Determinants of physiological uptake of 18F-fluorodeoxyglucose in palatine tonsils

Distribution and predictors of F-18-FDG uptake values of non-malignant cervical lymph nodes in pediatric patients

BACKGROUND

F-18-flurodeoxyglucose (FDG) PET/CT is routinely used for staging, evaluation of response to treatment and follow-up of most pediatric malignancies. Cervical lymph nodes can be involved in some pediatric malignancies, but increased uptake in non-malignant cervical lymph nodes is not exceptional in this population. The aim of the present study is to identify predictors of the maximum uptake in non-malignant cervical lymph nodes in the pediatric population.

METHODS

191 FDG PET/CT studies of pediatric patients without malignant involvement of cervical lymph nodes were retrospectively reviewed. The maximal Standard Uptake Value in the hottest cervical lymph node (SUVmaxCLN), as well as demographic, technical and imaging variables were recorded. The predictive effect of those variables on SUVmaxCLN was estimated using linear regression models.

RESULTS

Increased FDG activity in cervical nodes was observed in 136/191 studies (71%). The mean SUVmaxCLN was 2.2 ± 1.3. Ipsilateral palatine tonsil SUVmax, mean liver uptake, and treatment status were all statistically significant predictors of SUVmaxCLN. However, in multivariate regression analysis, only ipsilateral palatine tonsil SUVmax was found to be significant. In addition, SUVmaxCLN was greater than the mean liver uptake in 50% of all studies. This proportion was higher in younger children, reaching 77% of studies of children younger than six years.

CONCLUSION

SUVmax in ipsilateral palatine tonsil is a strong predictor of the maximal uptake value of non-malignant cervical lymph nodes in children. The intensity of uptake in non-malignant cervical lymph nodes is frequently higher than liver uptake in children, and this tendency increases for younger patients.

TRIAL WAS REGISTERED

In the internal hospital registry under TRN 0209-22-HMO on date 23.04.2022.

Imaging Approach for Cervical Lymph Node Metastases from Unknown Primary Tumor

Neck swelling due to lymph node (LN) metastasis is one of the initial symptoms of head and neck cancer, and in some cases, the primary tumor is not clinically evident. The purpose of imaging for LN metastasis from an unknown primary site is to identify the primary tumor or detect its absence, which leads to the correct diagnosis and optimal treatment. The authors discuss diagnostic imaging approaches for identifying the primary tumor in cases of unknown primary cervical LN metastases. The distribution and characteristics of LN metastases may help locate the primary site. Unknown primary LN metastasis often occurs at nodal levels II and III, and in recent reports, these were mostly related to human papillomavirus (HPV)-positive squamous cell carcinoma of the oropharynx. Another characteristic imaging finding suggestive of metastasis from HPV-associated oropharyngeal cancer is a cystic change in LN metastases. Other characteristic imaging findings such as calcification may help predict the histologic type and locate the primary site. In cases of LN metastases at nodal levels IV and VB, a primary lesion located outside the head and neck region must also be considered. One clue for detecting the primary lesion at imaging is the disruption of anatomic structures, which can help in identifying small mucosal lesions or submucosal tumors at each subsite. Additionally, fluorine 18 fluorodeoxyglucose PET/CT may help identify a primary tumor. These imaging approaches for identifying primary tumors enable prompt identification of the primary site and assist clinicians in making the correct diagnosis. ^© RSNA, 2023 Quiz questions for this article are available through the Online Learning Center.

Incidental FDG-Avid Focuses in Palatine Tonsils on PET/CT

OBJECTIVES

The management of incidental findings of FDG-avid tonsils on PET/CT (IFT) is unclear. We aimed to explore the prevalence of malignancy in IFT, identify risk factors for malignancy, and calculate optimal cutoffs of maximum standardized uptake values (SUV_max ) to discriminate between benign and malignant lesions.

METHODS

All patients who were tonsillectomized at our institution because of IFT from October 2011 to December 2020 were included. Patients undergoing PET/CT due to suspected tonsillar disease or cancer of unknown primary were excluded.

RESULTS

In total, 77 patients were included, of which 11 (14%) of them had IFT malignancy. Dysphagia (p = 0.019) and alcohol abuse (p = 0.035) were associated with malignancy. Absolute SUV_max cutoff (≥9: sensitivity 100%; specificity 53%) was superior to SUV_max side-to-side ratio (≥1.5: sensitivity 64%; specificity 70%) to discriminate between benign and malignant lesions.

CONCLUSION

We recommend tonsillectomy for patients with IFT displaying SUV_max  ≥ 9.0, ratio ≥ 1.5, or symptoms or findings suggesting malignancy.

