PET scan in Respiratory System

Dr Srujana Vitta
2nd year Post Graduate
Government General Chest Hospital
• What is PET scan?
• Principle of PET scan?
• What is the PET CT?
• How to perform PET study?
• Role of PET scan in pulmonology
• Infecatious and inflammatory conditions
• Malignant conditions
Introduction
• Imaging plays a vital role in the diagnosis, staging, and therapeutic
assessment in patients with various pulmonary diseases.
• PET stands for POSITRON EMISSION TOMOGRAPHY
• PET is a nuclear medical imaging technique which uses small amounts
of radioactive compounds (TRACERS) and has unique ability to
quantitate metabolic processes in vivo
• It is a non invasive and painless imaging modality.
History
• 1950 first PET scan was used for
detecting brain tumour with
sodium iodide as tracer
• 1978 – First PET commercial
scanner was introduced
• Initially used mainly for
research purpose
• First approval 1998 for FDG scan
• In year 2000, Hybrid scanner
PET CT was introduced
Principle
• PET detects hyper metabolism in cells as a proxy for the presence of
cancer
• Fludeoxyglucose F18 (FDG) is a positron-emitting radiotracer
(radiolabelled glucose analogue) that is selectively taken up by the
metabolically active cells
• Mechanism of cellular uptake and initial phosphorylation of 18 FDG is
similar to that of glucose
Principle
• Once 18 FDG is phosphorylated to FDG-6-phosphate it can not pass
through complete glycolytic cycle and it will be trapped with in the
cell
• The trapped 18 FDG undergoes radioactive decay by releasing a
positron which subsequently collides with an electron to produce two
high energy photons in a so called annihilation reaction
• The photons travel in opposite directions and are detected by a ring
scanner and are processed by a computer into image
Principle
• The amount of intra cellular 18 FDG is proportional to glucose uptake
and therefore to the metabolic activity of the tissue

• Malignant cells have increased glucose transport and metabolism due

to accelerated cell proliferation and increased hexokinase activity.
NORMAL UPTAKE IN WHOLE BODY ABNORMAL UPTAKE IN WHOLE BODY
PET SCAN PET SCAN
Integrated PET and CT
• PET images show functional information, they provide limited
anatomical data leading to difficulty in exact localization of the lesions

• PET/CT was designed to provide the solution to these shortcomings

• In a PET/CT scanner, the PET and CT tomographs are housed in a

single gantry with a single patient bed and workstation.
Integrated PET and CT
• Upon reconstruction, both the PET images and the CT images are
displayed side by side and overlaid (fused).

• CT scan gives a more precise localization and interpretation of the

hypermetabolic lesions, due to the availability of anatomical
landmarks
Patient Preparation
• Exercise should be avoided for 24 hours prior to the study
• Fast for 4-6 hours prior to the study
• Blood glucose level <120 mg/dl preferred and upto 200mg/dl maybe
acceptable
• Tracer administration intravenously (18 FDG is the most commonly
used) followed by an interval of 60 minutes before the scan to allow
the tracer to travel all over the body
• DIABETIC PATIENTS??
 Tracers in PET scan
Nuclide Half life Trcaer Application

0-15 2 mins Water Cerebral blood flow

C-11 20 mins Methionine Tumour protein synthesis

N-13 10 mins Ammonia Myocardial blood flow

F-18 110 mins FDG Glucose metabolism

Ga-68 68 mins DOTANOC Neuroendocrine imaging

Rb-82 72 secs Rb-82 Myocardial perfusion

Standard uptake value (SUV)
• SUV is semi-quantitative index of glucose utilization that is obtained
by normalizing the accumulation in the abnormal lesion to the
injected dose and patient body weight

• SUV is calculated using the following formula:

SUV = mean lesion activity/[injected dose/body weight(g)]

• SUV >2.5 is considered suspicious for malignancy

 PET Scan in Pulmonology

Malignant conditions Non malignant conditions

• Lung cancer • Infections
• Mesothelioma • Inflammatory conditions
PET in Lung malignancies
PET has significant implications in the management of
• LUNG CANCER
• MESOTHELIOMA
Role in lung cancer
• Characterisation of pulmonary nodules
• Staging and identification of occult distant metastasis
• Planning radiotherapy
• Monitoring for recurrence after completion of treatment
Solitary pulmonary nodule
• Solitary pulmonary nodule is defined as a round or oval radiographic
opacity in lung parenchyma that measures up to 3cm in size and is not
accompanied by mediastinal lymphadenopathy and atelectasis
• Commonly identified on chest radiographs and CT scans
• 40-50% of SPN are malignant
• PET provides accurate non invasive diagnostic assessment of SPN
without the morbidity and costs associated with invasive tissue
sampling
Solitary pulmonary nodule
• Patients with positive PET scan requires further evaluation as these
lesions are considered malignant until proven otherwise.

