British Journal of Radiology (2009) 82, e3-e7
© 2009 British Institute of Radiology
doi: 10.1259/bjr/96087474
British Journal of Radiology 82 (2009),e3-e7 ©2009 The British Institute of Radiology
False-positive positron emission tomography appearance with 18F-fluorodeoxyglucose after definitive radiotherapy for cancer of the mobile tongue
Y ARIJI, DDS, PhD,
N FUWA, MD, PhD1,
T KODAIRA, MD, PhD1,
H TACHIBANA, MD1,
T NAKAMURA, MD, PhD1,
Y SATOH, MD1 and
E ARIJI, DDS, PhD2
1 Department of Radiation Oncology, Aichi Cancer Center Hospital, 1-1 Kanokoden, Chikusa-ku, Nagoya 461-8681 and 2 Department of Oral and Maxillofacial Radiology, Aichi-Gakuin University School of Dentistry, 2-11 Suemori-dori, Chikusa-ku, Nagoya 464-8651, Japan
Correspondence: Eiichiro Ariji, Department of Oral and Maxillofacial Radiology, Aichi-Gakuin University School of Dentistry,2-11 Suemori-dori, Chikusa-ku, Nagoya 464-8651, Japan. E-mail: ariji{at}dpc.agu.ac.jp
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Abstract
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18F-fluorodeoxyglucose–positron emission tomography (FDG–PET) is an effective tool for evaluating the results of radiotherapy. However, some false-positive appearances caused by physiological or pathological accumulation are reported. We report on three patients who showed a high accumulation of FDG in the lingual muscles but had no recurrent tumour after definitive radiotherapy for the mobile tongue. All patients had squamous cell carcinoma of the tongue and received interstitial radiotherapy with small sources. High uptake was seen in the lingual muscles without recurrence or inflammation, based on physical and MR examinations. This false-positive appearance is thought to relate to ill-balanced high activity of the lingual muscles after definitive radiotherapy.
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Introduction
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For the evaluation of head and neck cancers, positron emission tomography (PET) has been widely used along with the administration of 18F-fluorodeoxyglucose (18F-FDG), which preferentially accumulates in tissues with an active glucose metabolism [1–7]. PET scan with FDG (FDG–PET) is effective for the staging of previously untreated cancer and in the assessment of recurrence after treatment because it has higher sensitivity than sectional imaging modalities such as CT or MRI [8]. Recently, to compensate for the lack of anatomical information in PET scans alone, an apparatus combining PET and CT (PET–CT) has been developed and appears to be effective with FDG administration in the diagnosis and staging of head and neck malignancies [3, 4]. One of the most effective uses of FDG–PET is to evaluate the results of radiotherapy [5–7]. It allows a non-invasive assessment of whether a tumour disappears or recurs after radiotherapy. However, physiological accumulation and artefacts in some tissues can cause false-positive appearances [9, 10]. Uptake in the salivary glands and maxillofacial skeletal muscles is reported to be a possible cause of misinterpretation in the diagnosis of head and neck cancers [3, 4, 9]. We report three cases showing false-positive appearances on FDG–PET after definitive radiotherapy with small source implantation for cancer of the mobile tongue.
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Case report
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All three patients were asked to fast and not to chew gum for at least 6 h before the PET examination. 150 MBq 18F-FDG was administrated intravenously 1 h before examination. The equipment used was a whole-body PET–CT scanner (Discovery LS; General Electric Medical Systems) and a PET scanner (Advance Nxi; General Electric Medical Systems, Milwaukee, WI). No patient had a history of diabetes.
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Case 1
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A 23-year-old Japanese man was referred to our hospital because of a tumour arising on the left border of the tongue. The tumour was clinically staged as T2N2cM0 (Stage IV) and was histologically diagnosed as squamous cell carcinoma. On the pre-treatment T1 weighted MR image with administration of gadolinium-diethyltriaminepentaacetic acid (Gd-DTPA) (Figure 1a
), a 37 x 20 x 35 mm mass with peripheral enhancement was found together with bilateral cervical lymph node swelling. The patient received three courses of systemic chemotherapy with 5-fluorouracil (5-FU) and cisplatin (CDDP), and underwent a total of 37.8 Gy external irradiation with 21 fractionations between the courses of chemotherapy. The field included the primary tumour and bilateral neck regions. 43.2 Gy was subsequently given to the primary site with 137Cs needle implantation. The PET–CT with FDG obtained 7 years and 3 months after the end of treatment revealed positive accumulation on the left side of the tongue (treated side) (Figure 1b). MR images were obtained 3 weeks after the PET–CT examination. On T1 weighted imaging with administration of Gd-DTPA, signal intensity was slightly increased in the area corresponding to the high uptake on PET–CT (Figure 1c). A T2 weighted MR image showed a decrease of signal intensity in the same area (Figure 1d). Although PET suggested tumour recurrence, there was no evidence of tumour growth based on physical examination and MR findings, which strongly suggested a fibrous change after irradiation. Moreover, there were no sign or symptoms of inflammation. Therefore, the patient was followed without treatment. 2-year follow-up after PET examination verified no recurrence. No tumour regression was observed upon periodic MR and physical examinations.
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Figure 1. (a) On the gadolinium-diethyltriaminepentaacetic acid (Gd-DTPA)-enhanced T1 weighted MR image before treatment, a relatively low-intensity area with peripheral enhancement is seen in the left side of the tongue. (b) Positron emission tomography (PET)–CT obtained 7 years and 3 months after treatment shows a high uptake area in the left side of the tongue. (c) Gd-DTPA-enhanced T1 weighted MR image obtained 3 weeks after the PET examination shows a slight increase in signal intensity in the left side of the tongue. (d) A T2 weighted MR image at the same time showed a decrease in signal intensity, indicating a fibrotic change.
