal view, (b) transverse view. No treatment for cancer was prescribed between the two scans. The scan revealed that the previous FDG avid lesion had disappeared.
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Discussion
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The usefulness of FDG PET in the detection of residual/recurrent tumour after radiotherapy in NPC patients had been discussed. The reported sensitivity and specificity of FDG PET for detecting recurrence is superior to CT (92.9–100% vs 72% and 64–96% vs 88%, respectively) [8, 10, 11, 13]. Therefore, FDG PET scan is regarded a better tool in post-irradiation follow-up, especially for those who have inconclusive CT/MRI findings. However, the pitfalls of FDG PET imaging, including low glucose uptake malignant tumours and significant accumulation of FDG-6-phosphate in some inflammatory conditions, should be taken into consideration. In the present case, both MRI and FDG PET suggested a local recurrence at the basal skull area, but the histology showed necrosis tissue only. This result indicated a delayed progression of ORN on MRI and a post-irradiation change in FDG PET.
In the past, standard uptake value (SUV) was used as an absolute reference to discriminate benign from malignant lesions [11, 14]. However, there were many exceptions. A dual time point FDG PET imaging was then considered reliable [15, 16]. The washout of FDG-6-phosphate is thought to be delayed from the malignant lesions compared with benign ones. Therefore, the retention index ([delayed SUVmax minus early SUVmax] divided by early SUVmax) would be positive in a malignant lesion. In this case, the retention index of the lesion was 3.9%, and it was classified as a malignant lesion.
False positive results by FDG PET in post-therapeutic NPC patients had been reported due to chronic ulceration, granulation tissue, and inflammatory changes [10, 12, 16]. To the best of our knowledge, ORN has not been reported before. Wong et al had defined ORN as the slow-healing, radiation-induced ischaemic necrosis of bone associated with soft tissue necrosis occurring in the absence of primary tumour necrosis, recurrence, or metastasis [17]. Therefore, the possible mechanism for false positive ORN in such cases may be still due to inflammatory process.
In the present case, the symptom of intermittent severe headache pointed to the possibility of skull base recurrence. ORN may also cause the same symptom. When ORN was diagnosed histologically, the patient received hyperbaric oxygen therapy which was considered to be treatment for ORN, not for cancer [18]. This false positive result was further confirmed by the negative second FDG PET scan 2 months later.
In conclusion, the FDG PET images in NPC patients with possible local recurrence should be interpreted carefully. History taking and clinical follow up may help in avoiding misinterpretation of ORN as local tumour recurrence in FDG PET. For the clinicians, despite all the clinical symptom/signs and diagnostic images indicating local recurrence, histological proof is very important to decide on the further management strategy. FDG PET plays a role in whole body survey including regional lymph nodes. More cases are needed to understand the real power of FDG PET in detection of local residual/recurrent tumours in the skull base area.
Received for publication April 14, 2003.
Revision received June 16, 2003.
Accepted for publication July 14, 2003.
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Abstract
Introduction
