NUCLEAR MEDICINE AND RADIOTHERAPY🗸

NUCLEAR MEDICINE AND RADIOTHERAPY

Functional imaging technique.

Uses a radioisotope bound to a ligand.

Ligand carries isotope to target organ.

Isotope emits radiation from site.

Gamma camera captures images.

Measures radiation amount → shows hypofunctioning or hyperfunctioning organ.

Common Radioisotopes

Tc99m → Scintigraphy.

F18-FDG → FDG PET

Scintigraphy

Detector: Gamma camera.

Hotspot → Increased uptake, appears black.

Coldspot → Decreased uptake, appears white.

SPECT (Single-photon emission CT) Sestamibi (3D) >> Tc 99 MiBi Scan (2D)

3D scintigraphy

Type of CT imaging.

Provides better resolution than planar scintigraphy.

Best type: SPECT Sestamibi

P → Photon
Localises parathyroid gland.
Sestamibi SPECT has better resolution than Sestamibi alone

HMPAO SPECT → Used for cerebral perfusion.

Mnemonic: Hambaoo → dance → blood perfuse into brain

PET (Positron Emission Tomography)

Uses positron-emitting radioisotopes.

Organ-specific scans are available depending on clinical need.

Parathyroid

For hyperparathyroidism, Sestamibi scan/Tc99 MiB1 scan is used.

History of stones/moans/groans.

Thyroid Scan

Half-life of Isotopes:

Iodine 123:

Half-life: 13.2 hours.
Use: Diagnosis.
Mnemonic: Hours are going like 1, 2, 3 → 13.2 hrs

Iodine 125:

Half-life: 60.14 days.
Use: Implant Brachytherapy

e.g., Carcinoma prostate

Mnemonic: 1 + 5 = 6; 12 x 5 = 60

Iodine 131:

Half-life: 8.04 days.
Use: Radioablations.
Therapeutic effect is due to β rays.

Emits β plus gamma rays.
Not β alone

Mnemonic: 3 → 8 → β

Renal Scans

Technetium-99m

DMSA:

Mnemonic: Morphology → Scar → Static
Used for kidney scars or morphological changes.
Detects if the kidney is located ectopically.
It is a static scan.
no GFR or tubular function

DTPA:

Gives the function/physiology of the kidney (GFR).

MAG3:

Mnemonic: Magnificient
Helps in evaluating GFR and tubular function.

Bone Scan

Tc99m is used in Scintigraphy.

Half-life: 6 hours.
Mnemonic: 9 → 6
Emits gamma rays.

In bone scans, it is bound to MDP (Tc99m - MDP).

MDP: Methylene Diphosphonate
Bone → bisphosphonate → diphosphonates

Bone Scintigraphy Uses

Hot spots

↑ uptake = new bone formation = Osteoblastic

Fracture → new bone callus
Osteomyelitis → periosteal reaction
Osteoblastic metastasis

Cold spots

↓ uptake
Multiple myeloma → no new bone formation

Not useful for Lytic metastasis

Better detected with:

PET (sodium fluoride)

Mnemonic: Pet (PET) nu bone pain (Bone lysis) varumbo Sofayil (Sodium fluoride) kidathum

MRI

Superscan

Diffuse ↑ uptake → Very high bone uptake

No uptake in soft tissues or kidneys

Non-visualisation of kidneys

Causes:

Diffuse osteoblastic metastasis

e.g., prostate cancer
osteoblastic breast cancer

Metabolic disorders

e.g., hyperparathyroidism

especially secondary hyperparathyroidism with osteosclerosis

Tc99 Pertechnetate

Tc99 Pertechnate scan

Warthin’s

Meckel’s → 2 mucosa → CHORIOSTOMA

Scan of choice
Detects ectopic gastric tissue
Pancreas
Stomach

Pertechnetate is taken up by:

Thyroid
Stomach
Salivary gland

Thyroid Cancer:

Shows decreased uptake (cold nodule)

Salivary gland tumors:

Show cold spot

Exception:

Warthin's tumour → hot spot

Focal Nodular Hyperplasia (FNH) → hotspot

Warthin → Is on a war → hot

Radioisotope	Key Findings / Notes
Tc99m-MDP (methylene diphosphonate)	Bone Scan Hot Spots: Mets, Bone tumors, Metabolic bone disease. Cold Spots: Multiple Myeloma.
Tc99m-HIDA	Acute Cholecystitis Bile leaks: Sensitive (fail to localise the site). ↳ To rule out EHBA Gold standard: Intra-op Cholangiography.
Tc99m Sestamibi	PTH Adenoma
Tc99m Sulphur colloid scan	Hot Spot ↳ Kupffer cells → Focal Nodular Hyperplasia (FNH) • Sulphur - Kupfer
Tc99m pertechnate	* Meckel's Diverticulum * Warthin's tumor
Tc99m DMSA	Static morphology (Scar)
Tc99m DTPA / MAG3	ObStruction → Functional / Dynamic