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Case 2
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A 37-year-old Japanese man presented with a tumour arising in the left side of the anterior portion of the tongue (squamous cell carcinoma, T3N0M0, Stage III). On the Gd-DTPA-enhanced T1 weighted image before treatment, peripheral enhancement was seen in the anterior portion of the tongue (Figure 2a). PET–CT showed a high uptake area in the left side of the tongue corresponding to the enhanced area on the T1 weighted MR image (Figure 2b). The patient underwent two courses of systemic chemotherapy with 5-FU and nedaplatin. External irradiation gave totals of 30.6 Gy and 66.6 Gy to the primary tumour and neck, respectively. After external irradiation, interstitial implantation with a 137Cs needle (50 Gy) was performed to the primary site. High uptake was still visible on PET–CT obtained 3 years after treatment (Figure 2c). The patient was followed up without treatment because no evidence of recurrence was found upon physical and MR examination. Follow-up 1.5 years after the PET examination showed no recurrence. No tumour regression was observed upon periodic MR and physical examinations.
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Figure 2. (a) On the gadolinium-diethyltriaminepentaacetic acid (Gd-DTPA)-enhanced T1 weighted MR image before treatment, peripheral enhancement is seen in the anterior portion of the tongue. (b) Positron emission tomography (PET) before treatment shows a high uptake area in the left side of the tongue, corresponding to the enhanced area on the T1 weighted MR image. (c) High uptake is observed on PET–CT obtained 3 years after treatment.
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Case 3
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A 36-year-old Japanese man presented with a squamous cell carcinoma of the right side of the tongue (T3N0M0). The initial MR examination showed an enhanced tumour on the right side of the tongue (Figure 3a). The patient received two courses of systemic chemotherapy with 5-FU and CDDP. Between the courses, combined treatment with continuous superselective intra-arterial infusion of carboplatin and 45 Gy of external irradiation was performed. These treatments were followed by 198Au grain implantation, which delivered a total of 40 Gy to the primary site. On Gd-DTPA-enhanced T1 weighted images taken 15 months after the end of treatment, a slightly high-intensity area was found on the right side of the tongue, indicating fibrosis after radiotherapy (Figure 3b). However, PET–CT showed a high uptake area on the right side of the tongue (Figure 3c). No recurrence could be found based on MR or physical examinations during 17 months follow-up after the PET examination.
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Figure 3. (a) The gadolinium-diethyltriaminepentaacetic acid (Gd-DTPA)-enhanced T1 weighted MR image before treatment showed an enhanced tumour on the right side of the tongue. (b) On a Gd-DTPA-enhanced T1 weighted image taken 15 months after the end of treatment, a slightly high-intensity area is seen at the right side of the tongue. (c) Positron emission tomography (PET)–CT after treatment shows a high uptake area in the right side of the tongue.
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Discussion
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Although FDG–PET/PET–CT is known to be effective for evaluating the results of radiotherapy for head and neck cancer [4–7], false-positive appearances have been reported with various causes, such as physiological accumulation [3, 9] and high uptake in inflammatory lesions [11]. This is the first report addressing the false-positive uptake in the lingual muscle region after definitive irradiation. All three patients reported here had relatively advanced tumours and had received an interstitial implant of small sources. The high accumulation of FDG on the PET–CT images was located in the lingual muscles, which were close to the tumour treated and the site of small source implantation. Inflammation after radiotherapy was a possible cause of this appearance. However, the interval between the end of therapy and the PET scan was sufficiently long (at least 15 months) to reduce the inflammatory change after irradiation. Moreover, clinical examination and MR findings did not support the possibility of inflammation. Several physiological and pathological accumulations, other than that due to inflammation, have been reported for the lingual muscles [12–16]. It is well known that high accumulations are observed in the skeletal muscles after exercise [17–19]. In tense patients, contraction of skeletal muscle increases this accumulation [20]. It may occasionally be unclear whether cervical lymph node metastases or the sternocleidomastoid muscle shows high uptake. Chewing gum before injection of FDG elevates glucose metabolism in the tongue and masticatory muscles, resulting in high uptake on PET images [12, 15, 16]. It is recommended that the patients should not chew gum for at least 4 h before PET examination [15]. Glucose metabolism is also elevated in the muscles of crying children [14]. Such uptake can be seen symmetrically in all lingual muscles. Moreover, the three patients presented here were asked not to chew gum and not to talk excessively before examination. Therefore, voluntary exercise should not have been a cause of these high uptakes. Although abnormal appearances are occasionally seen in patients with diabetes, the three patients showed normal glucose levels and did not use insulin. Asymmetric high accumulation in the tongue has been reported in patients with cranial nerve XII paralysis [13]. Increased lingual muscle activity is caused in the opposite side by decreased glucose metabolism in the affected side as a result of denervation of cranial nerve XII. However, this accumulation traces almost the entire area of the unaffected side of the tongue [13].
Conversely, definitive radiotherapy occasionally causes severe muscle atrophy with the disappearance of tumour volume and residual fibrosis, resulting in a limitation in tongue movement [21, 22]. In such cases, the balance of lingual muscle activity may be broken and altered.
Together, the high uptake of FDG in these three patients probably relates to ill-balanced high activity of the lingual muscles after definitive radiotherapy. Radiologists should take this false-positive accumulation of FDG into account when interpreting PET images of patients who have received definitive radiotherapy for the mobile tongue.
Received for publication June 30, 2007.
Revision received October 1, 2007.
Accepted for publication December 7, 2007.
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Abstract
Introduction