LEVEL OF EVIDENCE

3 Laryngoscope, 132:2370-2378, 2022.

PET/CT Variants and Pitfalls in Head and Neck Cancers Including Thyroid Cancer

PET/CT imaging is a dual-modality diagnostic technology that merges metabolic and structural imaging. There are several currently available radiotracers, but ¹⁸F-FDG is the most commonly utilized due to its widespread availability. ¹⁸F-FDG PET/CT is a cornerstone of head and neck squamous cell carcinoma imaging. ⁶⁸Ga-DOTA-TOC is another widely used radiotracer. It allows for whole-body imaging of cellular somatostatin receptors, commonly expressed by neuroendocrine tumors and is the standard of reference for the characterization and staging of neuroendocrine tumors. The normal biodistribution of these PET radiotracers as well as the technical aspects of image acquisition and inadequate patient preparation affect the quality of PET/CT imaging. In addition, normal variants, artifacts and incidental findings may impede accurate image interpretation and can potentially lead to misdiagnosis. In order to correctly interpret PET/CT imaging, it is necessary to have a comprehensive knowledge of the normal anatomy of the head and neck and to be cognizant of potential imaging pitfalls. The interpreter must be familiar with benign conditions which may accumulate radiotracer potentially mimicking neoplastic processes and also be aware of malignancies which can demonstrate low radiotracer uptake. Appropriate use of structural imaging with either CT, MR or ultrasound can serve a complimentary role in several head and neck pathologies including local tumor staging, detection of bone marrow involvement or perineural spread, and classification of thyroid nodules. It is important to be aware of the role of these complementary modalities to maximize diagnostic accuracy and patient outcomes. The purpose of this article is to outline the basic principles of PET/CT imaging, with a focus on ¹⁸F-FDG PET/CT and ⁶⁸Ga-DOTA PET/CT. Basic physiology, variant imaging appearances and potential pitfalls of image interpretation are presented within the context of common use cases of PET technology in patients with head and neck cancers and other pathologies, benign and malignant.

Tonsillar Metastasis From Colon Cancer Detected on 18F-FDG PET/CT Imaging

A 54-year-old woman with smoking history who was diagnosed with invasive adenocarcinoma of the rectosigmoid colon with subsequent surgical resection and chemotherapy was presented. The patient developed local recurrence and pulmonary metastases that required further chemotherapy and surgical resection. During follow-up, the patient had F-FDG PET/CT scan showing asymmetric hypermetabolic tonsils with activity greater on the right compared with the left. The patient underwent bilateral tonsillectomy, and pathology revealed moderately to poorly differentiated adenocarcinoma in the right tonsil, consistent with metastasis from known colonic primary.

Improved cancer detection in Waldeyer's tonsillar ring by 68Ga-FAPI PET/CT imaging

Purpose

In cancer of unknown primary (CUP), positron emission tomography/computed tomography (PET/CT) with the glucose analog [¹⁸F]FDG represents the standard imaging approach for localization of the malignant primary. Frequently, however, [¹⁸F]FDG PET/CT cannot precisely distinguish between small occult tumors and chronic inflammation, especially in Waldeyer’s tonsillar ring. To improve the accuracy for detecting primary tumors in the Waldeyer’s tonsillar ring, the novel PET tracer [⁶⁸Ga]Ga-FAPI-4 for specific imaging of fibroblast activation protein (FAP) expression was used as a more specific target for cancer imaging.

Methods

Eight patients with suspicion of a malignant tumor in Waldeyer’s tonsillar ring or a CUP syndrome were examined. PET/CT scans with [¹⁸F]-FDG and [⁶⁸Ga]Ga-FAPI-4 were performed for pre-operative tumor localization. After surgical resection, histopathological and immunohistochemical results were compared to PET/CT findings.

Results

Histopathology revealed a palatine or lingual tonsil carcinoma in all patients. In case of lymph node metastases smaller than 7 mm in size, the [¹⁸F]FDG PET/CT detection rate of cervical lymph node metastases was higher than that of [⁶⁸Ga]FAPI PET/CT, while both tracers identified the primary tumors in all eight cases. The size of the primary and the lymph node metastases was directly correlated to the respective FAP expression, as detected by immunohistochemistry. The mean SUV_max for the primary tumors was 21.29 ± 7.97 for ¹⁸F-FDG and 16.06 ± 6.29 for ⁶⁸Ga-FAPI, respectively (p = 0.2). The mean SUV_max for the healthy contralateral tonsils was 8.38 ± 2.45 for [¹⁸F]FDG and 3.55 ± 0.47 for [⁶⁸Ga]FAPI (p < 0.001). The SUV_max ratio of [⁶⁸Ga]FAPI was significantly different from [¹⁸F] FDG (p = 0.03). Mean TBR_max for the [⁶⁸Ga]Ga-FAPI-4 tracer was markedly higher in comparison to [¹⁸F]FDG (10.90 vs. 4.11).

