Physics Exam 2

SPecial Procedures

What is intra-fraction motion?

• motion that occurs throughout the fraction

What is inter-fraction motion?

• motions or variations in positioning between fractions (daily imaging accounts for this)

Types of treatments that most commonly use motion management due to respiration

1. Lung

2. breast

3. esophagus

4. liver

5. pancreas

What are the five methods of motion management?

1. abdominal compression (paddle/belt system)

2. integration of motion management in treatment planning (4D gating)

3. gated treatments

4. breath hold

5. tracking system (SGRT)

What is abdominal compression?

• compression of the abdomen and diaphragm that results in a forced shallow breathing technique

• this limits the amount of air a patient can inhale, therefore reducing how much the tumor can move

Types of systems used for abdominal compression

• paddle system

• compression belt system

• body frames with a vacuum bag

Where are compression systems usually placed?

• right below the xiphoid process (pointy end)

What is 4D scanning?

• A type of scan that uses over sampled CT to capture scans at all points within a breathing cycle

What is the benefit of a 4D CT scan?

• It shows tumor motion throughout the entire breathing cycle, improving target accuracy during planning.

How does pitch relate to the scanning process in 4D CT gating?

• pitch is the table movement per rotation.

• A pitch less than 1 means the same location is captured in multiple slices.

How does cine relate to the scanning process in 4D CT gating?

• allows for longer imaging per slice and captures multiple breathing cycles

What two ways is 4D CT scan used to create a patient’s treatment plan?

1. Internal target volume (ITV)

2. Gated treatment

What is Internal Target Volume?

• defining an internal target volume so that the area treated encompasses the tumor’s full range of motion throughout the entire breathing cycle

• no matter where tumor move it will be treated to the prescribed dose

When is the ITV used?

• when there is significant internal motion of the target (lung, liver, pancreatic)

2 advantages of using ITV for treatment

• Gating is not required (shorter treatment time)

• includes the entire range of motion of the tumor, reducing the risk of missing the tumor during treatment.

2 Cons of using ITV for treatment

• may include more normal tissues that are absolutely necessary

• increased dose to healthy tissue

What is Gated treatments (2)?

• Treatment where the patient is only treated during certain phases of the respiratory cycle

• treatment is only delivered when the target is in the treatment volume

In Gated treatments, how is the respiratory cycle tracked?

• tracked using external devices that correspond with internal motion

3 Pros of gated treatments

• has smaller margins

• less irradiation to healthy tissue

• more accurate targeting of the tumor

3 Cons of gated treatments

• Longer treatment times (biggest disadvantage)

• Dependent of patient’s breathing pattern

• increased patient cooperation required

Difference between gated treatments and breath hold treatments

• gated treatments may be done with the patient breathing normally (beam will only turn on during specific breathing phases), while breath-hold treatments require the patient to hold their inhale or exhale (esophagus)

What is SGRT used for?

• uses multiple cameras to create a 3D reconstruction of the patient’s surface that is linked to the coordinate system of the treatment machine

5 Pros of SGRT

1. real-time patient monitoring

2. improves positioning accuracy

3. reduces set up errors

4. reduces the need for permanent marks

5. helpful for breath hold treatments

2 Cons of SGRT

• Does not give us precise information about the internal anatomy

• requires patients to be exposed throughout treatment

What are the two methods of non-respiratory motion management

• electromagnetic (EM) transponders

• gold fiducials

What are electromagnetic (EM) transponders?

• glass-encapsulated circuits placed in the tumor/target and tracked by an electromagnetic array localization system

• mainly for the prostate

What are gold fiducials?

• seeds placed within the target and can be localized using CBCT and intrafraction KV images

Benefits of using gold fiducials

• use images to track fiducials thoughout treament

• helps account for motion of these seeds and ensures accurate delivery

• if seeds move, you can stop radiation and realign to the fiducials before resuming treatment

What 3 treatments use non-respiratory motion management?

• prostate

• lung

• liver

What is Stereotaxis?

• A technique in neurosurgery and radiation therapy that uses a 3D Cartesian coordinate system to accurately locate brain regions

• uses a fixed external frame (fiducials) to match internal targets.

Explain the differences between SRS, SRT, and SBRT.

