Molecular Imaging Practice Questions

Q: A 58-year-old woman with gastrinoma undergoes both FDG-PET and 68Ga-DOTATATE PET scans. FDG-PET shows minimal uptake (SUVmax 2.1) while DOTATATE scan shows intense uptake (SUVmax 45). What does this pattern indicate about tumor biology?

Well-differentiated slow-growing tumor. ***Well-differentiated slow-growing tumor*** - High **DOTATATE uptake** indicates dense expression of **somatostatin receptors (SSTRs)**, which is a hallmark of well-differentiated neuroendocrine tumors. - Low **FDG uptake** (low SUVmax) reflects a low rate of glucose metabolism, signifying a **low-grade (G1/G2)** tumor with a slow proliferation rate. *High grade aggressive tumor* - Aggressive, high-grade neuroendocrine carcinomas (G3) typically show high **FDG avidity** because they rely heavily on glycolysis for energy. - These tumors often lose their **somatostatin receptor expression**, leading to low or absent uptake on a **DOTATATE scan**. *Necrotic tumor with inflammation* - **Necrosis** generally presents as a photopenic (cold) area in the center of a lesion on PET imaging, not intense DOTATATE uptake. - **Inflammation** would typically result in increased **FDG uptake** due to high metabolic activity in activated leukocytes, rather than isolated high DOTATATE avidity. *False positive DOTATATE scan* - Intense uptake with an **SUVmax of 45** is highly specific for SSTR-rich tissues and is considered diagnostic for neuroendocrine pathology in this clinical context. - A **gastrinoma** is a known neuroendocrine tumor (NET) that consistently expresses these receptors, making a false positive highly unlikely.

Q: A 65-year-old man with known lung cancer undergoes FDG-PET scan. The scan shows intense FDG uptake (SUVmax 8.5) in the primary lung mass and a 1.2 cm mediastinal lymph node with SUVmax 4.2. What is the most appropriate interpretation?

Both primary and nodal metastasis. ***Both primary and nodal metastasis*** - In lung cancer staging, a **Standardized Uptake Value (SUVmax)** greater than 2.5 in mediastinal lymph nodes is highly suspicious for **metastatic involvement**. - The node's SUVmax of 4.2 relative to the primary tumor's high uptake (8.5) strongly indicates **metabolically active disease** at both the primary and nodal sites. *Primary tumor with false positive node* - **False positives** on PET often occur due to granulomatous disease or infection, but an SUVmax of 4.2 in a known cancer context is more likely **metastatic**. - Without history of **sarcoidosis** or active infection, the metabolic activity in the lymphatic basin of the primary tumor is considered malignant until proven otherwise. *Dual primary malignancies* - Two separate primary malignancies would typically involve different anatomical sites or different **histological features**, rather than a primary and its draining node. - The presence of a mediastinal node in a patient with a known lung mass is the classic presentation of **regional spread** (N staging), not a second primary site. *Primary tumor only, node is inflammatory* - While inflammation can cause **FDG avidity**, an SUVmax of 4.2 is significantly high and less typical for purely reactive or **incidental inflammatory** nodes. - Relying on an inflammatory interpretation without biopsy would risk **understaging** the patient's lung cancer, as the node meets the metabolic criteria for malignancy.

Q: How does 68Ga-DOTATATE molecular imaging differ from FDG-PET in mechanism of tumor detection?

It binds to somatostatin receptors. ***It binds to somatostatin receptors*** - **68Ga-DOTATATE** is a radiolabeled somatostatin analogue that specifically targets cells overexpressing **somatostatin receptors (SSTR)**, particularly subtype **SSTR2**. - This receptor-ligand binding allows for highly specific imaging of **neuroendocrine tumors (NETs)**, which is fundamentally different from metabolic pathways. *It targets metabolic activity* - This describes the mechanism of **18F-FDG PET**, which relies on **glucose metabolism** and the upregulation of **GLUT transporters** in malignant cells. - While effective for aggressive cancers, many low-grade neuroendocrine tumors have low metabolic rates and may be **FDG-negative**. *It measures blood flow* - Measuring blood flow is characteristic of certain **perfusion scans** or dynamic contrast-enhanced imaging, not the molecular targeting of DOTATATE. - Radiopharmaceuticals like **15O-water** or certain technetium-based agents are used for perfusion studies rather than receptor-specific mapping. *It detects DNA synthesis* - Agents that detect DNA synthesis, such as **18F-FLT**, target the enzyme **thymidine kinase-1** to visualize cellular proliferation. - While provide information on tumor growth, this is distinct from the **somatostatin receptor expression** targeted by **68Ga-DOTATATE**.

Question 1

A research team is developing a new radiotracer for imaging hypoxia in tumors. They need to select between 18F-labeled and 64Cu-labeled versions of the same molecule. Considering half-lives (18F: 110 min, 64Cu: 12.7 hours), positron ranges, and clinical applicability, which choice and rationale is most appropriate?

Accepted Answer

18F for better spatial resolution despite requiring on-site cyclotron

Answer

64Cu for longer imaging window despite inferior image quality

Answer

64Cu because shorter positron range improves resolution

Answer

18F because longer half-life allows delayed imaging

Question 2

A 58-year-old woman with gastrinoma undergoes both FDG-PET and 68Ga-DOTATATE PET scans. FDG-PET shows minimal uptake (SUVmax 2.1) while DOTATATE scan shows intense uptake (SUVmax 45). What does this pattern indicate about tumor biology?

Accepted Answer

Well-differentiated slow-growing tumor

Answer

High grade aggressive tumor

Answer

Necrotic tumor with inflammation

Answer

False positive DOTATATE scan

Question 3

A 65-year-old man with known lung cancer undergoes FDG-PET scan. The scan shows intense FDG uptake (SUVmax 8.5) in the primary lung mass and a 1.2 cm mediastinal lymph node with SUVmax 4.2. What is the most appropriate interpretation?

Accepted Answer

Both primary and nodal metastasis

Answer

Primary tumor with false positive node

Answer

Dual primary malignancies

Answer

Primary tumor only, node is inflammatory

Question 4

How does 68Ga-DOTATATE molecular imaging differ from FDG-PET in mechanism of tumor detection?

Accepted Answer

It binds to somatostatin receptors

Answer

It targets metabolic activity

Answer

It measures blood flow

Answer

It detects DNA synthesis

Molecular Imaging — MCQs

Molecular Imaging — MCQs

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