Conclusion

Non-invasive imaging of FAP expression by [⁶⁸Ga]FAPI PET/CT resulted in a better visual detection of the malignant primary in CUP, as compared to [¹⁸F]FDG imaging. However, the detection rate of lymph node metastases was inferior, presumably due to low FAP expression in small metastases. Nevertheless, by offering a detection method for primary tumors with the potential of lower false positive rates and thus avoiding biopsies, patients with CUP syndrome may benefit from [⁶⁸Ga]FAPI PET/CT imaging.

Clinical Significance of Increased FDG Uptake in the Waldeyer Ring and the Nasopharynx Region Identified by PET-CT in Postchemotherapy Follow-up in Patients With Lymphoma: When Should We Perform Biopsy?

PURPOSE

To identify whether fluorodeoxyglucose (FDG) uptake in the Waldeyer ring (WR)/nasopharyngeal (NP) region by positron emission tomography-computed tomography (PET-CT) was physiologic or pathologic in the follow-up of lymphoma patients receiving postchemotherapy treatment.

PATIENTS AND METHODS

We retrospectively examined FDG uptake in the WR/NP region in 534 patients with lymphoma as assessed by PET-CT used for both diagnosis and follow-up.

RESULTS

Forty-nine patients had FDG uptake in the WR/NP region by PET-CT performed after completion of a chemotherapy regimen. Biopsy was performed for 11 of these patients in whom the uptake was considered to be pathologic, and results indicated the presence of reactive follicular hyperplasia. It was considered to be physiologic in 38 patients. PET-CT was repeated after 1 year, and no significant difference was identified between the standardized maximum uptake values (SUV_max; P = .107). The initial diagnosis of 20 patients was made via biopsy performed in the WR/NP region. The SUV_max for the FDG uptake in these patients, asymmetry, SUV_max of the coexisting lymphadenopathies in the neck, and FDG uptake with a counterpart finding by CT were significantly higher than other groups (P = .047, .001, and .005).

CONCLUSION

When deciding whether to resample after treatment completion, it should be taken into account that, in addition to the SUV_max of the lesion, asymmetry, and the SUV_max of the coexisting lymphadenopathy in the neck, a crucial criterion is whether the FDG uptake has a counterpart finding by CT.

Normal FDG uptake in the adenoids and palatine tonsils in children on PET/MRI

BACKGROUND

Information about the normal [F-18]2-fluoro-2-deoxyglucose (FDG) uptake in the adenoids and palatine tonsils in children is not available.

OBJECTIVE

The purpose of this study was to report the range of standardized uptake values (SUVs) in the normal adenoids and palatine tonsils in children, assess for the degree of asymmetry between the right and left tonsils and evaluate for the correlation of SUVs between the adenoids and tonsils.

MATERIALS AND METHODS

Pediatric patients who had had an FDG positron emission tomography (PET)/magnetic resonance imaging (MRI) brain study in our institution from January 2018 to March 2019 were identified. Patients with a history of malignancy, adenoidectomy and/or tonsillectomy, incomplete imaging coverage of Waldeyer ring and the presence of artifact on PET/MRI were excluded. Two pediatric radiologists independently measured the mean and maximum SUVs of the right tonsil, left tonsil and the adenoids. Range, mean and standard deviation were calculated for all measurements. Ratios of SUV of the left to right tonsils and the adenoids to the tonsils were calculated. The paired t-test and Pearson's correlation test were used for statistical analysis with a P-value <0.05 considered to be significant.

RESULTS

Sixty-one PET/MRI brain scans were performed in our institution during the study period. After reviewing for exclusion criteria, 41 patients were included in the study (mean age: 10.1 years, range: 2-17 years; 19 boys and 22 girls). The mean SUV was 5.30±1.57 in the right tonsil, 5.25±1.53 in the left tonsil and 4.56±1.90 in the adenoids. The maximum SUV was 8.47±2.22 in the right tonsil, 8.45±2.18 in the left tonsil and 7.59±2.94 in the adenoids. The difference between the SUVs of the right and left tonsil was not statistically significant (P=0.69 for mean SUV and P=0.90 for maximum SUV). There was a statistically significant moderately positive correlation between the FDG uptake in the adenoids and the right and left tonsil for both mean and maximum SUV (r=0.36-0.41; P=0.008-0.022).

CONCLUSION

There is a wide variation of FDG uptake in the normal tonsils and adenoids in children. Uptake in the right and left tonsils is not significantly different. There is a moderately positive correlation between the FDG uptake in the adenoids and the tonsils.