Sulphur colloid scan

Hot spots in liver lesion:

In FNH (Focal Nodular Hyperplasia)
Not Hepatocellular carcinoma (HCC)

Reason

Sulphur colloid taken up by Reticuloendothelial system (Kupffer cells)
HCC → hepatocytes present, no Kupffer cells
FNH → has Kupffer cells, shows uptake

Other areas showing uptake

Spleen → Splenosis identified
Macrophages and occult abscesses

Cardiac Scans

Preferred Scan	Purpose	Key Findings
Thallium scan	Ischemia / Perfusion	• Ischemic areas → no Thallium uptake Stress and Rest test: ↳ ischemia induced → no uptake • At rest → uptake present • ie, Reversible defect → ischemia
Tc99 Pyrophosphate scan	Infarct detection	• Infarct shows hotspot • Mnemonic: Hot → Fire → Pyrr
MUGA scan ↳ Multiple Gated Acquisition	LV function	• Evaluates LV function (Ejection fraction)
Cardiac MRI	LV function test	• Most accurate • Gold standard for LV function
PET scan	Cardiac viability	• Differentiates hibernating myocardium vs scar/infarct • Compares metabolism & perfusion

HIDA Scan

Tests for the biliary tract

HIDA shows uptake/excretion in biliary tract

HIDA enters biliary tract
Blocked gallbladder → HIDA will not enter
Leads to non-visualisation of gall bladder
HIDA = nuclear scan for biliary tract

Inference

Presence in gallbladder, biliary tract, duodenum, intestine

Patent biliary tract

Biliary atresia:

HIDA not seen in biliary tract

Sulphur colloid scan findings:

Hot spots in liver lesion:

In FNH (Focal Nodular Hyperplasia)
Not Hepatocellular carcinoma (HCC)

Reason

Sulphur colloid taken up by Reticuloendothelial system (Kupffer cells)
HCC → hepatocytes present, no Kupffer cells
FNH → has Kupffer cells, shows uptake

Other areas showing uptake

Spleen → Splenosis identified
Macrophages and occult abscesses

MIBG

Used for pheochromocytoma

IOC for

Adrenal Pheochromocytoma → MRI
If not adrenal → Whole-body MIBG scan

DOPAPET can also be done

PET Scan

IOC → for mets

Requires a positron-emitting radioisotope

Tc99m cannot be used → emits gamma rays

Dedicated positron emitter used:

F18

Most common radioisotope
Half-life: 110 minutes = 2 hours

Most common ligand:

FDG (Deoxyglucose)

FDG (Deoxyglucose) & Cancer

Basis: Warburg effect → increased glucose uptake in cancer cells

False Results

False negative:

Low-grade tumors

Low-grade typical carcinoid
Bronchoalveolar carcinoma
Mnemonic: Typical car () nte backil (BAC) → False negative pet ooddi

High blood glucose

Cancer cells take glucose instead of FDG
Blood glucose should be normal before the test

False positive:

Infections → TB
Inflammations

Physiological Uptake (Normal high uptake sites)

Excretory system

Urinary bladder
Kidneys
Urinary tract

Bilateral supraclavicular areas (brown adipose tissue)

Principle of PET

Annihilation coincidence circuit

Mnemonic: Put PET in AC
PET radionuclide emits a positron (β⁺)
Positron travels a short distance in tissue
Positron meets an electron from normal body tissue
Positron + electron → annihilation
Entire rest mass converts to energy
Rest mass energy of one electron or positron = 511 keV
Energy is released as two gamma photons
Each photon has 511 keV
Total energy released = 2 × 511 keV = 1022 keV
Photons move in opposite directions (180°)
PET detects both photons at same time
Dual photon peak at 180°

When FDG is given:

F18 emits a positron.
The positron reacts with an electron in the body and they annihilate each other.
They coincide on the detector.
If they coincide at the same time, they are detected.
This is called a coincidence circuit.

PET-CT

Fusion / hybrid imaging

Used for:

Cancer staging
Assessing treatment response
Detecting recurrence

Radiotherapy

Use of radiation for treatment

Most common radiation: X-rays

Linear accelerator produces X-rays

Most common machine: LINAC

Types of Radiotherapy

Teletherapy

Source at a distance

Machines:

Cobalt machine (uses Cobalt-60)

LINAC

produces X-rays → M/c used

electrons

Brachytherapy

"Brachy" = short distance

Radiation source → close / on / in the body / in tumour

↓↓ surrounding organ damage

Not effective in large tumors

In the body

Implant brachytherapy

Temporary
Permanent (short half-life)

Mnemonic CIGYRP

Systemic Radiotherapy

Oral / Injections

Example: Radioiodine ablation of thyroid using I 131

I 131: β + gamma emitter
Uses β-emitting isotopes

β stays in the body

Isotopes in Radiotherapy

Type	Examples / Notes
Pure β emitters (Mnemonic: PSY)	Phosphorous 32 (P32) Strontium (for bone metastasis) Yttrium
Permanent brachytherapy implants (Mnemonic: CIGYRP → CIGAR P)	Cs 131 (NOT 137) I 125 Gold Yttrium Radon 222 Palladium
Mixed β + γ emitter	I 131

Remote Afterloading:

A concept in Brachytherapy.