• Lesions that do not have increased FDG uptake are usually benign.
These patients can be followed with sequential imaging

• Sensitivity and specificity of FDG PET in detecting benign and

malignant pulmonary nodules range from 95% and 80% respectively
Staging of Lung cancer
• Once diagnosis of lung cancer has been made accurate staging is
essential
• Staging helps in differentiating those in whom tumour is resectable
(stage I to IIIA) from those in whom tumour is not resectable (stage
IIIB or IV)
• In patients with known NSCLC the results of staging both within and
outside the thorax are key in determining operability
Staging of Lung cancer
• The negative predictive value of FDG PET is sufficiently high that a
negative mediastinum on PET may preclude mediastinoscopy and a
positive mediastinum on PET should be further assessed to exclude
false positive results

• PET is useful in identifying optimal site for mediastinal lymph node

biopsy and selection of additional invasive methods for sampling
lymph nodes inaccessible by mediastinoscopy
Staging of Lung cancer
• Although CT and occasionally MRI is used to evaluate hilar and
mediastinal nodes the accuracy of detecting nodal metastasis is
approximately 60%

• Normal sized lymph nodes may harbour malignant cells and enlarged
nodes may be benign

• PET is more sensitive and specific than CT alone with accuracy

reported to be greater than 80%
FDG PET for extrathoracic metastasis
• 40% with NSCLC have distant metastasis at presentation most often in
the adrenal gland, bones, liver or brain

• Adrenal glands: PET has high sensitivity (>92%) and specificity (80-
100%)

• Bones: PET has high sensitivity (90%) but higher specificity (98%)

• PET can detect bone metastases before reactive bone formation takes
place or prior to development of gross anatomical abnormalities
Planning Radiotherapy
• The use of anatomic imaging (CT or MRI or X RAY) alone leads to
inadequate radiation coverage and a higher chance of local
recurrence
• PET/CT guided RT improves radiation dose to the tumour and
metastases and reduced dose to the adjacent normal tissue
• Thus for radiation planning integrating PET CT appears to be more
accurate than CT alone in defining tumour extent
Post Treatment Followup
• In patients with residual parenchyma abnormalities following
radiotherapy PET scan can be used to distinguish between
persistent or recurrence and radiation fibrosis

• PET is more sensitive in measuring the effects of anti cancer therapy

and it can be used for early response assessment

• PET has a role in restaging after induction therapy in multimodality

approaches for locally advanced lung cancer
Prognostic Value
• In patients treated for lung cancer PET offers prognostic value that
correlates strongly with survival rate
• Patients with positive PET results after treatment have a significantly
worse prognosis than those with negative results
• Lack of clearance for mediastinal lymph node or unchanged FDG
uptake in the primary tumour after therapy usually denote a poor
outcome
Mesothelioma
• It is a malignant tumour that originate from mesothelial cell surface of
pleura
• PET can differentiate between benign and malignant mesothelioma
• FDG PET helps in identify best biopsy target within rind of foci with
most hyper metabolism
• FDG PET is also useful in identifying the extent of disease locally and
in the contralateral lung and detecting occult extra thoracic
metastases
PET CT in non oncological conditions
• In acute inflammation or infection of the chest FDG uptake primarily
occurs by activated neutrophils as their metabolism dependent on the
glycolysis resulting elevated uptake of glucose

• PET CT permits quantification of radioactivity throughout the lungs

and in air spaces and the interstitium thus enabling study of the
behaviour of inflammatory cells in their native microenvironment
Tuberculosis and non tuberculosis
mycobacteria
• Tuberculosis can involve any organ by hematogenous spread,
lymphatic spread or contiguity
• Two different patterns on FDG PET scanning seen in TB
Pulomonary pattern – more localised pattern
Lymphatic pattern – more intense systemic infection
• Due to large number of activated inflammatory cells with high
glycolytic rates active TB lesions usually display an intense FDG uptake
Tuberculosis and non tuberculosis
mycobacteria
• Important role of this imaging technique is its ability to assess early
treatment response.
• Morphological changes often take significantly longer to be
detectable than molecular changes.
• Thus, the ability to identify active tuberculosis lesions earlier than by
conventional radiology has an important impact on patient
management particularly relevant in certain clinical settings such as in
severely immunosuppressed patients and those co-infected with HIV
Tuberculosis and non tuberculosis
mycobacteria
• FDG-PET allows the rapid assessment of pulmonary and
extrapulmonary tuberculosis simultaneously, leading to time saving
and cost-effectiveness.
• However, PET images should be interpreted with caution because of
the lack of specificity and inability to clearly distinguish
granulomatous disease from malignancy based on standardized
uptake values
• Histological confirmation is needed in most cases
Role of PET in other lung conditions
• Like active granulomatous processes, other infectious diseases and
active fibrotic lesions can mimic malignancy, leading to false-positive
PET scans; thus, the role of this imaging technique in this setting is
limited
• Ability to quantitate FDG uptake, FDG-PET could be potentially useful
in monitoring the infectious or inflammatory processes, and in
determining treatment efficacy
Increased FDG PET uptake in some
benign pulmonary conditions
• Infections:
Lung abscess
Bacterial pneumonia, actinomycosis, histoplasmosis, invasive
aspergillosis, blastomycosis
• Inflammatory lesions:
Sarcoidosis
Vasculitis wegners granulomatosis, takayasu arteritis,
Pneumoconiosis (silicosis, coal workers – fibrosis)
SUMMARY
• PET scan is a painless and non invasive imaging modality that helps in
understanding in vivo metabolism of the tissue
• PET and CT are often integrated for better understanding anatomical
lesions exhibiting high metabolism
• PET helps in characterisation of pulmonary nodules, staging of lung
malignancy, planning of radiotherapy and in early identification of
recurrence
• In non malignant conditions PET helps in monitoring the infectious or
inflammatory processes, and in determining treatment efficacy
References
1. Fishman’s pulmonary diseases and disorders. 6th edition
2. Capitanio et al. PET/CT in nononcological lung diseases: current
applications and future perspectives
3. Murray and nadal’s textbook of respiratory medicine. 7th edition
 THANK YOU

Next seminar on 03.04.2024 by Dr Fayaz on Traube’s space