• SRS treats brain conditions in a single session.

• SRT treats brain conditions in 2-5 fractions.

• SBRT targets extracranial tumors (lung, prostate, spine, liver) in 1-5 fractions.

What is the accuracy tolerance for SRS/SRT treatments?

• 1 mm accuracy

4 Systems that are used for SRS/SRT/SBRT treatments

• GammaKnife

• Linac

• CyberKnife

• gamma pod

How does GammaKnife deliver stereotactic treatments?

• uses Co-60 sources that deliver radiation via non-coplanar gamma ray

• used to treat brain, skull, C-spine (1 TX)

• may headframes or is frameless

Explain the use of a head frame for stereotactic simulation and treatments (3)?

• A stereotactic head frame is a rigid frame attached to the skull

• Completely immobilizes the head and provides a precise 3D coordinate system.

• It allows doctors to target brain lesions with very high accuracy while protecting normal tissue.

How does the linac deliver stereotactic treatments (4)?

1. uses a flattening-filter-free (FFF) beam to deliver a significantly higher dose rate that is non-flat and more forward-peaked.

2. requires high-precision immobilization

3. CT sim scan should be thin slices (less than 2.5 mm)

4. For SBRT their is an addition of motion management techniques and PET/ MRI

What is the purpose of SRS cones?

• Used when lesions are extremely small

• SRS cones bring the collimator diaphragm closer to the patient, increasing the source-to-diaphragm distance and decreasing geometric penumbra.

How do flattening-filter-free (FFF) beams change the treatment (3)?

• uses a pencil photon beam that is non-flat (significantly high dose)

• reduces peripheral dose with sharper penumbra

• reduces treatment time while allowing for a higher overall dose

How does CyberKnife deliver stereotactic treatments (3)?

• A radiosurgery system that can treat anywhere in the body, but mainly SRS

• uses a 6 MV linear accelerator with a robotic arm and 2 ceiling mounted x-rays to deliver small beams at predefined positions

• imaging obtained in real time to adjust for patient motion

What is GammaPod (4)?

1. used for early-stage breast cancer as a form of APBI

2. uses Co-60 sources

3. patient in prone, breast goes in the vacuum cup and treated with multiple beam angles

4. 1- 5 fractions

What is HyperArc?

• advanced automated SRS/SRT that uses noncoplanar VMAT and a single isocenter to deliver quick high dose treatments at varied couch angles

• better for multiple brain mets

Gantry and MLC movement for Hyperarc

• The gantry is continuously rotating in noncoplanar arcs while the MLCs continuously modulate

Gantry and MLC movement for Dynamic Conformal Arcs (DCA)

• MLCs are stationary and conform to the shape while the gantry moves throughout treatment

• used in place of VMAT (used for small single lesions)

QA procedure for SRS/SRT treatments

• Winston-Lutz test

What is the Winston-Lutz test and how does it work (3)?

• a QA test that evaluated the integrity of the connection between the imaging system used to plan the patient’s treatment and dose delivery (test the x, y, z system)

• Images are collected at different gantry, collimator, and couch angles (verify Iso)

• Data analyzed to determine accuracy of isocenter placement (1mm)

10 Major differences between tomotherapy and C-arm linear accelerators

1. in tomotherapy the linac is mounted on a CT like gantry (rotates in full circle around pt)

2. the 6D couch moves slowly through the aperature

3. Both the gantry and couch move throughout treatments

4. beam is delivered in a helical motion

5. tomotherapy uses same MV sources for daily imaging and treatment

6. tomotherapy allows for daily CT images to guide treatment

7. tomotherapy can irradiate up to a 160 cm long field (treat multiple tumors in one plan without repositioning)

8. No possibility of collision

9. CT gantry can move faster than a c-arm gantry allowing treatment to a higher dose rate

10. can only treat using a single field (6 MV)

For image-guided RT, what type of imaging was used in the field historically?

• Ultrasound-guided imaging for localization of soft tissue structures in the abdomen, pelvis, and breast

• kV units mounted on Cobalt-60 machines

For image-guided RT, what types of imaging are currently used in the field?