A robotic system places the implant into the patient's body.
This avoids unnecessary radiation exposure to the person handling it.

Agents Used for Radiotherapy

X-rays and gamma rays:

Significant skin deposition → skin redness.

Electrons:

Not much penetration

Used for skin tumours like Cutaneous T cell lymphoma and Mycosis fungoides.

Protons:

Cyclotrons → used to make Protons

Provide targeted therapy.

They are directed at the tumour, not the skin.

Proton → Bragg Peak

vs Xray superficial deposition graph

A proton beam is targeted on the tumour.

Bragg peak:

Focal, local increase in radiation deposition

Peak deep into tissue

Usually as distance ↑↑ → peak ↓↓

Pencil beam targeted therapy can be given with protons.

LINAC

Has an electron beam;

output particles are X-rays.

Can rotate around the patient.

It is a Linear accelerator.

Important Radioisotopes in Radiotherapy

Mnemonic: Io Iri Co cesi ra

Radioisotope	Half-life	Note
Radium 226	1600 years	First radioisotope used in humans.
Cesium 137	30 years	ㅤ
Cobalt 60	5.26 years	Cobalt machine is used for Teletherapy and Brachytherapy.
Iridium 192	74.2 days	Used for Brachytherapy
Iodine 125	60.2 days	ㅤ

NOTE

Half-life of Tc99m: 6 hours.
Half-life of F18: 110 minutes.

Stereotactic Radiosurgery (SRS)

Gamma knife (Stereotactic Radiosurgery (SRS))

A type of Stereotactic Radiosurgery (SRS).

Gamma rays are focused to ablate the lesion.

Invented by neurosurgeon Lars Leksell.

Uses the Leksell frame.

Localized based on coordinates.

Use Cobalt 60

Used for localized cancers

not with metastasis

Used for

Brain lesions:

Vestibular schwannoma.
Trigeminal neuralgia.
AV malformations.
Pituitary microadenoma.
brain lesions that are inaccessible by surgery.

Cyberknife:

Stereotactic Body Radiotherapy

Uses LINAC and X-rays

Advantage:

Can be used for the rest of the body

(unlike Gamma knife for the brain only).

No frame is needed.
Localised inaccessible lesions
Cannot use for those with mets

Ex: Inoperable Stage 1 Lung Tumor

Mnemonic: Exes (X ray) Stand in Line (Linac) → and Cyberattack (Cyberknife) → Dont frame (No frame)

Prophylactic Craniospinal Irradiation

Indications: SMALL

Cancer with high chance of spreading to spinal cord.

Medulloblastoma
Ependymoma
ALL
Small cell carcinoma Lungs

First hormone deficiency after head & neck radiation:

Growth hormone deficiency.

Thyroid cancer due to radiation:

Papillary carcinoma.

Fractionated Radiotherapy

Dose divided into multiple fractions.

If one fraction fails, another may kill cells.
Reoxygenation occurs after 1st dose;

oxygen is essential for radiotherapy action.

Types:

1. Conventional/Regular fractionated

One fraction /day → for 5 days

2 days gap

Dose: 1.8–2 Gray/fraction

2. Hyperfractionated

Two fractions/day.

Without gap

Aggressive tumors ⇒ Small cell lung cancer

3. Hypofractionated

All Radiation at once

For bone mets

Radiosensitivity and Radioresistance

Law of Bergonie:

Actively dividing cells → radiosensitive.
Non-dividing cells → radioresistant.

Radiosensitivity by tissue/cell/organ/cycle:

Category	Most Radiosensitive	Most Radioresistant
Cell type	Undifferentiated/ rapidly dividing	Quiscent/ Not dividing
Cell cycle phase	Mitosis (G2/M)	S phase
Organ	Gonads	Vagina (pelvic organ)
Tissue	Bone marrow	Neurons
Blood cell	Lymphocytes	Platelet
Eye	Lens	Sclera
Tumor	Ewing Sarcoma	Osteosarcoma
Testicular tumor	Seminoma	ㅤ

Radiosensitive Tumors

Mnemonic: WELMS

Wilms' tumor.
Ewing's tumor.
Lymphoma.
Multiple myeloma.
Seminoma (Testicular).

Ovarian counterpart to seminoma: Dysgerminoma.

Radioresistant Tumors

Mnemonic: MORPH

Melanoma.
Osteosarcoma.
RCC.
Pancreatic cancer.
HCC.

Inverse Square Law

Distance vs radiation intensity.

As distance ↑↑, radiation intensity ↓↓

If distance is doubled → radiation is 1/2² = 1/4th.
If distance is tripled → radiation is 1/3² = 1/9th.