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18F-FDG PET/CT Normal Variants, Artefacts and Pitfalls in Thyroid Cancer

  • Arun Sasikumar
  • Alexis Corrigan
  • Muhammad Umar Khan
  • Gopinath Gnanasegaran
Chapter
Part of the Clinicians’ Guides to Radionuclide Hybrid Imaging book series (CGRHI)

Abstract

Please provide affiliation details for the following authors: Arun Sasikumar, Alexis Corrigan, Muhammad Umar Khan.

6.1 Introduction

18F-FDG is widely used as a PET tracer in UK oncologic clinical practice. 18F-FDG has the potential to be a specific and sensitive test with clear indications in the management of thyroid malignancy [1]. PET/CT is a whole body technique, and the normal distribution, common artefact, variant and pitfalls of 18F-FDG PET/CT have been extensively described [2, 3, 4]. In this chapter we will give an overview of the more commonly seen variants and artefacts, particularly within the context of assessment of thyroid malignancy.

The biodistribution of FDG can vary according to fasting state, level of muscular exertion, various medications and the length of the uptake period after injection [5]. Rigorous attention should be given to the preparation of patients prior to 18F-FDG PET/CT [6, 7]. Diabetic patients in particular require careful preparation.

18F-FDG accumulation is not specific to cancer cells, and awareness of normal biodistribution, variants and common benign pathology that may mimic malignancy is required for accurate interpretation of PET/CT examinations.

Activated inflammatory processes also show increased glycolysis and consequently FDG uptake [5, 8].

6.2 Head and Neck

6.2.1 Technical: Artefacts and Pitfalls

In the assessment of thyroid and other head and neck malignancies, the major technical challenge is limiting voluntary patient movement in the head and neck region. Motion artefacts commonly arise from either motion of the head during the PET acquisition or from movement that occurs between the two acquisitions. Motion during the CT acquisition is less common as the CT acquisition is relatively quick. It is usual in many centres to perform PET imaging in the assessment of head and neck malignancy with a specific head restraint to improve patient comfort and compliance, which also helps to limit motion. If local movement is severe, repeat local imaging may be required (Table 6.1).
Table 6.1

Common technical problems affecting 18F-FDG PET/CT in the head and neck region

 

Appearance

Reason

Comment

Voluntary motion

Blurred PET image

PET image represents average of all motion

Communication, support and restraints.

Repeat local view

Recent intervention (biopsy or surgery)

Locally increased or decreased uptake

Inflammatory reaction, haematoma, seroma

Delay PET imaging, if possible. Correlate with medical record

Artefact from metalwork or dental amalgam

Locally increased or decreased uptake

AC error due to CT metal artefact

Review non-attenuation corrected imaging

Local CT artefacts from dental implants or fillings can cause difficulties both related to localisation and beam hardening artefact, resulting in inappropriate attenuation correction (Fig. 6.1). Review of other imaging modalities and the non-attenuation correction imaging, respectively, can mitigate these problems.
Fig. 6.1

Beam hardening artefacts due to dental fixtures

6.2.2 Thyroid: Variants, Artefacts and Pitfalls

The incidence of thyroid malignancy is related to a variety of risk factors. In general age and sex are common risk factors for a variety of cancer types. When analysing studies, particularly for older patients, the reporter must also be alert to the potential for detection of additional incidental malignancy.

Incidental pathologies have been found in 25% of PET studies performed for a range of indications [9] (Fig. 6.2). The clinical impact of incidental findings needs to be considered in the context of patient care. Reviewing previous imaging, patient’s history, clinical and biochemical investigations and, often, histopathologic examination may all be required to enable confident analysis of a lesion. An awareness of the incidence of these pathologies in the local population is essential. Decisions on the most appropriate patient management are often enlightened by a multidisciplinary review.
Fig. 6.2

Incidentally detected intensely FDG concentrating well-defined enhancing lesion in the left parotid gland (yellow arrow) likely benign tumour (Warthins/pleomorphic adenoma)

In the context of known thyroid malignancy, focal thyroid uptake at the primary lesion relates to lesion size and pathological subtype [10]. Incidental uptake at other sites within the thyroid should also be treated with suspicion and, if clinically relevant, should be further assessed at US and with fine-needle aspiration cytology [11].