• Portal images and Radiographic images (kV x-ray imager and MV electronic portal image device)

• Cone beam CT (CBCT)

Difference between kV images and MV images

• kV images have better contrast, while MV images allow for imaging with MLCs in treatment position (portal verification)

What is adaptive radiation therapy?

• a form of radiation therapy that tracks changes in the tumor and surrounding anatomy, reoptimizing the treatment plan along the course of treatment

What does adaptive radiation therapy account for?

• daily changes in tumor size, shape, or position caused by:

  • tumor shrinking

  • weight loss

  • daily fluctuations

What is Offline ART?

• adjusting the treatment plan between treatment sessions (most common); DAYS

What is Online ART?

• adjusting the treatment plan before the treatment session; patient is on the table; quick (minutes)

• relies more on computational algorithms and automation

What is Real-time/dynamic ART (3)?

• adjusting the treatment plan continuously throughout treatment (seconds)

• involves imaging consistently throughout treatment

• relies more on computational algorithms and automation

3 Advantages of Adaptive Radiation Therapy

• more conformal targeting of tumors

• reduces dose to healthy tissue (smaller margins)

• minimizes side effects

What is MRI-guuided RT?

• form of RT that uses MRI linacs to deliver more conformal, high-dose volumes

• imaging completed before and continuously throughout treatment

3 Pros of MRI-guided RT

• better visualization of tumors and nearby organs

• allows for dose escalation (few sessions)

• margin reduction (sparing healthy tissue)

4 Cons of MRI-guided RT

• longer treatment times (30-45 mins)

• expensive

• magnetic field limitations

• Poor visualization of bone

For what type of treatments do MR linacs provide benefit?

• tumors in or near soft tissue (abdomen, pelvic, brain, head and neck)

• tumors in or near mobile tissue or those susceptible to respiratory movement

What type of adaptive RT is used with MR linacs?

• Online ART

What are the MRI images used for prior to treatment (3)?

• localize the tumor

• contour target/OAR

• assess daily anatomical changes (online ART)

What are the MRI images used for throughout to treatment (3)?

• real-time tumor tracking

• monitoring organ motion

• image based respirtory gating/motion management

What are the general parts of a MRI machine (3)?

• large magnet and coil system

• couch

• computer processing system

The MRI bore is surrounded by 3 coils, what are they?

• radio-frequency coils

• imaging coils

• electromagnetic coils

MRI coils are cooled using what?

• liquid nitrogen

• helium

Magnetic field for diagnostic MRI is ___tesla; for MR-GRT is ____

• diagnostic MR: 1.5T-3T

• MR-GRT: 0.5T-1.5T (large bore)

For MRI, what 6 things do we need to screen for?

1. biomedical implants

2. ferrous foreign bodies

3. ancillary equipment (oxygen tank, IV pole)

4. medical conditions (pregnancy, claustrophobia)

5. diagnostic or surgical procedures

6. topical or externally applied items

In MRI screening, what are 6 examples of topical externally applied items?

1. tattoos

2. medication

3. patches

4. body piercing jewelry

5. monitoring devices (pacemaker)

6. clothing

For MRI screening, why do we screen for pregnancy?

• Although MRI does not use ionizing radiation, the use of

  gadolinium contrast can be harmful to the fetus.

What is Zone I in MRI?

• area accessible to the general public

What is Zone II in MRI?

• more supervised area where patients are screened

What is Zone III in MRI?

• area restricted to MRI personnel outside of the treatment room (console area)

What is Zone IV in MRI?

• scanning/treatment room

What is thermal safety in MRI?

• where RF energy is converted into heat in the body

• if specific absorption rate is too high patients can experience burns (due to weight, MRI type, area scanned)

Two ways thermal safety is maintained in MRI

• large conductive loops should be avoided; no crossing arms or legs

• electrically conductive materials should be removed before scanning

Why should hearing protection be worn during scanning in MRI?

• The scanner produces very loud noises, which can be damaging

What is tracked for localization in the treatment of PET-guided RT?

• PET signals emitted by beamlets that act as fiducials.

What is injected daily for treatments in PET-guided RT?

• Fluorodeoxyglucose (FDG)

For intraoperative RT, when is radiation delivered?