Increased uptake throughout the thyroid (Fig. 6.3) may represent a benign thyroiditis and may correlate with subclinical autoimmune thyroiditis and relevant auto-antibodies or may be associated with Graves’ disease [12]. Thyroiditis may also involve one or more areas of the thyroid gland sparing the rest of the gland and hence warrants caution while interpreting relatively focal FDG uptake involving the part of the thyroid gland (Fig. 6.4).
Fig. 6.3

Diffusely increased heterogeneous FDG uptake in asymmetrically enlarged (R > L) lobes of the thyroid gland

Fig. 6.4

A case of bilateral breast carcinoma. FDG PET/CT revealed moderate heterogeneous FDG-avid hypodense lesions in the enlarged right lobe of the thyroid gland. She underwent bilateral mastectomy with right hemithyroidectomy, and frozen section was done which revealed thyroiditis in the right lobe. Hence surgery was restricted to right hemithyroidectomy, and final histopathology confirmed it to be thyroiditis

Following surgery to the thyroid gland, residua l uptake in the thyroid bed and related to the surgical approach, uptake in the lateral/central compartments of neck can persist for 6–12 months [13]. Routinely FDG PET/CT imaging is delayed for at least 3 months following surgery, though low-level uptake is often seen, usually quite uniformly distributed throughout the surgical bed. Foreign material, e.g. surgical stitch, can cause a granulomatous reaction and result in very localised intense activity, which can be precisely correlated to the surgical material on the CT. False-positive uptake can also be caused by recent intervention, for example, nodal biopsy or procedures like tracheostomy (Fig. 6.5). Clear clinical information and multidisciplinary discussion can help to clarify this situation.
Fig. 6.5

A case of oropharyngeal carcinoma posttreatment with tracheostomy tube in situ. Note the near symmetrical intense FDG uptake in bilateral strap muscles of the neck (red arrows). Surrounding the tracheostomy tube, reactive FDG uptake is noted (yellow arrow) with no morphological abnormalities in CT part

6.2.3 Head and Neck: Variants, Artefacts and Pitfalls

Normal variant head and neck uptake is routinely seen on Table 6.2. FDG uptake at the salivary glands can be moderate but is generally quite symmetrical [14] (Fig. 6.6). Localised intense uptake within the parotid gland is usually interpreted as benign, related to pleomorphic adenoma or Warthin’s tumour [15], though the parotid gland can contain lymph nodes (Fig. 6.7) and in the assessment of the thyroid malignancy should be dismissed with caution (Fig. 6.8). Rarely parotid metastasis has also been reported (Fig. 6.9). Review of previous or follow-up imaging is often helpful; alternatively, if there is ongoing uncertainty, definitive assessment is with ultrasound-directed biopsy or fine-needle aspiration cytology. Increased FDG accumulation can also be seen in granulomatous disorders of the salivary glands, such as sarcoidosis, where they tend to be bilateral, symmetrical and often related to typical features in the chest [16].
Fig. 6.6

Physiological tracer uptake in bilateral submandibular (red arrows) and parotid (blue arrows) glands. Also note the physiological tracer uptake in the mylohyoid muscles (yellow arrows)

Fig. 6.7

Moderate focal FDG uptake in bilateral intra-parotid lymph nodes

Fig. 6.8

FDG PET/CT done for evaluation of disease status in a TENIS (thyroglobulin-elevated negative iodine scan) patient showing focal intense FDG uptake in a homogeneously enhancing well-defined lesion in the left parotid gland (red arrow)

Fig. 6.9

FDG PET/CT scan done for suspected recurrence evaluation in a treated case of left renal cell carcinoma. Intensely FDG-concentrating large lesion in the left parotid which was later confirmed as metastasis

Table 6.2

Common variant and incidental pathology in the assessment of the head and neck region with 18F-FDG PET/CT

 

Appearance

Cause

Comment

Waldeyer’s ring

Variable, may be intense, usually symmetrical

Physiological but may be increased secondary to URTI

Common variant

Salivary gland

Low grade can be increased in systemic inflammatory disorders

Physiological

Usually symmetrical

Paro tid

Focal single or multiple FDG avid lesions. Frequently correspond with soft tissue density lesions identifiable on CT

Most commonly benign pleomorphic adenoma or Warthin’s tumour

The parotid also contains lymphatic tissue. US-guided biopsy should be considered

Brown fat

Involves neck and occasionally paraspinal and retroperitoneal fat

Thermogenesis may be particularly prominent in winter

Ensure a suitably warm uptake area

Thyroid focal

FDG-avid lesion within the thyroid, which may relate to hypo, iso- or hyperdense CT lesion

Represents an incidental thyroid malignancy in approximately 1/3 of cases [11]

Focal lesion should be assessed with US and FNAC

Thyroid diffuse

Diffusely increased thyroid uptake

May be normal variant or represent thyroiditis

Correlation with thyroid biochemistry

Vocal cord

Variable. Usually moderate and symmetric

Due to phonation during uptake period

Caution with unilateral vocal cord palsy

Muscular uptake in the head and neck region is a common finding due to muscle activation. Uptake at the floor of mouth, localising to genioglossus muscle, is a common finding and relates to normal muscle tone from maintaining upper airway patency in the supine position [17]. Physiological FDG uptake is often seen in the floor of the mouth and can be identified from the symmetry and absence of morphological changes in CT (Fig. 6.10). Uniform arytenoid and vocal cord activity is related to phonation during the uptake period (Fig. 6.11) and can be limited by maintaining a quiet, restful uptake area. Rarely pathological uptake can be present in the vocal cord region and can be identified by the asymmetry and presence of morphologically identifiable lesion in CT (Fig. 6.12). Uptake in the musculature of the neck is a not infrequent finding, commonly at the longus coli/longus capitis muscles at the anterior cervical spine (Figs. 6.5, 6.13, and 6.14). This can be minimised by ensuring patient comfort during the uptake and imaging periods. Altered local muscle dynamic as a result of previous neck dissection or nerve palsy can cause marked asymmetry in the degree of muscular and also vocal cord uptake; usually close correlation with the CT study and clinical history can allow the reporter to dismiss this as a benign pitfall.
Fig. 6.10

Symmetrical physiologic uptake in the floor of the mouth (mylohyoid muscle) marked as asterisk

Fig. 6.11

Symmetrical FDG uptake in bilateral vocal cords (no morphological changes in CT part)

Fig. 6.12

Focal abnormal intense FDG uptake is noted in an ill-defined nodular thickening at the right false vocal cord. Note the nature of difference in pathological and physiological tracer uptake in the region of vocal cords and the associated CT changes

Fig. 6.13

Symmetrical FDG uptake in the strap muscles of the neck (red arrows) and in longus capitis muscle (yellow arrows)

Fig. 6.14

Symmetrical intense FDG uptake in bilateral pterygoid muscles due to muscle activity

Lymphoid activation at Waldeyer’s ring is a frequent finding and can be intense; it is normally symmetrical, though asymmetry can be seen as a normal variant [18] (Fig. 6.15). Particularly in the context of morphological abnormality, asymmetric activity can mimic incidental squamous tumour or lymphoma. Dedicated local imaging can help to evaluate further, though direct inspection may be required.
Fig. 6.15

Different patterns of FDG uptake in the Waldeyer’s ring of lymphoid tissues (bold red arrows) in three different patients

Uptake within thermogenic (brown) fat is usually limited to the cervical regions but can extend to paraspinal and even retroperitoneal regions [19] (Fig. 6.16). The symmetry and CT appearances are typical, and generally this is straightforward to recognise. This can be minimised by maintaining patient warmth prior to and during the PET acquisition [20].
Fig. 6.16

Extensive brown fat uptake in a 19-year-old case of papillary carcinoma thyroid (TENIS). Brown fat uptake is noted in bilateral cervical, shoulders, paravertebral and retrocrural regions

The more common sites of thyroid metastases to the lung and bone [21] are well evaluated with 18F-FDG PET/CT. FDG uptake at new or morphologically evolving pulmonary nodules usually represents metastatic disease, though this can be mimicked by a variety of benign pathologies as a result of local immunologic response. In the UK, bacterial pneumonia, tuberculosis, inflammatory conditions, radiation pneumonitis, pneumoconiosis, cryptogenic organising pneumonia, pleurodesis and eosinophilic granuloma are more often seen, though in some communities cryptococcosis, paragonimiasis or histoplasmosis are increasing in prevalence [22]. Often the characteristic morphology will suggest a benign cause. Further medical and radiological review can also help to identify these inflammatory causes. FDG-avid emboli from paravenous injections of FDG are relatively rare and usually can be recognised due to the lack of CT correlate [23].

Confidence in the diagnosis of skeletal metastases can be improved with close review of the morphology of the FDG-avid focus. Thyroid metastases to the bone are typically multiple, lytic and well defined, occasionally with an extra-osseous soft tissue component and increase in size with time.

Key Points

  • In the context of known thyroid malignancy, focal thyroid uptake at the primary lesion relates to lesion size and pathological subtype.

  • Diffuse increased uptake throughout the thyroid may represent a benign thyroiditis and may correlate with subclinical autoimmune thyroiditis and relevant auto-antibodies or may be associated with Graves’ disease.

  • Following surgery to the thyroid, residual uptake in the thyroid bed and related to the surgical approach can persist for 6–12 months.

  • Routine FDG PET/CT imaging is delayed for at least 3 months following surgery.

  • The common sites of thyroid metastases to the lung and bone are evaluated with 18F-FDG PET/CT.

  • FDG uptake at new or morphologically evolving pulmonary nodules usually represents metastatic disease.

  • Thyroid metastases to the bone are typically multiple, lytic and well defined, occasionally with an extra-osseous soft tissue component and increase in size with time.

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Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  • Arun Sasikumar
    • 1
  • Alexis Corrigan
    • 2
  • Muhammad Umar Khan
    • 3
  • Gopinath Gnanasegaran
    • 4
  1. 1.Consultant and Head of the Department, Department of Nuclear MedicineSt Gregorios International Cancer Care CentreParumalaIndia
  2. 2.Consultant in Radionuclide RadiologyMaidstone HospitalMaidstoneUK
  3. 3.Al-Jahra HospitalAl-JahraKuwait
  4. 4.Department of Nuclear MedicineRoyal Free London NHS Foundation TrustLondonUK

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