• directly to the tumor/tumor bed during surgery (may be used when entire tumor can’t be surgically removed)

What is the main benefit IORT?

• allows a high dose of radiation to be delivered directly to the tumor bed during surgery, while normal surrounding tissues are moved or shielded, reducing radiation exposure to healthy structures.

• improved therapeutic ratio

What types of radiation are used in intraoperative treatments?

• electrons

• HDR brachytherapy

• orthovoltage x-rays

The most commonly used type of particle therapy is ___

• proton therapy

5 Advantages of using proton therapy

1. Bragg’s Peak effect allows maximum dose at the tumor depth

2. low integral dose

3. It has a low entrance dose and no exit dose

4. can escalate the dose with fewer side effects.

5. narrow lateral penumbra

3 Disadvantages of using proton therapy

• Proton therapy requires long treatment times because the proton beam can only be delivered to one treatment machine at a time.

• QAs for proton therapy are more complex.

• more expensive

5 Parts of a cyclotron

1. ionized hydrogen source

2. spiral path (dees)

3. magnet

4. radiofrequency gaps

5. oscilator

4 Parts of a synchrotron

1. ionized hydrogen source

2. circular path

3. magnet

4. radiofrequency cavities

4 Similarities between a cyclotron and synchrotron

• Both accelerate charged particles

• Both use magnets and magnetic fields to bend keep particles in path

• Both use electric fields (RF cavities, synchrotron; gaps in dees cyclotron) to accelerate particles.

• both produce proton beams

6 Characteristics of a cyclotron

1. Has a spiral path

2. creates a continuous beam with a fixed energy level;

3. requires external degraders/energy selectors to modulate energy

4. Used for protons and neutrons

5. More compact in size

6. produces secondary neutrons due to degrader/selector

6 Characteristics of a synchrotron

1. Has a circular path

2. Creates a pulsed beam with variable energy levels

3. Doesn’t need a degrader; uses a shaper beam

4. used for protons and carbon

5. Larger and more complex

6. lower levels of secondary neutrons (no energy selector/degrader)

What is the Braggs peak effect?

• a sharp increase in radiation dose at the desired tumor depth, followed by a rapid drop off.

What is the main therapeutic issue with the Bragg Peak effect?

• The peak has a very narrow range (3mm to 20 mm); as a result, it doesn’t cover the entire tumor.

What are two ways to fix the Braggs peak effect issue?

• creating Spread-out Bragg peak (SOBP) using passive scattering or an active scanning

What is the passive scattering approach, and how does it fix the Bragg’s peak issue (3)?

• Uses mechanical devices like rotating modulator wheels to spread out the peak range (SOBP)

• Creates a uniform dose

• The beam is then shaped by MLCS or a brass aperture (neutron contamination)

What is the active scanning approach, and how does it fix the Bragg’s pea’s peak issue (2)?

• uses magnets to vary the proton beam's energy, allowing the tumor volume to be “painted” to different depths and locations (pencil beam scanning).

• enables intensity modulated proton therapy (IMPT)

The energy range for proton therapy is approximately

• 50-250 MeV (monoenergetic)

Two reasons why proton therapy is beneficial for pediatric patients?

• It has a reduced integral dose, reducing the risk of secondary cancers later in life

• Bragg peak effects spare healthy tissue, lowering the risk of growth defects, cognitive effects, or organ damage.

Compare integral dose and RBE for protons vs photons

• Photons: high intergral dose; low RBE integral

• Protons: low integral dose; high RBE (1.1)

4 Treatment planning and delivery considerations for protons

1. Immobilization devices have to be uniform (braggs peak can be altered by slight internal/external changes)

2. anatomic changes, such as weight loss, may require replanning

3. must account for tissue density changes

4. prostate localization (endorectal balloon is more reliable than bladder filling)

4 Quality assurance (QA ) of protons

• more intensive (still has safety and mechanical tolerance checks)

• requires daily energy-dependent checks and beam range verifications

• PT with active scanning requires checking location, size and depth of thousand spots

• may require daily patient specific QA (highly sensitive to tissue and density variations)

What is FLASH- RT?

• involving ultra-high dose rate proton or electron radiation to a target

• delivered in bursts or “flashes” lasting less than a second